Development of a Miniature, Continuous Measurement,
Stochastic Perturbation Gas Chromatograph
by
MASSACHUSETTS INSTITUTE
OF TECHNOLOGY
Eli Paster
AUG 15 2014
B.S., University of Colorado at Boulder (2004)
S.M., Massachusetts Institute of Technology (2010)
LIBRARIES
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Mechanical Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2014
@ Massachusetts Institute of Technology MMXIV. All rights reserved.
Signature redacted
A u th or . . . . . . . . . . . ....
.. ...
.. .. ...
.. .. . ... . . . . ... .. . . . . . . . . . . . .. .. ...
Department of Mechanical Engineering
March 31, 2014
Signature redacted
C ertified by .......................
............
Ian W. Hunter
Hatsopoulos Professor of Mechanical Engineering
J)Thesjs
Supervisor
Signature redacted'
A ccepted by ............................
......
David E. Hardt
Chairman, Department Committee on Graduate Theses
Development of a Miniature, Continuous Measurement,
Stochastic Perturbation Gas Chromatograph
by
Eli Paster
Submitted to the Department of Mechanical Engineering
on 31 March, 2014 in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy in Mechanical Engineering
Abstract
Gas chromatography is one of the most widely used analytical chemistry techniques for
separating and analyzing chemical compounds. Chromatographic methods are used to identify
constituent species within a compound and determine the purity and relative concentrations
of those species. Current gas chromatographs are heavy, bench top instruments that require
large capital expenditures, kilowatt power sources, and trained technicians. Additionally,
traditional chromatographic measurements are non-continuous.
The first part of this thesis explores the application of stochastic system identification
techniques applied to chromatography to enable continuous chromatographic measurements,
multiplexing of instrument components, and the ability to optimally tune instrumentation
parameters and reduce chromatogram noise. The second part of this thesis explores the
development of a miniaturized, standalone gas chromatograph. A handheld, low-cost gas
chromatograph has been developed over the course of five device generations, through the
implementation of localized heating techniques, on-demand gas generation, and the
integration of electrical, mechanical, and chemical processes into a compact volume.
Characterization of the device shows comparable operating parameters and performance to
equivalent bench top instruments at 0.5% total cost and 0.03% total volume. These
contributions reduce the barrier-to-entry for performing high quality chemical measurements,
and enable more widespread use of chromatography in monitored, closed-loop, remote
operation and automated systems.
Thesis Supervisor: Ian W. Hunter
Title: Hatsopoulos Professor of Mechanical Engineering
2
In Loving Memory of Zhang Mingsheng
1941 -2012
3
Acknowledgements
Doctoral work is a roller-coaster ride, filled with ups and downs, successes and failures,
and insights and blunders that in the end, give rise to a coherent understanding. I am
grateful to have encountered so many encouraging, brilliant, and supportive people along
this path.
My thesis committee, composed of Professor Ian Hunter, Professor Steven Leeb, and
Professor Timothy Swager, has been exceptional. Their versatility, their recommendations,
their insights, and their guidance have been an integral part of this work, and beyond.
From the BioInstrumentation Lab, I would like to thank Ian Hunter, who has created
an environment and an ethos in which the exploration of non-traditional approaches to
traditional systems is the norm, and where it is expected and encouraged to challenge the
boundaries of what is possible.
I would like to thank Jean Chang, Ellen Chen, Cathy Hogan, Brian Hemond, Adam
Wahab, Ashin Modak, Mike Nawrot, Alex Ohayon, and Alex Gabella for their
camaraderie as friends and fellow researchers. Additionally, I would like to thank Kate
Melvin, Leslie Regan, and Joan Kravit, who work behind the scenes to keep the lab and
the department running like clockwork.
Outside of MIT, I would like to thank Dr. Jubal Hamernik, who has without
hesitation offered guidance, allowing me to successively complete my academic studies. I
would also like to thank Zorba and Penny Paster, and Dan and Kay Barry for their
support.
Finally, for all of the late nights, the weekends, the times when research took
precedence, and the unwavering support, I thank my wife, Zhang Xia: per aspera, amor
omnia vincit.
4
Contents
Chapter 1.................................................................................................................14
Introduction.............................................................................................................14
1.1 M otivation...................................................................................................
14
1.2 Outline of Thesis .........................................................................................
17
1.3 Developm ent of Gas Chromatography.........................................................
17
1.4 Instrum entation and Processes ....................................................................
21
1.4.1 Instrum entation ....................................................................................
21
1.4.2 Separation.............................................................................................
23
1.4.3 Detector Types.....................................................................................
28
1.5 Stochastic Perturbation Chrom atography ..................................................
30
Chapter 2.................................................................................................................37
Chrom atographic Sim ulations...............................................................................
37
2.1 Simulation Constructs ................................................................................
37
2.2 Traditional Chrom atography Sim ulation Results ........................................
43
2.3 Stochastic Perturbation Chromatography Simulation Results....................46
5
2.3.1 Stochastic Signal Generation and Implementation................................
46
2.3.2 Reducing Noise.....................................................................................
49
2.3.3 M ultiplexing Possibilities ......................................................................
51
Chapter 3.................................................................................................................
56
Bench Top Implementation .................................................................................
56
3.1 Instrumentation...........................................................................................
56
3.2 Results............................................................................................................62
Chapter 4.................................................................................................................65
Design Considerations ...........................................................................................
65
4.1 Design Elements .........................................................................................
65
4.1.1 Injection Port.......................................................................................
66
4.1.2 Column .................................................................................................
70
4.1.3 Detector .................................................................................................
71
4.1.4 Gas Sources and Flow Control.............................................................
73
4.1.5 Sample Injection System .......................................................................
75
4.1.6 Therm al Components...........................................................................
77
4.2 Summ ary .....................................................................................................
6
78
Chapter 5.................................................................................................................80
Prototype Development of Micro Gas Chromatograph (pGC) .............................
80
5.1 Generation I 1iGC.......................................................................................
80
5.2 Generation II pGC.......................................................................................
87
5.3 Generation III pGC .....................................................................................
98
5.3.1 Flame Ionization Detector Design.........................................................
99
5.3.2 Injection Port M iniaturization Design....................................................
102
5.3.3 Intermediate Fittings .............................................................................
106
5.3.4 Pressure Transducers .............................................................................
107
5.3.5 Column Configuration............................................................................
108
5.3.6 M echanical Layout.................................................................................
111
5.4 Generation IV pGC ......................................................................................
113
5.4.1 Im proved FID Design.............................................................................
113
5.4.2 M odular Heater Development ................................................................
116
5.4.3 Capillary Column Heating .....................................................................
118
5.4.4 Im proved Electrolyzer ............................................................................
122
5.4.5 Layout M odifications and PCB Integration ...........................................
124
7
5.4.6 Electronics .............................................................................................
128
5.4.7 Assembly................................................................................................
129
5.5 Generation V IiGC .......................................................................................
131
Chapter 6................................................................................................................
137
11GC Perform ance Characterization.......................................................................
137
6.1 Design and Specifications Summary .............................................................
137
6.2 Basic Flow and Operation ............................................................................
139
6.3 Therm al Performance ...................................................................................
140
6.4 Stochastic Perturbation Results ...................................................................
145
Chapter 7...............................................................................................................
147
Conclusion .............................................................................................................
147
References..............................................................................................................
150
8
List of Figures
Figure 1. Commercially available gas chromatographs ..............................................
20
Figure 2. Portable gas chrom atographs.......................................................................
21
Figure 3. Basic components of a gas chromatograph ..................................................
23
Figure 4. The chromatographic separation process ....................................................
25
Figure 5. Exam ple gas chromatograph......................................................................
26
Figure 6. Schematic representation of the traditional injection method used in gas
chrom atography ...................................................................................................
31
Figure 7. Schematic representation of stochastic system identification applied to gas
chrom atography ...................................................................................................
Figure 8. Exam ple Gaussian curve.............................................................................
33
38
Figure 9. Flame ionization detector signal simulations of benzene..............................44
Figure 10. Chromatography simulation of multiple chemicals ....................................
45
Figure 11. Random binary signal input..........................................................................
47
Figure 12. Simulated binary stochastic input and output ..........................................
48
Figure 13. Impulse response from binary stochastic mass flow perturbations ............ 49
Figure 14. Noise comparison of chromatographic methods........................................51
Figure 15. M ultiplexing possibilities...........................................................................
9
52
Figure 16. Superimposed FID responses from multiplexed samples. ..............................
54
Figure 17. The impulse responses of two samples injected simultaneously ................
54
Figure 18. Variance Accounted For of various stochastic perturbation chromatography
simu lation s..............................................................................................................55
Figure 19. Autom ated injection system .......................................................................
58
Figure 20. Bench top implementation of stochastic perturbation chromatography........60
Figure 21. Input and output signals from bench top gas chromatograph using stochastic
binary perturbation.............................................................................................
62
Figure 22. Single solute chromatogram, determined using stochastic perturbation
meth o d s ..................................................................................................................
63
Figure 23. Chromatogram of relative concentrations for two-analyte sample measured
using the stochastic perturbation method............................................................
64
Figure 24. Typical injection port types found on gas chromatographs.......................68
Figure 25. Full bridge thermal conductivity detector.................................................81
Figure 26. Suspended thermistor thermal conductivity detector................................83
Figure 27. Custom vertical alignment mount.................................................................84
Figure 28. Four cell thermistor-based TCD with PCB integration............................85
Figure 29. First generation pG C ................................................................................
Figure 30. Single solute (pentane) chromatogram, determined using stochastic
10
86
perturbation methods on the first generation pGC..............................................87
Figure 31. Typical, commercially available TCD filament ..........................................
88
Figure 32. Type 24E, 28 V bulb filament thermal response ........................................
90
Figure 33. Rotary attachment for bulb removal.........................................................91
Figure 34. Filament manufacturing process ................................................................
92
Figure 35. Copper block T CD .....................................................................................
93
Figure 36. Stacked TCD heating system....................................................................94
Figure 37. G eneration II pG C .....................................................................................
97
Figure 38. Thermal image of the second generation 11GC during heating ...................
98
Figure 39. Preliminary prototype of flame ionization detector.....................................101
Figure 40. Thermal image of flame ionization detector ................................................
101
Figure 41. Schem atic of injection port .........................................................................
103
Figure 42. Injection port com ponents...........................................................................
106
Figure 43. Flame ionization detector built from off-the-shelf fittings...........................
107
Figure 44. Quartz capillary heating configuration........................................................109
Figure 45. Quartz capillary heat distribution..............................................................
109
Figure 46. Transient heating and passive cooling curves for 330 pm quartz capillary
colum n surrounded by NiCr wire..........................................................................
110
Figure 47. Board m ounting com ponents ......................................................................
112
11
Figure 48. Third generation pGC mechanical layout ...................................................
112
Figure 49. Chromatogram from the third generation pGC ..........................................
113
Figure 50. Flame ionization detector schematic ...........................................................
114
Figure 51. Flam e ionization detector............................................................................
115
Figure 52. Injection port m odular heater .....................................................................
116
Figure 53. Concentric heater fabrication steps.............................................................
117
Figure 54. Photo and thermal image of guided coil heating scheme ............................
119
Figure 55. Photo and thermal image of Delrin-PTFE guided column..........................
121
Figure 56. Photo and thermal image of polyimide sheathed column............................
122
Figure 57. Eight cell polymer electrolyte membrane electrolyzer .................................
123
Figure 58. Eight cell polymer electrolyte membrane electrolyzer output pressure, shown
as a function of total input current.......................................................................
124
Figure 59. Fourth generation 1iG C ...............................................................................
125
Figure 60. PCB layout of fourth generation pGC ........................................................
127
Figure 61. Proportional-integral (PI) and pulse-width-modulated (PWM) heating
control schem e ......................................................................................................
129
Figure 62. Populated PCB from the fourth generation piGC....................
130
Figure 63. Final assembly of the fourth generation liGC .............................................
131
Figure 64. Populated PCB from the fifth generation pGC...........................................
133
12
Figure 65. PCB layout of fifth generation 11GC ...........................................................
134
Figure 66. Fifth generation pG C ..................................................................................
135
Figure 67. Front panel of LabVIEW program..............................................................
135
Figure 68. Chromatogram of a well-known American bourbon..................
136
Figure 69. Gas flow patterns for the fifth generation pGC ..........................................
140
Figure 70. Heating and cooling performance of the injection port ...............................
141
Figure 71. Injection port heating rates........................................................................
142
Figure 72. Closed loop performance of injection port temperature...............................
143
Figure 73. Colum n heating rates..................................................................................
144
Figure 74. Closed loop performance of column temperature ........................................
145
Figure 75. Fifth generation pGC noise signal...............................................................
146
Figure 76. Single solute (pentane) chromatogram, determined using stochastic
perturbation methods on the fifth generation pGC...............................................
146
Figure 77. Handheld, continuous measurement, stochastic perturbation gas
chrom atograph ......................................................................................................
13
149
Chapter 1
Introduction
1.1 Motivation
For millennia humans have observed their surroundings so as to better understand
the world in which they live. From the early botanical observations of ancient cultures
[1], to the scientific societies and discoveries of post-Renaissance Europe [2], we have
maintained an unending curiosity for knowledge and understanding.
In recent decades, the ushering of the digital era has enabled us to move into a
computational age of data-based decision-making and development. Traditional methods
of discovery are being uprooted by massive computational and statistical undertakings,
made possible in part by the ability to inexpensively create, organize, and analyze swaths
of distributed information from various sources. In areas ranging from medicine [31 to
agriculture [41, from education [5] to political strategy [6], we are reducing the cost and
increasing the accessibility of techniques previously limited to large-scale simulations [7].
Along similar progressive paths has been the democratization of technology, a trend
14
that has enabled scientific measurements and data collection to reach the hands of a much
greater audience [8] who may then aid in both the power of collection and the analysis of
their own immediate surroundings. Information has become a commodity [9], valued
equivalently to what was once reserved for precious metals and far away spices [10]. This
intrinsic value of information stems from enabling us to better decide, for ourselves, for
our societies, and for our future.
A key limit to introducing enabling technologies into society is the need to
significantly reduce their adaptation barrier-to-entry
by reducing the cost, size,
maintenance, and ease of use of such technologies. As high-tech instrumentation becomes
democratized, and technologies previously reserved only for cutting-edge scientific
laboratories become user-friendly, we will see a broader adaptation of these devices in
everyday life, bringing benefits not only to the individual, but also to communities,
countries, and global consortiums.
This work anticipates the societal trend toward data-driven distributed measurement
systems, by enabling a powerful means of performing chemical analysis on everything
from the gasoline that one pumps daily into an automobile, to the quality of water passing
through one's faucet or running through a nearby stream. This thesis describes the
development of a stand-alone, miniaturized, low-cost gas chromatograph and the
implementation of nontraditional perturbation techniques that enables continuous
15
chromatographic measurements.
Gas chromatography is used everywhere from analyzing wine and coffee [11], to
petroleum [121 and amino acids [13]. It is commonly implemented as a quality control
process in pharmaceutical manufacturing, chemical processing, and food science. For the
individual, one might analyze their favorite food flavors. For the farmer, one might place
low-cost gas chromatographs near their feedstock water supply, their crop supplies, and
their harvest storage to monitor pesticide levels. For extant processing lines with
controlled manufacturing methodologies, a miniature gas chromatograph may serve as a
form of automated quality control, so that manufacturing variances can be quickly
identified and remediated in real-time. A low-cost gas chromatograph may be a
commonplace in the doctor's office, where a simple exhalation of breath can provide an
early diagnosis of lung cancer [141.
Under these premises, this thesis seeks to reduce a gas chromatograph to a handheld
size, reduce the cost by designing a system that is suitable to mass manufacturing and
economies of scale, and enable high sensitivity capabilities along with the possibility of
implementing more advanced chemical analyses via stochastic system identification
techniques. These aims will enable gas chromatograph units to be employed within health
and home monitoring systems, and at locations and within industries that are otherwise
prohibitively limited by space, cost, or ease-of-use requirements.
16
1.2 Outline of Thesis
This thesis is divided into distinct, yet interrelated chapters. The introductory chapter
discusses a brief history of chromatographic development, a survey of the basic
instrumentation and chemical processes that govern the chromatographic process, and a
discussion of stochastic perturbation techniques as applicable to chromatography. Chapter
2 builds off previous models of gas chromatography by simulating stochastic perturbation
chromatography and investigating the benefits of this methodology. Chapter 3 describes
the first implementation of stochastic perturbation chromatography on a retrofitted bench
top gas chromatograph. Chapters 4, 5, and 6 describe the design considerations,
prototypical development, and characterization of a miniature gas chromatograph.
Conclusions and recommendations for further exploration in the field may be found in
Chapter 7.
1.3 Development of Gas Chromatography
The development of chromatographic methods dates back to the turn of the
2 0 th
century, when the British Nobel-Prize winning Chemist Sir William Ramsay1 published
1 Ramsey won the Nobel Prize in 1904 for the discovery of neon, argon, krypton, and xenon, and establishing
them as a new periodic category: noble gases.
17
a method for separating volatile gases and vapors by passing them through a column of
cooled cocoanut charcoal [15]. Around the same time period, Russian-Italian botanist
Michael Tsvet designed a method of liquid-adsorption column chromatography by
separating plant pigments into colored bands and correctly identifying the cause of
separation as an adsorption process [16]. Tsvet is credited with coining the term
chromatography (color writing) as is now used in analytical chemistry2
Over the next century, the fundamental principles of chromatography were expanded
into a variety of chromatographic instruments and techniques. A taxonomic scheme
typically used to classify these discoveries separates chromatography into planar and
column chromatography, referring to the geometry through which separation occurs
during a chromatographic process. Column chromatography is often sub-classified into
liquid and gas methods, based on the state of the carrier medium that passes through the
column. This work will focus on one of these sub-categories: gas chromatography.
Gas chromatography (GC), first published significantly in 1952 [17], is used to
determine the purity of a particular substance or the relative concentration of species
within a chemical mixture. Gas chromatographs also serve as a conduit for simulated,
small-scale distillation, enabling the separation of a sample before it is input into a mass-
2 The
word chromatography had previously been used as a reference to artistic techniques and painting.
18
spectrometer, a Fourier transform spectrometer, a thermogravimetric analyzer or other
analytical instruments for further analysis.
Commercial chromatographs have been developed over the past six decades,
burgeoning into a billion dollar industry [18]. Despite their widespread use, bench top gas
chromatographs still remain niche products that are rarely found outside of scientific or
industrial laboratories. One reason for their limited deployment is that the majority of
operating GC's require high capital expenditures, and need to be operated by a skilled
technician under controlled environmental conditions. Several commercially available gas
chromatographs are shown in Figure 1.
Over the course of GC development, major innovations have occurred through the
improvement of sensing and control techniques, the invention of new detection methods,
the advancement of column chemistry used for separation, the creation of multidimensional chromatography to improve the resolution of an instrument, and innovative
approaches to sample preparation. Automation has also significantly reduced operator
errors. Chromatographic column lengths can vary anywhere from 1 m to 100 m, and GC's
can be configured to operate using multiple detectors in both series and parallel, with
automated sample handing and injection, and programmed flow and temperature control.
For the current state-of-the-art, gas chromatography measurements can detect substances
down to a range of parts per billion (ppb).
19
Intswr
A
B
C
Figure 1. Commercially available gas chromatographs from: (A) Agilent, Inc.; (B) Perkin
Elmer, Inc.; and (C) Shimadzu, Inc.
Miniaturized chromatographs are far-less common than bench top versions. Smaller
gas chromatography units that have been developed previously (Figure 2) range in cost
from 2,000 to 50,000 USD. Portable systems have limited battery lifetimes, limited
operating parameters, and often lower sensitivities than bench top alternatives [19].
Additionally, these units often require an external gas tank, thereby increasing the mass
and volume constraints. For portable GC units that operate with air as the carrier gas,
the range of measureable analytes is restricted. A performance summary of both bench
top and portable GC's is shown in Table 1.
20
A
C
B
Figure 2. Portable gas chromatographs from: (A) Vernier Software & Technology, LLC;
(B) Inrag, AG; and (C) Inficon, Inc.
1.4 Instrumentation and Processes
1.4.1 Instrumentation
The basic components found in a gas chromatograph (Figure 3) include an injection
port, a column, an axillary gas supply, flow control hardware, a detector, a column oven,
and a data acquisition system. Samples typically enter the instrument through the
injection port via a microliter syringe. The injection port heats or vaporizes the sample,
after which it is flushed out of the injection port and through the column. The column of
a gas chromatograph is usually placed within a temperature-controlled oven. Upon exiting
the column, the chemical components pass through a detector and are identified based on
their retention times. Retention times will vary, depending on carrier gas flow rate,
column temperature, column-analyte reaction rates, and column length.
21
Table 1. Summary of performance specifications for bench top and portable gas chromatographs
Size
Mass
Resolution
Model
Operating
Carrier
Column
Gas
Type
Temperature
(C)
Detector
Power
Type
(mm)
(kg)
Perkin
Elmer
770 x 260 x 280
49
ppb
External
(various)
Various
50-450
Various
2 kW
Agilent
(7890)
860 x 580 x 540
45
ppb
External
(various)
Various
50-450
Various
3 kW
515 x 440 x 530
30
ppb
External
(GC-2010
PLUS)
Vaiu3040Vros
Various
30-450
Various
2k
2 kW
Vernier
108 x 133 x 191
MEMS
Chemicapacitive
72 W
MEMS
TCD
160 W
Shimadzu
PLUS)(various)
1.3
Hundreds of
Inficon
(3000
11GC)
155 x 364 x 413
16.6
10 ppm
Air
11m/ MXT-1
mn
30-120
Internal
(various)
Capillary
columns
(propriety)
30-180
Packed
Photovac
390 x 270 x 150
6.8
50 ppb
Internal
Columns
30-105
(various)
PID
90 W
ECD
(propriety)
Inrag
(IGraphX)
310 x 290 x 100
3.5
100 ppm
External
(various)
22
Capillary
columns
(propriety)
30-350
MEMS
TCD
60 W
cj
VF
C
Figure 3. Basic components of a gas chromatograph, including the: (A) carrier gas supply,
(B) injection port, (C) microsyringe, (D) column, (E) detector, and (F) data acquisition.
The boxed enclosure contains the column oven and other supporting hardware.
1.4.2 Separation
The key chemical process that takes place during chromatography is the separation
process. Separation occurs as the axillary gas supply, commonly referred to as the carrier
gas, transports the sample through the column. A chromatographic column is typically
coated with a thin film or packed with solid particles of a particular chemical substance.
These coatings or packed particles are referred to as the stationary phase. The
interaction between the mobile phase (the carrier gas and sample) and the stationary
phase causes chromatographic separation. At the exit of the column, the sample separates
into solutes or analytes.
23
The carrier gas is usually an inert, unreactive gas such as nitrogen or helium that
travels at a constant rate through the column. The different chemical constituents of the
sample travel with the mobile phase through the column at different rates, depending on
their interactions with the stationary phase.
The rate of travel of each analyte in a given sample is determined by the partition of
each analyte between the mobile and the stationary phases. Partitioning is often described
in terms of the distribution constant, Kc, which is also referred to as the partition
coefficient. Kc is a representation of the tendency of an analyte in a sample to be attracted
to the stationary versus the mobile phase. It is commonly represented as the ratio of
concentrations in the stationary and mobile phases as,
Kc = [C]
[cIM,
(1.1)
where [C] refers to concentration, and subscripts s and m refer to the stationary and
mobile phases respectively. K, is a temperature dependent, thermodynamic value [20].
Partitioning may occur either by adsorption of an analyte with the surface of the
stationary phase, absorption of an analyte with the bulk of the stationary phase, or a
combination of the two. Different stationary phases and samples types govern if the
separation process occurs through absorption, adsorption, or a combination of the two.
Analytes with high partition coefficients pass through the column more slowly,
24
because of their tendency to distribute themselves more in the stationary phase. The
amount of partitioning determines each analyte's relative retention time, the time
difference between sample injection and outlet detection, for a particular analyte. A
schematic representation of the partition chromatographic separation process is shown in
Figure 4.
Detector Chromatogram
Direction of mobile-phase flow
B
A
Concentration of solute in
B
mobile phase
A
-YConcentration of solute in
stationary phase
B
A
BB
A
BB
B
Fraction of bed length
Figure 4. The chromatographic separation process, adapted from [21].
For a sample that is initially composed of more than one analyte, each analyte will
ideally have a unique partition coefficient and therefore, a different retention time. The
25
difference in retention times for each respective analyte results in a separation of the
sample in the mobile phase as it elutes from the column. The power of chromatography
and its development lies in this separation ability. Depending on the column configuration,
thousands of samples can be separated from a single instrument [22]. An example output
from a moderate number of separated peaks, often referred to as a chromatogram, is
shown in Figure 5.
3
4
I,2
6
7,8
10
12
51%
I
u
5
10
I
I's
20
-U
-T
25
1
30
1
U-
I
a
35 40 45
Time (min.).
I
a
a
a
I
I
50
55
60
65
70
75
80
Figure 5. Example gas chromatograph, adapted from [17]. This chromatogram shows the
different analytes found in fatty acids. Each peak represents a different analyte, and can
be expressed in terms of a retention time that is related to the instrument's parameters
and column type.
Retention times can also conceptually be represented as retention volumes. In this
regard, the retention volume is the volume of carrier gas needed to elute a given analyte.
The retention time, tR and the retention volume, VR, are related to each other by the
column flow rate, Fc, and the equation,
26
VR =CR
-FC
(
(1.2)
Equation 1.2 assumes a constant flow rate. If the flow rate is not constant, the
retention volume will be the integral of the time-dependent flow rate.
In theory, the retention volume can then be related to the distribution constant by
the equation,
(1.3)
VR = KC V +VM,
where VM is the volume of the mobile phase. For a derivation of this relationship, see
previous summaries by Cazed and Scott [23].
As the peaks from a chromatogram represents different analytes (or a combination of
analytes if full separation has not occurred), each peak's height or area is proportional to
amount of analyte within a given compound. In other words, the integrated area under
each analyte curve will correspond to the relative concentration of each analyte with
respect
to the
total sample.
This characteristic
of the chromatogram
makes
chromatography a powerful tool for determining constituent chemical ratios, chemical
adulteration, and chemical purity.
Relative areas are typically analyzed after a chromatogram is complete, through the
process of normalization. For a given analyte area, A,, its relative concentration, A, is
given by,
27
Ac
-
\2:=' lAi)
100
(1.4)
where there are n total peaks within a chromatogram and i is a counting index.
1.4.3 Detector Types
In gas chromatography, there are a wide variety of detectors available depending on
the desired type of measurement to be taken. Chromatographic detectors vary from simple
resistive element systems to elaborate spectroscopic instruments. Detector choice will
determine what types of analytes can be detected within a sample, and how these analytes
are treated upon exiting the chromatograph. Certain detectors are non-destructive,
enabling a separated sample to be further analyzed in additional instruments upon exiting
the chromatograph. Other detectors permanently alter or destroy the sample during the
measurement process. Many detectors are selective to a particular type of analytes, such
as halogens or sulfur compounds, while other detectors measure non-selective properties
such as thermal conductivity or gas density. A brief summary of common gas
chromatograph detectors is shown in Table 2.
28
Name
Table 2. Gas chromatograph detector types
Destructive
Selectivity
Thermal conductivity
Universal
No
Gas density
Universal
No
Flame ionization
Organics
Yes
Photoionization
Aromatics
Yes
Helium ionization
Volatile inorganics
No
Thermionic
N, P
Yes
Flame photometric
S, P
Yes
Plasma atomic emission
Metals, X, C, 0
Yes
Electron capture
X
No
Nitrogen-phosphorous
N, P, X
Yes
Chemiluminescent
S
Yes
N=nitrogen, P=phosphorus, S-sulfur, X=halogens
The two most common detectors found in commercial gas chromatographs are the
flame ionization detector (FID) and the thermal conductivity detector (TCD), both of
which have sensitivities over a wide range of concentrations. Thermal conductivity
detectors operate by measuring the differential thermal conductivity of the outflowing
analyte gases with respect to the carrier gas. TCD's have sensitivities on the order of 10
PPM with linearity over a range of 10 4 . FID's have sensitivities up to 50 PPB with
linearity over a range of 106. A major difference between these two detectors is their
implementation as concentration or mass flow rate measurements.
Concentration
detectors, such as the TCD, measure the concentration of an analyte in a carrier gas.
Mass flow rate detectors, such as the FID, measure the absolute amount of an analyte,
29
irrespective of the volume of the carrier gas.
It is sometimes beneficial to operate multiple detectors simultaneously, as they can
work to complement each other, depending on the types of analytes that are being
analyzed. Thermal conductivity measurements are non-destructive, while FID's require
that the analyte be ionized via a flame. In many cases, both TCD's and FID's will operate
in series, with the TCD measuring the analytes before they are destructively measured in
an FID. A common solution to destructive detectors is to split the flow of the analytes at
the beginning of a detector, and redirect a portion to another instrument.
1.5 Stochastic Perturbation Chromatography
One of the drawbacks in the current art of chromatography is that unlike other
chemical analysis techniques such as spectroscopy and selective chemosensory systems,
chromatographic measurements cannot be performed continuously. Continuous methods
in chromatography could enable both the ability to sample a system as it changes and
detect those changes in real-time. Traditional chromatography, however, is a serial process
that involves the injection of a sample, followed by a delay period during which the sample
interacts with the column, followed by a series of recorded, time-delayed peaks that
produce a chromatogram. A schematic representation of the traditional injection method
30
is shown in Figure 6.
Impulse Response
Sample Input
Gas Chromatograph
Chromatogram
Figure 6. Schematic representation of the traditional injection method used in gas
chromatography.
The total length of the chromatographic process depends on the retention times of
the analytes. Retention times can vary, depending on the instrument parameters and the
partition properties of the analytes and the column, from several minutes to several hours.
For samples that have relatively short retention times, it is possible to obtain
chromatograms in short, successive batches over time. For samples with longer retention
times, however, the gas chromatograph is occupied until all of the analytes have passed
through the column from a given injection.
There have been several attempts to develop continuous chromatographic methods.
The most common technique is to stagger or separate the sample introduction by a known
time-delay, using the traditional injection method, such that a series of staggered inputs
with a similar time delay will be output to a series of staggered chromatograms with the
same known time delay [241. This method, however, can cause erroneous measurements if
31
column saturation occurs, or if analyte peaks overlap between samples. A second method
involves passing a chromatographic medium, such as a column, through a continuous gas
[25].
An alternative method to performing traditional chromatography involves applying
techniques commonly used in control theory that involve the identification of unknown
systems. One form of this technique, often referred to as stochastic system identification,
involves the stochastic perturbation of a system's input in order to determine the impulse
response of that system.
The impulse response of a system is the system's response to an infinitely short pulse
input of unit area. For a linear system, the impulse response contains sufficient
information to identify and understand how that system will react to a given input. In
theoretical chromatography, the injection time of the sample is supposed to be as short
as possible, similar in signal processing to an impulse input. Therefore, under linear
conditions, the resulting chromatogram from a gas chromatograph can be considered the
impulse response of the system.
The definition of the system, in the case of gas chromatography, includes the process
of vaporizing a sample in the injection port, passing the sample through the column, and
measuring the analyte output at the detector. The input to the system, in its most basic
state, is the sample itself. The output is the detector signal. Under the assumption that
32
the system is linear 3 [26], it is possible to replace the traditional, single injection impulse
input with a random sequence of binary inputs, and, after performing a series of
mathematical operations, obtain an impulse response that is equivalent to a traditional
chromatogram.
A
schematic
representation
of
this
stochastic
perturbation
chromatographic process is shown in Figure 7.
Sample Input
Impulse Response
Gas Chromatograph
Chromatogram
Figure 7. Schematic representation of stochastic system identification applied to gas
chromatography.
Although chromatographic system identification techniques are not strictly limited to
binary inputs, the use of a stochastic binary input enables fast calculations and an efficient
distribution of input frequencies [271. Also, because the input is stochastic, it does not
matter when in time the input and output signals are recorded for determining the impulse
response. Therefore, one can determine the impulse response, for arbitrarily defined,
3 Very small and very large sample injections exhibit non-linear behavior, but moderate injections tend to
be linear, if the detector is within its linear range as well.
33
moving windows, and therefore continuously compute the system's impulse response.
The equations governing stochastic system identification involve a sequence of
operations. Consider the variable P as representing the system input, the variable 0
representing the output, the variable n representing the number of data samples, the
variable At representing the time interval between each successive data sample, the
variable h representing the impulse response or chromatogram, and the variables i and j
representing indices. Once the autocorrelation of the input function (1.5) and a crosscorrelation of the output and input functions (1.6) are determined, a Toeplitz matrix can
be formed (1.7) from 1.6. The impulse response (1.8) of the system is then product of the
inverse of the time interval, the inverse Toeplitz matrix, and the cross-correlation matrix
[28].
CppZj
=(Pi_ -Pi)
(1.5)
CPO =
=1 (Pi_;
(1.6)
GPO
(1.7)
Tipi -CPO.
(1.8)
-
Ti= CPPpj
h=
The resulting impulse response and the mathematical characteristics of the impulse
response curve correspond to a traditional chromatogram. The method, as will be shown
34
in upcoming sections, works for both single analyte samples and multiple-analyte
compounds.
Stochastically perturbing a gas chromatograph can provide several benefits, the most
prominent being that it enables the continuous perturbation and determination of
chemical components within a sample, the relative concentrations of those components,
the retention times of chemicals that interact with a chromatographic medium, and the
sensitivity and correlations between the chemical components of a mixture and the various
apparatus parameters of a chromatograph.
Sample input is not the only parameter of the system that may be perturbed.
Modulated parameters may also include the column temperature and pressure 4 . If
additional variables are perturbed, such as the column temperature or the system pressure,
then the respective impulse responses for those conditions can also carry additional
information about the properties of the chemical mixture and the individual components
of
that
mixture.
For
example,
in
traditional
chromatography,
operating
the
chromatograph at different temperatures can cause the constituent chemicals to elute at
different rates, thereby causing a shift in retention times, peak widths, and heights. If the
The partition coefficient, which ultimately governs the retention times, is a non-linear function of
temperature. Stochastic temperature perturbations would therefore be carried out using non-linear
stochastic system identification techniques.
4
35
impulse response of the system as a function of temperature is determined, then the
retention rate peak shifts can also be determined and the chromatograph can be optimized
for ideal peak separation.
Another advantage of stochastic perturbation chromatography is the fact that
spurious sources of noise present when performing chromatographic analysis can be
greatly reduced. The operations performed to determine the impulse response are based
on correlation methods. Therefore, in cases where a system's noise is uncorrelated with
the input, the noise's contribution to the final impulse response will be negligible.
The employment of stochastic perturbation techniques also allows for input parameter
and system tuning through input shaping. For example, if a chromatogram with a broad
stochastic set of frequencies is found to have a large response for a given input spectrum,
the probability distribution function of the random binary input can be tailored such that
more power is distributed to the most responsive portions of the system [27]. Additionally,
although this method of stochastic perturbation is currently being applied to gas
chromatography, it can, both conceptually and in practice, be applied to liquid, solid,
HPLC, paper, and ion exchange chromatography.
36
Chapter 2
Chromatographic Simulations
In order to evaluate the stochastic perturbation approach to chromatography, a
computational simulation was created based on the basic input and output equations that
govern gas chromatography. The simulation was then compared and calibrated against
experimental chromatography results. After verification, the simulation was expanded to
include system identification techniques as applied to gas chromatography.
2.1 Simulation Constructs
Gas chromatography simulations are often performed using a series of equations,
based on van Deemter [29], Golay [291, and Kovits retention indices [30]. These models
help elucidate chromatographic behavior and the properties of chemicals. In this
simulation, plate theory was used as the theoretical construct for predicting the detector
output in the time domain.
The first step in the simulation was to approximate the system response, due to
37
'\
partitioning, in the time domain. The partitioning that governs the chemical kinetics
between the mobile and stationary phases is a statistical process, in which individual
molecules can be assumed to act independently of one another. The resulting retention
time, and the shape of the elution curve are an accumulated reflection of the sorption and
desorption interactions of each individual molecule. In its ideal form, the elution curve
can be approximated as a Gaussian curve, whose mean represents the retention time. A
representative curve is shown in Figure 8. In practice, chromatographic peaks can show
signs of broadening, asymmetry, and overlap, depending on sampling techniques and
instrumentation parameters [31]. Specific cases like band broadening that differ from the
ideal chromatographic response will not be addressed in this simulation.
Tmients to points of Inflection
0.399
1.0
-Il
I
I
I
0.20
**-
0.\
I
/
I'OD
3
/2
0.0
1
0
1
2\
3
Figure 8. Example Gaussian curve, adapted from [35].
38
Plate theory is based on the construct that a chromatographic column is composed
of a series of discrete individual plates. One of the key assumptions of plate theory is that
a solute is always in equilibrium between the mobile and stationary phases for a given
plate5 . Working with the equilibrium equation (1.1) from the previous chapter, it can be
rewritten as,
Xs = KXM,
(2.1)
where Xs and XM are the concentrations of the stationary and mobile phases respectively,
and K is the distribution coefficient. Equation 2.1 expresses the equilibrium relationship
between the stationary and mobile phases for a single plate. If this equation is
differentiated, the result is,
dXs = KdXM.
(2.2)
The concentrations, XM and XS can be written in terms of mass, such that,
(2.3)
,
Xs =
and
XM = M
V
(2.4)
where ms and mm are the masses of the stationary and mobile phases respectively, and V
In practice, the mobile and stationary phases are never in equilibrium. The division of a column into
discrete plates allows this discrepancy to be overcome.
39
is the plate volume.
Since each plate has two adjacent neighbors, the change in mass between two plates
can be determined by the plate index, i, rewriting equation 2.4, such that,
m
=
(XM (i_1) - XM (i))V.
(2.5)
Differentiating equation 2.5 results in,
dm = (XM (i-1) - XM (i))dV-
(2.6)
The change in mass between two plates can also be written in terms of the change in
concentration of the mobile and stationary phases, or,
dm = VsdXs(i) + VMdXM(i).
(2.7)
Substituting equation 2.3 into 2.7 results in,
dm = (VM + KVs)dXmui).
(2.8)
If the representations for the change in mass from equations 2.6 and 2.8 are equated, and
algebraically manipulated, an expression for the changes in concentration and volume can
be written as,
dXM(i)
dV
_
XM (i-1)-X (i)
(VM-KVs)dXm(i)*
(2.9)
In gas chromatography, it is often convenient to express the plate volume not in terms
40
of length cubed, but in terms of the plate volume. Plate volume, Vp, can be written as,
Vp = VM + KVs.
(2.10)
Using this new definition of plate volume, it is mathematically convenient to define a new
variable, v, as the ratio of the conventional column volume to the plate volume, or
V
(2.11)
V =V.V
VM+KVs
Differentiating equation 2.11 results in,
dv =
dV
VM+KVS
(2.12)
Substituting equation 2.12 into equation 2.9 results in a simplified representation of the
changes in concentration and volume as,
dXM(i) = XM
(i-1) - XM (i).
(2.13)
Equation 2.13 can be solved through integration [23]. Due to the discrete nature of plate
theory, the resulting concentration of a solute at a given plate i can be written as,
Xoe-vvi
(2.14)
,
Xyggi
= I,
where Xo is the initial concentration.
Equation 2.14 represents the analyte distribution along a given column. The equation
41
takes a Poisson form, but as the number of plates becomes large 6 , the function closely
represents a Gaussian curve. This can be shown by defining the term w as the number of
plate volumes from the elution peak maximum, such that,
w = v - i.
(2.15)
The transformation from the Poisson to the Gaussian form requires the implementation
of Sterling's theorem [231. The resulting elution curve, in Gaussian form can then be
written as,
(v-i)2
X0
XMi) =f2-7-e
(2.16)
2i.
The Gaussian form of the elution curve can be transformed into various variables of
interest. One such transformation is to rewrite the elution curve in the time domain [32],
resulting in,
A(tR-t)
Xm(i) = Xoe
LH
2
(2.17)
where A is a constant, L is- the column length, and H is the height equivalent of a
theoretical plate. The plate height, column length, and number of plates are all related
by the function,
6 Columns
typically have large plates counts, where i>>100.
42
N = L.L
H
(2.18)
From equation 2.17, we see that the retention time shifts the Guassian function, while
the length of the column and the theoretical plate height control the width of the elution
curve, such that a longer or more efficient column results in sharper peaks. The retention
time, tR, will be dependent on instrument and column parameters such as the column
type, column temperature, and column flow.
In the upcoming simulations, equation 2.17 was used as the time domain
representation of the elution curve. Flow modeling was based on the assumption of
Poiseuille flow [331, with a constant pressure drop across the column. Typical column
diameters, lengths, and inlet pressures were used in determining the flow rate. The input
sample mass was based on typical ranges found in the literature for bench top
chromatography (0.1 pL to 50 pL). In order to calibrate and verify the system, the
retention times of the simulated analytes were compared to calibration data taken on an
Agilent 6890 gas chromatograph.
2.2
Traditional Chromatography Simulation
Results
The simulation constructs were first verified by simulating single analyte injections
and observing the detector output. The simulation worked such that a larger sample input
43
resulted in an increase in the output accordingly (Figure 9). If the column length was
modified, or the flow rate or temperature changed, the retention times shifted accordingly
as well. It was assumed that all injections remained below the saturation threshold of the
system
8x
7
10-7
-10 uL
7
-1
uL
6
I
-
V
0
50
100
150
Time (s)
200
!
250
A
300
Figure 9. Flame ionization detector signal simulations of benzene, using 1 pL and 10 PL
injection volumes.
To simulate multi-analyte compounds, it was assumed that none of the analytes
interfered with the sorption rates of the others, and that the output detector worked on
the principle of superposition. For multi-solute systems with two or more analytes,
7
The simulation constructs do not account for column or detector saturation.
44
modification of the gas chromatograph parameters is often done in order to induce peak
separation8, as the retention time is a temperature-dependent property that is unique to
each analyte (being derived from the partition coefficient). An example multiple analyte
sample is shown in Figure 10.
-7
810
-phenol
7-
-3-phenyl
propanol
-acetophenone
-p-chlorophenol
6
-benzene
5
S3
V
3-
21
C0
50
100
150
Time (s)
200
250
300
Figure 10. Chromatography simulation of multiple chemicals. Each analyte has a different
retention time, based on the column and instrument parameters of the simulation.
8 In chromatography, if the resolution of an instrument is low, or separation does not occur, the detector
output will not be able to differentiate two overlapping peaks. In cases where partial overlap occurs, it is
sometimes possible to infer the relative concentrations of the individual peaks, by recognizing that the
contributing peaks are formed from a summation of the individual analytes. This method of post
chromatogram analysis, however, is limited to two-analyte systems, and requires previous knowledge of the
retention times.
45
From the
initial simulation,
the
retention
times,
peak
shapes,
and peak
transformations moved according to experimental observation, as a function of column
length, pressure, and analyte type. Although a variety of chromatographic principles could
be applied to enhance the realisticness of the simulation, it was decided that these primary
qualities would sufficiently model the chromatographic process in order to evaluate if
stochastic perturbation gas chromatography was possible.
2.3 Stochastic Perturbation Chromatography
Simulation Results
After the simulation was verified for the conventional form of chromatography,
stochastic system identification techniques were applied to examine the possibility of
continuously sampling and computing chromatograms.
2.3.1 Stochastic Signal Generation and Implementation
To simulate the stochastic input, a random sequence of white noise that had a
Gaussian probability density function was hard-limited to a binary signal. A numerical
low-pass filter was then applied to the hard-limited sequence, similar to methods
previously described [36]. The time constant of the filter was based on the heuristic
understanding that the smallest time intervals of the binary output should be at least 25
times smaller than the system response and within the bandwidth of the input controller.
46
A plot of a simulated example random binary sequence is shown in Figure 11.
1.2
1
0.8
-
0.6
c 0.4
ED
LI
0.2
0
-0.2
0
1-
2
3
4
I
I
5
6
Time (s)
7
8
9
10
Figure 11. Random binary signal input. Only a portion of the total signal duration is
shown.
To determine the simulated time domain response of the gas chromatograph detector,
the random binary input sequence was convolved with the discrete form of the time
domain elution equation (2.17). An example of the resulting, simulated detector output,
for a single analyte convolved with the binary input is shown in Figure 12.
47
-input_
-Output
5
CD
3
CL
0
2
CL
0
if
1f
-F~1
LI
0
2
4
6
8
10
Time (s)
12
14
16
18
20
Figure 12. Simulated binary stochastic input (red) and chromatograph detector output
(black). The lag between the input and the output signals, visible during the first 2
seconds, is due to the dead volume lag associated with the column. The actual lag was 40
seconds, but the first 38 seconds are not shown. The input and output have been scaled
so that they can be viewed simultaneously.
The impulse response of the system was then determined by implementing the
equations discussed in section 1.5. A plot of the impulse response determined through
stochastic system identification is shown in Figure 13.
48
2010
-IIII
18F
16
<14
C,,
C:12 K
0
10 H
8
E 6
4
2
II
0
20
I
40
60
80
100
Time (s)
120
140
160
180
200
Figure 13. Impulse response computed from binary stochastic mass flow perturbations
through a GC column with a flame ionization detector. The retention time and elution
peak height are identical to the simulated traditional chromatogram.
2.3.2 Reducing Noise
One of the advantages of stochastic perturbation techniques is the ability to reduce
noise inherent in a system. Sources of noise in a chromatographic system may include
temperature fluctuations, excess chemicals, and unclean columns [341. While these types
of noise can be reduced through proper maintenance, if the system is to be operated
outside of a controlled laboratory environment [351, it is preferred that spurious sources
of noise contribute minimally to the overall chromatogram. To deal with noise inherent
in the system, many commercial gas chromatographs have built-in filters that are either
software or hardware based. Reducing noise without filtering offers the advantage of
49
increasing an instrument's detection limit.
To prove the utility of stochastic techniques, noise simulations were performed using
both'the traditional and stochastic perturbation methods. In these simulations, it was
assumed that system noise was rooted in the detector itself, and had no correlation with
the input sequence. It was also assumed that the noise was random, white, and did not
affect the chemical kinetics between the stationary and mobile phases. Finally, it was
assumed that the noise was manifested only in the detector output, and was equal to 5%
.
of the total detector signal9
Under these assumptions, a white noise signal with a limited amplitude was
superimposed on the detector output signal of both the traditional chromatogram and the
signal response from stochastic perturbation. The chromatogram from the stochastically
perturbed sequence was then determined, as discussed in the previous section. A
comparison of the noiseless chromatogram and noisy detector chromatograms using both
traditional and stochastic perturbation methods is shown in Figure 14.
9 A noise source that has an amplitude equal to 5% of the total detector signal is highly uncommon for
flame ionization detectors. The simulated noise signal was chosen, however, to show the benefits of using
stochastic techniques in high-noise environments. In cases, however, where large temperature or
environmentally chemical fluctuations might exist, such as oil and gas discovery, large fluctuations in
environmental noise may exist.
50
-7
2cx10
18- -Noisy Detector (Traditional Processing)
-Noisy Detector (Stochastic Processing)
'16 --- No Noise
C
i
14-
O12
~10
8I
E4
I
0
0
M
10
20
30
Time (s)
50
40
60
Figure 14. Noise comparison of traditional chromatography in the presence of noise and
stochastic perturbation chromatography in the presence of noise. Note the drastic
reduction in noise between the traditional method (magenta) and the stochastic method
(green), as compared to the ideal, theoretical chromatogram (black).
2.3.3 Multiplexing Possibilities
Another advantage to stochastic perturbation gas chromatography is the ability to
multiplex columns and samples, and to perturb multiple instrument parameters at once.
The stochastic methods discussed in this work are based on the correlation between the
input and the output signals. Therefore, if there is a system with multiple inputs and
those inputs are independent of each other 10 , the resulting impulse responses relating the
10 Ideally, multiple inputs will not just be independent of each other, but orthogonal.
51
output with each respective input will be unique. This enables, for example, the ability
to perturb the sample input, the column temperature, and the column pressure
simultaneously, as long as the three input signals are independent. This also enables the
possibility of multiplexing 1 several columns or several samples simultaneously. A
schematic of multiplexing possibilities is shown in Figure 15.
Pressure Temperature
Modulation Modulation
Property r
Modulation
Gas Chromatograph
Cohunin 1
IAW A[, J
Sample 1
3
IFIJ_1FJ1PI I
Sample 2
OK
Column 2
3
Columin
Sample n
q
i
n-q-r + n*q
Figure 15. Multiplexing possibilities. Each sample is injected according to its own,
independent binary sequence.
11
It should be noted that this technique, while multi-dimensional in describing the system inputs, is entirely
different from N by N multi-dimensional chromatography. In the latter case, the output of a column is
sampled into an additional column, resulting in multiple separation processes. N by N chromatography is
used to improve the resolution of a chromatographic system, or of a particular subset of analytes.
52
If each of the system inputs is independent of the others, the system will yield a
certain number of impulse responses, N, according to the equation:
2.1
N = n - q(1+ r),
where n is the number of independent samples, r is the number of instrument parameters
that are modulated, and q is the number of columns. The (1 +r) derives from the fact that
a chromatogram can be obtained, even if no instrument parameters are modulated apart
from the sample mass.
Multiplexing was integrated into the simulation based on the injection of two different
samples with different retention times and elution characteristics. The samples were
injected simultaneously into the same column, according to their respective binary input
sequences. Both samples shared the same detector. A plot of the detector output, and the
contributions that each sample made to the total output, are shown in Figure 16.
The chromatogram for each respective sample input was computed by the same
methodology as described in section 2.3.1, resulting in two chromatograms, one for each
stochastically perturbed sample input. The respective chromatograms
from each
independent sample, super-imposed, are shown in Figure 17. Data from the original
theoretical elution curves were compared to these results, to verify accurate retention
times, areas under the curves, and peak widths. A larger baseline noise was observed when
using the multiplexing method, but the baseline noise was only observed in areas where
53
no elution response existed.
8x
10-9
-Contribution
-Contribution
7
from Sample 1
from Sample 2
Response
66) -Total
C
05-
4--
05
CD
-
(D03-
2
1
10
3
6
5
4
8
7
10
9
Time (s)
Figure 16. Superimposed flame-ionization detector responses from multiplexed samples.
The total response of the detector (black) is observed, while the two contributive signals
from U75
sample one (blue) and sample two (green) are plotted for illustrative purposes.
9X 108 -Sample
1
-Sample
7-
2
.6-
c. 5 --
-
-CO 3
E 21
-0
C0
100
150
Time (s)
260
250
300
The impulse responses of two samples injected simultaneously and
independently into the same gas chromatograph. The respective elution curves were
determined by independently processing the total gas chromatograph detector output for
Figure
17.
each respective input. As the sampling time increased, the baseline noise dropped to zero.
54
Simulations were also run with two single-analyte samples, multiplexed on a single
column, and two dual-analyte samples multiplexed on two columns simultaneously.
Results from these various scenarios were quantified by taking the Variance Account For
(VAF) of the simulated signals, in comparison with the theoretical expectations. Results
of the VAF's are shown in Figure 18. The VAF's suggest that computing the impulse
response from more complex samples (i.e. with more analytes) requires more data than
with simple samples. Additionally, multiplexing samples, or multiplexing columns,
introduces more variance into the system. In all cases, however, the VAF's of the
simulations were found to approach unity over time. Simpler samples, however,
approached unity significantly more quickly.
1
0.9
L_
0
U-
0.80.7-
-0
0.60.5-
0
0.4-
-Single Analyte
-Dual Analyte
-Two Samples Multiplexed
-Two Samples + Two Columns Multiplexed
0.30.20.10I
0
200
400
600
800
Normalized Chromatogram Length
1000
Figure 18. Variance Accounted For of various stochastic perturbation chromatography
simulations. The normalized chromatogram length refers to the length of data taken,
divided by a length equivalent to one chromatogram using the traditional injection
method.
55
Chapter 3
Bench Top Implementation
3.1 Instrumentation
To prove the validity of the simulation results that applied stochastic system
identification techniques to chromatography, the process was also implemented on a
retrofitted Agilent 6890 gas chromatograph. In order to enable stochastic perturbations,
the injection port on the instrument was modified by replacing the traditional, automated
syringe injection system with a computer-controlled nanoliter injection system.
The computer-controlled injection system was composed a linear actuator, an airtight
syringe, a cooling system and a controller. A Zaber T-LA60A Series linear actuator with
a built in controller was used to control the syringe piston's position. This actuator was
chosen based on a combination of its force output (15 N), its stroke length (60 mm), and
its small step size (0.1 pm). The typical, total stroke length of a microsyringe is around
40 mm. Coupling the Zaber system to a typical syringe resulted in an injection control
volume, dependent on the syringe volume, as low as to 2.5 picoliters as shown in Table 3.
56
Table 3. Syringe injection volumes
Max
VMex
1 IpL
10 1L
100 pL
1
250 11L
500 piL
1000 pL
Min
VMen
2.5 pL
25 pL
0.25 nL
0.625 nL
1.2 nL
2.5 nL
Volume
Volume
The actuator and syringe were mounted on a custom frame that included a vertical
collar that held the syringe at a specified height that prevented it from moving during
injections. Two lateral cooling blocks maintained the needle and the base of the syringe
at a constant temperature, offsetting any heat conduction originating from the injection
port. The Zaber actuator was coupled to the syringe only through mechanical contact, as
the backpressure on the syringe plunger from the injection port caused the piston to
maintain contact with the distal end of actuator shaft. The mounting system for the
injection controller is shown in Figure 19.
The Zaber actuator was controlled by a custom-built script written LabVIEW. The
system operated under closed-loop control at a maximum bandwidth of 200Hz. During
operation, the position of the actuator was recorded, via LabVIEW, from the actuator's
built-in encoder.
57
1ml syringe
I -
E.
Aluminum
cooling
block
Figure 19. Automated injection system, composed of the: (A) Zaber linear actuator, (B)
support and mounting hardware, (C) microsyringe, (D) thermoelectric leads for cooling
system, and (E) syringe tip.
After mounting the injection system on the bench top device, samples were injected
into the gas chromatograph at stochastic intervals, according to a randomly generated
58
binary array. A value of one from the binary array indicated an injection of a specified
amount. A value of zero indicated no injection.
The actuator was tested under two different injection-operating configurations. The
first configuration involved controlling the Zaber actuator to deliver a constant volume
injection over a specified time interval. In the second configuration, the linear actuator
moved at a constant speed for a specified duration. In both cases the shortest interval
was 50 ms, based on the baud rate and response time of the controller. No noticeable
differences in the performance 12 of the actuator were observed under both operating
conditions (constant linear displacement or constant speed).
The Agilent 6890 GC was fitted with a 30 m capillary column (HP-5ms), and operated
under the conditions shown in Table 4.
Injection
Injection
Port PortPort Port
Pressure
Temperature
Table 4. Agilent 6280 settings
Column
Column
Temperature
Flow
Deetr
Split
Vetnto
Vent
Helium
200 0C
213 kPa
150 0C
14.8
FID
20:1
mL/min
Performance was quantified by comparing the position of the actuator from the encoder with an externally
mounted linear variable differential transformer (LVDT, Shaevitz MHR 005 Series). The LVDT was
sampled at 1 kHz.
12
59
The linear actuator system was mounted vertically onto the injection port of the
Agilent bench top GC, as shown in Figure 20. The syringe needle height was adjusted
such that the needle tip was at approximately the same height as a standard automated
injection. A new septum was used for each test, and tests were limited to 1.5 hours or
less. Prolonged piercing of the septum while the injection port was held at an elevated
temperature caused permanent damage to the septum over time. Although the septum
was replaceable, some of the longer duration tests (greater than 2 hours) showed a modest
drop in injection port pressure, or reduced signal responses for the same input. Both of
these observations were indications that some of the volatile sample was escaping through
the injection port septum. Data from these initial tests were discarded.
Controllers
Figure 20. Bench top implementation of stochastic perturbation chromatography. Two
personal computers control the gas chromatograph and injection port system. The inset
(red outline) shows an enlarged view of the syringe entering the injection port.
60
The stochastic sample perturbation system was tested with both single analyte
samples and multi-analyte samples, to validate that the stochastic system identification
process yielded the same results as the traditional GC injection methodology. An example
plot of the input and output signals from the bench top setup is shown in Figure 21.
From initial tests, it was observed that at the beginning, the FID signal moved above
the baseline, and remained above the baseline until after the last perturbation passed
through the column. It was possible to keep the lower limit of the signal response closer
to zero13 , but that required smaller injection volumes or a longer time constant when
filtering the input signal. Both of these methods in turn reduced the amplitude of the
signal.
Instead of targeting a small baseline, the injection volume was modified such that the
output signal took full advantage of the dynamic range of the detector. At too small a
volume, the signal to noise ratio was higher. At too large a volume, the detector became
saturated. The injection volume, therefore, was determined experimentally after the
4
.
filtered binary sequences were determined
1 Flame ionization detectors will not drop below zero, unless the entirely polarity of the ionization bias is
reversed.
1 Any set of binary sequences created according to the methods described in Chapter 2 should be sufficiently
similar, if they are filtered with the same time constant, to work with different injection volumes.
61
-
~ut
np
tput
98-
4'
-
6 -I
3 -111?
0
10
5
15
Time (min)
Figure 21. Input (red) and output (blue) signals from bench top gas chromatograph using
stochastic binary perturbation. The input signal indicates the state of the injector, while
the output signal was recorded directly from the flame ionization detector.
3.2 Results
A plot of the chromatogram obtained on the bench top system using stochastic
perturbation, computed from the sample show in Figure 21, is shown in Figure 22. The
sample was composed of pentane using the instrument parameters shown in Table 4.
Stochastic perturbation occurred over an eight-minute period. Each single injection was
equivalent to a sample volume of 9 nL. The impulse response was determined according
to the methods discussed in section 1.5. The retention time, peak width, and peak height
62
corresponded with the traditional chromatographic methodology.
9nn,
180
-
160
()
140
CO 120100-
.
CL
80604020
0
~&V~AA
~
50
0
Time (s)
150
100
Figure 22. Single solute chromatogram, determined using stochastic perturbation methods
on an Agilent 6890 gas chromatograph.
A
second experiment
was also performed to determine whether
stochastic
perturbation worked with samples that contained more than one analyte, and, if the
relative areas of the peaks in a multi-analyte sample could be accurately determined using
the stochastic perturbation method. In this regard, a controlled, two-analyte sample was
tested using a 1:4 ratio of pentane to heptane. The instrumentation conditions were the
same as with the single injection experiment.
The resulting chromatogram is shown in Figure 23. From the plot, the areas of under
the retention
curves were determined using numerical
63
integration. The relative
concentrations were found to be 19.4% and 80.6%, within the experimental errors of the
GC instrument and in the same proportion to the traditional GC method.
16C
I
I
II
*Heptane
*Pentane
1 40+
1201000
0L
C.
80-
C-
60-
E 40200
-2 1
0
I
40.5
41
I
41.5
Time (s)
II
42
42.5
43
Figure 23. Chromatogram of relative concentrations for two-analyte sample measured
using the stochastic perturbation method. The areas under each elution curve correspond
to the calibrated, relative concentrations of heptane and pentane.
64
Chapter 4
Design Considerations
4.1 Design Elements
Once it was determined that stochastic perturbation gas chromatography was possible,
implementation
of a miniaturized gas chromatograph began. Before designing a
miniaturized gas chromatograph to meet the objectives discussed in the introduction, the
major components and operational characteristics of a high-performance bench-top gas
chromatograph were analyzed.
These components included the:
1. Injection port
2. Column
3. Detector
4. Gas sources and flow control
5. Sample injection control
65
6. Thermal components and control
After this analysis, a series of parameters, restrictions, and objectives guided the
progressive designs through five device generations of the micro gas chromatograph (pGC).
Design principles, high volume manufacturability considerations, component costs and
ease of assembly were all considered, and are discussed in the proceeding sections.
4.1.1 Injection Port
Sample input, either in the solid, gaseous or liquid state, typically enters a sealed gas
chromatograph through the injection port. For solid samples, the material of interest is
often dissolved into a solute, preferably one that adds little or known chemical analytesi5
The dissolved sample is then passed through the injection port, vaporized, and passed
through the column. Liquid samples follow the same process, whereby the sample is flash
vaporized before entering the column1 6 . For gaseous samples, the main requirement is
that the injection port maintains the necessary temperature and pressure so that no
condensation occurs.
If the sample enters as a liquid, flash vaporization is the preferred method of inducing
5
Solutes can vary by detector so that they do not affect the resulting chromatogram.
16 Allowing the sample to pass through the column in the liquid state can cause column saturation, or short-
term and long-term degradation of the stationary phase.
66
a phase change. In order to induce flash vaporization, the injection port temperature must
be kept above the boiling point of all of the solutes in the sample 7 . At the same time,
the injection port walls must be kept below a temperature at which chemical
decomposition or chemical rearrangement may occur 18 . The injection port thermal control
is therefore guided by,
boiling point < injection port temperature < sample degradation.
Injection ports are commonly configured as on column ports or separate mechanical
ports, as shown schematically in Figure 24. For both on column and separate component
configurations, a self-sealing, high temperature polymer septum typically separates the
injection port from atmospheric conditions. For best results, the injection duration should
be as short as possible, and the septum seal should be broken for as short a time as
possible, so as to avoid contaminating the sample with air or other environmental species.
Additionally, it is preferred that the syringe needle through which most injections are
made never comes into direct contact with the stationary phase of the column, because
A general rule of thumb is to maintain the injection point at 50 *C above the maximum boiling point of
the analytes.
18 For most commercial GC's, the injection port temperature operates between 50 0C and 450 'C.
17
67
of both the fragility of the stationary phase and the possibility of contamination.
In the on column configuration, the sample is injected directly into a vacant space at
the start of the column, before encountering the stationary phase. Vaporization takes
place directly on the column. Flow and column purging are typically controlled with a
sealed fitting on the column end. On column ports provide a reduced part count to the
overall instrument and typically have low dead space volumes. Typically they operate
exclusively in the splitless injection mode, and are therefore limited in their capabilities
to serve as pre-concentrators or pre-diluters.
ON COLUMN INJECTION PORT
SEPERATE INJECTION PORT
L
Septum
--
+ Purge Outlet
Carrier
Purge Outlet
Syringe Needle
Inlet
Syringe
Needle
I
Septum
_.
Vaporization]
Chamber
Split Outlet
Carrier
Inlet
Vaporization
Glass Liner
Chamber
Column
-
Column
Ji
Figure 24. Typical injection port types found on gas chromatographs: the separate
injection port was adopted from [35]. The on column injection port was adopted from [36].
For separate mechanical injection ports, mixing chambers, additional temperature
68
controls, and some aid in both the vaporization process and sample protection are common.
Separate injection ports have larger dead volumes than on column ports, but are able to
operate in splitless or split mode.
On column injection ports simplify and reduce the part count, but they also simplify
the capabilities and versatility of the gas chromatograph. For example, it is often desired
to keep the injection port temperature constant, even if the column temperature is
modulated. An on column injection port may restrict injections to steady state
temperature modes if it is not fitted with an independent heaiter. Additionally, on column
injections work well for larger, packed columns, but poorly for capillary columns. In
considering this, on column injection ports tend to limit the column type that could be
used in the miniature gas chromatograph. For these reasons, a separate injection port was
chosen.
A separate injection port allows for the independent control of the injection port and
the column temperature, thereby enabling the instrument to operate under a broader
range of thermal conditions. A separate injection port also enables the possibility of sealing
a permanent injection line to the system. In place of a consumable, self-sealing polymer,
it will be possible to create a design based on a ferrule system that seals either an injection
syringe or a supply line.
In terms of micro gas chromatograph (1 iGC) development, the first and second
69
generation ipGC's used standard fittings as injection ports. In generation III, a dedicated
injection port was designed and fabricated. In generations IV and V, thermal control and
sealing improvements were made to the injection port.
4.1.2 Column
Different column types within gas chromatographs offer different advantages and
disadvantages. From large diameter packed columns to sub-millimeter capillary columns,
the geometric restrictions of a column and the pressure drop across a column can vary by
orders of magnitude [361. It was therefore decided that the system would be designed to
operate within the range of typical capillary and moderate diameter packed columns (250
pm to 1.69 mm). Larger column diameters were eliminated from consideration because of
their weight, volume, and high flow rate requirements.
One of the major drawbacks of current portable GC systems is that the column
selections for these devices are limited. Typically, portable systems either make the
columns non-replaceable, or use their own proprietary manufacturing to produce or
integrate the column [37]. Table 5 lists the catalogue of column types for currently
available portable GC's.
The design presented in this thesis sought to take full advantage of the past five
decades of column chemistry development by building a platform similar to a bench top
chromatograph, in which the column can be obtained from third party vendors. Limited
70
column selections would result in a limited number of applications to which the PGC
could be applied. Therefore, the only restriction on the column design for the 11GC was
9
.
that it must have a relatively small minimum bend radius
Table 5. Column options for portable gas chromatographs
Vernier
Inficon
Photovac
Inrag
IGraphX
Bench top
Comparison
(3000 micro GC)
Capillary columns
Yes
Yes
No
Custom
Yes
Packed columns
No
No
Yes
No
Yes
Number of different
chemical column
compositions
1
12
3
7
150+
Throughout the development of the pGC, column types were changed to show both
the ability of the system to operate with a variety of columns, as well as reduce the size
of the final instrument. In terms of development, 1.69 mm diameter stainless steel columns
were used in generation I, 3.750 mm stainless steel packed columns were used in generation
II, 330 pm fused silica capillary columns were used in generation III, and 350 Pm stainless
steel capillary columns were used in generation IV and generation V.
4.1.3 Detector
Most portable GC units use a micro-electro-mechanical system (MEMS)-based
19
The bending radius should preferably be less than 50 mm.
71
thermal conductivity detector, because of its small size and simplicity. MEMS-based
TCD's have higher sensitivities than their macroscopic equivalents [38] due to scaling
effects, and also offer the advantage of being universal detectors [39] that can be operated
using a single gas supply stream.
Electron capture devices (ECD) are also sometimes found in portable GC's, but their
use is usually limited to environmental monitoring applications for detecting halogens.
Considering these limitations, along with the required use, monitoring, and transportation
of radioactive substances, an ECD was not developed.
One of the greatest disparities between bench top and portable gas chromatographs
is the absence of flame ionization detectors in portable systems. FID's are the most
common bench top detector unit, because of their high sensitivity, their wide range of
linearity, and their robustness. One postulation for this discrepancy is that FID's are less
common in portable units because of the need to transport hydrogen. As discussed in the
upcoming section, a solution to the hydrogen transportation issue is proposed so that an
FID can be integrated into the pGC system. Considering this, and the current art of
portable GC's, a TCD was chosen as a candidate detection system developed for the PGC
in generation I and generation II. A portable, fully integrated flame ionization detector
was used in generations III, IV, and V.
72
4.1.4 Gas Sources and Flow Control
The carrier gas source is one of the most important components that affect the quality
of a measurement in gas chromatography. Ideally, the carrier gas will be a highly pure,
inert gas. Contaminants to the carrier gas cause decomposition and corrosion of the
column's coating, as well as build-up within the detector, leading to decreased system
sensitivity. An additional challenge associated with carrier gases is the support needed for
external, pressurized gas tanks. Most portable systems use a small canister of carrier gas
concealed within the instrument's housing [40], adding significant mass and volume to the
units. Certain portable systems use air as the carrier gas [41], but this greatly limits the
accuracy of the instrument and the type of chemical that can be analyzed.
Common carrier gases include helium, hydrogen, and nitrogen. Carrier gas choice is
typically guided by the type of detector used: for thermal conductivity detectors, helium
is often the first choice because of its high thermal conductivity; for flame ionization
detectors, helium, nitrogen, or hydrogen is often preferred; for less common detectors like
an electron capture detector, oxygen-free nitrogen is preferred. Of all of the candidates
discussed, air is the only readily available carrier that requires a simple pump. Helium
cannot be produced using portable equipment, and nitrogen extraction from air is an
energy-intensive and bulky operation [42].
This thesis proposes to use hydrogen as the carrier gas, generated via electrolysis
73
within the 11GC instrument. This design choice will enable the production of highly pure
hydrogen without the need to carry or refill an external tank. Using a closed gas
production system will significantly reduce contamination, and will make replacing the
carrier gas as easy as filling the instrument up with deionized water
A disadvantage to using hydrogen is its flammability hazard. For the scale of the
pGC, however, this is of less concern because a) the hydrogen may be combusted upon
passing through the detector if an FID is implemented, b) the quantity of hydrogen
produced is small, around 25 mL/min, and c) hydrogen sensors are now available as lowcost, solid-state devices [431, allowing for safety mechanisms to be put in place to monitor
for leaks and cut off hydrogen production in the event of an emergency.
Constant carrier gas flow is a critical factor in being able to reproduce chromatograms,
and being able to use the chromatograph for identification purposes. The control of the
carrier flow rate will affect the accuracy of measurement, as the retention times are based
on the linear velocity of the carrier gas with respect to the inlet and outlet of the column.
Flow rates depend on the column diameter, length, and temperature, and may vary
anywhere from tens of microliters to several milliliters per minute [44]. If the samples are
to be injected stochastically, there is also the possibility of slight fluctuations in column
pressure due to the pressure caused by flash vaporization of the sample. Furthermore,
74
20
changes in the column temperature will give rise to changes in the flow rate
To address these issues, either the varying flow rate must be take into account during
calculations of the chromatogram, or constant flow must be maintained in real-time via
closed loop control of the carrier gas pressure. Both approaches may be applied to the
1iGC
system, depending on the magnitude of such fluctuations. One possibility is to
provide flow control by adjusting the output of the hydrogen electrolysis source [45]. A
second possibility is to add an additional reservoir and micro-pump to control the carrier
gas pressure. Both of these options were considered.
The design permits, if required by the end user, to allow for external gas tanks or
feeds to replace on-board hydrogen production. In generation I through generation III,
external tanks were used for flow control. In generation IV and generation V, integrated
hydrogen generation was used for the detector gases by including polymer electrolyte
membrane (PEM) electrolyzers. If scaled, the PEM system could provide sufficient
pressure for the carrier gas as well.
4.1.5 Sample Injection System
Sample injection control is crucial to both bench top and portable GC systems.
The flow rate will decrease as the column temperature increases due to the increased viscosity of the
carrier gas.
2
75
Although bench top systems typically allow for manual control, in the laboratory
environment the use of an automatic sample handler and injector is common. Automation
reduces the risk of external contamination, and also ensures, when comparing samples,
that the injection volume and injection duration remain consistent. The speed and
accuracy of sample injection is incredibly important for resolving the peaks of a gas
chromatograph [46], as slow injections will result in poor peak separation, while inaccurate
injections will bias the results.
In the portable GC design, the sample will be injected into the column as a binary
(on/off) sequence, because of the use of stochastic perturbations of the sample input when
implementing system identification methods. For these reasons, the accuracy, speed, and
repeatability of the automated injections are critical.
For the current design, the smallest injection volumes must be in the nanoliter range,
while the total injection amount is preferably, at least 6 orders of magnitude higher. The
restriction on the minimum injection amount was discussed previously, and comes from
the potential pitfalls associated with column saturation, detector saturation, and poor
vaporization [47]. Oversaturating the column will result in samples that pass through the
column without separating. Oversaturating the detector will result in an inability to
quantify the peaks of the chromatogram. Typically, saturation limits will be determined
by the column and detector characteristics, and therefore, will have to be considered after
76
a column and detector are chosen.
For generations I through V, the Zaber linear actuator injection system that was
discussed in section 3.1 was used. If needed, it would be possible to switch to a smaller
linear actuator such as a squiggle motor [48], and obtain the same positioning resolution,
albeit with a shorter total stroke length. In the case where the pGC is used for monitoring
systems continuously, the injection system could be replaced with a valve-controlled direct
line to the sample of interest.
4.1.6 Thermal Components
The thermal system that regulates the temperature of various components of a gas
chromatograph is critical to both the consistency of the measurement and the overall size
of the instrument. In bench top gas chromatographs, the column oven typically occupies
60% to 70% of the total volume. To address this size constraint, two heating schemes
have been developed that will work for any column type discussed in the previous section.
For metallic columns, joule heating will be applied directly to the columns. For fused
quartz columns, resistive wire elements will act as heaters, causing heat transfer through
both conduction and IR absorption. In both heating schemes, the objective is to keep
heating as localized as possible and within the temperature range of most columns (25 C
to 450 *C). Localized heating will reduce the unintentional heating of other components
while improving the thermal response time. For generation I, no heating system was used.
77
For generations II through V, joule heating was implemented.
4.2 Summary
In summary, the pGC should be guided by the following design components:
1. Injection Port
a. Separate component
b. Operating temperature range of 25 0C to 500 0 C
c. Chemically inert
d. Adaptable to capillary and packed columns
e. Sealed through compression fittings (without septum)
2. Column
a. Maximum bend radius of 50 mm
b. Maximum diameter of 3.175 mm
3. Detector
a. TCD or FID
4. Gas Sources and Flow Control
a. Adaptable to multiple gas types
b. On-board hydrogen production preferred
5. Sample Injection System
78
a. High bandwidth (greater than 50 Hz)
b. Long stroke length (preferably near 50 mm)
c. Small step size
6. Thermal Components and Control
a. Operating temperature range of 25 C to 500 0 C
b. Replaceable and localized
c. Controllable down to 0.1 C
d. Small volumetric footprint
7. Structural and Mechanical Hardware
a. Robust enough to withstand transportation and vibration
8. Power Requirements
a. Battery operated
b. Comparable with other portable gas chromatographs
79
Chapter 5
Prototype Development of Micro
Gas Chromatograph (pGC)
5.1 Generation I pGC
The first generation micro gas chromatograph was based on a proof-of-concept
thermal conductivity detector.
Thermal conductivity detectors
are concentration
dependent detectors, such that the response of the detector is proportional to the
concentration of the mobile phase with respect to the reference carrier gas. Scaling and
size reduction for thermal conductivity detectors exploits this phenomena, and can result
in increased levels of sensitivity [49] and improvements in signal to noise ratios [501. The
basic configuration of a TCD is shown in Figure 25.
80
SAMPLE GAS
V,
V
REFERENCE GAS
Figure 25. Full bridge thermal conductivity detector. The reference gas and sample gas
pass through opposing legs of the bridge, improving the overall sensitivity of the device.
Thermal conductivity detectors are often operated in a full bridge configuration,
under differential conditions in which the thermal conductivity of the carrier gas is
measured against the thermal conductivity of the eluents. These two gas streams are
passed through diagonal bridge sensors to double the sensitivity of the measurement [51].
Table 6 lists the thermal conductivities of a variety of gases.
Although TCD's can operate as single cell systems, the lack of a reference stream
measurement makes these systems susceptible to errors, especially as environmental
conditions fluctuate. At the very least, a two-cell configuration is recommended, while a
four-cell configuration yields the best results.
In differential mode, there are various methods that can be used to measure the
thermal conductivity, including the use of sensitive thermistors [52], hot wire filaments
[19], ultrasonic detectors [531, and optical devices [54]. The most common technique for
commercial bench top TCD's includes the use of hot-wire filaments. In recent years
81
1.
however, MEMS TCD's have shown great promise to replace hot-wire filament TCD's,
operating under the same measurement principle but at a reduced volumetric scale. For
this thesis, the high capital and temporal costs involved in developing and manufacturing
a MEMS TCD lead to exploring alternative methods of producing a low-cost TCD out of
off-the-shelf components.
Table 6. Thermal conductivity of various gases
Thermal
Conductivity
W/m-K (300 K)
Helium
0.142
Hydrogen
0.168
Methane
0.030
Oxygen
0.024
Propane
0.015
Nitrogen
0.024
The first TCD prototype was based off a highly sensitive thermistor. The thermistor2 1
was chosen based on its fast response time (500 ms), small size (360 jim), and low-cost.
Typical thermistor-based TCD designs are bulky, due largely to the mechanical mounts
21
Honeywell 111 Series thermistor
82
that hold the thermistor in place. In the first generation design, the total volume was
minimized by suspending the thermistor between two adjacent stainless steel tubes, as
shown in Figure 26. The tube tips were coated with a thin layer (1 pm) of Parylene@ so
as not to add a resistive path across the thermistor leads.
_iA
Figure 26. Suspended thermistor thermal conductivity detector. Enlargement on the left
shows a 360 pm diameter thermistor, ready to be sealed within glass tubes. The photo on
the right shows concentrically aligned stainless steel tubes with inlet and outlet union
fittings. Gold foil served as the thermistor's electrical leads.
Initial tests with the single cell TCD system showed that the thermistor was highly
sensitive to small fluctuations in flow and changes in gas type. Consequently, a four-cell
version was built, based on an integrated mechanical printed circuit board (PCB) design.
83
One of the challenges associated with the suspended thermistor design was being able
to ensure proper vertical alignment of the adjacent tubes. To accomplish this with offthe-shelf fittings, a series of custom hanging mounts where manufactured that maintained
the center of the fittings with the vertical top plane of the PCB. An example alignment
mount is shown in Figure 27.
Figure 27. Custom vertical alignment mount.
A four legged bridge configuration was then integrated into the parallel mounting of
four suspended thermistor cells, as shown in Figure 28.
All tubing for the four-cell TCD was 1.69 mm, type 304 stainless steel. Tube diameter
specifications were chosen in order to minimize the dead volume within each leg of the
detector. Symmetry in the design came from the preferred identical flow requirements of
each cell. Two T-shaped adapters were used for the reference and mobile phase streams.
Two trim capacitors were also used to adjust for any inherent resistive biases in the
84
thermistors.
I
L]i
10 mm
Figure 28. Four cell thermistor-based TCD with PCB integration.
A combined setup of the first generation iGC with the full bridge detector is shown
in Figure 29. Two 1.69 mm diameter columns, each of 0.3 m in length, were created such
that the path from the carrier gas separator to the TCD thermistors was the same for
both the separating column and the reference column. Both columns were lightly packed
with desiccated TIDE
® detergent,
which has been shown act as a simple separating
agent [29]. For the injection port, a simple T-shaped connector with a septum seal was
used.
85
Figure 29. First generation piGC, composed of the: (A) helium gas tank, (B) injection port
controller and data acquisition system, (C) carrier gas pressure monitor, (D) injection
port actuator, and (E) full bridge TCD.
The system was tested with 1 Ill stochastic injections of pentane over a 6-minute
period. Results from the first series of tests are shown in Figure 30.
The first generation 1iGC showed that it was possible to use stochastic methods with
a TCD, and that miniaturization using low-cost thermistors was possible. The PGC
detector, however, showed considerable drift over a long period of time. Because of the
high sensitivity of the thermistors, the system was also noisy in the presence of small
thermal fluctuations from the surrounding environment. Both of these sources of noise
contributed to the overall noise of the system, resulting in poor results as evident from
the large baseline noise visible in Figure 30. If a longer sample was obtained, the baseline
86
noise would be invariably reduced, as shown in the simulations. However, because the
piGC is designed to operate in both stochastic and tradition injection modes, the high
noise level was found to be unacceptable. Taking these results into consideration, along
with the mechanical fragility of the thermistors, a redesign of the TCD can be found
generation II, discussed in the upcoming section.
6
A-
4
CU
2
313-
0
-2
0
5
Time (s)
Figure 30. Single solute (pentane) chromatogram,
perturbation methods on the first generation 1iGC.
10
determined
using
stochastic
5.2 Generation II pGC
The second generation micro gas chromatograph prototype took the concepts from
the first generation thermal conductivity detector, and addressed several of the issues
associated with reducing drift and keeping the sensory unit at a constant temperature.
87
Due to the fragility of the small thermistors in the previous generation design, the TCD
sensor system was changed from a thermistor based sensor to a hot wire filament design.
Typical hot wire systems are bulky (see Figure 31), and costly, where a single filament
can range anywhere from 100 to 500 USD.
To reduce these costs, it would be possible to create filaments using raw materials
(such as thinly drawn tungsten wire), but making electrical contact with custom filaments
is challenging. However, miniature light bulbs often have finely drawn wire, are readily
available, and cost nearly one-tenth the price of a regular filament while offering similar
performance.
10 mm
Figure 31. Typical, commercially available TCD filament.
Several off-the-shelf miniature light bulbs were examined as possible candidates for
TCD filaments. A summary of their properties is shown in Table 7. Scanning electron
microscopy images in conjunction with micrometer measurements were used to estimate
the filament diameters.
88
Table 7. Properties of various light bulb filaments
70
Filament Diameter
(Im)
100
Cost
(USD)
2
#24X
70
100
1.5
#74
13.5
175
1.2
6418LL
3.4
2.2
5.5
211-2
1.2
3.5
2.5
Bulb Type
Resistance (n)
#24E
Thinner filament diameters were desirable due to the fact that thin filaments are
more resistive for a given length of the same material, and therefore will exhibit a larger
change in resistance. Also, the sensitivity of a filament increases as its size decreases [38],
determined by its heat capacitance and overall mass [39]. The #24 Series bulbs were
found to be the best candidates for TCD filaments, because of their resistive properties,
their filament sizes, and their overall cost.
Certain precautions should be taken when creating hot-wire systems to prevent
overheating and ensure that over excitation of the filaments is avoided. For the #24 Series
bulbs, the thermal-electrical behavior was observed by applying successively larger
amounts of power to a test filament while monitoring its thermal characteristics (see
Figure 32). A type A40 FLIR camera recorded surface temperature. The filaments were
tested until the filament burnt out from overheating.
89
600
0
500C)
400
0~
300
U>)
2001001
'aI
0
.
. . .
I .
.
E
0.4
0.2
Power (W)
0.6
Figure 32. Type 24E, 28 V bulb filament response.
The results show an approximate logarithmic relationship between the bulb's surface
temperature and the input power. The nonlinear change in temperature as a function of
input power suggests the bulb behaves in a similar manner to a diode. What is more
critical to note, however, is that the bulb temperature needs to remain below 250 C in
order to avoid causing permanent damage to the filament.
In order to prepare an off-the-shelve bulb to be used in the TCD, the filament's
metallic casing was first carefully removed by soaking the bulb in acetone to break down
the casing adhesive. The glass outer bulb structure of the filaments were then removed
by using a custom built, miniature, 3-axis attachment in conjunction with a diamond
band saw. The rotating bulb, when cut with the band saw, cleanly removed of the bulb
90
shell (Figure 33). The remaining bulb housing and filament were then rinsed and soaked
in ethanol for 12 hours and deionized water for an additional 12 hours. The bulb filaments
were then air-dried for another 24 to 36 hours. The resulting, exposed filament is shown
in Figure 34A.
Figure 33. Rotary attachment for bulb removal, composed of the: (A) diamond blade saw,
(B) DC motor and gearbox, (C) motor speed controller, and (D) dual axis stage
attachment.
In practice, hot wire filaments are either plug fitted or thread fitted onto the thermal
conductivity detector housing, because they must be able to withstand high temperatures,
and also maintain a seal under moderate pressures. Due to the relatively large volume
that threaded systems require, a series of plugs were designed and molded (see Figure 34)
through the use of high-temperature silicone (Smooth-On, Mold Max 60). Silicone was
91
chosen for its elastic properties, its high temperature resistance, and its relative chemical
inertness. The fitting dimensions of the silicone molds were determined experimentally,
resulting in light press fits between the TCD housing and the filament bulb structures.
The TCD housing was tested up to 100 kPa (well above the actual operating pressure of
the TCD) and at 250 C, in order to ensure the fittings maintained their seal under
elevated temperatures and pressures. Pressure measurements were monitored using an
absolute pressure transducer 22
5 mm
bi
A
B
C
Figure 34. Filament manufacturing process. After the bulb housing is remove (A), silicon
molds (B) were made. The final filament was then plugged into the TCD housing, shown
as a frontal view (C).
The housing for the TCD was made out of high purity copper because of its high
thermal conductivity. The bulbs were arranged in a linear configuration, such that the
22
Omega PX303 Series
92
measurement filaments were as close to each other as possible. Ferruled fittings were used
for the input and output gas streams. The TCD housing, and filament configuration
within the housing are shown in Figure 35.
Figure 35. Copper block TCD. Three views of the copper housing (above), and a CAD
drawing and actual assembly (below), show the layout of the TCD.
The housings of thermal conductivity detectors are typically held well above room
temperature, and preferably at or above the column temperature so as to avoid
condensation of the eluents. For thermal conductivity detectors, the stability of the TCD
93
housing significantly affects the quality of the signal [20].
The copper block TCD was therefore controlled by a multi-stage, stacked heating
scheme that included two outer stainless steel plates, two nickel-chromium (NiCr)
polyimide heaters, and two 10 W thermoelectric coolers (TEC). The TEC's were added
so that active cooling could take place, if necessary. Thermal control was developed and
built for the system through the use of a specialized integrated circuit (IC)2 3 that had
built-in
proportional-integral-derivative
(PID)
and pulse-width-modulated
(PWM)
control. The outer resistive heaters were controlled by a PID-PWM control scheme
developed in LabVIEW. A photo of the stacked TCD heating system is shown in Figure
36.
Figure 36. Stacked TCD heating system.
The second generation micro gas chromatograph was the first apparatus to include
23
Maxim 1979 IC
94
direct joule heating. The column2 4 was heated by applying a high power, DC signal across
its length. In order to accomplish this, the sample and the reference columns had to be
electrically isolated from all other components. Isolation was implemented by sealing the
columns between the carrier gas source and the thermal conductivity detector through
the use of high temperature Teflon ferrules. The columns were carefully aligned in the
axial direction such that the ferrules served as electrical insulators between the carrier
gas fittings and the TCD housing. An image of the assembled second generation 1 IGC is
shown in Figure 37.
Once sealed and isolated, a high current DC voltage was applied to the ends of the
column. Due to the thickness of the tubing, a high power amplifier was needed in order
to heat the column significantly. An example thermal image of the heat distribution is
shown in Figure 38.
The filament designed TCD worked significantly better than the Generation I TCD.
The bridge output was more stable, due in part to the active temperature control of the
TCD housing, and the more stable nature of the filaments.
The second generation pGC, however, still underperformed when compared to a bench
RESTEK-80442: Packed Column, 10% Rtx-1 Silcoport W 100/120, 2 mm ID
95
top device, and while the cost of the system was drastically reduced2 5 , the size of the TCD
did not scale well. One of the limiting scaling factors was the filament bulb plug housing,
which could not be eliminated without having to create custom-built filaments. Another
scaling factor was related to the TCD thermal control system. It would have been possible
to use smaller resistive heaters and TEC's, but these components also had limited heating
and cooling capabilities that were not sufficient for maintaining the TCD at the necessary
temperature. Additionally, from a volumetric and power consumption perspective, the
second generation TCD was not on an effective path towards creating a truly portable
gas chromatograph. Even at a reduced size, the TCD housing would still have required a
significant amount of power to heat it, its mass would have been large, and the required
supporting hardware would have limited the smallest possible footprint of the device.
It was therefore decided that the third generation device would be based on a flame
ionization detector, a smaller column, and an improved injection port. The device
mounting techniques, thermal systems, and control schemes would be taken from
generation I and generation II, and applied to the next generation device.
25
The entire TCD, at high volumes, cost less than 20 USD.
96
Figure 37. Generation II 1 iGC. Red dashed lines indicate carrier gas flow. Blue dashed
lines indicate mobile phase (carrier gas and vaporized sample) gas flow. The main
components include the: (A) thermoelectric controller, (B) NiCr heater control electronics,
(C) thermal conductivity detector, (D) reference column, (E) separation column, (F)
carrier gas pressure monitor, and (G) injection controller.
97
Figure 38. Thermal image of the second generation pGC during heating. The injection
port, on the left, reached 193 C. The internal temperature of the TCD was measured at
200 0 C, while the outer steel plate was at 112 0 C.
5.3 Generation III pGC
The third generation micro gas chromatograph was a transformational period in which
miniaturization took precedence. It was decided that a miniaturized flame ionization
detector may stabilize the signal output. The large pressure transducers of generation I
and generation II were replaced by miniaturized equivalents. The heating scheme
remained the same, but the packed column from the second generation device was replaced
with a general-purpose capillary equivalent. Finally, a miniaturized injection port design,
based on bench top versions, was designed and built.
98
5.3.1 Flame Ionization Detector Design
The second type of miniature detector that was built for the pGC was a flame
ionization detector. Unlike a thermal conductivity detector, FID's can only be used to
measure organic substances. In an FID, two electrodes are kept a constant distance apart,
over which a high bias is applied (typically greater than 150 V). The sample and the
carrier gas from the column output are then mixed with a hydrogen-air or hydrogen gas
stream. This hydrogen and sample mixture is then passed through a combustion jet, which
causes the sample to vaporize and ionize. The ionization current is approximately
proportional to the carbon content of the sample.
The flame ionization detector for the third generation 1iGC was built to prove both
the stability and usability of a miniaturized detection system while using an integrated
hydrogen source. Hydrogen was generated via three polymer electrolyte membrane cells
operating in parallel, to give an approximate flow rate of 21 ml/min.
To determine an approximate jet diameter that would accommodate a fast enough
flow rate for a sustained flame, a series of needles with diameters ranging from 1.27 mm
to 0.46 mm (18 to 26 gauge) were tested. A flame was able to be maintained at diameters
above 0.72 mm (22 gauge) for the amount of hydrogen flow produced. It was observed
that the smaller the jet tip diameter, the more stable the jet. At gauges above 26, however,
the jet tip was so small that the combustion portion of the flame remained inside the tip.
99
Since it was preferred that combustion of the analytes took place immediately upon
exiting the tip, such that the exhaust did not build up on the tip walls, a 0.57 mm (24
gauge) needle was chosen for the first FID.
In the initial tests, a 1.69 mm stainless steel tube was used as the collector electrode,
and was spaced 5 mm away from the jet tip in an open atmosphere. A high voltage bias
(200 V) between the proximal and distal electrodes was maintained using a precision high
voltage power source (Agilent B2962A). The proximal electrode was held at ground, while
the current, monitored in the circuit, was converted to a voltage using a resistor. Results
from the FID's steady state signal with helium as the carrier gas showed a noise level that
was indistinguishable regardless of flow pressure (0 kPa up to 50 kPa). Photos and a
thermal image of the initial FID test apparatus are shown in Figure 39 and Figure 40.
The on-board flame ionization detector housing was made from a 1.69 mm brass union,
whose outlet line was replaced with a 0.57 mm stainless steel tube.
100
Figure 39. Preliminary prototype of flame ionization detector.
Figure 40. Thermal image of flame ionization detector.
101
5.3.2 Injection Port Miniaturization Design
In previous designs the injection port was simply a sealed T-shaped fitting with a
septum top. The previous injection ports, however, exerted high backpressures on the
syringe piston, often resulting in bubbles entering the syringe during the duration of an
injection sequence. Previous injection ports also had poor flow characteristics, due in part
to their oversized vaporization chambers. Finally, previous injection ports had been heated
exclusively through joule heating, and were limited to around 150 *C. To raise the
injection port temperature above 150 C required significant power consumption, and still
resulted in poor flash vaporization. For these reasons, the third generation micro gas
chromatograph
required
a redesigned
injection
port,
based
on the
functional
characteristics of a bench top port, with a reduced volumetric footprint.
Both planar and cylindrical injection port geometries were considered, the former of
which would have been simpler to manufacture, but more difficult to seal. A cylindrical
design was chosen, based on the premise that concentric heaters could be applied
externally. Concentric heaters would enable a constant radial temperate profile to exist
within the injection port, eliminating edge effects that may be associated with a planar
design.
The injection port design for the third generation ptGC was based on a flash
vaporization tube, an upper and low mixing chamber, and the ability to easily seal and
102
modify the inlet and outlet gases. A schematic of the components is shown in Figure 41.
Carrier gas (He)
High temperature
seal separating upper
and lower chambers
Syringe
njecte
sample
Vaporized sample
and carrier gas
Vaporization chamber
(up to 500 OC)
Figure 41. Schematic of injection port.
Flash vaporization was designed to take place in the center of the port, occurring in
a quartz cylinder lightly stuffed with glass wool. Quartz was chosen because of its chemical
inertness and its ability to withstand rapid temperature fluctuations. The quartz tube
103
was held in the axial direction by a high temperature Viton@ washer, allowing the tube
to rest between the upper and lower portions of the injection port. The washer also served
as a seal that separated the input sample from the mixed sample. A small amount of
radial spacing was allowed on both the upper and lower portions of the injection port to
encourage mixing within both chambers before passing the carrier gas and sample to the
column.
There were two input lines and one output line connected to the injection port. The
carrier gas input line was a permanently attached 1.69 mm stainless steel tube. For the
purpose of allowing for various types of sample inputs, including a syringe or a valve
operated gas or liquid supply stream, the sample input line was a ferrule-based axial
connection, whereby the only requirement for the input line was that it be of a tubular
geometry less than 1.69 mm. Similarly, to accommodate for different types of columns,
the output line of the injection port was a ferrule-based system. The upper and lower
portions of the injection port were manufactured in two parts, to accommodate for the
internal flash vaporization tube.
The upper and lower portions were sealed through the implementation of a flanged
design and a high temperature graphite washer 2 6 , as shown in Figure 42. Graphite was
26 The graphite washer was produced through a combination of high-speed vertical drilling for the flange
holes, and low force, wire EDM machining for the internal and external outlines.
104
chosen as the flange seal because of its relative chemical inertness and its ability to
maintain a seal under high temperatures. Although a flanged design is less desirable from
the perspective of manufacturing, due to the increased part count and assembly
requirements, the two-piece design allowed for the placement of an interior quartz tube
and the use of ferruled fittings, while maintaining a large interval vaporization chamber.
A large vaporization chamber was desired, as the volumetric change of some liquids to
the gaseous state can be as high as 1:100.
Although the column temperature may be modulated during operation of the PGC,
the injection port chamber is typically held at a constant temperature. Thermal stability
of the injection port, aided in part by the thermal mass of the port, will result in improved
vaporization consistency. Therefore, the chamber was made from 316 stainless steel,
chosen for its mechanical strength, chemical inertness, and high heat capacity. A stainless
steel port also allows for the reduction of the wall thickness within the device, without
compromising the injection port's ability to maintain high pressures and withstand
mechanical vibrations during storage or use.
105
A
B
DI
E
D
F
CO
10 mm
Figure 42. Injection port components, including the: (A) upper injection port housing, (B)
lower injection port housing, (C) Viton @ washer, (D) quartz tube, (E) brass ferrule, and
(F) graphite washer.
5.3.3 Intermediate Fittings
The third generation design replaced the bulky ferrule fittings with permanently
sealed, miniature three way unions made from type 304 stainless steel. The unions were
located at both inlet gas lines, such that an external pressure transducer could monitor
both the carrier gas pressure and the FID gas pressure. Sealing between the pressure
transducers and the unions was made through a combination of light press fits and hightemperature epoxy. A photo of the FID and union are shown in Figure 43.
106
5 mm
Figure 43. Flame ionization detector built from off-the-shelf fittings.
5.3.4 Pressure Transducers
The pressure transducers for the third generation micro gas chromatograph were
chosen based on performance, size, mechanical structure, chemical compatibility, and cost.
As part of an integrated system, the sensors had to be able to measure possible carrier
gases (helium, air, nitrogen) and the flame ionization gas (hydrogen). It was preferable
that the sensors be both compact and low cost. A variety of MEMS-based pressure
transducers met these requirements, but many of them required the fabrication of custom
sealed housings in order to be integrated into the system. Additionally, many of the
standalone pressure sensors had plastic housings. Metallic housings were preferred, both
to be able to withstand the volatile chemicals that entered the injection port, and to
reduce the risk of contaminating the inlet gas streams. Additionally, metallic housings
would be more suitable to withstanding temperature fluctuations.
Based on these conditions, the Omega PX72 Series pressure sensor was chosen. The
107
PX72 Series is a gauge sensor capable of measuring helium, air, and hydrogen, with a high
degree of linearity ( 0.5% FS),
a relatively high storage temperature
(125 0C),
temperature compensation up to 80 C, and a full metal housing. In generation III, the
pressure transducers were horizontally mounted on custom fittings to monitor the inlet
gas pressures.
5.3.5 Column Configuration
In the third generation micro gas chromatograph, the packed, large diameter stainless
steel column was replaced by a quartz capillary system. Thermal control of the column
was performed using inductive and IR heating. Because of the low mass of quartz that
needed to be heated, and the physical dimensions of the column (less than 400 Pm
diameter), rapid heating and cooling was possible. Heating was performed by wrapping a
0.23 mm (32 gauge) NiCr wire at an approximate 2.5 mm pitch along the length of the
column2 7 , as shown in Figure 44.
The thermal performance of the column was tested with aid of a FLIR A40 thermal
camera. The surface temperature of the NiCr wrapped quartz capillaries were monitored
at a rate of 60 Hz while a series of successively larger step inputs were applied across the
2
Agilent HP-5 capillary column, with a 0.33 mm O.D.
108
heating element. An example thermal distribution from heating is shown in Figure 45.
Figure 44. Quartz capillary heating configuration. Quartz columns were wrapped in NiCr
wire.
250
200
150
100
Figure 45. Quartz capillary heat distribution. Blue indicates a temperature 25 C. Red
indicates a temperature of 350 C.
109
Heating and cooling times for the column depend on the column mass, the pitch of
the wrapped NiCr wire, the input power, and the environmental
characteristics
surrounding the column. Heating and cooling performance of the quartz capillary system
were quantified by measuring the change in surface temperate as a function of input power.
Input power was calculated from the product of input voltage and the resulting current.
The results, shown in Figure 46, show a nonlinear component to the heating slew rate.
Power (W)
180
160
13.725
12.663
140
11.44
10.41
120-
9.3225
8.175
7.11
6.1
5.0225
3.975
100
80
60
3.025
180
160
140\
o
e
120
Z
100
Ce
)
80
S
60
s
2.094
40
1.2735
40
0.601
20
0. 5
1
Time (s)
1.5
2
20
2
4
6
Figure 46. Transient heating (left) and passive cooling (right) curves for 330 Jm quartz
capillary column surrounded by NiCr wire with a pitch of approximately 2.5 mm. Each
curve represents a different step input of a prescribed power.
For each power input, the column cool down temperature was also measured. It was
noticed that, while heating rates were relatively fast for this system (~400 'C/sec), the
cooling rates were significantly slower (~100 'C/sec). Cooling rates were improved by
110
placing the column in a helium atmosphere (data not shown) and taking advantage of
helium's high thermal conductivity. However, this approach was not pursued further
because of the impracticality of maintaining the column within a helium atmosphere,
while at the same time minimizing the volume and mass of the overall pGC.
5.3.6 Mechanical Layout
All of the mechanical components were mounted onto a 60 mm acrylic plate, using a
set of custom manufactured aluminum mounts (see Figure 47). The mounts were designed
to keep the components elevated from the mounting plate and vertically aligned with each
other. A photo of the third generation lIGC is shown in Figure 48. The column, not shown,
had a minimum bend radius of 50 mm and was wrapped below the mounting plate. The
third generation iiGC was tested using stochastic perturbation techniques, in order to
ensure that a chromatogram could be obtained, before further design modifications were
made. The impulse response tests from the third generation pLGC are shown in Figure 49.
The results show a clear reduction in baseline noise, when compared with previous
generations.
111
a, .,
10i
10 mm
Figure 47. Board mounting components.
Distal Electrode
FID Jet
(Proximal Electrode)
4
Hydrogen
Pressure Monitor
Carrier Gas
T-Fitting
Carrier Gas
Pressure
Monitor
Hydrogen SupplyT-Fitting
FID
Injection
Port
10 mm
Figure 48. Third generation 11GC mechanical layout.
112
301
25
200
L 15
1
0-
E
5-
0-
-105
10
15
20
25
Time (s)
Figure 49. Chromatogram from the third generation piGC.
5.4 Generation IV pGC
Building on the functionality and performance of the flame ionization detector in the
third generation micro gas chromatograph, the fourth generation instrument was further
refined to include PCB integrated electronic controls, modular heaters, a reduced footprint,
and an improved FID chamber. These components are discussed in detail below.
5.4.1 Improved FID Design
A custom-built housing for the flame ionization detector was built for the fourth
generation device, in order to reduce the size of the detector and maintain the flame and
electrodes within an enclosed environment. The housing for the detector was made from
113
316 stainless steel with two external fittings. The front-end fitting was a ferrule system
connected directly to the column. The center housing served as the inlet port for the oxyhydrogen stream, and as a small dead volume mixing chamber for the column eluents and
the combustion gas. The back-end fitting connected the mixing chamber to the jet nozzle.
A schematic representation of the FID is shown in Figure 50.
Hydrogen gas (H)
Proximal electrode
Ask
I
Separated
sample
and carrier gas
Distal
electrode
& exhaust
+ 200 V
+
e-
C+
C
Ground
Figure 50. Flame ionization detector schematic.
The back-end fitting contained step-down diameter rods, compressed through a ferrule
fitting. A high temperature Teflon ferrule was used to seal the jet, so that it would be
electrically isolated from the FID housing and the column. At the jet output, a stainless
114
steel electrode 2 8 was placed in mechanical contact with the jet tip. A second, hollow
electrode 2 9 was concentrically placed approximately 5 mm away from the first electrode.
The second electrode also served as the exhaust vent of the system.
Both proximal and distal electrodes were isolated from the FID housing by means of
hollow ceramic fittings that were permanently affixed to the housing. A photo of the FID
jet is shown in Figure 51.
5 mm
Figure 51. Flame ionization detector, composed of the: (A) hydrogen gas inlet line, (B)
capillary column inlet, and (C) jet tip.
28
29
proximal electrode
distal electrode
115
5.4.2 Modular Heater Development
A key component to flash vaporization is maintaining the injection port at a high
temperature. In previous generations of the micro gas chromatograph, the injection port
was heated by direct joule heating. For the fourth generation piGC, a series of modular
heaters were developed in order to improve both the heating efficiency of the system and
the upper temperature limit of the injection port.
A concentric heater design was developed with the interest of creating low-volume,
low-mass modular heaters for the injection port. The design was based off of three
components: a copper sleeve, aluminum-nitride (Al-N) coatings, and nickel-chromium wire,
shown in layers in Figure 52.
Figure 52. Injection port modular heater, shown in layers.
To fabricate the heaters, an interior copper sleeve was custom made using wire EDM
machining, allowing for a minimal gap to exist between the injection port and the heater
sleeve. Copper was chosen because of its high thermal conductivity. The copper sleeve
116
was then coated externally with aluminum nitride (Al-N), a high electrical resistance,
high thermal conductance material. Aluminum nitride has a thermal conductivity almost
equaling that of aluminum, one to two orders of magnitude higher than most ceramics,
thereby ensuring the thermal response of the heater was rapid, and was capable of
inducing uniform concentric heating30 . After coating the copper sleeve, NiCr wire was
coiled around the heater, and then an additional layer of Al-N was applied. Between each
step, the coatings were step cured up to 500 0 C over a 36-hour period. The final result
was a rapid response, high temperature modular heater, with a wall thickness of 1.25 mm.
(
The three stages of the fabrication process are shown in Figure 53.
A
B
C
5 mm
Figure 53. Concentric heater fabrication steps. A copper sleeve (A) is coated with
aluminum nitride (B) and then wrapped with NiCr wire and coated with another layer of
aluminum nitride (C).
It should also be noted that the concentric design ensured a uniform heating profile in the radial
direction.
30
117
The modular heaters were both compact and able to maintain the injection port at
temperatures well above the necessary temperature. For most samples, the injection port
remained at less than 450 C. The heater was designed to reach up to 900 oC 3 , based on
the maximum temperature of the individual heater components, shown in Table 8.
Table 8. Operating temperature of heater components
NiCr (22 AWG)
Aluminum Nitride
Copper
Max Temperature
1085
1650
900
(OC)
5.4.3 Capillary Column Heating
Column heating from the third generation pGC instrument showed promise as a fastheating capillary system. The portability of the system was limited, however, by the
fragility of the column and its relatively large minimum bending radius. It was therefore
decided that if needed, the pGC could accommodate quartz columns, but future designs
would focus on more mechanically robust stainless steel capillary columns.
With metallic capillary columns, heating could be controlled through direct joule
heating, as was performed with the second generation device. Similar to the second
generation heating scheme, the coil would have to be electrically isolated between each
31
The maximum heater temperature will actually be far below 900 *C, as dictated in part by the column
operating temperature.
118
individual winding. Three approaches were taken to accomplish electrical isolation
between each successive coil winding.
The first approach was to wind the column along grooved tracks of a guiding structure,
while keeping each respective track separated. For example, for a 350 im column, a M2
screw with a pitch of 400 pm was sufficiently large such that as the column sat in each
individual groove and remained out of contact with each successive turn, as shown in
Figure 54. For this configuration, high temperature PEEK
@ screws
were used as column
guides.
10 mm
Figure 54. Photo (left) and thermal image (right) of guided coil heating scheme.
32
PEEK® screws can operate at continuous temperatures up to 250 0C.
119
This scheme only allowed for one row of coil windings, whose total length was
determined by the bend radius, the total screw height, and the screw pitch. In this
configuration, the screws were also found to bend over time from the compression of the
wrapped column. Metallic screws could not be used because of their electrical conductivity.
Ceramic screws or threaded bars allowed for another possibility. However, ceramic screws
were found to be prohibitively expensive for large-scale systems, and too brittle for a
portable system.
A second means of coil isolation was performed by creating an array of high
temperature Delrin-PTFE guides, as shown in Figure 55. In this configuration, the coil
was wound through a 20 by 3 array of 400 pm holes. This design allowed for several
concentric windings to be wrapped, thereby greatly increasing the maximum allowed
column length. This system, however, had thermal limitations based on the Delrin-PFTE
structure. Additionally, the friction between the guides and the coil became successively
larger with each winding, making this configuration difficult to assemble.
120
10 mm
Figure 55. Photo (left) and thermal image (right) of Delrin-PTFE guided column.
A third design involved the sheathing of the column with a high temperature material
such as fiberglass or braided ceramic. A sheath would insulate the column from itself, so
that joule heating would occur over a single resistive element, and would provide
insulation to the column from ambient conditions that may cause temperature
fluctuations. Under this design, three types of insulation were tested: polyimide sheathing,
braided fiberglass, and high temperature braided ceramic. Polyimide sheathing (Figure
56) worked well in terms of electrical insulation, but worked poorly as a thermal
insulator 33 . A second sheathing of high temperature braided ceramic was also tested. This
Polyimide also allowed for the possibility of direct column coating, if the system was mass-produced, in
a similar manner to how motor wire is fabricated.
3
121
sheathing performed well in electrically and thermally isolating the column. The cost,
however, was prohibitively expensive. An intermediate solution was found by using
fiberglass sheathing that could be maintained at temperatures up to 316
0 C,
while
providing good thermal isolation and costing one-tenth the price of the ceramic alternative.
10 mm
Figure 56. Photo (left) and thermal image (right) of polyimide sheathed column. The
thermal image shows that polyimide sheathing provides good electrical insulation but poor
thermal insulation.
5.4.4 Improved Electrolyzer
In the fourth generation pGC, an improved electrolyzer was built so that the system
could operate at higher flow rates. The electrolyzer was based on a scaled version of the
third generation design, with the addition of an automatic refill system and upper and
lower manifolds for the hydrogen and oxygen (see Figure 57). Hydrogen was generated
122
from a battery of eight off-the-shelf polymer electrolyte membrane (PEM) electrolyzers.
Each of the eight cells was 25 mm by 25 mm, and the electrolyzer consumed up to 10 W
(total) under steady state conditions. An 80 mm diameter disk, or two, stacked 28 mm
diameter disks would yield the equivalent surface area required to switch from a multicell to a single-cell electrolyzer. The FID system was tested only using the hydrogen
output of the electrolyzer, but could also be switched to an oxy-hydrogen source and still
provide the necessary gas flow for combustion.
-Refill Tank
PEM Cells
Hydrogen
Manifold
Figure 57. Eight cell polymer electrolyte membrane electrolyzer.
The hydrogen output pressure of the eight-cell system was also measured, to verify
123
that the electrolyzer would output a high enough pressure to mix properly with the mobile
phase eluents. A plot of the pressure versus the input current is shown in Figure 58. A
second order exponential was fit to the curve, and the output pressure was then computed
as a function of PEM cross-sectional area. The results suggest that the PEM's can produce
hydrogen at a rate of 11.2 L/m 2 , with an output pressure of 1.4 MPa/m 2 , at a power
.
consumption rate of 7.2 kW/m2
7
6
5
CU)
C,)
4
3
0D 2
S
C
0
2
4
Current (A)
6
8
Figure 58. Eight cell polymer electrolyte membrane electrolyzer output pressure, shown
as a function of total input current.
5.4.5 Layout Modifications and PCB Integration
In the fourth generation pGC, an improved mechanical layout was implemented in
124
order to reduce the planar footprint of the system. Initially, the pressure transducers were
changed from horizontal, custom mounts to vertically oriented, self-mounting components.
This allowed for the elimination of the pressure transducer mounts and the horizontal
space that they occupied. It also allowed for the pressure transducer housings to serve as
vertical and horizontal alignment structures. Due to the rearrangement of the transducers,
the 3-way unions from the third generation were replaced with custom, stainless steel
hexagonal fittings. The injection port mount was also changed by replacing the single,
center-mounted flange holder with two, bookend holders, allowing for the ability to use
the pressure transducer, instead of the mount, as an axial alignment tool. The new
arrangement is shown in Figure 59.
Column
High Voltage
Converter
Hydrogen
Pressure Monitor
FID
Housing
\FID Jet
(Proximal Electrode)
Carrier Gas
T-Fitting and
Pressure Sensor
10 mm
Figure 59. Fourth generation 1 iGC.
125
The mechanical mounting plate from the previous instrument generation was replaced
with a PCB. Printed circuit board manufacturing in high volumes is a practical means of
obtaining tight tolerance mechanical components at a low price. The layout of the PCB
was designed around the injection port and the FID mounts, and served as the alignment
structure and the mechanical base of the device. From the mechanical side, four major
alignments occurred. Both the injection port and the flame ionization detector were
restricted to rotational and single axis, translational motion through the use of elevated
concentric mounts. These two cylindrical housings were then connected to the helium
input and hydrogen input lines respectively, which further constrained their rotational
and lateral motion. All parts were fixed in the vertical direction, by attachment either
directly to the PCB, or from the mechanical PCB mounts.
The supporting system circuitry was designed around the basic operating functions
of the device. After determining the mechanical layout, any available free space was used
for the necessary supporting electronic components that performed thermal control and
instrumentation measurement. The PCB support circuitry involved several base level
operations, including: temperature sensing, signal amplification, power supply regulation
for the heaters, pressure sensors, and supporting integrated circuits, high voltage boost
conversion, and low-noise signal amplification. The board communicated with a PC that
monitored and controlled everything, through a series of parallel input-output (I/O) lines.
126
Several physical restrictions on the PCB existed. The mechanical mounts for the flame
ionization detector and the injection port restricted the lateral free space available. The
bodies of the pressure transducers consumed a significant portion of the PCB surface
area3 4 . Traces were placed sparingly under the high voltage supply, since switching
regulators tend to generation noise that can add spurious signals to routing paths. An
image of the PCB layout is shown in Figure 60.
Figure 60. PCB layout of fourth generation pGC.
There was a design tradeoff here, as the pressure transducers served a dual electro-mechanical function
as part of the monitoring system and the mechanical alignment system.
3
127
5.4.6 Electronics
The support electronics for the fourth generation ptGC included heating circuits for
thermal control, support electronics for the flame ionization detector, pressure transducer
regulators, and a series of regulators for the IC's and other electronic components.
The heater amplifiers were based off a driver-MOSFET pair that was designed to
meet the bandwidth and power requirements of the modular heaters. Because the heaters
were resistive elements, the thermal response time of the heating system was dependent
on the operating voltage input and the maximum allowable current of the restive elements.
In this case, the current draw from the resistive elements was significantly less than both
the maximum current of the MOSFET's and the drivers, thereby allowing the heaters to
be operated at the maximum value of the input voltage. The MOSFETS were rated up
to 30 V (12 A).
The high voltage bias for the flame ionization detector was generated by a 5 V to 200
V boost converter35 . For the detection circuit that measured the ion current between the
two FID electrodes, a low input bias 36 current precision amplifier was used (LMP 7721).
3
36
UMHV Series from HVM Technology, Inc.
3 femtoamps
128
Heating control was performed using a proportional-integral (PI) scheme coupled with
a PWM transformation. Heater temperatures were monitored using type K thermocouples,
coated in Al-N to prevent electrical shorting. The thermocouple voltages were converted
to temperatures using an eighth order polynomial according to NIST [55]. The thermal
feedback was then processed through a proportional-integral (PI) control scheme written
in a custom script in LabVIEW. The controller output for each independent heater was
then converted to a PWM signal. The resulting PWM signals drove independent driverMOSFET pairs, which subsequently output the amplified signals to the respective
modular heaters. A schematic of the control scheme is shown in Figure 61. Feedback gains
for each individual heater were tuned experimentally.
s~s -
PW M
HEAERR
T
TYPE K
THERMOCOUPLE
Figure 61. Proportional-integral (PI) and pulse-width-modulated (PWM) heating control
scheme.
5.4.7 Assembly
After the PCB was populated (see Figure 62), the fourth generation pGC was
assembled onto the PCB substrate. The final version shown in Figure 63 includes a 4m,
129
polyimide-sheathed column. The fourth generation ptGC had a 45 mm diameter, a 25 mm
height, and a mass of 50 g (see Figure 63).
Although the fourth generation assembly was characterized by a compact and efficient
use of space, it was found in practice that the modular heaters (not shown in the figure)
were heating up nearby components, including the high voltage boost converter and the
carrier gas pressure transducer housings. Although no noticeable noise was observed in
those respective signals, it was decided to increase the footprint slightly so as to avoid
permanently damaging the components. Minor issues in the fourth generation device
design also included the input power line standoffs, which were insufficient to handle the
power requirements of the system. Minor modifications addressing these issues would be
incorporated into the fifth generation pGC.
TOP
BOTTOM
10 mm
Figure 62. Populated PCB from the fourth generation pGC.
130
10 mm
Figure 63. Final assembly of the fourth generation pGC.
5.5 Generation V pGC
Modifications were made to the fifth generation pGC to address some of the heating
and electrical overload issues experienced in the fourth generation pGC. To accommodate
for the thickness of the modular heaters and their surrounding heat signature, the board
diameter was increased from 45 mm to 50 mm.
In addition to increasing the board size, thermal standoffs were added to the PCB
design to address the issue of localized heating that occurred in the NiCr heater wires.
Tests performed with NiCr showed that directly soldering NiCr wire to the PCB created
significant, localized hot spots. While the temperatures at these hotspots remained below
131
the maximum PCB tolerance,
temperature
extended
exposure
and temperature
fluctuations could cause damage to the PCB over time. Additionally, fluctuation in the
board's temperature could add an additional source of noise to the system, in terms of IC
performance, and the performance of supporting components. For these reasons, small
brass standoffs where added to the PCB, such that the resistance of the standoffs was
significantly less than the resistance of the NiCr wire, allowing the majority of the heat
to dissipate within the standoff and not the PCB.
The input, output, and power lines were also modified between the fourth and fifth
generations. In the fourth generation, two 1 mm pitch I/O ports were located on the back
of the PCB (Figure 62), adding 5 mm to the overall thickness of the device. During testing,
the input power lines exceeded their maximum current rating, and some of the surface
mounted connections melted. To eliminate this effect, the power lines were moved to a
separate standoff, while I/O ports were replaced by a micro HDMI jack, which reduced
the overall height of the ports while maintaining the number of signal lines. Photos of the
fifth generation PCB are shown in Figure 64.
132
10 mm
BOTTOM
TOP
Figure 64. Populated PCB from the fifth generation p.GC.
The PCB board layout was also changed between the fourth and fifth generations.
The fifth generation design moved away from a double side populated PCB (see Figure
65). All electrical components were moved to the bottom layer so that the PCB provided
a thermal shield from the top layer's heated components.
133
Figure 65. PCB layout of fifth generation pGC.
The final mechanical assembly of the fifth generation was similar to the fourth
generation. The same mounts, pressure transducers, and fittings were used. Input lines
for the carrier gas and the FID gas were permanently affixed to the three-way fittings
with high-temperature epoxy. A photo of the fifth generation pGC is shown in Figure 66.
The column
in the fifth generation device was moved to a 7 mm tall cylinder that was
affixed to the bottom of the PCB.
3
RESTEK-71815: MXT-1 Cap. Column, 20 m, 0.18 mm ID
134
dColumn
Prsure Monitor
hV
a
Convet
Hosing
Carrier Gas
Pressure Mantrr
mIcro44DMI 1/0
Injection Pr
FID Distal Electrocle
10 mm
Figure 66. Fifth generation 1iGC. The sheathed column (not visible) is wrapped in a coil,
underneath the device.
Figure 67. Front panel of LabVIEW program that controls and monitors the system
parameters, controls the injection port system, and records the input and output signals.
A superimposed thermal image of the system (bottom left) shows the heat distribution of
the concentric injection port heaters.
135
All of the component signals were integrated into a custom-built LabVIEW program.
The front panel virtual instrument that was used to control the heaters, monitor the
pressure transducers, control the injection port and acquire the FID output is shown in
Figure 67.
The fifth generation pGC was able to generate chromatograms using both stochastic
and traditional injection techniques. One example of the device operating using traditional
injection techniques is shown in Figure 68, where an injection of bourbon was passed
through the system, while the column temperature was modulated from 50 0 C to 300 0 C.
An example of the pGC operating using stochastic injection techniques will be discussed
in the upcoming section.
I
I
I
I
I
I
I
I
4.5
4
3.5F
3
cc 2.5F
2
1.5
1
0.5
U
i .ii
IL
0
2
4
6
8
10
Time (min)
12
14
16
18
Figure 68. Chromatogram of a well-known American bourbon.
136
20
Chapter 6
pGC Performance Characterization
6.1 Design and Specifications Summary
The final design specifications for the fifth generation pGC are shown in Table 9.
Table 9. Performance specifications of the fifth generation PGC
50 mm (diameter) x 32
Size
mm (height)
Mass
50 g
Temperature Range
25 0 C to 450 0 C
Column Type
capillary and micropacked
Power Consumption
up to 50 W (dependent on
operating temperature)
Detector Type
flame ionization detector
Operating Pressure
700 kPa
Max Heating Rate
207 'C/min
Measuring Time
sample dependent (seconds
to minutes)
137
The total cost breakdown of the pGC, shown in Table 10, is an estimation of the
device cost for an initial run of 2500 units. The cost does not included machining costs
for the mechanical components, or any assembly costs. From the initial breakdown,
however, we see that the largest contributors to the cost are the pressure transducers, the
high voltage converter, and the PEM, accounting for approximately 74% of the total cost.
The column cost, which is a significant investment for bench top gas chromatographs,
actually scales with size.
Costs can be reduced further if some of the propriety technologies are removed. For
example, the pressure transducers and the high voltage converter are currently sourced
from proprietary suppliers, but their designs are simple enough to be manufactured inhouse. Cost savings with regards to the PEM are more difficult to reduce. Inexpensive
PEM's are rare, and tend to perform poorly. Regarding the cost breakdown, the estimated
cost of the fifth generation pGC is about 0.5% the cost of a bench top equivalent, and
around 10% the cost of the cheapest portable GC available on the market.
138
Table 10. Cost breakdown of fifth generation pGC
4
Stock metal
3
PCB
30
Components
1
Insulation
50
High voltage
converter
4
Ferrules
38
Column
70
Pressure
transducers
PEM
105
TOTAL
$305
6.2 Basic Flow and Operation
The flow pattern of the pGC involves two gas inputs, one liquid input, and two mixing
chambers. A diagram of the input and output flows is shown in Figure 69. Initially, the
sample is input into the injection port in the liquid or gas state, upon which is it
immediately vaporized and mixed with the carrier gas. The mixture is then passed through
the capillary column where its eluents are separated. The output of the column is then
fed into the flame ionization detector housing, where it is mixed with hydrogen gas,
supplied by the electrolyzer. The hydrogen-sample-carrier gas mixture is then passed
through the flame ionization detector jet, ionized, and measured. The exhaust gases are
then vented.
139
Hydrogen gas
Carrier gas (He)
Sample
Figure 69. Gas flow patterns for the fifth generation piGC.
6.3 Thermal Performance
The external and internal temperatures of the heating port were monitored to verify
a proper operating system and ensure stable internal temperatures during injection. Two
type K thermocouples were embedded within the modular heaters on the front and back
of the injection port. An additional type K thermocouple was placed inside the injection
port. Example heating and cooling curves for the injection port operating in open loop
are shown in Figure 70. The heat distribution of the surface temperature of the injection
port was observed to be constant. The internal temperature of the injection port was
140
found to lag behind the external heater temperatures, in both heating and cooling modes.
At steady state, however, all three temperatures reached the same temperature, within
0.75 0 C.
-
140
120-
100-
-Front Injection Port
-Rear Injection Port
-Internal Injection Port
140
Time (min)
140
120
100-
-Front Injection Port
-Rear Injection Port
-Internal Injection Port
1
40
-
1-082
0
a. 60-
E
40-
20-
5
10
Time (min)
15
20
Figure 70. Heating (above) and cooling (below) performance of the injection port.
141
The injection port heating rate was also determined by subjecting the injection port
heaters to a series of voltage step inputs. Figure 71 shows the thermal performance of the
heaters, for various power inputs, using the same temperature-monitoring scheme.
500
I
450-
-loW
400-
20W
-40W
350*-300250L
E 200..
150 10 0 -
.............
......-
50 -
00
0.5
1
15
2
25
3
3.5
4
4.5
5
Time (min)
Figure 71. Injection port heating rates. The temperature was measured inside the injection
port.
From the figure, we see a power relationship between the input power and the thermal
heating rate38. It should be noted that although the injection port heating system is
relatively slow, the injection port is designed to be operated at steady state. The thermal
inertia of the system, therefore, can be advantageous as it will make the injection port
38 10
W (16 OC/min), 20 W (34 'C/min), 40 W (80 'C/min)
142
less susceptible to environmental variations. Figure 72 shows the closed loop performance
of the injection port, using the PI-PWM control scheme discussed previously. The time
to reach steady state approaches ~2 min. At steady state, the observed ripple was caused
partially by the tuned PI parameters, and may also have been related to the accuracy of
the thermocouples (+1%).
150
-internal Inj. Port
-Setpoint
-100
E
50-
0
0.2
0.4
0.6
0.8
1
Time (min)
1.2
1.4
1.6
1.8
Figure 72. Closed loop performance of injection port temperature.
The thermal performance of the column was also measured by mounting a type K
thermocouple directly onto the column's external surface using a small amount of
143
aluminum nitride adhesive. The heating rates 39 of the column (Figure 73), for various
power inputs, were found to be significantly higher than those that were observed for the
injection port, due in part to the column's low mass.
-
-
-
150
-18W
-8.3W
-4.62W
0
-100
0.
C0
I
0.1
I
I
I
I
0.2
0.3
0.4
0.5
Time (min)
0.6
07
0.8
0.9
Figure 73. Column heating rates.
The time constant of the closed loop heating system (Figure 74) was also shorter,
allowing the column to reach operating temperature 40 within 30 to 60 seconds, depending
on the set point and input power.
39
4.62 W (49 0C/min), 8.3 W (88 0C/min), 18 W (207 *C/min)
4 An ambient temperature of 25 0C is assumed.
144
r-
100r-
c
90
80
______
___
70
0
60
L
50
E 40
30
-Internal Inj. Port
-Setpoint
20
100
0.1
0.2
0.3
0.4
I6
0.5
0.6
I.7
0.7
0.8
1_____9
0.9
1
1
Time (min)
Figure 74. Closed loop performance of column temperature.
6.4 Stochastic Perturbation Results
The fifth generation pGC was found to have an extremely stable output FID signal.
The lower level noise oscillation was around 20 mV peak-to-peak, compared to a 5 V
signal, as shown in Figure 75. The noise was stable over time, meaning that it could be
filtered out when using traditional injection methods, and could be greatly reduced when
using stochastic methods. The FID response of the system was observed to be higher than
that of the bench top system, in the sense that smaller amounts of sample were needed in
order to induce a response. This was due to the fact that the injection port was being
operated in splitless mode. A chromatogram from a stochastic sequence of pentane
injections, using the fifth generation device is shown in Figure 76. The chromatogram was
145
comparable to the same injection made using the traditional injection mode, with a similar
retention time and peak height.
n r%,2
U
00 .02
0
.04-n
(0
I
_n n
I
2
1
3
4
5
Time (s)
6
7
9
8
10
Figure 75. Fifth generation pGC noise signal.
8
-
7
I
I-
6
C/)
0
C/)
5
4
3
E
-
Cln
1
I
0
-1
0
20
40
60
80
100
120
140
Time (s)
Figure 76. Single solute (pentane) chromatogram,
perturbation methods on the fifth generation 1 GC.
146
determined
using stochastic
Chapter 7
Conclusion
This thesis explored the use of stochastic perturbation system identification
techniques, as applied to gas chromatography, and the implementation of those techniques
on both bench top and miniature gas chromatographs. A handheld gas chromatograph
was developed (Figure 77), over the course of five device generations. Characterization of
each prototype led to successively smaller, and more stable designs.
The stochastic perturbation techniques, as shown through the simulations and the
various implementations, were found to reduce noise in chromatographic measurements
and improve the processing capabilities of a single gas chromatograph. With the addition,
for example, of multiplexing methods, it was possible to analyze multiple samples on the
same gas chromatograph, simultaneously and continuously. Along the same lines, it was
possible to analyze a single sample on multiple columns, simultaneously. This type of
parallel processing expands the versatility and adaptability of gas chromatographs, so
that they can be configured to adapt to and monitor multiple environmental variables at
the same time.
147
The five generations of prototypical development documented over the course of this
thesis resulted in a handheld GC that cost less than 1% of its bench top equivalent, at a
fraction of the volume and mass. At a manufacturing price around 300 USD, an
instrument as powerful as a gas chromatograph may have the opportunity to see more
widespread deployment, in factories, in small business, and in environmental monitoring.
The applicability of stochastic methods in the miniaturized design also enabled continuous
sampling and processing, thereby removing a trained technician from the equation and
allowing a 11GC to function as a standalone monitoring device.
In future work, more exploration into stochastic perturbation chromatography is
recommended, along with the investigation into the nonlinear effects of temperature
perturbation. Currently, any samples that require a temperature ramp must be processed
by the pGC using the traditional injection method. In terms of hardware and software
design, the pGC could benefit from thermal isolation mounts, and a separate
microcontroller for monitoring and controlling the device. Future development should also
consider the possibility of gaseous injections, and the miniaturization of hardware that
will be able to control stochastically modulated gas inputs.
148
/
Figure 77. Handheld,
chromatograph.
continuous
measurement,
149
stochastic
perturbation
gas
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