STUDIA UNIVERSITATIS BABES-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
NEW CONCEPTS AND SENSING MATERIALS FOR THERMAL
CONDUCTIVITY DETECTORS IN GAS CHROMATOGRAPHY
Adrian Bot, Rodica Turcu, Izabella Peter, Viorel Cosma, Vasile Surducan
National Institute for Research & Development of
Isotopic and Molecular Technologies, POBox 700, 3400
– Cluj-Napoca, România.
We report two new detector designs, which improve the sensitivity
of TCD in order to make it suitable for trace analysis and with capillary
columns:
- a half bridge "classic" thermal conductivity detector consisting by two
microcells – sample measuring and carrier reference – with conducting
polymers based temperature-sensitive resistors;
- a new concept of differential thermal conductivity detector based on
pyroelectric effect, consisting by two cavities in a single block, each with
a polyvinylidene fluoride (PVDF) pyroelectric transducer.
The thermal conductivity detector (TCD) was the first detector used in gas
chromatography and, despite the general dissatisfaction with the problem of his
relative low sensitivity, it still remains, after more that 50 years, one of the most
commonly used detectors, even though more sensitive and specialized detectors
have been developed. The advantage of the TCD lies in the detection of gases such
as CS2, H2S, SO2, CO, NO, NO2 and CO2 in gas-solid chromatographic analysis on
packed columns. Some of the advantages of TCD are its simplicity, stability,
versatility and low cost; one of his best features is the ease of quantitative analysis.
The internal volume of the thermal conductivity detector is relatively large,
thus the necessity to use high flow rate of column effluent (15÷50 ml/min); this
represents an incompatibility with the new modern opened capilary columns, wich
operates at lower flow rate [1, 2].
The thermal conductivity detector with polypyrrole ribbon filaments
The TCD consists of a metallic block containing a cavity through which
the gas flows. A heated element is positioned upon the thermal conductivity of the
gas. For practical considerations a differential method is usually used that requires
two cavities and two heated elements. Only carrier gas is passes trough one cavity
and the column effluent through the other. The two most commonly used detector
transducers are resistive wires (metallic filaments) and thermistors (beads of
metallic oxide). Their operation is similar except that filaments have a positive
coefficient of resistance and thermistors have a negative coefficient of resistance .
The choice between thermistor or filament is usually based on working temperature
considerations - thermisors for ambient or sub-ambient and filaments for higher
temperatures. Once the decision has been made to use filaments, the selection is
based on the corrosiveness or oxidation characteristics of the materials to be
ADRIAN BOT, RODICA TURCU, IZABELLA PETER, VIOREL COSMA, VASILE SURDUCAN
analyzed. Filaments are fabricated from a variety of metals, the most common
being tungsten; other materials are nickel, rhenium-tungsten and gold platted
tungsten.
Based upon our experience in obtaining and manufacturing for different
applications of conducting polymers, we have designed a new TCD with
polypyrrole ribbon filaments used as temperature-sensitive transducers.
Table 1.
Physical properties of some materials used as temperature-sensitive transducers
Thermistor
Polypyrrole
Physical properties
Tungsten Nickel
Density
10 -3 δ [kg m-3]
19,3
8,9
3,5
1,6
Specific heat
10-2 cp [J kg-1 K-1]
1,33
4,44
7,5
11
Thermal conductivity λ [W m-1 K-1]
173
90
30
0,3
Electrical resistivity
10-8 ρ [Ω m]
5,4
6,9
≈ 60.000
≈ 12.500
Temperature coefficient
α [K-1]
0,0048
0,0068
- 0,16
- 0,0039
The Table 1. presents some of the physical properties of films made by
naphtalensulfonate doped polypyrrole [3, 4] compared with actually used materials.
As we can see, the polypyrrole film has a relative low temperature coefficient,
which represents a disadvantage, but the assembly of properties allow appropriate
design solutions which guaranties the good performances of the detector.
a.
b.
Fig.1. The detection cell with polypyrrole ribbon temperature-sensitive resistors (a) and
the electrical scheme of the detection circuit (b).
The Figure 1.a. presents a transverse section of the TCD block (1) with the
two cavities - one for the column effluent and the other for reference gas. The
temperature –sensitive elements are made by polypyrrole film – 0,015x1,4x16 mm
ribbon (3); the resistance legs are made by gold plated Kovar Ø0,6 mm (7) and are
electrical insulated from the cell block by Teflon passage rings (4), which assure
also the pneumatic insulation of the cavities – the cell must be perfect gastight.
NEW CONCEPTS AND SENSING MATERIALS FOR THERMAL CONDUCTIVITY DETECTORS
The two polypyrrole ribbon filaments (as good as possible electrically and
mechanically matched) are connected into two arms of a Wheatstone Bridge
(Fig.1.b.); two conventional resistors (R) comprise the remainder of the bridge.
When the thermal conductivity of the gas in one of the cavities changes as a result
of the sample being eluted from the column, the temperature and the resistance of
the detector element in that cavity change and an imbalance of the bridge appear.
An instrumentation amplifier (Ainst) amplifies the small signal. The readout can be
a potentiometric recorder or a DAQ system. The matching in/out is done by the
digital resistive attenuator (Ratten). With the same gas passing trough both cavities
the network is balanced by the offset potentiometer (Roff) so the electrical output is
zero.
Polypyrrole films (PPY) were obtained by electrochemical polymerization
in galvanostatic conditions. Among the different types of ions used for in-situ
doping polymerization of PPY we selected the following organic ions which results
in stability and good mechanical properties of the polymer: p-toluensulfonate (TS-)
and naphtalensulfonate (NS-). The ions concentration in the synthesis solution was
varied in the range 0.01-0.1 M. The electropolymerization was carried out on
stainless steel electrodes by using current densities in the range 0.11- 4 mA/cm2.
The as-synthesized PPY films were peeled off from the electrode, washed and
dried. Flexible freestanding PPY films with good mechanical properties and
thickness in the range 10-20m were obtained. The electrical conductivity of PPY
films measured by the standard four contacts methods by using painted silver
contacts has values in the range 37- 92 S/cm.
1,35
ENS1
ENS4A
ETS10i
ETS11A
ETS8
ETS9
1,30
Normalized resistance [R/R20]
1,25
1,20
80 S/cm
65 S/cm
37 S/cm
92 S/cm
85 S/cm
87 S/cm
1,15
1,10
1,05
1,00
0,95
0,90
0,85
-70
-60
-50
-40
-30
-20
-10
0
10
20
30
40
50
60
70
80
O
Temperature [ C]
Fig.2. The slope of the electrical resistance vs. temperature for some polypyrrole films.
ADRIAN BOT, RODICA TURCU, IZABELLA PETER, VIOREL COSMA, VASILE SURDUCAN
A better stability of the electrical properties was obtained for PPY doped
with NS- ions as compared with TS- doped ones, due to the compactness of the
resulting structure for NS- doped polymer. The temperature dependence of the
electrical resistance for PPY films doped with both types of ions (TS and NS)
shows a reversible behavior in the temperature range -100  +120 C; at higher
temperatures (140<T<250 C) irreversible changes of the polymers properties or
the films degradation could appear.
From the figure 2 one can see that the slope of the resistance vs.
temperature dependence is higher for the samples doped with NS- ions as compared
with TS- doped ones. This means that the activation energy of the interchains
charge transport process is higher for NS doped PPY. This fact can be attributed to
the structural differences between the two doping ions (NS and TS respectively)
which strongly influence the PPY morphology and consequently the interchains
distances. The control of the resistance vs. temperature variation can be done
mainly by two synthesis parameters: the nature and the concentration of the doping
ions. In order to obtain a strongly variation of the resistance vs. temperature for
PPY films, the synthesis should be performed with lower concentrations of NS doping ions.
Table 2.
Estimated performances of polypyrrole ribbon filaments compared with metallic filaments
and thermistor beads
Performance
Weight (without legs)
m [μg]
Lateral surface
Sl [mm2]
Rezistance @ 25OC
R [Ω]
Resistance variation at 1mW power
input
ΔR1mW/R [%]
Power consumed at 100OC P100 [W]
Power losses trough legs ΔP 100 [W]
Excellence coefficient of sensitivity
Sl x ΔR1mW
Excellence coefficient of power
consumption
1 / (P 100 +ΔP100)
Ø0,033mm
Thermistor
bead
Ø1,2mm
Polypyrrole
ribbon
0,015x1,4x
16 mm
2,36
32,1
25
2,20
3,9
8000
0,504
42
100
0,0105
0,00649
0,0965
0,00704
0,89
0,18
1,16
0,12
0,48
0,08
0,41
0,02
0,278
0,208
0,280
0,296
0,93
0,78
1,78
2,32
Tungsten
filament
Nickel
filament
Ø0,025mm
3,45
26,5
40
We have presented in the Table 2 some of the estimated performances of
the ribbon polypyrrole filaments compared with metallic wires and thermistor
beads; we have defined two excellence coefficients – one of sensitivity and the
other of power consumption – which allow a better comparison between the new
design and the classic TCD's.
The new device can operate at cell temperatures between –100  +115OC
and we estimate an increase of sensitivity more than ten times compared with hotwire filaments or thermistor beads made TCD. Moreover this detector has a few
advantages related to the low temperature differential between sensitive resistors
NEW CONCEPTS AND SENSING MATERIALS FOR THERMAL CONDUCTIVITY DETECTORS
and cell block and low power consumption, which allow a major decrease of cavity
volume, thus the using of lower column effluent flow rate (0,5÷2 ml/min).
The conducting polymer has a superior chemical resistance and doesn't
degrade to different chemical agents – especially corrosive or oxidizing conditions.
The thermal conductivity detector with pyroelectric transducers
One of the most sensitive thermoelectric transducers at this moment is the
pyroelectric sensor, still it has two disadvantages. First, all the pyroelectric
materials are also piezoelectric, thus a major source of electric noise due to
mechanical vibrations. Second, the pyroelectric sensor is a dynamic transducer,
i.e. it can measure only a change of temperature not a stable one.
Considering the distribution of samples in the column effluent a dynamic
process (usually a Gaussian distribution), we have designed a thermal conductivity
detector based on pyroelectric effect, consisting by two cavities in a single block
(Figure 3) each with a polyvinylidene fluoride (PVDF) pyroelectric transducer (8).
Fig.3. The detection cell with polyvinylidene fluoride pyroelectric transducer (in this
section on can see only one of two identical cavities with transducers)
The temperature of the base block of the detector (2) is controlled with a
principal thermoelectric module (10); the temperature difference between the wall
of the microcells (4) and the substrate of the pyroelectric sensors is done with a
second thermoelectric module (3). The Peltier elements assure the best temperature
stability, which is the essential condition for the measuring performances of this
detector. The temperature of the base and the cell blocks are measured with the
integrated semiconductor sensors (7). The heat pumped by the thermoelectric
modules is evacuated by the forced air convection cooled finned heat sink (1). The
assembly of the detector is thermally insulated with the neoprene foam cover (9).
When a component of the analyzed sample flow trough the measuring cell,
the difference between its thermal conductivity and that of the carrier gas produce a
ADRIAN BOT, RODICA TURCU, IZABELLA PETER, VIOREL COSMA, VASILE SURDUCAN
40
60
90
30
50
80
20
40
70
10
30
60
0
20
-10
10
-20
0
-30
-10
-40
-20
10
-50
-30
0
-60
-40
-10
-70
-50
-20
-60
-30
-80
0
10
20
30
40
50
60
70
80
90
50
40
30
20
Current mode signal [pA]
Voltage mode signal [mV]
Temperature variation [mK]
variation of heat transfer, and, as a consequence, a temperature variation of the
pyroelectric sensor which will generate an electrical charge (Figure 4).
100
Time [sec]
Fig.4. The theoretical electrical response – voltage mode and current mode - of the
pyroelectric PVDF transducer (25μm, 0,2cm2) at a chromatographic mode thermal
excitation (four components with Gaussian distribution)
The resulted small current signal (tens of pA to nA) is voltage converted
trough an electrometric amplifier. The reference cell serves to compensate the
piezoelectric noise and the parasite signals due to the residual variation of the
detector temperature and the variation of the flow rate. After the analog digital
conversion, the electric signal is numerical processed and integrated in order to
obtain the real chromatogram.
This pyroelectric thermal conductivity detector (PYTCD) can operate at
two temperature ranges: –35  +70OC and +50  +125OC, depending upon the
class of thermoelectric modules used. The PVDF sensors are gold-sheathed and the
detector has a good chemical resistance. We estimate an increase of sensitivity
more than hundred times compared with usual TCD's, at a temperature differential
of just a few degrees. The thermal slew rate is very good and the volume of cell
cavities is minimal - tens of μl - thus this detector can operate at very low flow rate
of column effluent.
Conclusions
To create an appropriate image of the technical performances of the new
thermal conductivity detectors, we have presented in the Table 3. some of the
estimated characteristics, compared with usually metallic wires filaments and
thermistor beads based TCD's [5, 6, 7, 8]. On can see the improvements of the
sensitivity and linearity, but mostly the major decreasing of the internal volume.
NEW CONCEPTS AND SENSING MATERIALS FOR THERMAL CONDUCTIVITY DETECTORS
Table 3.
Compared performances of the four types of thermal conductivity detectors
Technical characteristics
Working temperature range
[OC]
Sensitivity
Linearity
Thermal time constant
[sec]
Internal volume
[ml]
Gas flow rate
[ml min-1]
Metallic
wire
filaments
Thermistor
beads
Polypyrrole
ribbon
filaments
50÷450
-100÷50
-100÷115
10-6
10-4
0,2
4
15÷60
10-7
10-2
0,5
0,25
2÷8
10-7
10-4
0,1
0,088
0,5÷2
Pyroelectric
transducers
-35÷70
50÷120
10-8
10-5
0,01
0,018
0,25÷1
All those performances make the new thermal conductivity detectors –
with polypyrrole ribbon filament and with pyroelectric PVDF transducers - suitable
for trace analysis and use with new modern capillary opened columns.
References :
1.
2.
3.
4.
5.
6.
7.
8.
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