Lecture X
Gas Chromatography
Outline
GC Theory
What are the separations?
Instrumentation
Applications
Conclusions
Brief request for next week’s lecture
FT-IR.
Gas Chromatography (GC)
This method depends upon the
solubility and boiling points of organic
liquids in order to separate them from
a mixture. It is both a qualitative
(identity) and quantitative (how much
of each) tool.
GC Theory
 An inert gas such as helium is passed through the column as a
carrier gas and is the moving phase. A sample is injected into a
port which is much hotter than the column and is vaporized. The
gaseous sample mixes with the helium gas and begins to travel with
the carrier gas through the column. As the different compounds in
the sample have varying solubility in the column liquid and as these
compounds cool a bit, they are deposited on the column support.
However, the column is still hot enough to vaporize the compounds
and they will do so but at different rates since they have different
boiling points. The process is repeated many, many times along the
column. Eventually the components of the injected sample are
separated and come off of the column at different times (called
"retention times").
 There is a detector at the end of the column which signals the
change in the nature of the gas flowing out of the column. Recall
that helium is the carrier gas and will have a specific thermal
conductivity, for example. Other compounds have their own thermal
conductivities. The elution of a compound other than helium will
cause a change in conductivity and that change is converted to an
electrical signal. The detector, in turn, sends a signal to a strip
chart recorder or to a computer. Detectors come in several
varieties, for example, thermal detectors, flame-ionization and
electron capture detectors.
Additional Information
From
http://www.shu.ac.uk/schools/sci/che
m/tutorials/chrom/gaschrm.htm
Theory
In order to understand GC, need to focus
on the general principles of separations
which has its roots in solvent extraction.
The theory of solvent extraction are used
to explain all forms of chromatography i.e.
HPLC, LC, GC, EP, and TLC.
Consider two solvents S1 and S2 and solute
X that is in S1. The partition between the
two phases.
X(S1)

X(S2);
GC Theory
Partition coefficient K is an
equilibrium constant is
K = [X]S1/[X]S2. Suppose that solute
X in V1 (water) is extracted with V2
(CCl4). Let m be the moles of X in
the system and let q be the fraction
of X remaining in phase 1 at
equilibrium. The molarity in phase 1
(water) is therefore qm/V1.
GC Theory
The fraction of total solute transferred to
phase 2 (CCl4) is (1-q) and the molarity in
phase 2 is
(1-q) m / V2. Then
K = ((1-q) m / V2)/ q m V1.
For GC:
K = Cs/Cm where Cs is the concentration in
the stationary phase (column) and Cm is the
concentration in the mobile phase (gas).
Suppose I2 in H2O is 1 and I2 in CCl4 is 0.
After shaking and letting settle, I2 in H2O is
0.0014 and I2 in CCl4 is 0.9986. Therefore
K = 7 x 102. Fraction (q) remaining after the 1st
extraction is:
q = V1/(V1+KV2). Fraction remaining after n
extractions is
qn = (V1/(V1+KV2))n. It is more efficient to do
several small extractions than one large one. This
extraction can be used to describe GC where the
liquid is the mobile phase and the column is the
stationary phase. Each extraction is a theoretical
plate.
Theory of separation
Water + I2
Water + I2
Water + I2
CCl4 + I2
CCl4
CCl4
+ I2
Start
Shake 1
Shake 2
Theory GC
As the gas moves the solute (analyte)
through and over the stationary
phase, the solute will be in equilibrium
with the gas and the solid phase.
Since there is a mobile phase, the
separation will appear as a
chromatogram showing the separation
of the analytes.
Diffusion in and out of column pores
Solute in and out of packing by diffusion.
GC Theory
Analyte
To detector
Column + packing
Time
Separations
To detector
Time 1
Time 2
Time 3
Capacity Factor (k’)
•
While inside the column, a retained component spends part of its
time on the stationary phase and part time in the mobile phase
 • When in the mobile phase, solutes move at the same speed as the
mobile phase
 • this means that all solutes spend the same amount of time in the
mobile phase (to)
 • the amount of time the solute spends on the stationary phase is
equal to tR- to(adjusted retention time, t’R)
 •the ratio t’R/ to is the capacity of the column to retain the solute (k’)
tR
to
t’R
k’ = (tr - t0) / t0
Inject
.
Unretained
Solute
k’ = (t’r ) / t0
GC Process
Column Efficiency (N)
Solutes are placed on an GC column in a narrow band
 • Each solute band spreads as it moves through the column due
to diffusion and mass transfer affects
 • The later eluting bands will spread more
 • Peak shape follow a Gaussian distribution
to
t1
t2
Band spreading eventually causes peaks to merge into the baseline. We want to
minimize band spreading as much as possible.
Chromatogram nomenclature
GC Peak parameters
Typical separation
GC Peak fitting
GC General
The chromatogram shows the order of
elution (order of components coming off the
column), the time of elution (retention
time), and the relative amounts of the
components in the mixture. The order of
elution is related to the boiling points and
polarities of the substances in the mixture.
In general, they elute in order of
increasing boiling point but occasionally the
relative polarity of a compound will cause it
to elute "out of order". This is analyzing
your sample.
Elution Order








Compound
Boiling Point (0C)
pentane
36
hexane
69
cyclohexane
80
isooctane
99
toluene
110
4-methyl-2-pentanone 117
octane
126
Example Chromatogram
The observed elution pattern appears
below. Notice the reversed elution of
toluene and 4-methyl-2-pentanone.
GC Chromatogram
Column performance
Plate height H = L / N where L is the length of
the packed column and N is the number of
theoretical plates.
For example: A solute with a retention time of
407 s has a width at the base of 13 s on a 12.2
m long column.
N = 16 * 4072/ 132 = 1.57 X 104.
H = 12.2 / 1.57 X 104 = 0.78 mm;
Van Deemter equation:
H = A + B / x + C x. All three terms A, B, and
C contribute to band broadening.
A is for multiple paths, B / x is due to
longitudinal diffusion,
C x is due to equilibration time.
Peak performance
Longitudinal diffusion is important
because diffusion takes place along
the axis of the column and
contributes to peak broadening. The
faster the linear flow, the less time
spent in the column and the less
diffusional broadening occurs.
Separation efficiency
Efficiency of separations will depend on:
1. Elution times of the peaks,
2. Broadness of the peaks.
2 = 2Dm t = 2DmL /x.
Plate height due to long diff:
HD = 2Dm /x = B / x where Dm is the
diffusion coefficient of the solute in the
mobile phase and t is time and L is column
length.
Column efficiency
C x comes from the finite time required
for a solute to equilibrate between the
mobile phase and the stationary phase.
Although some solute is stuck in the
mobile phase, most of it will move on and
elute. The plate height for finite
equilibration time i.e. mass transfer is:
'
2
kd
Hs  '
 x  C x
2
(k  1) DS
More column stuff
Ds = is the diffusion coefficient of
the solute in the stationary phase, d
is thickness of the stationary phase,
and k’ is the capacity factor.
Effected by temperature and
thickness of stationary phase.
GC Instrumentation
GC Instrumentation
 Carrier gas
The carrier gas must be chemically inert. Commonly used gases
include nitrogen, helium, argon, and carbon dioxide. The choice of
carrier gas is often dependant upon the type of detector which is
used. The carrier gas system also contains a molecular sieve to
remove water and other impurities.
 Sample injection port
 For optimum column efficiency, the sample should not be too large,
and should be introduced onto the column as a "plug" of vapour slow injection of large samples causes band broadening and loss of
resolution. The most common injection method is where a
microsyringe is used to inject sample through a rubber septum into
a flash vapouriser port at the head of the column. The temperature
of the sample port is usually about 50°C higher than the boiling
point of the least volatile component of the sample. For packed
columns, sample size ranges from tenths of a microliter up to 20
microliters. Capillary columns, on the other hand, need much less
sample, typically around 10-3 mL. For capillary GC, split/splitless
injection is used. Have a look at this diagram of a split/splitless
injector;
Instrumentation
 Detectors
 There are many detectors which can be used in gas
chromatography. Different detectors will give different
types of selectivity. A non-selective detector responds to all
compounds except the carrier gas, a selective detector
responds to a range of compounds with a common physical or
chemical property and a specific detector responds to a
single chemical compound. Detectors can also be grouped into
concentration dependant detectors and mass flow dependant
detectors. The signal from a concentration dependant
detector is related to the concentration of solute in the
detector, and does not usually destroy the sample Dilution of
with make-up gas will lower the detectors response. Mass
flow dependant detectors usually destroy the sample, and the
signal is related to the rate at which solute molecules enter
the detector. The response of a mass flow dependant
detector is unaffected by make-up gas.
Components of GC:
Column, oven, injector, and detector.
These parameters (HETP, etc) are
affected by the various components of the
instrumentation. Perhaps the column is the
most important component of the GC. With
it, different separations can be
accomplished.
See Figure 27-1 Pg 703 of text for
instrumentation.
GC Instrumentation
Column temperature
For precise work, column temperature must be
controlled to within tenths of a degree. The
optimum column temperature is dependant upon the
boiling point of the sample. As a rule of thumb, a
temperature slightly above the average boiling
point of the sample results in an elution time of 2
- 30 minutes. Minimal temperatures give good
resolution, but increase elution times. If a sample
has a wide boiling range, then temperature
programming can be useful. The column
temperature is increased (either continuously or in
steps) as separation proceeds.
Mobile Phase (gas)
GC Under the hood
GC Column and Oven
Typical GC (dual column)
Sample Injections
Next, the sample injection system. Here it is
important that the sample be injected onto the
column as a plug and of a suitable size. Also, the
injector should provide consistent and reproducible
injections. See Figure 27-3, Pg 704. The microsyringe is used to load the sample onto the column.
The syringe should be clean and accurate and gas
tight. The syringe is injected through a rubber
septum. The septum should be replaced after
many injections to insure gas tightness onto the
column. An auto sampler can be used to inject the
samples. Typical volumes range from 0.2 to 20
Ls. With capillary columns it is necessary to use
a splitter. (See Figure.) A suitable solvent is also
necessary for the proper separations and
injections.
GC Injector
Carrier Gas
This is the mobile phase and should be pure
gas so as not to react with the column or
analyte. Gas is usually He, Ar, N2, or H2.
Choice will depend on the type of detector
used. He and H2 give better resolution
(smaller plate height) than N2. Pressure is
also important and as expected the system
comes with regulators. Can you find where
in GC equations that are dependent on
pressure?
Columns
The column is the most important component of
GC. Here is where the separations take place.
All the various equations we discussed above are
dependent on properties of the column. There are
four types of columns: wall-coated open tubular
(WCOT), support coated open tubular (SCOT),
micropacked, fused silica open tubular (FSOT), and
packed column. The FSOT column is the most
flexible. Open tubular is also capillary. Particle
size is important because the efficiency of GC
column increases rapidly with decreasing particle
size of the packing material.
Column
The column sits in a temperature controlled
environment that is 0.50. Temperature is
very important in GC. Can you remember
what equations are affected by
temperature? See page 706 Fig. 27-5 for
temperature effects on separations.
Normally, one does a temperature program
to get the various analytes off the column
for better separations (resolutions).
Columns
 There are two general types of column, packed and capillary (also
known as open tubular). Packed columns contain a finely divided,
inert, solid support material (commonly based on diatomaceous
earth) coated with liquid stationary phase. Most packed columns are
1.5 - 10m in length and have an internal diameter of 2 - 4mm.
 Capillary columns have an internal diameter of a few tenths of a
millimeter. They can be one of two types; wall-coated open tubular
(WCOT) or support-coated open tubular (SCOT). Wall-coated
columns consist of a capillary tube whose walls are coated with
liquid stationary phase. In support-coated columns, the inner wall of
the capillary is lined with a thin layer of support material such as
diatomaceous earth, onto which the stationary phase has been
adsorbed. SCOT columns are generally less efficient than WCOT
columns. Both types of capillary column are more efficient than
packed columns.
 In 1979, a new type of WCOT column was devised - the Fused
Silica Open Tubular (FSOT) column;
FSOT column
Capillary column
Detectors
How is the analyte detected? Several detectors
are available for GC.
FID (flame ionization detector) is the most widely
used detector. See figure 27-6, Pg 707. Based
on the production of ions when compounds are
burned then detecting the current produced from
the ionization. What compounds can not be
detected with this detector?
TCD (thermal conductivity detector). Operates
on the changes in the thermal conductivity of the
gas stream brought about by the presence of
analyte molecules. See Figure 27-7 on page 708.
He is the carrier gas most often used with this
detector because it has a high thermal
conductivity.
Detection
 The effluent from the column is mixed with hydrogen and
air, and ignited. Organic compounds burning in the flame
produce ions and electrons which can conduct electricity
through the flame. A large electrical potential is applied at
the burner tip, and a collector electrode is located above the
flame. The current resulting from the pyrolysis of any
organic compounds is measured. FIDs are mass sensitive
rather than concentration sensitive; this gives the advantage
that changes in mobile phase flow rate do not affect the
detector's response. The FID is a useful general detector
for the analysis of organic compounds; it has high sensitivity,
a large linear response range, and low noise. It is also robust
and easy to use, but unfortunately, it destroys the sample.
Flame Ionization Detector
Detectors continued
ECD (electron capture detector). Uses Ni-63 as a
radiation source to cause ionization of the
substance using N as the carrier gas. Detector is
sensitive to functional groups containing
electronegative species such as halogens, quinones,
peroxides, and nitro groups. Hence, very good
detector for environmental analysis where
pesticides need to be measured. See Figure 27-8
page 709.
AED (atomic emission detector). It is a AA unit
using MIP that accepts the output from the GC.
See Figure 27-9, Page 709.
Mass spectrometer
Different GC detectors
PDD (pulsed discharge detector)

The VICI PDD (pulsed discharge detector) utilizes a stable, low powered, pulsed DC
discharge in helium as an ionization source. Performance is equal to or better than
detectors with conventional radioactive sources.
In the electron capture mode, the PDD is a selective detector for monitoring high
electron affinity compounds such as freons, chlorinated pesticides, and other halogen
compounds. For this type of compound, the minimum detectable quantity (MDQ) is at
the femtogram (10-15) or picogram (10-12) level. The PDD is similar in sensitivity and
response characteristics to a conventional radioactive ECD, and can be operated at
temperatures up to 400°C. For operation in this mode, He and CH4 are introduced just
upstream from the column exit.
In the helium photoionization mode, the PDD is a universal, non-destructive, high
sensitivity detector. The response to both inorganic and organic compounds is linear
over a wide range. Response to fixed gases is positive (increase in standing current),
with an MDQ in the low ppb range.
The PDD in helium photoionization mode is an excellent replacement for flame ionization
detectors in petrochemical or refinery environments, where the flame and use of
hydrogen can be problematic. In addition, when the helium discharge gas is doped with
a suitable noble gas, such as argon, krypton, or xenon (depending on the desired cutoff
point), the PDD can function as a specific photoionization detector for selective
determination of aliphatics, aromatics, amines, as well as other species. (Click here for
an ionization potential chart in .pdf format.)
PDD detector
Applications
Pesticides
 Analysis of pesticide residues in soil, water, and food is
crucial for maintaining safe levels in the environment. The
PDD in the ECD mode is highly selective for monitoring
electron capturing compounds such as chlorinated pesticides
and other halogens. This chromatogram illustrates the
sensitivity of the non-radioactive PDECD for such compounds.
Sample: Pesticide calibration mix Detector mode: Electron
capture Detector temp: 330°C Column: 25 m x 0.32 mm x
25 µm, HP-5 Column temp: 150°C to 300°C at 10°C/min
Sample volume: 1 µL, 10:1 split Discharge gas: Helium, 30
mL/min Dopant gas: 5% methane in helium, 2.4 mL/min
Attenuation: 1
Pesticide separations
Retention time (sec)
Headspace gas chromatography analysis
Headspace GC (HSGC) analysis employs a
specialized sampling and sample introduction
technique, making use of the equilibrium
established between the volatile components
of a liquid or solid phase and the gaseous /
vapor phase in a sealed sample container.
Aliquots of the gaseous phase are sampled
for analysis.
Headspace sampling
Headspace sampling
HSGC
Examples of HSGC are the forensic analysis
of blood and urine alcohol levels, quality and
production control of diesel fuel and beer
constituents. Aromatic flavors and trace
volatiles in foods and soft-drinks are also
readily analyzed. and HSGC analysis of volatile
free fatty acids produced by bacteria,
particularly anaerobic bacteria, enables a
fingerprint of the particular microorganisms
to be obtained, which assists in the
identification of the bacteria.
Food analysis
Analysis of foods is concerned with the assay of
lipids, proteins, carbohydrates, preservatives,
flavours, colorants and texture modifiers, and also
vitamins, steroids, drugs and pesticide residues and
trace elements. Most of the components are nonvolatile and although HPLC is now used routinely for
much food analysis, GC is still frequently used. For
examples, derivatization of lipids and fatty acid to
their methyl esters(FAMEs), of proteins by acid
hydrolysis followed by esterification (N-propyl
esters) and of carbohydrates by silylation to produce
volatile samples suitable for GC analysis.
GC Food
 GC quality control analysis of food products can confirm the
presence and quantities of the analytes For example, fruits,
fruit derived foodstuffs, vegetables and soft drinks, tea and
coffee, were analyzed for their polybasic and hydroxy acid
contents (citric, maleic acids) as TMS derivatives.
 All food and beverage products on sale today must be
carefully assayed for contamination with pesticides,
herbicides and many other materials that are considered a
health risk. The analysis of food involves separating and
identifying very complex mixtures, the components of which
are present at very low concentrations. GC is the ideal
technique for use in food and beverage assays and tests.
Furthermore, the origin of many herbs and spices can often
be identified from the peak pattern of the chromatograms
from their head space analysis.
Food and Cancer
Chemicals that can cause cancer have a
wide variety of molecular structures and
include hydrocarbons, amines, certain
drugs, some metals and even some
substances occurring naturally in plants and
molds. In this way, many nitrosamines have
carcinogenic properties and these are
produced in a number of ways such as
cigarette smoke. GC can be used to
identify these nitro-compounds in trace
quantities.
Drugs
 There are still numerous GC applications involving both
quantitative and qualitative identification of the active
components and possible contaminants, adulterants or
characteristic features which may indicate the source of the
particular sample. Forensic analysis frequently users GC to
characterize drugs of abuse, in some cases the
characteristic chromatographic fingerprint gives an indication
of the source of manufacture of the sample or worldwide
source of a vegetable material such as cannabis.
 Analytical procedures, chromatographic methods and
retention data are published for over 600 drugs, poisons and
metabolites. These data are extremely useful for forensic
work and in hospital pathology laboratories to assist the
identification of drugs.
Pyrolysis gas chromatography
Pyrolysis GC (PGC) is used principally for the
identification of non-volatile materials, such
as plastics, natural and synthetic polymers,
drugs and some microbiological materials. The
thermal dissociation and fragmentation of the
sample produces a chromatogram which is a
fingerprint for that sample. The small
molecules produced in the pyrolysis reaction
are frequently identified using a GC-MS
system and information on molecular
structure for identification is also obtained.
Metal chelates and inorganic materials
Although inorganic compounds are generally
non-volatile, GC analysis can be achieved by
converting the metal species into volatile
derivatives. Only some metal hydrides and
chlorides have sufficient volatility for GC.
Organometallics other than chelates, which
can be analyzed directly, include boranes,
silanes, germanes, organotin and lead
compounds.
Environmental analysis
Environmental pollution is an age-old trademark of
man and in recent years as technology has
progressed, populations have increased and standards
of living have improved. So the demands on the
environment have increased, with all the attendant
problems for the ecosystems. Combustion of fossil
fuel, disposal of waste materials and products,
treatment of crops with pesticides and herbicides
have all contributed to the problems. Technological
developments have enabled man to study these
problems and realize that even trace quantities of
pollutants can gave detrimental effects on health and
on the stability of the environment. There is a vast
amount of literature on the use of GC for studying a
wide variety of these problems.
GC application
 Every year many new substances are synthesized that differ
radically from the natural products that exist in biosystems.
The Environmental Protection Agency is empowered to control
water pollution and the production, use and disposal of toxic
chemicals.
It follows that detailed studies must be made of their effect
on the environment and their method of movement through
the ecosystem. Many of the compounds are not
biodegradable and will thus progressively pollute the
environment. There are a number of tragic examples of
which DDT (dichlorodiphenyltrichloroethane) and the PCBs
(polychlorinated biphenyls) are well known instances. The
materials of interest are present in environmental samples at
very low concentrations and are often to be found among a
myriad of other compounds from which they must be
separated and identified. It follows that GC, with its
inherent high sensitivity and high separating power, is one of
the more commonly used techniques in the analysis of
environmental samples.
Various configurations:
Column: capillary
Autosampler
Computer controlled for data acquisition
and analysis of peaks. Some come with
their own compressor for the air supply.
Sensitivity is very important because of
detection limits. More expensive system
will use a MS for the detector to do
GC-MS. Can do headspace analysis for
volatiles in water etc.
GC Companies
ThermoQuest
Agilent (Hewlett Packard)
Perkin-Elmer
Buck Scientific
Thermo Nicolet
Applications
Go to PDF files
Summary
GC separations are based on a
partitioning between the analyte with
the mobile phase (gas) and the
stationary phase (column).
Remember your equilibrium constants?