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Chem. 331
Chapter 27
Gas Chromatography
Introduction:
In gas chromatography (GC), the sample is vaporized and injected onto the head of a
chromatographic column. Elution is brought about by the flow of an inert gaseous mobile
phase. In contrast to most other types of chromatography, the mobile phase does not
interact with molecules of the analyte; its only function is to transport the analyte through
the column.
Gas liquid chromatography is based upon the partition of analyte between a gaseous
mobile phase and a liquid phase immobilized on the surface of an inert solid.
Gas-Liquid Chromatography
Retention Volumes:
To take into account the effects of pressure and temperature in gas chromatography, it
useful to use the retention volumes. The specific retention volume, Vg is defined as
follows:
Vg 
JF (t r  t m )
 273

W
 Tc 
where, J = the pressure drop factor, F = average flow rate, tr and tm = retention times W =
mass of stationary phase and Tc = column temperature in Kelvin.
Instruments for GC
The schematic of a gas chromatograph is shown below:
Carrier Gas Supply:
Carrier gases, which must be chemically inert, include helium, argon, nitrogen,
carbon dioxide, and hydrogen. The choice of gases is often dictated by the detector used.
Associated with the gas supply are pressure regulators, gauges, and flow meters. In
addition, the carrier gas system often contains a molecular sieve to remove water or other
impurities.
Flow rates are normally controlled by a two-stage pressure regulator at the gas cylinder
and some sort of pressure regulator or flow regulator mounted in the chromatograph.
Inlet pressures usually range from 10 to 50 psi, which lead to flow rates of 25 to 150
mL/min with packed columns and 1 to 25 mL/min for open-tubular columns. Flow rates
can be established using a soap-bubble meter.
Sample Injection System:
Column efficiency requires that the sample be of suitable size and be introduced
as a plug of vapor; slow injection of oversized samples causes band spreading and poor
resolution. The most common method of injection involves the use of a micro-syringe to
inject a liquid or gaseous sample through a silicone rubber diaphragm or septum into a
flash vaporizer port located at the head of the column. For ordinary analytical columns,
sample sizes vary from a few tenths of a microliter to 20microliters. Capillary columns
require much smaller samples (~0.001microliters)
Column Configurations and Column Ovens:
Two general types of columns are encountered in gas chromatography, packed and
open tubular, or capillary. To date, the vast majority of gas chromatography has been
carried out on packed columns. Chromatographic columns vary in length from less than
2m to 50m or more. They are constructed of stainless steel, glass, fused silica, or Teflon.
In order to fit into and oven for thermostating, they are usually formed as coils having
diameters of 10 to 30cm.
Column temperature is an important variable that must be controlled to a few tenths of a
degree for precise work, so the column is ordinarily housed in a thermostatted oven.
The optimum column temperature depends upon the boiling point of the sample and the
degree of separation required. Roughly, a temperature equal to or slightly above the
average boiling point of a sample results in a reasonable elution time. For samples with a
broad boiling range, it is often desirable to employ temperature programming, whereby
the column temperature is increased either continuously or in steps as separation
proceeds. The figure below shows the improvement brought about by temperature
programming. In general, optimum resolution is associated with minimal temperature;
the cost of lowered temperature, however, is an increase in elution time and therefore the
time required to complete an analysis.
Detectors
Characteristics Of The Ideal Detector:
1. Adequate sensitivity.
2. Good stability and reproducibility.
3. A linear response to analyses that extends over
several orders of magnitude.
4. A temperature range from room temperature to
at least 400degC.
5. A short response time that is independent of flow rate.
6. High reliability and ease of use.
7. Similarity in response toward all analyses.
8. Nondestructive of sample.
Flame Ionization Detector:
The flame ionization detector (FID) is one of the most widely used and generally
applicable detectors for gas chromatography. The effluent from the column is mixed with
hydrogen and air and then ignited electrically. Most organic compounds, when pyrolyzed
at the temperature of a hydrogen/air flame, produce ions and electrons that can conduct
electricity through the flame. A potential of a few hundred volts is applied across the
burner tip and a collector electrode is located above the flame. The resulting current is
then directed into a high impedance operational amplifier for measurement. Functional
groups such as carbonyl, alcohol, halogen, and amine, yield fewer ions or none at all in a
flame. In addition, the detector is insensitive toward noncombustible gases such as H2O,
CO2, SO2, and NOx. These properties make the flame ionization detector a most useful
general detector for the analysis of most organic samples, including those that are
contaminated with water and oxides of nitrogen and sulfur. The flame ionization detector
exhibits a high sensitivity, large linear response range, and low noise. It is generally
rugged and easy to use. A disadvantage of this detector is that it destroys the sample. A
diagram of a typical FID is shown in below.
Thermal Conductivity Detector:
A very early detector for gas chromatography is based upon
changes in the thermal conductivity of the gas stream
brought about by the presence of analyte molecules. This
device is sometimes called a katharometer. The sensing
element of a katharometer is an electrically heated element
whose temperature at constant electrical power depends
upon the thermal conductivity of the surrounding gas. The
heated element may be a fine platinum, gold or tungsten wire or, alternatively, a
semiconducting resistor. The advantage of the thermal conductivity detector is its
simplicity, its large linear range, its general response to both organic and inorganic
species, and its nondestructive character. A limitation of the katharometer is its relatively
low sensitivity.
Thermoionic Detector:
The thermoionic detector (TID) is selective toward organic compounds containing
phosphorous and nitrogen. It is similar in structure to the flame detector.
Electron-Capture Detector:
Electron-capture detector (ECD) operates in
much the same way as a proportional
counter for measurement of X-radiation.
Here the effluent from the column passes
over a beta-emitter, such as nickel-63 or
tritium (adsorbed on platinum or titanium
foil). An electron from the emitter causes ionization of the carrier gas (often nitrogen)
and the production of a burst of electrons. In the absence of organic species, a constant
standing current between a pair of electrodes results from this ionization process. The
current decreases however in the presence of those organic molecules that tend to capture
electrons. The electron-capture detector is selective in its response, being highly sensitive
toward molecules that contain electronegative functional groups such as halogens,
peroxides, quinones, and nitro groups. Electron-capture detectors are highly sensitive and
possesses the advantage of not altering the sample significantly. On the other hand, their
linear response range is usually limited to about two orders of magnitude.
Atomic Emission Detector (AED):
The newest commercially available gaschromatographic detector is based upon
atomic emission. In this device, the
eluent is introduced into a microwaveenergized helium plasma that is coupled to
a diode-array optical emission
spectrometer. The plasma is sufficiently energetic to atomize all of the elements in a
sample and to excite their characteristic atomic emission spectra.
GC COLUMNS AND STATIONARY PHASE
Packed Columns:
Present day packed columns are fabricated from glass, metal (stainless steel, copper,
aluminum), or Teflon tubes that typically have lengths of 2 to 3 m and inside diameters of
2 to 4mm. These tubes are densely packed with a uniform, finely divided packing
material, or solid support, that is coated with a thin layer of stationary liquid phase. The
tubes are formed as coils having diameters of roughly 15cm.
Solid Support Materials:
The solid support in a packed column serves to hold the stationary phase in place so that
as large a surface area as possible is exposed to the mobile phase.
The ideal support consists of small, uniform spherical particles with good mechanical
strength and a specific surface area of at least 1m/g. In addition, the material should be
inert at elevated temperatures and be uniformly wetted by the liquid phase.
Particle Size Supports:
The efficiency of a gas-chromatographic column increases rapidly with decreasing
particle diameter of the packing. The pressure difference required to maintain a given
flow rate of carrier gas, however, varies inversely as the square of the particle diameter;
the latter relationship has placed lower limits on the size of particles employed in gas
chromatography, because it is not convenient to use pressure differences that are greater
than about 50psi.
Open Tubular Columns:
Open tubular, or capillary, columns are of two basic types, namely, wall-coated
open tubular (WCOT) and support-coated open tubular(SCOT). WCOT columns are
simply capillary tubes coated with a thin layer of the stationary phase. In SCOT columns
the inner surface of the capillary is lined with a thin film of a support material, such as
diatomaceous earth. This type of column holds several times as much stationary phase as
does a wall-coated column and thus has a greater sample capacity.
Inverse Gas Chromatography:
In an inverse gas chromatography this technique is used to characterize the surface and
properties of solid materials. This whole concept is the same as Gas chromatography the
only difference is that in IGC the column is packed with powder and the vapor probes are
injected. (All in all the opposite of GC).
This is good for:

Dispersive and Polar Surface
Energies

Heats and Entropies of Sorption

Acid/Base Interactions

Phase Transitions

Sorption Isotherms

Permeability, Solubility and Diffusion

Micropore and Mesopore Distributions

Competitive (Multicomponent) Adsorption
ADSORPTION ON COLUMN PACKINGS OR CAPILLARY WALLS:
A problem that has plagued GC has been the physical adsorption of polar or polarizable
analyte species such as alcohols or aromatic hydrocarbons, on the silicate surfaces of
column packings or capillary walls. Adsorption results in distorted peaks, which are
broadened and often exhibit a tail. It has been established that adsorption is the
consequence of silanol groups that form on the surface of silicates by reaction with
moisture. The SiOH groups on the support surface have a strong affinity for polar
organic molecules and tend to retain them by adsorption. Support materials can be
deactivated by silanization with dimethyl chlorosilane (DMCS).
STATIONARY PHASE
Desirable properties for the immobilized liquid phase in a gas liquid chromatographic
column include:
1.
2.
3.
4.
low volatility
thermal stability
chemical inertness
solvent characteristics such that k’ and alpha values for
the solutes to be resolved fall within a suitable range.
Generally, the polarity of the stationary phase should match that of the sample
components. Commonly used stationary phases are listed in below.
Stationary Phase
Polydimethyl siloxane
Common
Maximum
Trade Name
Temperature (C)
OV-1, SE-30
350
Common Applications
General-purpose nonpolar phase;
hydrocarbons; polynuclear aromatics;
drugs; steriods; PCBs
Poly(phenylmethyldimethyl)
OV-3, SE-52
350
siloxane (10% phenyl)
Poly(phenylmethyl) siloxane
Fatty acid methyl esters; alkaloids; drugs;
halogenated compounds
OV-17
250
Drugs; steriods; pesticides; glycols
OV-210
200
Chlorinated aromatics; nitroaromatics;
(50% phenyl)
Poly(trifluoropropyldimethyl)
siloxane
Polyethylene glycol
alkyl-substituted benzenes
Carbowax 20M
250
Free acid; alcohol; ethers; essential oils;
glycols
Poly(dicyanoallyldimethyl)
OV-275
240
siloxane
Polyunsaturated fatty acids; rosin acids;
free acids; alcohols
Bonded and Cross-linked Stationary Phases:
The purpose of bonding and cross-linking is to provide a longer-lasting stationary phase
that can be rinsed with a solvent when the film becomes contaminated. With use,
untreated columns slowly lose their stationary phase due to bleeding in which a small
amount of immobilized liquid is carried out of the column during the elution process.
Cross-linking is carried out in situ after a column is coated with one of the polymers
listed in above
Chiral Stationary Phases:
There has been recent developments of stationary phases which can separate chiral
compounds. One method is to form a derivative of the analyte with an optically active
reagent that forms a pair of diasteromers that can be separated on an achiral column. The
alternative method is to use a chiral liquid as the stationary phase.
Film Thickness:
Film thickness primarily affect the retentive character and the capacity of a column.
Thick films are used with highly volatile analytes, because such films retain solutes for a
longer time and thus provide a greater time for separation to take place. Thin films are
useful for separating species of low volatility in a reasonable time.
APPLICATIONS OF GC:
Qualitative Analysis
Gas chromatographs are widely used as criteria for establishing the purity of organic
compounds. Contaminants, if present, are revealed by the appearance of additional peaks.
The Retention Index:
The retention index, I, was first proposed by Kovats in 1958 as a parameter for
identifying solutes from chromatograms. The retention index for any given solute can be
derived from a chromatogram of a mixture of that solute with at least two normal alkanes
having retention times that bracket that of the solute. That is, normal alkanes are the
standards upon which the retention index scale is based. By definition, the retention
index for a normal alkane is equal to 100 times the number of carbons in the compound
regardless of the column packing, the temperature, or other chromatographic conditions.
The retention index system has the advantage of being based upon readily available
reference materials that cover a wide boiling range.
Quantitative Analysis:
The relative concentration of an analyte is proportional to the peak area obtained on the
gas chromatogram. Thus the GC can be calibrated with several standards and a
calibration curve is obtained, then the concentration of the unknown analyte can be
determined using the peak area.
INTERFACING GC WITH SPECTROSCOPIC METHODS
When GC is coupled with other separation methods, both serve as a powerful tool for
identifying the components of complex mixtures. A popular combination is GC/MS.
This interface have been used for the identification of hundreds of components that are
present in natural and biological systems. For example, these procedures have permitted
characterization of the odor and flavor components of foods, identification of water
pollutants, medical diagnosis based on breath components, and studies of drug
metabolites. Another popular interface is that of GC/FTIR. Below is a diagram of
GC/MS a instrument.
GAS-SOLID CHROMATOGRAPHY
Gas-solid chromatography is based upon absorption of gaseous substances on solid
surfaces. Distribution coefficients are generally much larger than those for gas-liquid
chromatography. Consequently, gas -solid chromatography is useful for the separation of
species that are not retained by gas-liquid columns, such as the components of air,
hydrogen sulfide, carbon disulfide, nitrogen oxides, and rare gases. Gas-solid
chromatography is performed with both packed and open tubular columns. For the latter,
a thin layer of adsorbent is affixed to the inner walls of the capillary. Such columns are
sometimes called porous layer open tubular columns, or PLOT columns. Two types of
adsorbents are molecular sieves and porous polymers.
Molecular Sieves:
Molecular sieves are aluminum silicate ion exchangers, whose pore size depends upon
the kind of cation present. The sieves are classified according to the maximum diameter
of molecules that can enter the pores. Commercial molecular sieves come in pore sizes of
4, 5, 10, and 13 angstroms. Molecular sieves can be used to separate small molecules
from large.
Porous Polymers:
Porous polymer beads of uniform size are manufactured from styrene cross-linked with
divinylbenzene. The pore size of these beads is uniform and is controlled by the amount
of cross-linking. Porous polymers have found considerable use in the separation of
gaseous polar species such as hydrogen sulfide, oxides of nitrogen, water, carbon dioxide,
methanol, and vinyl chloride.
Important Terms to Know:
Stationary phase
 The stationary phase consists of the support, which is either a porous small-particle material for
packed columns or the inner walls of a glass or fussed-silica capillary column. The stationary
liquid is either fixed merely by adhesion or by chemical bonding in order to produce a
homogeneous coating.
Mobile phase.
 In GC an inert gas e.g. He N2 H2 acts as the mobile phase, which effects the transport of the solutes
through the column.
Elution
 By transportation in the mobile phase the solutes migrate with a velocity through the separation
column which depends on the partition coefficients of the solutes between the given stationary
phase and mobile phase.
Solutes
Sample components that are being separated.
Detector
 The detector generates a continuously recorded electrical signal during the chromatographic
separation, which is proportional to the concentration of the separated solute in the mobile phase.
There are universal and specific types of detectors.
Chromatograph
y
 A term for methods of separation based upon the partition of analyte species between a stationary
phase and mobile phase.
Distribution constants, Partition coefficients
An equilibrium constant for the distribution of a solute between two immiscible phases.
Retention times
The time between sample injection and arrival of the analyte peak at the detector.
Gaussian Distribution
 A distribution in which small departures from a central result occur more frequently then large
ones and in which positive a negative variation occur with equal frequency.
GC Quiz Question
The following data were obtained by gas-liquid chromatography on a 40cm packed
column
Compound
Air
Methylcyclohexane
Methylcyclohexene
Toluene
TR min
1,9
10.0
1.09
13.4
Question 1
Calculate,
1a) the average number of plates from the data
1b) the standard deviation of the average in 1a
1c) the average plate height for the column
Question 2
W½ min
0.76
0.82
1.06
Calculate the resolution for,
2a) Methylcyclohexene and Methylcyclohexane
2b) Methylcyclohexene and toluene
2c) Methylcyclohexane and toluene
Question 3
If Cs and Cm for the column were 19.6 and 62.6 mL, respectively and a non retained air
peak appeared after 1.9 min calculate the
3a) Retention factor for each of the three compounds
3b) Distribution constant for each of the three compounds
3c) Selectivity factor for methylcyclohexane and methylcyclohexene
3d) Selectivity factor for methylcyclohexene and toluene
GC Quiz Answers
1a) N = 2.7 x 103
1b) s = 0.1 x 103
1c) H = 0.015cm
2a) 1.1
2b) 2.7
2c) 3.7
3a) k’1 = 4.3 k’2 = 4.7 k’3 = 6.1
3b) K1 = 14 K2 = 15 K3 = 19
3c) 1.11
3d) 1.28
References:
www.shahid.ic24.net
http://www.shu.ac.uk/schools/sci/chem/tutorials/chrom/gaschrm.htm
http://www2.arnes.si/~uljffarmac3/theory.htm
http://www.scimed.co.uk/products/product25.htm