Gas Chromatography
Gas chromatography is a type of chromatography
used in chemistry for analyzing and separating
compounds that can be convert to gas without
decomposition. Uses of this technique include testing
the purity of a Gaseous substance and separating the
different components of a mixture. Sometimes, it
may help in identifying the compound. It can also be
used to prepare pure compounds from a mixture. Gas
chromatography is similar to fractional distillation, as
both processes separate the mixture components
primarily based on their vapor pressure differences.
Principle of GC
Gas chromatography is based on the principle of
partition(differential distribution) of an volatile
compound(gas) in two phases - a liquid phase
covering the adsorbent surface and a gaseous phase
of the eluting gas. With a fixed set of parameters
(length and diameter of column, temp., nature and
flow rate of the eluting gas etc.)
Compounds A and B interact with the stationary phase through intermolecular
forces.
A interacts more strongly with the stationary liquid phase and is retained
relative to B, which interacts weakly with the stationary phase. Thus B spends
more time in the gas phase and advances more rapidly through the column
and has a shorter retention time than A.
Typically, components with similar polarity elute in order of volatility. Thus
alkanes elute in order of increasing boiling points; lower boiling alkanes will
have shorter retention times than higher boiling alkanes.
Sample is injected into the injection port. Sample vaporizes and is forced
into the column by the carrier gas ( = mobile phase which in GC is
usually helium)
Components of the sample mixture interact with the stationary phase so
that different substances take different amounts of time to elute from
the column.
The separated components pass through a detector. Electronic signals,
collected over time, are sent to the GC software, and a chromatogram is
generated.
Components of a Gas Chromatograph
Gas Supply: E.g. N2 or He
Sample Injector: syringe or septum
Column: Tubing packed with small uniform size, inert
support coated with thin film of nonvolatile liquid
Detectors: - Thermal conductivity (TC)
- Flame ionization detector (FID)
- Electron Capture (ECD)
- Photo Ionization (PID)
Gas-Supply
Carrier gases, which should be chemically inert and
non reactive, include Helium, Nitrogen, and
Hydrogen. Along with the gas supply there are
pressure regulators, gauges, and flow meters. In
addition, the gas carrier system often contains a
molecular sieve to remove water and other
impurities.
Sample Injection System
Column efficiency depends upon that Sample should
be of suitable size and introduced as a “plug” of
vapor. Slow injection of oversized samples causes
poor resolution and band spreading .
The most common method of sample injection
involves the use of micro syringe to inject a liquid or
gaseous sample through a self-sealing, silicone-rubber
diaphragm or septum into a flash vaporizer port
located at the head of the column.
Schematics of sample injection
Column Configurations
Two types of columns are commonly used in gas
chromatography
• Packed and open tubular
• Capillary type
Chromatographic columns vary in length from <2 to
50 m or more. They are made up of stainless steel,
glass, Teflon or fused silica. In order to fit into an oven
for thermo stating, they are usually formed as coils
having diameters of 10 to 30 cm.
Column Ovens
Column temperature is a very important variable that
must be controlled for precise result. Thus, the
column is housed in a thermo stated oven. The
optimum column temperature depends upon the
evaporation point of the sample and the degree of
separation required.
Usually a temperature more than or equal to average
boiling point of a sample results in a reasonable
elution time (2 to 30 min). For samples with a broad
boiling range, it is desirable to employ temperature
program, where the temperature is increased either
continuously or in steps as the separation proceeds.
Detection Systems
Properties of the Ideal Detector: The ideal detector
for Gas Chromatography should have following
characteristics:
• Good stability and reproducibility.
• A linear response to solutes that extends
over several orders of magnitude.
• A temperature range from 25oC room
temperature to at least 400oC.
• A short response time that is independent of
flow rate.
• High reliability and ease of use.
• Similarity in response toward all solutes or a
highly selective response toward one classes of
solutes.
• Should be Nondestructive toward sample.
Flame Ionization Detectors (FID)
The flame ionization detector is the widely used and
most applicable detector 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.
Thermal Conductivity Detectors(TCD)
An old age detector for Gas Chromatography, and one
that still have wide application, is based upon changes
in the thermal conductivity of the gas brought about
by the presence of analyte molecules.
The sensing element of this detector is an electrically
heated element whose temp. at constant electrical
power depends upon the thermal conductivity of the
surrounding gas.
Heated element - Fine platinum, gold, or tungsten
wire or a semiconducting thermistor.
• The advantage of the thermal conductivity
detector is its simplicity, its large linear dynamic
range, its response toward both organic and
inorganic species, and its nondestructive nature,
which permits collection of solutes after
detection.
• A limitation is its relatively low sensitivity (~10-8 g
solute/mL carrier gas).
*Other detectors exceed this sensitivity by
factors as large as 104 to 107.
Electron-Capture Detectors(ECD)
The electron-capture detector has become one of the
most widely used detectors for environmental samples
because this detector selectivity detects halogen
containing compounds, such as pesticides and
polychlorinated biphenyls.
The effluent from the column is passed over a  emitter,
usually nickel-63. An electron from the emitter causes
ionization of the carrier gas 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
markedly, however, in the presence of those organic
molecules that tend to capture electrons.
Advantage: The electron-capture detector is selective
in its response. It is highly sensitive to molecules
containing electronegative functional groups such as
halogens, peroxides, quinones, and nitro groups.
Limitation: It is insensitive toward functional groups
such as amines, alcohols, and hydrocarbons.
*An important application of the electron-capture
detector has been for the detection and
determination of chlorinated insecticides.
Atomic Emission Detectors (AED)
The atomic emission detector is used commercially. In
this device the eluent is introduced into a microwaveenergized helium plasma that is coupled to a diode
array optical emission spectrophotometer. The plasma
is sufficiently energetic to atomize all of the elements
in a sample and to excite their characteristic atomic
emission spectra.
Thermionic Detectors (TID)
The thermionic detector is selective toward organic
compounds containing phosphorus and nitrogen. Its
response to a phosphorus atom is greater than to a
nitrogen atom. Compared with the flame ionization
detector, the thermionic detector is approximately
500 times more sensitive to phosphorus-containing
compounds and 50 times more sensitive to nitrogen
bearing species. These properties make thermionic
detection particularly useful for detecting and
determining the many phosphorus-containing
pesticides.
GAS CHROMATOGRAPHIC COLUMNS
The choice of column depends on the sample and the
active measured. The main chemical attribute
regarded when choosing a column is the polarity of
the mixture, but functional groups can play a large
part in column selection. The polarity of the sample
must closely match the polarity of the column
stationary phase to increase resolution and
separation while reducing run time. The separation
and run time also depends on the film thickness (of
the stationary phase), the column diameter and the
column length.
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). Wall-coated
columns are simply capillary tubes coated with a thin
layer of the stationary phase. In support-coated open
tubular columns, the inner surface of the capillary is
lined with a thin film (~30 m) 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.
Packed Columns
Packed columns are fabricated from glass, metal
(stainless steel, copper or aluminum), or Teflon tubes
that typically have lengths of 2 to 3 m and inside
diameters of 2 to 4 mm. These tubes are densely
packed with a uniform, finely divided packing
material, or solid support, that is coated with a thin
layer (0.05 to m) of the stationary liquid phase. In
order to fit in a thermo stating oven, the tubes are
formed as coils having diameters of roughly 15 cm.
The Stationary Phase
Desirable properties for the liquid phase in a gasliquid chromatographic column include:
1. Low volatility (ideally, the boiling point of the
liquid should be at 100oC higher than the
maximum operating temperature for the column)
2. Thermal stability
3. chemical inertness
4. solvent characteristics such that k` and  values for
the solutes to be resolved fall within a suitable
range.
The retention time for a solute on a column depends
upon its distribution constant which in turn is related
to the chemical nature of the stationary phase
Common liquid phases for GC
Film Thickness
Commercial columns having stationary phases that vary
in thickness from 0.1 to 5m. Film thickness primarily
affects 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, thus
providing a greater time for separation to take place. Thin
films are useful for separating species of low volatility in a
reasonable length of time. For most applications with
0.26- or 0.32-mm columns, a film thickness of 0.26 m is
used. With mega bore columns, 1- to 1.5 m films are
often used.
Qualitative Analysis
• Gas chromatogram is used as criteria of purity for
organic compounds. Contaminants present are
revealed by the appearance of additional peaks.
• The areas under these peaks provide rough estimates
of the extent of contamination.
• The technique is also useful for evaluating the
effectiveness of purification procedures.
• Retention times should be useful for the identification
of components in mixtures.
• Gas chromatography provides an excellent means of
confirming the presence or absence of a suspected
compound in a mixture.
Quantitative Analysis
The detector signal from a gas-liquid chromatographic
column has wide use for quantitative and semi
quantitative analyses. An accuracy of 1% relative is
attainable under carefully controlled conditions.
Reliability is directly related to the control of
variables.The nature of the sample also plays a part in
determining the potential accuracy
Combining Gas Chromatography
with Spectroscopic Methods
Gas Chromatography/Mass Spectrometry
(GC/MS)
The flow rate from capillary columns is generally low
enough that the column output can be fed directly
into the ionization chamber of the mass spectrometer.
For packed columns and mega bore capillary columns
however, a jet separator must be employed to remove
most of the carrier gas from the analyte.
Advantages of Gas Chromatography
• Requires only very small samples with little
preparation
• Good at separating complex mixtures into
components
• Results are rapidly obtained (1 to 100 minutes)
• Very high precision
• Only instrument with the sensitivity to detect
volatile organic mixtures of low concentrations
• Equipment is not very complex (sophisticated
oven)
Applications