2. 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.
3. Principle of Gas chromatography
Principle of 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.)
4. 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)
5.
6. 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.
7. 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.
9. Column Configurations
Two types of columns are commonly used in gas
chromatography
Packed
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.
10. Packed column:
Packed columns contain a finely divided, inert, solid support material
coated with a thin layer of liquid stationary phase.
Most packed columns are m in length and have an internal diameter of
2 - 4mm.
They are made from glass, metals, or Teflon.
Capillary column (open tubular):
All the GC studies in the early 1950s were carried out on packed
column.In the late 1950s capillary column were constructed that much
superior in speed and column efficiency (≈ plates).
11. Capillary column
Capillary columns have an internal diameter of a few
tenths of a millimeter.They were constructed of stain-less
steel, aluminum, copper, plastic, or glass.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, onto which the stationary phase has
been adsorbed.SCOT columns are generally less efficient
than WCOT columns, but both types are more efficient
than packed columns.
12. 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.
13. 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 400°C.A short
response time that is independent of flow rate.High
reliability and ease of use.
14. 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.
15.
16. 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.
17.
18. Electron-Capture Detector(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.
20. 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)
21. Applcations of Gas Chromatography
Gas Chromatography can be used to determine the identity of herbal
products containing complex mixtures of similar compounds.
It is most suitable for the separation and analysis of volatile
compounds such as; essential oils and fatty oils.
It is used in the analysis of foods like; carbohydrates, proteins, lipids,
vitamins, steroids, drugs, pesticide residues, and trace elements.
Pollutants like; formaldehyde, carbon monoxide and DDT can be
analyzed.
It serves both qualitative and quantitative purposes.
It is used in the analysis of dairy products for rancidity.
