Gas chromatography is a technique used to separate, identify, and quantify components of a mixture. It works by selectively partitioning the components between a stationary and mobile phase inside a column. Components are then sequentially eluted from the column. First, a sample is injected into an inert gas stream and carried into a separation column. The various components interact differently with the column's stationary phase and are separated as they move through. A detector then measures the quantity of components exiting the column, allowing identification and quantification. Gas chromatography offers high separation efficiency, small sample needs, good selectivity for similar compounds, and a wide range of applications, though it is limited to analyzing volatile substances.

1. Gas Chromatography
By
Dr. Nidhi Gupta
Assistant Professor
M.M. College of Pharmacy
Maharishi Markandeshwar (Deemed to be University), Mullana,
Ambala, India

2. Introduction
Gas Chromatography or Gas Liquid Chromatography is a technique applied
for separation, identification and quantification of components of a mixture
of organic compounds by selective partitioning between the stationary
phase and mobile phase inside a column followed by sequential elution of
separated components. The technique is suitable for separation of
compounds having following characteristics :
•High volatility
•Thermal stability
•Low molecular weights

3. Principle
The sample solution injected into the instrument enters a gas stream which
transports the sample into a separation tube known as the "column." (Helium or
nitrogen is used as the so-called carrier gas.) The various components are
separated inside the column. The detector measures the quantity of the
components that exit the column. To measure a sample with an unknown
concentration, a standard sample with a known concentration is injected into
the instrument. The standard sample peak retention time (appearance time) and
area are compared to the test sample to calculate the concentration.

5. First, the sample is introduced into a stream of inert gas, or a carrier, which
is usually helium or argon. For a liquid sample, it needs to be evaporated
before being injected into the carrier. Sample components move through
the packed column at a rate affected by the degree of interaction of each
component with the stationary non-volatile phase. Substances that interact
more with the stationary phase are delayed and thus separated from
substances that interact less. When the components are eluted from the
column, they can be quantified and/or collected through the detector for
further analysis.
Procedure of Gas chromatography

6. Advantages of gas chromatography
1) High separation efficiency and analysis speed: for example, gasoline
samples can obtain more than 200 chromatographic peaks in 2 hrs. A general
sample analysis can be completed in 20 minutes.
2) Small sample consumption and high detection sensitivity: 1 ml of gas
sample consumption, 0.1 µl of liquid sample consumption, a few mg of solid
sample consumption. Proper detectors can detect impurities in the tens to a
few parts per million.
3) Gas chromatography has good selectivity and can be used to analyze
azeotropic mixtures and samples with close boiling points. For example, some
isotopes, cis-trans isomers, adjacent or intertrans isomers, optical
isomers, etc.
4) Wide range of applications, although mainly used to analyze gases and
volatile organic substances；under certain conditions, it can also be used to
analyze high boiling point substances and solid samples.

7. Disadvantages of gas chromatography
It can only be used to analyze volatile substances. Some highly polar
substances can be derived to increase their volatility for GC
analysis, but the process can be complex and may introduce errors
in quantitative analysis.

8. Thank you………..