Gas Chromatography
Gas chromatography (GC) is based on the repeated partition or
adsorption, between a mobile phase and a stationary phase, of components to be
separated. The mobile phase is always a gas known as the carrier gas. The
stationary phase can be either a solid or a liquid.
When the stationary phase is a solid then it is called gas-solid chromatography (GSC) or adsorption chromatography. The
components of the injected sample are adsorbed on the solid and the principle
of separation is based on the fact that the various components can be more or
less strongly adsorbed by the adsorbent.
The solid stationary phases used in gas chromatography are materials
such as silica gel, active carbon, aluminum oxide, molecular sieves.
When the stationary phase is a liquid then it is called gas-liquid chromatography (GLC) or partition chromatography. The
components of the injected sample form different concentrations in the liquid
stationary phase and in the gaseous mobile phase.
Like all chromatographic techniques, gas chromatography separates
mixtures by taking advantage of their components differential distribution
between two phases – one stationary and the other moving. The distinctive
feature of gas chromatography is the use of a gas as the moving, or mobile,
phase. It is usually called carrier gas. A sample
of the mixture to be separated is introduced into this gas stream just before
it encounters the stationary phase. The
components of the injected sample – the sample may be a liquid or a gas- are separated by elution and detected as they
emerge in the gas at the other end of the column (Fig. 1). They are
distinguished by the different times which they take to pass through the colum
– the retention times.
 |
| Fig. 1: Block diagram of a gas chromatography
system |
The hardware components used in
typical GC systems include an injector, a carrier gas, a column (stationary phase),
an oven, a detector and a recorder or information processor (Figure 1). Several
components have variable settings that can be used to optimize the analysis of
different sample types.
A GC-MS analytical instrument is shown in the following video. A mass spectrometer is the detector in this case:
Mobile Phase in Gas Chromatography
Which are the most common mobile phases
in gas chromatography?
The most common mobile phases
(carrier gases) for gas chromatography (GC) are He, H2 and N2 which have the advantage of being chemically inert
toward both the sample and the stationary phase. However, other gases such as Ar and CO2 have also
been used though much less frequently.
The choice of which carrier gas to
use is often determined by the instrument’s detector.
The gases can be obtained from commercial air suppliers,
conveniently compressed in cylinders and in a state of purity sufficient for
most purposes.
What
happens if the mobile phase gases are impure?
If the gases are impure then a very unstable and noisy baseline is
observed in the chromatogram.
The commonest impurity is water, and drying the gas with an in-line
molecular sieve trap is recommended.
Another impurity is oxygen which may cause oxidation of the
stationary phase particularly in high temperatures. The stationary phases are
usually organic, and the products of oxidation are usually volatile and cause a
noisy, drifting baseline when they reach the detector. The column life will
also be shortened and will not give good results.
The Injection System in Gas Chromatography – Sample
Introduction
Sample introduction is of primary importance in gas chromatography.
The performance of the sample introduction system is crucial for the overall
chromatographic performance.
What volume
of sample is injected to the gas chromatograph and how?
The sample volume injected is usually less than 1-2 μl of liquid or 5 cm3 of
gas. Injection, is usually achieved by means of a syringe inserted through a
self-sealing silicone rubber septum at the injection head.
 |
| Fig. 2: Injection head of a gas chromatograph. Injection is achieved by means of a syringe inserted through a self-sealing silicone rubber septum at the green-colored injection port. |
However, there are a number of problems inherent in the use of
syringes for injection, even when they are not damaged:
- Even the best syringes claim an accuracy of only 3%, and in unskilled hands, errors are much larger
- The needle may cut small pieces of rubber from the septum as it injects sample through it. These can block the needle and prevent the syringe filling the next time it is used. It may not be obvious of what happened.
- A fraction of the sample may get trapped in the rubber, to be released during subsequent injections. This can give rise to ghost peaks in the chromatogram.
- There may be selective loss of the more volatile components of the sample by evaporation from the tip of the needle.
There are also several problems with the splitting process since the
split may not be homogeneous. This may be because there is a poor mixing with
the carrier gas at the dilution stage or because the low relative molecular
mass components diffuse toward the vent more rapidly than those of higher
molecular mass.
In principle, there are four types of sample injection:
- Splitless injection
- Split injection
- Temperature-programmed injection
- On-column sample injection
When
splitless injection is used?
This method is useful for very dilute solutions. When splitless injection is used, the column is overloaded with
the solvent. For this reason, the temperature at the top of the column is kept
low (10-20 ◦C below the boiling point of the solvent), so that the
low-volatility components and the solvent condense. This condensation causes
the components to be focused. The method is not recommended for volatile
components, as these are eluted from the column with the solvent.
When split
injection is used?
Split sampling was the first
sample introduction system developed for capillary gas chromatography.
In the split injection technique, only
a part of the sample is delivered to the column. This method is used with
capillary columns. The sample is injected into the carrier gas stream through a
septum, vaporized in the vaporizing chamber, and then mixed with the carrier
gas. The gas then is divided into two streams by means of an infinitely
adjustable needle valve, which should be adjusted so that a very small
proportion of the sample is delivered to the column. This method is used when
concentrations in the sample are high because the capacity of a capillary
column is low.
Please see the following video from the Royal Society of Chemistry on gas chromatography using a flame ionisation detector (FID) with a brief mention of gas chromatography mass spectrometry (GCMS),
Relevant Posts
References
- D. Harvey, “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
- “Gas Chromatography”, J. Willett, John Wiley &Sons, 1987
- “High Resolution Gas Chromatography”, K.J. Hyver, P. Sandra, 3rd Edition, 1989
For further reading check the following:
ReplyDelete"Modern Practice of Gas Chromatography", R. L. Grob
"Analytical Gas Chromatography", W. Jennings
This is really good.
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Woww..all about gas chromatography. The video provided is very useful to understand the concept. A well explained blog post and it would be useful for students and customers who are in search of gas chromatography. The question answer session is very useful too. You can find details on the website of Chemtrol too.
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