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Gas Chromatography - Principles and Practice

BY: Shikha Sharma | Category: Others | Submitted: 2010-11-22 08:12:28
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Article Summary: "Gas Chromatography was originally suggested by Martin and Synge (1941). It is basically a separation technique (liquid or solid) in which the compounds of a vaporized sample are separated and fractionated as a consequence of partition between a mobile gaseous phase and a stationary phase held in column..."


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Gas Chromatography was originally suggested by Martin and Synge (1941). It is basically a separation technique in which the compounds of a vaporized sample are separated and fractionated as a consequence of partition between a mobile gaseous phase and a stationary phase held in column.

Partition takes place between a gas and liquid or gas and solid. Thus, according to nature of stationary phase, gas chromatography may be divided into two classes:

1. GSC (Gas Solid Chromatography)
Fixed phase or stationary phase consists of solid material such as granular silica.

2. GLC (Gas-Liquid Chromatography)
Fixed phase or stationary phase is a non-volatile liquid held as thin layer on a solid support.


Techniques of Gas Liquid Chromatography

The apparatus consists of the following main components:
a) A tank of carrier gas
b) An injection port of sample
c) The column
d) Detector with appropriate read out

Carrier Gas
A mobile phase (carrier gas) in GLC is usually Helium or Nitrogen. Although CO2 and H2 from tank sources have also been tried. H2 has the obvious disadvantage of explosion danger.
The most important requirements of a carrier gas are:
1. It should be inert
2. It should be available at low cost because large quantities are used.
3. It should allow the detector to respond in an adequate manner.
He or N2 fulfills all the above requirements and hence these gases are most commonly used as carrier gas.
• A high pressure gas cylinder (in which carrier gas is filled in compressed form) is used as carrier gas reservoir.
• The cylinder is also attached with pressure regulator to reduce and control the gas flow.
• A soap bubble meter is also an accurate device to produce the rate of carrier gas

The Injection Port
• The carrier gas is conducted the gas reservoir to a sample port injector.
• The injection port is heated to a temperature, which will ensure rapid vaporization but not thermal degradation of the solute.
• The port is constructed of a heavy mass that is maintained at an elevated temperature and contains a pliable septum through which samples are injected.
• The port is designed for instantaneous injection and vaporization of a sample so that the sample is introduced immediately into a column.
• Liquids are introduced near or as solutions with syringe of 0.1-100μl capacity.
• The liquid is drawn into the syringe a number of times to remove any air bubble and injected very rapidly into the gas stream.
• For gases, a gas tight syringe of 0.5-10μl capacity can be used.
• The solid may be dissolved in a suitable solvent and then injected a solution.
• It should be noted that the injection of sample, which cannot be vaporized at the operating temperature, must always be avoided.

Columns
• The column is the heart of the chromatography.
• The different solutes in the vaporized samples are separated from each other by virtue of their different interaction with column packing.
• As the solute emerges individually from the end of the column, it enters the detector.

Types of Column
a) Capillary Column

• It is fabricated from capillary tubing, the bore of which is coated with very thin film of the liquid phase.
• The column diameter is 1/16" or less.
• These columns are available in length up to 200m because of the fact that they have very low pressure drop.
• These columns however, have low sample capacities and these are used for partition.

Packed Column

• It is usually a stainless steel or copper tube packed with either a solid substance (GSC) or a liquid coating on an inert solid (GLC).
• The column is place din an oven such that the temperature can be easily controlled.
• The glass tubes have also been used especially for biological samples.
• The diameter of metal or glass tube is 0.25 inch.
• Length of the tube is 5-50 feet.
• Ordinarily, the tubes are folded or coiled so that helium can be conveniently fitted into a thermostat.

Terms used in GLC
Retention time
The maximum time required for the solute peak to reach the detector in GC is called retention time Rt.
Retention volume
It is the volume of gas required to carry a component maximum through the column.

Detectors
General requirements of GC detectors are:
• High sensitivity
• Physically suitable
• Capable of operable up to a maximum column temperature of 350˚C
• Ease of operation
• No response to undesired compounds
• Response to compounds for which analysis is required.
• An output signal which is a linear function
• Linear response extending to high concentration

Based on the above physical properties, detectors are of the following types:
1. Thermal conductivity detector
2. Electron capture detector
3. Cross section detection
4. Argon ionization detector
5. Gas density balance
6. Microwave excited discharge detector
7. Helium ionization detector
8. Flame thermocouple detector
9. Micro detector
10. Photo ionization detector

Factors affecting separation
1. Particle size and Surface area
The number of theoretical plates increases with a decrease in partial size or increase in surface area. In general, a 60/80 mesh particle size is used in a 0.25" column.

2. The rate of flow of carrier gas
The maximum efficiency can be obtained with an optimum flow rate, i.e., the rate of flow should neither be too low nor too high. The elute peaks will tend to be broad if the gas flow rate is too low and the peaks will not be resolved if the rate of flow of the gas is too fast.

3. Type and amount of stationary
This is a very important factor in determining the efficiency of the column. The best separation is obtained when the liquid stationary phase is structurally similar to the compounds being separated.
The amount of stationary phase also affects the efficiency of the column. As the concentration of liquid phase increases, the number of theoretical plates for the column also increases. Excessive liquid support can also cause trailing of peaks in the chromatogram.

4. Column length

The number of theoretical plates increases with an increase in the length of the column. But there is a practical limit to the length of the column because long column may create problems relating to the gas flow.

5. Column diameter
The number of theoretical plates increases with a decrease in the diameter of the column.

Applications of Gas Liquid Chromatography
1) Qualitative Analysis
• The retention time of a particular gas is constant for a given column, flow rate and temperature.
• The retention time data should be useful for the identification of components of mixtures.

2) Qualitative analysis
It is based either on peak height or on peak area and hence for known substances, quantitative determinations are generally performed on the recorded chromatogram.

3) Miscellaneous Applications
a) In petroleum industry: GLC has been used in analysis of crude petroleum products/fractions, gasolines, waxes, LPG etc.
b) Analysis of pesticides: Analysis of pesticides residues and determination of organophosphate in pesticides have successfully been carried out by using GLC.
c) In food industry: GLC has been employed to account for the color and flavor of food.

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