GAS CHROMATOGRAPHY

Gas Chromatography

Gas chromatography was developed in 1941 by AJP Martin and RLM Synge as a purely analytical method. But in 1952, RLM Synge got Nobel Prize for the discovery of gas chromatography.

Gas chromatography is used to separate and analyze mixtures of gases and volatile liquids or solids in their gaseous state.

Gas chromatography is basically a separation technique in which the compounds of vaporized sample are separated as a consequence of partition between mobile gaseous phase and a stationary phase held in a column.

Based on the nature of stationary phase, GC is of two types.

1. Gas Solid Chromatography                        2. Gas Liquid Chromatography

1. Gas Solid Chromatography:

In GSC, the stationary phase consists of an active solid absorbance such as granular silica, carbons, aluminium oxide

2. Gas Liquid Chromatography:

In GLC, separation occurs by partition between a mobile gas phase and a thin layer of non-volatile liquid coated on an inert support. The most common inert support is diatomaceous earth.

WORKING PRINCIPLE

The carrier gas passes through a flow regulator for the adjustment of flow rate of gases and enters into the sample injector. A little amount of the sample is introduced into the sample injector with the help of syringe (hypothermal). The sample injector is maintained at the high temperature than the boiling point of the highest boiling point of the sample in order to ensure rapid vaporization of liquid sample.

            The carrier gas entering the sample injector carries the vaporized sample to the thermostated column. The components of the sample pass through the column and are distributed between the stationary phase and mobile phase and pass down the column at different rates. The compounds (sample components) get partitioned between the stationary phase and the gaseous mobile phase and hence separated because of differences in their partition coefficient.

The carrier gas with the separated components now enters the detector which measures the changes in composition of carrier gas as it passes through it.

As each component reaches the detector, a peak is provided by the recorder. The separated components of the sample in the carrier gas register a series of signals which appears as succession of peaks above a baseline on the recorded chromatogram.

Identification of chromatogram is done by the use of peak called retention volume.

Retention volume: Retention volume is the volume of carrier gas that passes out of the column to the time peak maximum is obtained.

Figure 1: Retention time graph

V_R = t_R - F_c

Where                         V_R = Retention volume

t_R = retention time

F_c = flow rate of the carrier gas

Retention time: Retention time is the time from the point of the injection of sample to the time of emergence of separated components from the column.

The retention time and retention volume are specific for individual components.

BASIC COMPONENTS OF GAS CHROMATOGRAPHY

1.       A higher pressured cylinder containing carrier gas
2.       Flow regulator
3.       Sample injector
4.       Column
5.       Thermostated column
6.       Detector
7.       Recorder and data handling device

Figure 2: Optical Diagram of Gas Chromatography

Figure 3: Diagrammatic representation of Gas Chromatography

1. A HIGHER PRESSURED CYLINDER CONTAINING CARRIER GAS

Normally nitrogen and helium are the most commonly used carrier gases. The most common carrier gases are helium, hydrogen, nitrogen, argon and carbon dioxide. The most important requirement of carrier gas used in

a.       It should be inert.

b.      It should be available at the low cost because large quantities are used.

c.       It should allow the detector to respond in an adequate manner.

Helium or nitrogen gas fulfills all the above requirements and hence these are used most in carrier gas. For most analytical purposes, hydrogen gas is not used as carrier gas because of its explosion hazard and its reactivity toward unsaturated compounds.

2. FLOW REGULATOR

A simple flow meter is soap bubble flow meter. Soap bubbles are generated in the burette by squeezing a rubber bulb the connecting with the burettes are filled with the soap solution. The time required for soap bubble to move between two graduation on the burette can be measured which directly gives the flow rate of carrier gas. If flow rate of gas is slow, the eluted peaks will be broad. And if flow rate of gas is fast, eluted peak will not be resolved i.e. flow rate greatly influences column efficiency.

3. SAMPLE INJECTION

The sampling part or sample injection system is a small box which is electrically maintained (temperature control).

The amount of sample required for gas chromatography depends upon:

•        Nature and concentration of solutes present in the sample.
•       The size of the column and
•        Sensitivity of the detector

Generally a small amount of sample is loaded. The usual range is from 0.1-50 ml and for the solid samples milligram of fraction. For the solid sample, it is first dissolved in suitable solvents such as ether and methanol and injected as solution.                                                                                                                             

Devices by which required amount of sample that can be introduced into the carrier gas stream are:

1. By syringe
2. By ampule
3. By valve

A. Syringe method: Syringe technique is most commonly used technique for introducing gas and liquid sample into the carrier gas. Hypodermic syringes are available in many calibrated size which is inserted into the column through a replaceable rubber septum.

B. Ampule method: Viscous liquid and solid sample are weighed in a thin walled glass ampoules and are introduced into the carrier gas and then crushed. But this method isn’t commonly used.

C. Valve method: Valve methods are used for introducing the samples which are especially convenient to gaseous sample. It has two stop cocks (a pair of identical dual stop cocks). First stop cock no.1 is turned 90° and measured amount of the sample is filled in the reservoir. Then, it is turned to its original position. Stop cock no. 2 is then turned to 90° where a major quantity of sample is flushed into the column.

4. COLUMNS

Columns are made up from variety of material such as stainless steel, Cu, glass or plastic and may be coiled in U-shaped or W-shaped. Metal columns though expensive are prepared because they are stout, inert and possess good thermal properties.

Glass column are fragile and difficult to coil, so they are not commonly used.

Types of Column

      A. Partition column

      B. Adsorption column

A. Partition Column: Partition column is used in gas liquid chromatography. The partition columns are packed with inner support carrying a non-volatile liquid phase. Support material consists of celite, firebricks or glass bead. Liquids commonly used are silicon oil and grease, apieson oil and grease, squalance (C₃₀H₆₂).

Methods of Packing Column in GLC: The required amount of liquid is dissolved in a volatile solvent and then desired amount of inert solid support is mixed with it in an open container and volatile solvent is removed by evaporation.

The solid with its liquid coating has the appearance of free flowing sand which can be packed into a long tube with tapping or vibrating to promote even packing.

B. Adsorption column: They are used in gas solid chromatography. Several adsorbing material such as silica gel, activated carbon or aluminium oxide can be used in gas solid chromatography.

Method of Packing Column in GSC: While packing the column, one end is closed with glass wool plug and as adsorbent material is introduced into the column through the other end of column by tapping the column thoroughly.

5. THERMAL COMPARTMENT

The column is never operated at room temperature in case of gas chromatography. The temperature of column is controlled by using air baths or using vapor jacket or by using electrically heated blocks. The temperature of the column should not be as high as to vaporize the stationary phase. Normally a temperature equal to or slightly greater than the average boiling point of sample gives good elution period. If sample containing components with a broad boiling point is to be fractionated then it is desirable to increase the temperature of the column as the separation proceeds.

6. DETECTOR

The function of detector is to measure the small amount of separated components present in the gas steam leaving the column. Based on the physical properties of gases, detectors are of various types. For example;

•       Electron capture detector
•      Thermal conducting
•       Ionization detector
•       Flame ionization detector
•       Gas density detector

7. RECORDER AND DATA HANDLING DEVICES

The output from the detector is fed to a recorder which provides a chromatogram. So, recorder records or traces out a series of peak forming chromatogram. Most gas chromatography is collected to a mass spectrophotometer i.e. capable of identifying the angle represented by the peak for quantitative estimation; we can calculate the peak area by applying geometrical method. The area of the peak is the direct measure of concentration of each compound present in the sample. The peak area is the product of the peak height and the peak width at half peak height.

APPLICATIONS OF GAS CHROMATOGRAPHY

•     Gas chromatography can be used for analysis and separation of petroleum products, fatty acids, steroids, rubber and rubber products.
•    In cosmetic and perfume industries, gas chromatography is helpful in determining the composition of various cosmetics and to check the quality of ingredients used for preparing cosmetics.
•      An important use of gas chromatography is determination of elements such as carbon, nitrogen, oxygen and sulfur in inorganic metallic sample.
•       GSC can be used to analyze pesticide residues in soil sample, milk sample and different food products like honey etc.