Short notes on Gas Chromatography

Gas chromatography involves separation and analyses of different constituents of mixtures by a mobile gas phase passing over a stationary adsorbent. The technique is similar to column chromatography except that the mobile phase is replaced by a moving gas which is called the carrier gas.

Gas chromatography is a powerful tool for the analyses of organic materials. It is very handy for low levels of pesticides and other contaminants of the environment. Gas chromatography can be of two types: Gas-liquid Chromatography (GLC) and Gas-Solid Chromatography.

In gas liquid chromatography the separation is brought about by partitioning the sample between a mobile gas phase and a thin nonvolatile liquid layer coated on some inert solid particles while gas-solid chromatography is based upon selective adsorption of constituents of the sample on a solid of large surface area used as the stationary phase.

When a mixture of volatile material transported by a carrier gas is led through a column containing an adsorbent solid phase or more commonly an absorbing liquid phase coated over a solid material, each volatile component is partitioned between the carrier gas and the solid or the liquid.

Depending upon the retention time in the column, the volatile components emerge from the column at different times and are finally detected by a suitable detector. If the carrier gas used the rate of flow and the temperature of the column is kept constant the retention time (i.e., the time taken by each component of mixture to traverse through the column) for each constituent of the mixture will always be the same.

It usually shows a linear relationship with the boiling point of the compound and is characteristic for the constituents concerned under the given set of conditions for a given column. Thus, it is possible to identify the compound from its characteristic retention time on a particular column and under a given set of conditions. Quantitative estimation can be carried out from the extent of peak area recorded by the detector recorder system.

Apparatus used for gas chromatography is a simple tube of about 4 mm in diameter and about 120 cm to many metres in length. It is made of stainless steel or glass and is usually bent or coiled so that it could be accommodated in a small space. The tube or the column is packed with particles of some suitable adsorbent or in the case of gas-liquid chromatography the fixed phase is a non-volatile liquid coated over some solid support (particles of diatomaceous earth, crushed fire bricks etc.).

As mobile phase, the gases used may be Argon, Helium, Nitrogen or Hydrogen – Hydrogen is usually not preferred because of fire hazards. There has been a recent trend to use capillary-gas chromatography in which instead of the column a very thin capillary, about 0.25 mm in diameter and usually 50 metres in length made of glass, stainless steel or some organic polymer is employed. The inside of the capillary wall is coated with the stationary liquid phase. These capillary columns are superior to packed columns in terms of separation efficiency. They can separate upto several components from a single sample.

In order to maintain a constant rate of flow of the carrier gas, there is a flow meter and an adjustment device which regulates the flow of the carrier gas into the column. The sample is introduced through a self sealing silicon rubber partition into a chamber which is heated to bring about evaporation of the sample. The temperature of the chamber must not be so high as to decompose the sample.

Solid samples have to be dissolved in some solvent whereas gaseous samples require special sample introduction valves. The detectors, placed at the exit of separation chamber, detect and measure the small amounts of separated components present in the stream of the carrier gas leaving the column. Normally three types of detectors are employed in gas chromatography: thermal conductivity detectors, flame ionization detectors and electron capture detectors.

Thermal conductivity detectors are the most widely used detectors in gas chromatography. These detectors use heated metal filament (or thermisters which are made of some semi-conductor of fused metal oxides) to sense small changes in thermal conductivity of the carrier gas. Thermal conductivity of the carrier gas only gives an essentially constant signal. The presence of vapours of the different components of the mixture in carrier gas brings about changes in the thermal conductivity proportional to their amount in the stream. This brings about changes in the resistance of the filament which is measured. The recorder which records these changes is equipped with an automatic device which traces the magnitude of these changes on a graph sheet along with the retention time.

Flame ionization detectors are based on the measurement of electrical conductivity of gases. At normal temperatures and pressure, gases act as a bad conductor or insulators but when ionized they act as a good conductor of electrical current.

Gases and vapours as they emerge from the separation column are mixed with hydrogen and burned in air to produce a flame which ionizes the component molecules in the carrier gas. The burning jet is the negative electrode while the anode is usually a small loop of wire extending into the tip of the flame across which a small voltage is applied.

The ions produced are collected at the electrodes and a current is generated which is proportional to the number of the component molecules ionized. Only the carrier gas burning with hydrogen produces an essentially constant signal but when the components of the mixture being analysed emerge ionization occurs and a higher current is observed. The detector is equipped with an automatic recording device which records these fluctuations and transmits them to a graph sheet along with the retention time.

Electron capture detector is based on the phenomenon of electron capture of compounds having an affinity for free electrons. A (3-ray source is used to obtain slow electrons by ionization of the carrier gas (nitrogen is preferred) passing through the detector.

These electrons as they flow towards the anode under a fixed potential give rise to a steady current. When the component molecules of the mixture being analysed come out of the separation column and pass through the detector these electrons are trapped.

The net result is replacement of the electrons by negatively charged ions of much greater mass and corresponding reduction in the flow of electric current proportional to the concentration of electron capturing component in the carrier gas. These changes are detected by using a suitable circuit and recorded with the help of an automatic device which traces a graph along with the retention time.