Sample Introduction and The Injection Chamber

Sample Introduction:

The sample, usually in the form of a solution in the order of 0.5 μl, is introduced into the chromatograph with a microsyringe. Several types of syringe exist because of the diversity of injectors and columns. For gaseous samples, loop injectors similar to those in liquid chromatography can be used. In order to automatic the injection and improve reproducibility-simply changing the user can cause substantial deviations-manufactures provide autosamplers in which the syringe and injection procedure are totally automated. These autosamplers, which can handle several samples, are very reliable. They operate in a cyclic fashion, taking the sample, injecting it rapidly (0.2 second) and rising the syringe. The latter is very important to avoid cross-contamination of successive samples that have similar composition.

A typically 10 μl syringe frequently used in gas chromatography. A guide is used to protect the fragile piston. In some models (0.1 to 1 μl), the plunger enters the needle in order to deliver all of the sample and avoid dead volume effects.

For the analysis of very volatile samples, there is technique known as head space chromatography which can be used either in a static or dynamic mode.

Carrier Gas & Flow Regulation | Gas Chromatography

The mobile phase is a gas (helium, nitrogen or hydrogen) which can be purchased from industrial sources or generated on-site in the case of nitrogen and hydrogen since the flows are relatively small (1 to 25 ml/min depending on column type).

The carrier gas should not contain traces of water or oxygen because these are very deleterious to the stationary phase. Typically, a filtering system containing a drying agent and a reducing agent is used between the gas source and the chromatograph.

In gas chromatography, the nature of the carrier gas does not significantly alter the partition coefficient K between the stationary and mobile phase. However, the viscosity of the carrier gas and its flow rate have an effect on the analyte dispersion in the column (Van Deemeter equation) thus affecting the efficiency and sensitivity of detection. The pressure at the head of the column (several tens to hundreds of kPa) is stabilised either mechanically or through the use of an electronic device ensuring that flow rate (linear velocity of the gas) in the column is at its optimal value. When a temperature programme is used during analysis, the viscosity of the mobile phase is increased thus increasing the resistance to carrier gas flow. It is, therefore, desirable to correct the pressure to compensate for this effect.

In a chromatogram contains a peak for a compound that is not retained on the stationary phase, it is possible to calculate the mean linear velocity ū of the carrier gas in the column. It is also possible to determine ū0 at the outlet of the instrument at atmospheric pressure p0 by putting a flow meter at the end of column. The ratio between these two velocities is equal to the compression factor J, which is related to the pressure differential between the inlet and outlet, P/P0 (P is the pressure at the head of the column).

Gas Chromatography Introduction & Components of A Gas Chromatograph

What is Gas Chromatography?

Gas Chromatography (GC) is a common type of chromatography used in analytical chemistry for separating and analyzing compounds that can be vaporized without decomposition.

Introduction:

Gas chromatography (GC) is a widely used technique.Its applications, dating back to the 1940s, were essentially in the analyses of light fractions from petroleum refineries. The use of GC has grown due to its sensitivity, versatility, wide range of stationary phases, and speed with which new analyses can be developed. The technique has also gained great interest due to its case of automation. Because separation occurs in the gas phase, liquid and solid samples must first be vaporised.

This represents the main constraint of this technique; compounds that are analysed by GC have to be thermostable and sufficiently volatile. Usually the stationary phases used in GC are liquids; rarely solids. Applications of this technique are used in the fields of petrochemistry, pharmaceuticals, the environment flavour analyses and so on.

Components of A Gas Chromatograph:

The gas chromatograph is compound of several components. These components include the injector, the column and the detector.

 The mobile phase that transports the analytes through the column is a gas and is referred to as the carrier gas. The carrier gas flow, which is precisely controlled, allows great precision in the retention times.
The analysis starts when a small quantity of sample in liquid or gaseous state is injected. The dual role of the injector is to vapourise the analytes and to mix them uniformly in the mobile phase. Once the sample is vapourised in the mobile phase, it is swept into the column, which is usually a tube coiled into a very small section with the length that can very from 1 to 100 meters. The column containing the stationary phase is situated in variable temperature oven. At the end of the column, the mobile phase passes through a detector before it exits to the atmosphere. Some gaseous chromatograph models have their own power supply, permitting them to be used in the field. 



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