Gas Chromatography Essay

Gas chromatography is widely used for analytical purposes in the chemical world to separate and analyse volatile compounds. It is mostly widely known as being paired with a mass spectrometer as, whilst it is a useful technique for testing purity and separating and quantifying compounds, it is less useful for definitive characterisation or identification. In this technique, a sample, which must consist of sufficiently volatile liquids or solids, is injected through a septum into a heated port, which causes the components to rapidly evaporate. The column must then be hot enough to provide sufficient vapour pressure that elution time will be reasonable. The volatilised component will then be carried through the column by the mobile phase – consisting

The most widely used carrier gas is helium, which is used in the GOW-MAC Series 600 instrument used in this experiment. Although hydrogen gas gives the fastest separations, whilst having little impact on resolution, it was historically less commonly used due to concentrations of greater than 4% in air being potentially explosive [1]. Capillary chromatography, however, is not known to generate explosive conditions and, more recently, electrolytic generators have been used to produce high purity hydrogen without the risks associated with pressurised hydrogen gas. Solutes will diffuse more rapidly through hydrogen and helium gas than nitrogen gas, resulting in a small plate height, meaning greater resolution, and also a smaller mass transfer in van Deemter’s equation, which links the length of the separation column to the velocity of the mobile

They can potentially be used for polar analytes and will increase the maximum temperatures that can be used without bleed, which would have been of use in this experiment because it could be used to study polar alcohols, and it would mean that alcohols of longer chains and higher boiling points could be studied. As it was, this was not possible with the GOW-MAC Series 600 for this experiment because it had a maximum temperature of 250 oC and studying alcohols longer than butanol would take too long as the temperatures could not have been sufficiently raised to shorten retention times.

In a Gas Chromatograph, the resolution is proportional to the number of theoretical plates (N) – also known as the column efficiency – which is in turn proportional to the length of the column, meaning that the resolution is affected by column length:

Resolution=