3.4.11 Structure Determination - Chromatography
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Gas-liquid chromatography - GCL
GLC is a microscale chromatographic technique that can be used for both qualitative and quantitative analysis. It consists of a mobile phase of a gas such as helium, argon, nitrogen or hydrogen, (depending on the detection technique used) which flows over a liquid stationary phase absorbed onto a solid support.
GLC is used to determine temperature stable, volatile substances in microscale amounts.
The GLC column
The column is between 1 and 3mm wide and is packed with a fine mesh powder solid support, onto which has been absorbed the liquid stationary phase.
The sample is injected into the gas flow just before it enters the thermostatically controlled column.
Separation of the components is achieved by partition between the two phases, the mobile and stationary phases. Roughly speaking this means that an equilibrium is set up between a component from the mixture in the gas phase and dissolved in the stationary phase.
Component (gas phase) 
Component (stationary liquid phase) |
The more the tendency for the equilibrium to lie to the right hand side, the slower the component travels through the column.
Temperature control
The temperature can be thermostatically controlled to be constant over the whole column, or it can be set as a gradient. The choice of techniques depends on the substances being separated and their relative volatility.
Qualitative and quantitative analysis by GLC
The retention factor can be calculated from a knowledge of the length of the column, the gas flow rate and the time of appearance of a component at the detector. This retention factor can be used to identify components by comparison with known values.
For quantitative analysis, the amount of each component within the sample can be calculated from the area under the curve in the readout. By comparing this to the size of the sample, the amount of each component in the sample can be determined.
Uses of GLC
GLC is particularly useful in the analysis and determination of volatile compounds, i.e. those that vaporise easily without decomposition.
- analysis of urine samples from athletes for drugs,
- underground mine gases
- blood alcohol levels
Column chromatography
Column chromatography relies on gravity to draw a solvent through a column of the stationary phase, which may be alumina, silica or a zeolite resin. The procedure is very useful for separation of compounds for purification purposes. The compounds can be eluted with the solvent and collected separately.