Gas/liquid chromatography — common mistakes

When asked to predict the order of elution (the order components leave the column), first look at boiling points. The compound with the lowest boiling point is the most volatile and will have the shortest retention time. If boiling points are similar, consider polarity. The compound that is most 'like' the stationary phase in terms of polarity will be retained for longer.

The main difference is the mobile phase. In GLC, the mobile phase is an inert gas, and the sample must be volatile. In HPLC, the mobile phase is a liquid solvent under high pressure, which allows it to separate non-volatile compounds that cannot be analysed by GLC.

The oven maintains a precise and uniform temperature for the column. This is crucial because temperature directly affects the volatility of the components and their retention times. Controlling the temperature ensures that results are reproducible and allows for temperature programming (gradually increasing the temperature) to improve separation of complex mixtures.

GC-MS stands for Gas Chromatography-Mass Spectrometry. It's a hybrid technique where the outlet of a gas chromatograph is fed directly into the inlet of a mass spectrometer. The GC separates the mixture into its individual components, and then the MS analyses each component as it elutes, providing a mass spectrum. This gives a 'molecular fingerprint' that allows for much more definitive identification than retention time alone.