Chemistry, Third edition

CHROMATOGRAPHY 363

Gas chromatography (GC)

Gas chromatography (GC) is used for checking the purity or identity of volatile

liquid samples. In most experiments small quantities of sample are used and

mixtures are separated, and the components identified, rather than collected. The

sample is injected into the instrument, vaporized and carried in a stream of carrier

gas (helium or nitrogen) on to a chromatography column. The components of the

sample move through the column at different rates and reach a detector at different

times. When a component hits the detector an electric signal is sent to a chart

recorder, or VDU, which produces a graph, or chromatogram, of the different com-

ponents passing through the detector. The time taken for a compound to pass

through the apparatus and reach the detector is called the compound’s retention

time. The retention times of the components of a mixture can be compared with the

retention times of known compounds obtained under the same conditions. In this

way, the components of the mixture may be identified. The trace produced for each

component also gives a rough idea of the relative quantity of the component. The

areaunder each peak (notthe height of the peak) is proportional to the percentage of

that component present in the sample.

The chromatograph in the diagram (Fig. 19.21) shows three peaks. One of these

Fig. 19.21Gas chromatogram.

TheRfvalue is constant for a compound, on the same plate, if the temperature and

solvent are kept the same. If a spot from an unknown substance is developed on a TLC

plate together with a spot from a substance that is suspected to be the unknown, and

the two substances are found to have the same Rfvalue, they are probably the same

substance. A substance that just produces one spot when subjected to TLC is pure.

Rfvalues

Using the diagram of a TLC plate below,

write expressions, involving x,yandz, for:

(i)theRfvalue for component A;

(ii)theRfvalue for component B.

Exercise 19D