Question

What is the expected elution order in GC for each of the following homologous compounds? List your answer from first eluted to last eluted.

a. An alcohol sample that contains 1-butanol, ethanol, methanol, and 1-propanol.

b. Diesel fuel that contains benzene, ethylbenzene, and toluene.

c. A sample of butter that contains butyric acid and caproic acid (hexanoic acid)

Answer 1

a) The elution order will depend on the boiling points of the compounds chosen. The lower the boiling point of a compound, the more volatile will be the compound and hence shorter will the retention time be. The boiling point of a compound depends on the molecular mass or molecular weight of the compound. The higher the molecular weight, the higher will the boiling point be. Among the homologous alcohols given, the increasing order of molecular weight and hence the increasing order of boiling points are

methanol < ethanol < 1-propanol < 1-butanol

The order of elution will be

methanol > ethanol > 1-propanol > 1-butanol (the compound with the lowest molecular weight elutes faster and the compound with the highest molecular weight elutes last).

b) We need to apply the same principle as in (a) to predict the order of elution. The increasing order of molecular weights and hence the increasing order of boiling points will be

benzene < toluene < ethylbenzene

Therefore, the order of elution is benzene > toluene > ethylbenzene.

c) Butyric acid has a lower molecular mass (4 carbons) that caproic acid and hence butyric acid will have a lower boiling point than caproic acid (6 carbons). The boiling point of caproic acid will naturally be higher than that of butyric acid. Hence the order of elution is GC will be butyric acid > caproic acid in the sense that butyric acid elutes faster than caproic acid.