Instant Notes: Analytical Chemistry

Methods of identifying unknown solutes separated by chromatographic tech-

niques are described in Topic D2. In the case of gas chromatography, there are

four alternatives:

● Comparisons of retention times(tR) with those of known solutes under iden-

tical conditions, preferably on two columns of differing polarity to reduce the

chances of ambiguous identifications;

● Comparisons of chromatogramsof samples spikedwith known solutes with

the chromatogram of the unspiked sample;

● Calculation of a retention indexbased on a set of standards that can be

compared with published or in-house library data. For a homologous series

of compounds, the logarithm of the retention timeis directly proportional to

the number of carbon atoms. The Kováts system, based on the homologous

Qualitative
analysis

152 Section D – Separation techniques

0246

Minutes

81012 0246

Minutes

81012

(a) (b)

0246

Minutes

81012 0246

Minutes

81012

(c) (d)

Fig. 2. Pyrograms of four common polymers. (a) Polyethylene; (b) A polyester; (c) A fluoro-

carbon copolymer; (d) Nylon.