Instant Notes: Analytical Chemistry

carbon number by interpolation (Fig. 4). For an unknown solute, its Kováts

index can then be checked against data bases by computer searching;

● Interfacinga gas chromatograph with a mass or infrared spectrometer. This

enables spectral information for an unknown solute to be recorded and inter-

preted. Identifications are facilitated by searching libraries of computerized

spectra (Topics F3 and F4).

Methods used in quantitative chromatography are decribed in Topic D2 and

alternative calibration procedures are described in Topics A5 and B4. Detector

response factors must be established for each analyte as these can vary consider-

ably, especially where selective detectors such as the ECDor NPDare used.

Calibration can be with external standardschromatographed separately from

the samples, by internal standardization, standard addition or internal

normalization. An internal standard should have similar chromatographic char-

acteristics to the analyte(s); homologues or isomers are often the most suitable.

The use of internal standards, where peak/area ratios of analyte to internal

standard are calculated, is preferable because a major source of variability arises

from the very small volumes injected by microsyringe, and peak area ratios are

independent of the volume injected. Autoinjectors minimize this source of error,

and quantitative analysis by GCcan be expected to have an overall relative

precision of between 1 and 5%.

Quantitative
analysis

154 Section D – Separation techniques

Unknown

Log (retention time)

600 700 800 900 1000

860

(Retention index scale (C number x100))

Fig. 4. Kováts retention index plot for n-alkanes. Retention index for unknown interpolated

from its log(retention time) as 860.