Kovats indices and column polarity

Kovats indices (/-values) are based on the retention time of an analyte compared to retention times of the series of n-alkanes. For a particular GC phase, /-values are very reproducible from one column or from one GC to another. However, they are slightly affected by GC programming conditions. n-Alkanes have most affinity for non-polar phases and tend to elute more quickly from polar phases. In contrast, a polar analyte will elute more slowly from a polar phase and thus relative to the n-alkanes, its retention time and thus its /-value will increase as the polarity of the

GC phase increases. A measure of the polarity of a stationary phase is given by its McReynold's constant (Table 11.1), which is based on the retention times of benzene, n-butanol, pentan-2-one, nitropropane and pyridine on a particular phase. The higher the McReynold's constant the more polar the phase. Many stationary phases are described by an OV-number. The higher the number after the OV the more polar the phase.

Table 11.1 McReynold's constants

Phase

Chemical type

McReynold's constant

Squalane

Hydrocarbon

0

Silicone OV-1

Methylsilicone

222

Silicone SE-54

94% methyl, 5% phenyl, 1% vinyl

337

Silicone OV-17

50% methyl, 50% phenyl

886

Silicone OV-225

50% methyl, 25% cyanopropyl, 25% phenyl

1813

Carbowax

Polyethylene glycol

2318

/-values provide a useful method for characterising unknown compounds and tables of /-values have been compiled for a large number of compounds.3 Under temperature programming conditions, where the GC temperature rises at a uniform rate, e.g. 10°C/min, a plot of the carbon numbers of n-alkanes (where 1 carbon = 100) against their retention times is a straight line. Under isothermal conditions, where the column is maintained at the same temperature throughout the analysis, a plot of carbon number against the logarithm of the retention times of the w-alkanes is a straight line. Such calibration curves can be used to convert the retention time of a compound into an /-value.

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