Selectivity of liquid stationary phases

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. 

Associate the following /-values obtained on an OV-1-type column with structures of the local anaesthetics shown below, /-values 1555, 2018, 2323 and 2457. Note oxygen in an ether linkage is equivalent to ca 1 CHj unit. 

5551 aujeaozuaa LSK auieaeina -EZtZ aujeaeiaiuAxoJd aiOZ aujeDOJj usMSuy 

A very polar phase such as carbowax is generally only used for samples requiring a high degree of polar discrimination for adequate separation or retention. An example of this is in the analysis of fatty acids with differing degrees of unsaturation. On a non-polar column such as BPX-5, a series of C-18 acids such as stearic, oleic, linoleic and linolenic acids, which contain respectively 0, 1,2 and 3 double bonds, overlaps extensively. However, on polar columns such as carbowax they are separated. 

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