A comparative study of the gas chromatographic behavior of liquid crystals and polyester stationary phases using paraffinic hydrocarbons as standards

Abstract

SummaryTwo new categories of stationary phase have been studied: liquid crystals and polyesters. The liquid crystals were p,p'-azoxyphenetol, p(n-hexy1oxy)phenyl-p'-methoxy benzoate, and p-pentyloxy-p'-ethoxyazoxybenzene; and the polyester phases 1,4-butanediol succinate (LAC-860), ethylene glycol succinate (LAC--), diethylene glycol succinate (LAC-738), diethylene glycol adipate (LAC-296), and neopentyl glycol adipate (LAC-769).Abnormal chromatographic behavior, has been observed: an increase in specific retention volumes and an improvement in the separation of normal, branched, and cycloparaffins, depending on their structures, at high temperatures within a specified range. This phenomenon is the opposite of the behavior of traditional stationary phases. From the chromatograms obtained, plots of log 3 against r, and differential thermal analysis it is concluded that this abnormal behavior may be attributed to the penetration of solutes through the ordered structure of the mesophase and the rod-like molecules of the polyesters. Furthermore, the chromatographic behavior of some of the polyester (LAC-series) stationary phases in respect of temperature changes is similar to that of the nematic mesophases. 1 effect of temperature on the behavior of some hydrocarbons using cholesteric liquid crystal as the stationary phase. Grushka and Solsky [4] tried to expand the temperature range of the mesophase by using a mixed phase capable of forming a eutectic mixture , and also pointed out [5] changes in the height equivalent of a theoretical plate, H, as a function of changes in the temperature of the nematic crystal. Sojak et 8. 1. [6, 71 showed that liquid crystals as stationary phases in gas chromatography possess shapeselective separation properties for structural and geometrical isomers of hydrocarbons. Later, Sojak et al. [8] studied the separation of diastereomeric C8-C20 alkanes using mesogenic stationary phases (PBO and MEAB) in glass capillary columns. Previous work in our laboratory has shown abnormal chromatographic behavior for thiophenes @I, furans [lo], aldehydes [lll, and anilines [12] on some of the new series of polyester (LACseries) stationary phases; namely an increase in specific retention volumes and improvements in resolution at certain column temperatures. These critical temperatures were different for the various stationary phases and in some cases for the different types of compound. The present investigation deals with the comparative gas chromatographic behavior of liquid crystal and polyester stationary phases at different column temperatures using C5-C9 paraffins and some branched and cycloparaffins.