Abstract
Data presented herein show that the film thickness of a polar stationary phase, SPβ2380, affects column selectivity under isothermal analysis conditions. The expected linear relationship between film thickness and solute capacity factors (k), as predicted by the partition coefficient equation, was confirmed experimentally. However, selectivity, as determined by ECL values for the fatty acid methyl esters, does not remain constant. As the film thickness is increased from 0.10 ΞΌm to 0.20 ΞΌm, 0.30 ΞΌm, and 0.50 ΞΌm, there is an increase in effective column polarity. This increase mimics the polarity shift seen when comparing a 90% bisβcyanopropyl/10% cyanopropyl phenyl polysiloxane capillary column to a 100% bisβcyanopropyl polysiloxane column of the same film thickness (0.20 ΞΌm) under isothermal conditions. The increase in selectivity over the film thickness range studied is revealed by ECL values for an unsaturated C18:3 ester relative to an unsaturated C20:1 ester and saturated esters, C18:0(methyl stearate), (C20:)(methyl arachidate), and C22:0(methyl behenate). As the film thickness increased, the C18:3 ester eluted farther from the C20:0 ester and changed from eluting before the C20:1 ester to eluting after the C20:1 ester and closer to the C22:0 ester.
The effective polarity increase also reduced the retention time for all of the saturated FAMEs, compared to their respective unsaturated counterparts. Possible causes of the differences in selectivity among these columns include a lessening of the surface adsorption and increased partitioning with the thicker stationary phase film.