Effect of film thickness on the selectivity of cyanosilicone capillary columns.

Data presented herein show that the film thickness of a polar stationary phase, SP-2380, affects column selectivity under isothermal anal. conditions. The expected linear relationship between film thickness and solute capacity factors (k), as predicted by the partition coeff. equation, was confirmed exptl. However, selectivity, as detd. by equiv. chain length (ECL) values for the fatty acid Me esters, does not remain const. As the film thickness is increased from 0.10 mm to 0.20 mm, 0.30 mm, and 0.50 mm, there is an increase in effective column polarity. This increase mimics the polarity shift seen when comparing a 90% bis-cyanopropyl/10% cyanopropyl Ph polysiloxane capillary column to a 100% bis-cyanopropyl polysiloxane column of the same film thickness (0.20 mm) under isothermal conditions. The increase in selectivity over the film thickness range studied is revealed by ECL values for an unsatd. C18:3 ester relative to an unsatd. C20:1 ester and satd. esters, C18:0 (Me stearate), C20:0 (Me arachidate), and C22:0 (Me 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 satd. FAMEs, compared to their resp. unsatd. 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.