Application of polymethylphenylsiloxane coating to multicapillary columns

Abstract

Owing to their short length, fast multicapillary columns (MCC) have limited peak capacity, so for wider analytical opportunities there is a need for a set of columns with assorted stationary phases (SP) of different separating properties. The paper is devoted to the aspects of coating of MCCs with the following polymethylphenylsiloxane SPs: OV-5 (phenyl group content 5%), OV-7 (20%), OV-61 (31%), OV-17 (50%), and OV-25 (75 %). Particular emphasis is placed on the influence of the nature of capillary surface and type of solvent used for dissolving stationary phases on the performance of columns. 0.25 m long multicapillary tubes (MCT) were used as the base for MCC. Their surfaces were deactivated by hexamethyldisilazane (HMDS) (phenyl group content 0%), 1,1,3,3-tetramethyl-1,3-diphenyldisilazane (33%), 2,4,6,8tetramethyl-2,4,6,8-tetraphenylcyclo-tetrasiloxane (50%), 1,3-dimethyl-1,1,3,3-tetraphenyldisilazane (67%), and hexaphenyl-cyclotrisiloxane (HPTS) (100%). Because of the different ratios of methyl and phenyl radicals in the solvents, the modified surface has different polarity which increases with the rise of phenyl radical content. To improve the efficiency of MCC, the 'correlated' method of SP coating was used: greater amount of SP was impregnated into wider capillaries.
The case of OV-7 SP application shows that the use of chloroform as a solvent for SP results in the MCC with the efficiency of over 3000 theoretical plates (t.p.). Alternatively, benzene and toluene provide MCC with lower efficiency. During the investigation of the influence that the nature of capillary surface has on MCC performance it was found that coating a nonpolar surface (deactivated by HMDS) with all kinds of SPs resulted in less efficient MCCs. SPs with up to 33% phenyl group content (OV-5, OV-7 and OV-61) can be successfully applied to the surface that has been silanized by reagents with phenyl group content lower than 50%, with the MCC efficiency exceeding 3000 t.p. Applying OV-17-vinyl on the surface deactivated by a reagent with phenyl group content 50% leads to the formation of surface film that is stable in narrow temperature range. Greater phenyl group content in the reagent provides higher efficiency MCC. Applying OV25 only provides decent results (2000-2500 t.p.) when the surface is deactivated by HPTS.
So, the main finding is that the application of polymethylphenylsiloxane SPs: OV-5, OV-7, OV-61, OV-17 and OV-25 on MCT is only successful provided that the surface of capillaries is deactivated by reagents with phenyl group content higher than in the SP.