USE OF METHYL SPACERS IN A MIXED HORIZONTALLY POLYMERIZED STATIONARY-PHASE

Studies of mixed horizontally polymerized monolayers of octadecyl- (C-18) and methyl- (C-1) trichlorosilanes show that C-1 groups are valuable as spacers in this type of chromatographic stationary phase. Molecular models are presented that predict C-1 spacers to have less steric hindrance than propyl (C-3) spacers, which aids in the cross-linking of the siloxane monolayer. Si-29 NMR measurements reveal significantly greater cross-linking in the polymerization of the C-18/C-1 mixed monolayer compared to the C-18/C-3 mixed monolayer. Contact angle measurements for a pure C-1 monolayer on a nat silica surface indicate that the methyl groups are predominantly directed away from the silica substrate. The chromatographic retention behavior of aniline shows that the C-18/ C-1 monolayer has significantly less silanol activity than does the C-18/C-3 monolayer. As a critical test of silanol activity, the retention behavior of a set of cationic peptide standards shows that the C-18/C-1 monolayer has very low silanol activity and provides less peak asymmetry than does a monomeric phase made with the same high-quality silica gel (Zorbax-300RX-sil). The baseline resolution of a mixture of three cytochrome c genetic variants establishes that the C-18/C-1 stationary phase allows high column efficiency in addition to low silanol activity.