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 valuab le as spacers in this type of chromatographic stationary phase. Molecu lar models are presented that predict C-1 spacers to have less steric hindrance than propyl (C-3) spacers, which aids in the cross-linking o f the siloxane monolayer. Si-29 NMR measurements reveal significantly greater cross-linking in the polymerization of the C-18/C-1 mixed mono layer compared to the C-18/C-3 mixed monolayer. Contact angle measurem ents for a pure C-1 monolayer on a nat silica surface indicate that th e methyl groups are predominantly directed away from the silica substr ate. 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 re tention behavior of a set of cationic peptide standards shows that the C-18/C-1 monolayer has very low silanol activity and provides less pe ak asymmetry than does a monomeric phase made with the same high-quali ty 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 sila nol activity.