Differences in carbon 13 nuclear magnetic resonance spectra for monomeric and polymeric octadecyl derivatized silica column packings for liquid chromatography

Abstract Two different types of reversed-phase octadecyl derivatized silica (C 18 ) packings have been compared using cross-polarization, magic-angle spinning carbon 13 nuclear magnetic resonance spectrometry ( 13 C NMR) on dry solids and high-resolution 13 C NMR on suspensions in chromatographic solvents. Differences in the NMR spectra of the dry monomeric and polymeric C 18 packings confirmed the differences in the manufacturing procedures for endcapping the C 18 packings. The high-resolution studies on suspensions showed differences in peak shapes and areas for the packings. In pure methanol as opposed to pure chloroform the peaks due to the alkyl chain on the polymeric C 18 packing were almost lost in the baseline, while the peaks for the monomeric packings were decreased much less. In both cases, the decreases in the sizes of these peaks reflected decreased mobility of the groups in the alkyl chain and suggests that the chain is lying on the surface. The differences in the spectra for the packings provides a basis for rationalizing the differences in chromatographic retentions reported by others for polycyclic aromatic hydrocarbons.