PATCHING IN REVERSED-PHASE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHIC MATERIALS STUDIED BY SOLID-STATE NMR SPECTROMETRY

The distribution of silane chains over the silica gel surface in rever sed-phase high-performance liquid chromatographic (RP-HPLC) phases was investigated with special attention being paid to surface homogeneity : there might be areas with high coverage and areas with low coverage of silane chains, hence clustering or patching of the silane chains ca n occur. Two RP phases were studied before and after well conditioned ageing, together with four silylated silica gels, serving as models. T wo solid-state NMR techniques, namely(1)H-Si-29 dipolar dephasing Si-2 9 cross-polarization magic angle spinning NMR and C-13 spin-lattice re laxation in the rotating frame, were used. For the four model compound s with varying degrees of coverage only differences in the NMR time co nstants were observed between the maximally covered phase and the thre e less densely covered silica gels. This proves that silane chains on an RP phase with maximum coverage are restricted in their mobilities w ith respect to the less densely packed materials. For the two non-aged RP phases the silane chains are probably homogeneously distributed ov er the silica gel surface. Further, the non-aged RP phases were compar ed with their counterparts, aged under well defined experimental condi tions. After ageing, no differences were found between the original an d the aged phases, now indicating however, that patching had developed upon ageing.