A CP‐MAS NMR study of some deactivation methods in capillary gas chromatography

Abstract

The effect of temperature, water content, and the type of reagent on the silylation of fused silica capillaries was studied by 29Si and 13C CP‐MAS NMR. Fumed silica (Cab‐O‐Sil M5), which is essentially a highly dispersed vitreous quartz with a surface comparable to that of fused silica capillary columns, was selected as a model material. Hexamethyldisilazane (HMDS) and 1,2‐diphenyl‐1,1,3,3‐tetraphenyldisilazane (DPTMDS), which were used as silylation reagents, yielded trimethyl‐ and dimethylphenylsilyl surface groups respectively at lower temperatures (< 350°C and <250°C respectively). At higher temperatures, increasingly more dimethylsilyl groups are formed, with the silicon bound to two oxygen atoms. This process occurs for DPTMDS at a considerably lower temperature than for HMDS. The formation of silyl groups on the surface and the disappearance of hydroxyl groups are followed independently. The 13C NMR and GC‐MS of the reaction products showed that with DPTMDS, the formation of two Si‐O‐Si links is accompanied by a loss of phenyl groups rather than of methyl groups. After the Cab‐O‐Sil had been dried over P2O5, the formation of these double links occurred for HMDS only at temperatures above 460°C and for DPTMDS at 400°C. Thus we concluded that water supplies oxygen atoms for double Si‐O‐Si links (possibly crosslinks) necessary for efficient deactivation. This may explain the less successful silanization of fused silica capillaries because their water content is lower than that of glass capillaries.

A contribution to high‐temperature silylation

“…In the case of DPTMDS, these reactions start already at about 25OOC [7]. This is in accord with IR measurements.…”

“…In the same manner we may explain the results of Welsch [8] and Rutten et al [7]. We also believe that the main cause of the very good deactivation by HTS is that short dimethylsiloxy chains are obtainedwhich may bearterrninal triorganosiloxygroupsand thus provide for a better shielding of the glass surface.…”

“…In contrast, Rutten et a/. [7] showed by CP-MAS NMR analysis of fused silica (Cab-0-31) that a loss of phenyl groups starts around 250°C.At 35OOC there were no longer any phenyl groups at the surface. They explained the high degree of deactivation after HTS by the formation of two-fold orthree-fold anchored organosilyl groups on the surface.…”

“…We also believe that the main cause of the very good deactivation by HTS is that short dimethylsiloxy chains are obtainedwhich may bearterrninal triorganosiloxygroupsand thus provide for a better shielding of the glass surface. The high temperature in HTS with HMDS is necessary since reaction (2) and (3) start at temperatures around 35OOC [7].…”

Deactivation by polysiloxane and phenyl containing disilazane: A 29SI CP‐MAS NMR study after the formation of polysiloxane chains at the surface

“…From polysiloxane degradation experiments at temperatures between 4OOOC and 50O0C, as published before [7], it can be concluded, largely on 29Si NMR chemical shift arguments and on cross polarization behavior, that rather small fragments of polydimethyldisiloxysilanes are formed at the surface. However, no terminal groups were detected and only a relatively small, narrow signal in the dimethyldisiloxysilane region near -22 ppm was noticed, assigned on the basis of chemical shift arguments to longer, mobile polydimethylsiloxanes.…”