Deactivation with silazanes in chromatography, mechanism of the reaction and practical consequences in capillary GC and RP‐HPLC: A29Si CP‐MAS NMR study

Abstract

AbstractThe reaction of Cab‐O‐Sil, a highly dispersed vitreous quartz, with hexamethyldisilazane (HMDS) was studied in the temperature range 380–500°C, using 29Si solid state NMR and other techniques. Such studies are of importance in view of deactivation procedures of fused silica at high temperatures in capillary GC.The commonly accepted reaction equation: predominates only below ca. 400°C. Above ca. 400°C the intermediate cleavage product Me3SiNH2 reacts with surface silanol groups to form SioSiMe2NH2 + CH4. At higher temperatures these groups may ultimately form (SiO)3SiNH2 groups (analogously to the formation of bi‐and tridentate linkages starting from SiOSiMe3 groups), but (SiO)3SiNH2 groups are also directly formed at lower temperatures, simultaneously with the SiOSiMe3 groups, probably by reaction with siloxane bridges: .The reactions of the trimethylsilylamine part of HMDS with Cab‐O‐Sil were confirmed by an independent series of silylations using N,N‐dimethyltrimethylsilylamine instead of HDMS. The presence of amino groups as SiNH2 was confirmed by FT‐IR. This may be one of the reasons why very high temperature silylation with disilazanes does not provide a satisfactory deactivation of fused silica GC columns: the active SiOH groups are “replaced” by active SiNH2 groups. However, such a material may be of interest in LC. Silylation with silazanes at ca. 350°C in humid atmosphere and/or after extensive hydroxylation (leaching) of the surface should yield sufficiently deactivated surfaces with a rather well defined surface structure.