Stabilization of reversed phases for liquid chromatography : Application of infrared spectroscopy for the study of bonded-phase stability

Abstract

This paper examines the spectroscopic characterization of chromatographic silica gels at three stages: before bonding of an n-butyl group to the silica surface, after bonding, and after exposure to hydrolytic conditions expected to lead to some loss of the alkyl groups from the surface. Hydrolysis was achieved by circulating an aqueous acetonitrile solution containing 0.5% (v/v) trifluoroacetic acid at 27°C through a loosely packed bed of the n-butyl-bonded silica. This hydrolytic treatment results in a rapid partial loss of bonded phase over about 1 day, followed by a much more gradual loss over the next six days. The extent of hydrolysis depends upon the silica used and the initial degree of coverage of the bonded phase. The results show that a portion of the bonded sites of the bonded phase are hydrolyzed more rapidly under acidic conditions, and that higher initial bonded-phase coverages lead to shielding of the more reactive sites and thus less overall hydrolytic loss. Diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy was used to study three different silicas before and after hydrolysis of the bonded phase. On one of these silicas, rapid hydrolysis of the n-butyl bonded phase was associated with reemergence of isolated silanol groups on the silica surface. On the other two silicas, the same hydrolytic conditions led to much slower loss of coverage, and spectroscopy did not detect a reemergence of lone silanol groups. Although all three unbonded silicas apparently had similar physical and chemical properties, one of them was clearly different from the others in stability of a subsequently attached alkyl phase. This difference could only be detected by bonding, followed by hydrolysis. One possible explanation of these results is that different manufacturing conditions lead to a different distribution of silanol groups on the silica surface, which is not detectable by routine physical and chemical characterization methods, yet nevertheless affects bonded-phase stability.