Silica Surface Modification: in Search of More Hydrolytically Stable HPLC Stationary Phases
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Abstract A number of studies were done to develop hydrolytically stable HPLC stationary phases on superficially porous silica. The primary study used diazonium chemistry to generate a polymeric layer on the silica surface. Manipulations of synthetic conditions were performed to limit the polymer to a thin layer. Controlling the thickness of the polymer layer proved critical to chromatographic performance. Primary amine groups built into the polymer layer provide sites where further synthetic modification of the surface can be performed. Additionally, these amines were shown to influence the chromatographic properties of the phase via retention mapping. The polymer stationary phase exhibits reverse phase behavior, though highly organic mobile phases elicit some HILIC-like behavior. Under reverse phase conditions, the polymer was compared to a commercial phenyl column. The two columns showed similar retentivity for a diverse group of analytes. Lastly, the hydrolytic stability of the polymeric phases were tested via degradation in 0.5% TFA at 80°C. After 20 hours of exposure (2400 column volumes) optimized materials lost less than 10% of their initial retention. Additional studies sought to generate hydrolyticlly stable bonded phases through encapsulation of the silica support. Triethoxysilane was used to generate a self-assembled monolayer of type c silica. Despite numerous attempts to control the reaction, vertical polymerization of the silane could not be prevented. Catalytic hydrosilation of the type c silica using 4-vinylaniline yielded surface loadings of up to 6.4 µmol/m2. Encapsulation of the silica support with a carbonaceous layer was attempted by heating the polymer generated previously to 900°C under argon. Attempts to generate a carbonaceous layer on silica supports resulted in the formation of carbon black like materials as heating beyond the graphitic transition of the carbon was impossibe. Use of these materials as stationary phases results in high retentivity and poor peak shapes. At this time the materials generated are unfit for use in HPLC as a stationary phase.