STATIONARY PHASE FORMATION FOR CHEMICALLY MODIFIED CHROMATOGRAPHIC SUPPORTS.

Abstract

A new theory has been proposed for stationary phase formation of chemically modified chromatographic adsorbents. This theory consists of a model in which the bonded hydrocarbon moiety, silica substrate, and their respective solvation layers all participate in stationary phase formation. Stationary phase formation was found to be dependent on three parameters: (1) Solvent strength of the mobile phase components for the bonded organic moiety and the silica substrate; (2) the type of organic moiety covalently bound to the surface; and (3) the bound moiety density or surface coverage. Binary aqueous-organic mobile phases were investigated for LiChrosorb RP-8 and RP-18. For RP-8 the solica substrate played a more important role in stationary phase formation. Whereas, for RP-18 the longer bound hydrocarbon chain dominated stationary phase formation. With different organic modifiers in the mobile phase, the modifier with the larger solvent strength for the bound hydrocarbon was selectively enriched in the stationary phase solvation layer for RP-18. Ternary mobile phase systems were also investigated for RP-18. The second modifier was found to exert a large influence on stationary phase formation. Temperature's role in stationary phase formation was studied with a ternary mobile phase of 40/45/15 methanol, water, THF with RP-18. In this specific case, changing the temperature of the system did not impact on stationary phase formation. A new type of column structure was investigated. This structure involved a totally porous silica gel as compared to a column packed with totally porous silica microparticles. These silica gel columns were characterized both thermodynamically and kinectically. Under Normal Phase chromatographic conditions the silica gel column was found to have a higher selectivity but poorer efficiency for the separation of aniline from nitrobenzene than a packed column. The silica gel can be chemically modified by silane reaction and its bonded phase characteristics were investigated. The gel also showed ion-exchange properties which were investigated using sodium nitrite.