Syntheses of Stationary Phases for Reversed-Phase HPLC: Comparison of the Fluidized Bed Technique with the Organic Solvent Process

A series of chemically bonded stationary phases are prepared by fluidized bed and conventional organic solvent (toluene) bonding processes, respectively, using mono-, di-, and trifunctional n-octylchlorosilanes. Solid-state 13C cross-polarization—magic angle spinning (CP—MAS) nuclear magnetic resonance (NMR) spectroscopic studies show that the surface structure of the fluidized bed packings consists only of the expected silane skeleton. In addition to the silane signals, the organic solvent packings give unexpected —OCH3 absorbance at about 50 ppm, which arises from a possible side reaction between the methanol washings and the residual chlorine atoms. Chromatographic studies show that the fluidized bed packings possess higher plate numbers and superior retention characteristics for hydrocarbon solutes. This is probably caused by their higher carbon load and homogeneous distribution of the alkyl ligands on the silica surface as a result of better accessibility of the silane molecules to the surface silanol groups. In comparison with the organic solvent process, the fluidized bed technique produces a more reproducible packing regardless of the functionality of the silanizing agent.