Characterization of C-18-bonded liquid chromatographic stationary phases by Raman spectroscopy: the effect of temperature

This study represents the first time that both the mobile phase composition and the temperature are simultaneously controlled to examine silica-bonded octadecylsilyl (C-18) ligands spectroscopically at typical liquid chromato graphic (LC) mobile phase flow-rates and back-pressures. Raman spectroscopy is used to characterize the behavior of the C-18 bonded ligands equilibrat ed at temperatures from 45 to 2 degrees C in neat, single-component, mobile phase solvents including: water, acetonitrile, methanol, and chloroform. I n addition, the effect of stationary phase ligand bonding density is examin ed by using two different monomeric reversed-phase liquid chromatographic ( RPLC) stationary phases, a 2.34 and a 3.57 mu mol m(-2) Microporasil C-18 s tationary phase, under identical conditions. The direct, on-column, spectro scopic analysis used in this study allows direct evaluation of the temperat ure-dependent behavior of the bonded C-18 ligands. The temperature-dependen t ordering of the stationary phase ligands is examined to determine if the ligands undergo a phase transition from a less-ordered "liquid-like" state at higher temperatures to a more-ordered "solid-like" state at lower temper atures. A discrete phase transition was not observed, but rather a continua l ordering as temperature was lowered. (C) 2000 Elsevier Science B.V. All r ights reserved.