INFLUENCE OF ALKYL CHAIN-LENGTH ON THE STABILITY OF NORMAL-ALKYL-MODIFIED REVERSED PHASES .1. CHROMATOGRAPHIC AND PHYSICAL ANALYSIS

The Influence of the ligand alkyl chain length, chemically bonded at the surface of a silica substrate, on stationary phase stability in liquid chromatography practice Is reviewed. Several factors affecting long-term stability of modern reversed-phase high-performance liquid chromatography (RP-HPLC) phases are considered and their individual contributions are evaluated in this paper and the following paper In this Issue. The stationary phases under study were Identically modified on the same batch of silica substrate to eliminate differences in substrate properties and synthesis conditions. Modifications with ligand alkyl chain length between C1 and C18 were performed such that an approximately equal ligand density was obtained for the seven RP-HPLC phases studied. These n -alkyldtmethylsiloxysilane bonded phases were exposed to simulated aging experiments. A subsequent chromatographic characterization regarding changes in capacity, lipophilic and polar selectivity, and silica degradation related separation performance was carried out. Comparison with results determined by other characterization methods, like bulk analysis, elemental analysis, and solid-state 29Si cross-polarization magic angle spinning NMR revealed that with longer n-alkyl ligands gradually better substrate shielding properties were obtained. The ligand alkyl chain length affects the stability of reversed phases to a large extent. Short ligand modified phases changed drastically using relatively aggressive eluents. Organic modifier rich eluents combined with longer ligand modified phases are preferable for a longer lifetime of RP-HPLC columns, especially when aggressive eluents are used. © 1990, American Chemical Society. All rights reserved.