Influence of alkyl chain length on the stability of n-alkyl-modified reversed phases. 1. Chromatographic and physical analysis

Abstract

The Influence of the llgand alkyl chain length, chemically bonded at the &ace of a sllka substrate, on statlonary phase stabilrty In llquld chromatography practlce is revlewed. Several factor$ affecting long-term stability of modern reversed-phase htgh-performance llquld chromatography (RPHPLC) phases are consldered and their lndlvldual contrlbutlons are evaluated In this paper and the following paper In this Issue. Tho stationary phases under study were Identically modifled on the same batch of slllca substrate to ellmlnate dmerences In substrate properties and synthesis conditions. Modtflcatlons with llgand alkyl chain length between C1 and C18 were performed such that an approxknately equal llgand density was obtained for the seven RP-HPLC phases studled. These n-alkyldimethyldloxydlane bonded phases were exposed to sknulated aglng experlments. A subsequent chromatographk characterlzatlon regarding changes In capacity, llp6phlUc and polar sdectlvlty, and slllca degradatlon related separation performance was carried out. Comparison with results determined by other characterization methods, llke bulk analysla, elemental analysls, and solid-date %I crosspolarkatloll magic angle spinnhrg NMR revealed that with longer n-alkyl llgands gradually better substrate shleldlng propertks were obtahd. The llgand alkyl chain length affects the stabllrty of reversed phases to a large extent. Short llgand modM phases changed drastically uslng relatively aggresslve eluents. Organlc modHler rlch eluents comblnsd with longer @and modltled phases are preferable for a longer llfetlme of RP-HPLC columns, especially when aggressive eluents are used.