Loss of bonded phase in reversed-phase liquid chromatography in acidic eluents and practical ways to improve column...

Silica-based, reversed-phase liquid chromatographic (RPLC) stationary phases are very widely used to separate basic compounds in acidic eluents due to their high efficiency, good mechanical strength, and the versatile selectivity offered by different functional groups and the chemistry on the silica surface. However, the stability in acid of most silica-based stationary phases is poor, especially at elevated temperatures, due to hydrolysis of the siloxane bonds, which hold silanes on the silica substrate. This hydrolysis is commonly believed to be solely the result of catalysis by protons. However, we show that various metal cations (principally Fe3+/Fe2+, Ni2+, and Cr3+) released from acid corrosion of the stainless steel inlet frit greatly accelerate the hydrolysis of the siloxane bond. Furthermore, these metal cations, and not the high acidity per se, are mainly responsible for column instability. We show that removing the stainless steel inlet frit, or use of a titanium frit, greatly reduces or totally eliminates corrosion of the inlet frit and radically improves retention stability. The effects of various acids and types of organic modifier were also studied. These observations suggest a number of practical approaches that can significantly extend the lifetime of any RPLC stationary phase in acidic media at elevated temperature.