STABILITY OF SILICA-BASED, ENDCAPPED COLUMNS WITH PH-7 AND PH-11 MOBILE PHASES FOR REVERSED-PHASE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY

The goal of this study was to define practical conditions and limitati ons of using silica-based, endcapped bonded-phase columns in intermedi ate and higher pH environments for developing rugged HPLC methods. Bon ded-phase degradation in this pH range is a result mainly of silica su pport dissolution; covalently-bound silane ligands are hydrolyzed very slowly if at all from silica supports at intermediate and higher pH. Based on rates of silica support dissolution determined by chemical me asurements and comparable chromatographic studies, we now find that en dcapping alkyl-bonded stationary phases increases column longevity at pH 7, compared to non-endcapped columns. As previously determined for non-endcapped packings, we also find that the type of silica support d etermines the stability of bonded-phase packings, Silicas made by the sol-gel process are more resistant to dissolution than supports made b y a silicate-gel (xerogel) process. In addition, endcapping methods ap parently affect column stability, with double-endcapping methods appar ently superior to single-endcapping approaches. Degradation rates for several endcapped commercial bonded-phase C-8 columns were found to be quite variable in highly aggressive pH 7 accelerated-lifetime tests. Column stability in the pH 7-11 range is enhanced by using buffers oth er than phosphate in the mobile phase, and by excluding higher column temperatures. Certain silica-based endcapped bonded-phase columns can be used for developing rugged methods to at least pH 11 when used with organic buffers at less than or equal to 40 degrees C.