A Comparison of Silica C and Silica Gel in Hydrophilic Interaction Mode: The Effect of Stationary Phase Surface Area

Hydrophilic interaction chromatography (HILIC) was first described in 1990 although several interesting reviews focusing on general aspects of HILIC have been published [2-4] and it is becoming increasingly popular, (HILIC) is far from being completely understood.

To assess the effect of silica gel structure on retention in HILIC, a test system was developed using the quaternary ammonium ions benzyltrimethylammonium (BTM), benzyltriethylammonium (BTE) and benzyldimethylhexylammonium (BDM) as probes with tetramethylammonium acetate (TMA) as the counter-ion competing against the interaction of the test probes with ionised silanols in the stationary phases. Five columns were examined: 300 Å and 100 Å ACE silica gel columns, 100 Å and 60 Å Kromasil silica gel columns and a Cogent Silica- C column. Retention time was obtained for the test probes at different acetonitrile concentrations with all the mobile phase mixtures containing 10 mm tetramethylammonium acetate (TMAA) buffer at pH [6.00]. In all cases BTE, which has the intermediate ionic radius, was the most retained probe at 20% acetonitrile suggesting that retention on silica gel at low % acetonitrile was due to a balance between lipophilic interaction with siloxane groups and ion exchange. All phases gave “U” shaped plots for log k against percentage of acetonitrile with the steepest rise in retention occurring between 80 and 90% acetonitrile. BTM was the most retained probe at 90% acetonitrile and was most retained on the 60 Å Kromasil column and least retained on the 300 Å ACE silica gel columns.