Characterization of the acidity of residual silanol groups in immobilized artificial membranes

Residual silanol acidity and activity of one immobilized artificial membrane (IAM) column have been measured from the retention of LiNO3 in the column with a methanol/buffer (1 mM in Na+) (60:40, v/v) mobile phase buffered to different pH values. Just one type of silanol with p s s K a = 7.61 (near the pH limit recommended by the manufacturer) was found, although these silanols show large activity. The results obtained have been compared with those obtained previously for Resolve C18, Resolve Silica, Symmetry C18, Symmetry Silica, XTerra MS C18, underivatized XTerra, Lichrospher 100 RP-18, Purospher RP-18e, Luna C18 (2) and Chromolith Perfomance RP-18e, showing that the IAM column is similar to Luna C18 and Symmetry C18 in terms of silica quality, as measured by Li+ retention. A warning about the use of IAM columns for emulation of biological systems at physiological pH 7 is given because the ionized silanols may contribute to the retention of some drugs at this pH.