Comparison of the acidity of residual silanol groups in several liquid chromatography columns

Abstract

The silanol acidity of Waters Resolve C 18, Waters Resolve silica, Waters Symmetry C 18, Waters Symmetry silica, Waters XTerra MS C 18 and underivatized XTerra columns has been measured from the retention of LiNO 3 with a methanol/water (60:40) mobile phase buffered to different pH values. The Li + cation is retained by cationic exchange with the background cation of the mobile phase (Na +) through the ionized silanols. The number of active silanols increases in the order: XTerra MS C 18≪Symmetry C 18<underivatized XTerra≪Resolve C 18<Resolve silica≈Symmetry silica. XTerra MS C 18 does not present any residual silanol acidity up to s s pH 10.0 (pH in 60% methanol) as measured by LiNO 3. The underivatized XTerra packing and Symmetry C 18 present active silanols only at s spH values higher than 7.0. For the other three columns, two different types of silanols with different acidity ( s sp K a values about 3.5–4.6 and 6.2–6.8, respectively) have been observed. Symmetry C 18 shows evidence of the presence of active basic sites that retain NO 3 − by anionic exchange.