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

The silanol acidity of Waters Resolve C,,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 LiNO3 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 much less than Symmetry C-18 < underivatized XTerra much less than Resolve C-18 < Resolve silicaapproximate toSymmetry 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 LiNO3. The underivatized XTerra packing and Symmetry C-18 present active silanols only at pH values higher than 7.0. For the other three columns, two different types of silanols with different acidity ((s)(s)pK(a) values about 3.5-4.6 and 6.2-6.8, respectively) have been observed. Symmetry C,, shows evidence of the presence of active basic sites that retain NO3- by anionic exchange. (C) 2002 Elsevier Science B.V. All rights reserved.