Chromatographic silanol activity test procedures: the quest for a universal test

Reversed-phase chromatography is the most used and the most studied method of modem liquid chromatography. There is yet no ideal support available for preparing reversed-phase stationary phases, but the vast majority have his torically been and are still prepared on microparticulate silica. The silic a surface has a number of properties which make it attractive for derivatiz ation, including easily controlled particle size and porosity and mechanica l stability. There are several types of surface silanols which have their o wn unique properties that affect both chemical derivatization reactions and adsorptive interactions with solutes. The relative distribution of these d ifferent types of silanols may affect the characteristics of silica-based s tationary phases more than the absolute number of surface silanol groups. T he relative importance of each of these different types of silanols has not yet been unambiguously established. Free or isolated silanols, internally hydrogen-bonded vicinal silanols, and geminal silanols all have been implic ated as the primary reaction and adsorption sites. There are many different synthetic schemes that have been used to block the remaining silanols, and "deactivated" phases are very popular. Unfortunately, there is still no un iversally agreed upon method to measure the accessibility or interaction of these silanols with solute molecules. Many tests have been proposed, focus ing mainly on chromatographic probe molecules, but different tests run on t he same column will often show different interactions. We will briefly revi ew the surface chemistry of silica and focus on the multitude of tests that have been proposed. Our focal point will be silanol activity test; other a spects of column performance will not be included. Where possible, comparis ons among the methods will be made. (C) 2000 Elsevier Science B.V. All righ ts reserved.