Chromatographic silanol activity test procedures: the quest for a universal test.
Reversed-phase chromatography is the most used and the most studied method of modern liquid chromatography. There is yet no ideal support available for preparing reversed-phase stationary phases, but the vast majority have historically been and are still prepared on microparticulate silica. The silica surface has a number of properties which make it attractive for derivatization, including easily controlled particle size and porosity and mechanical stability. There are several types of surface silanols which have their own unique properties that affect both chemical derivatization reactions and adsorptive interactions with solutes. The relative distribution of these different types of silanols may affect the characteristics of silica-based stationary phases more than the absolute number of surface silanol groups. The 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 implicated 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 universally agreed upon method to measure the accessibility or interaction of these silanols with solute molecules. Many tests have been proposed, focusing mainly on chromatographic probe molecules, but different tests run on the same column will often show different interactions. We will briefly review 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 aspects of column performance will not be included. Where possible, comparisons among the methods will be made.