One Stationary Phase, Three Modes of Separation: Reversed Phase and Normal Phase Separations on the Same Column

The most commonly used support material for HPLC packings are spherical, micro porous particles of silica and these are amorphous (i.e. non-crystalline) in nature. The surface of the silica particles shows poor homogeneity, being a mixture of silanols (Si-OH) of varying degrees of activity, and siloxane groups (Si-O-Si). This makes subsequent surface modifications to produce bonded phases greatly reliant on the nature of the base silica. Early silicas, made from sodium silica sols, were referred to as type A silicas. The more recent ’ultra pure’ silicas (exhibiting much lower levels of transition metal ions and better surface homogeneity - resulting in more reproducible and better chromatographic performance) , made from organic sols, are referred to as type B silicas. In this article Joe Pesek and Maria Matyska discuss a type C silica which is designed to reduce problems associated with types A&B silicas when modified.