The silica surface and bonded phase of HPLC columns are responsible for adsorption. Partitioning takes place on the “quasi liquid phase layer” that forms around the surface of the Stationary Phase. The “Liquid Stationary Phase” is mostly formed by solvating it with water. This “hydration shell” and the subsequent organic phase that forms around the Stationary Phase contributes to the separation mechanism in conjunction with the underlying solid phase which can have different functionalities at work. Depending on the bonded phase and liquid phase that forms, the amount of partitioning will vary.

With Type-B silica, water is readily adsorbed onto the silica’s surface, which contains active silanol groups; this strong adsorption of water forms a durable “Hydration Shell” which is responsible for longer equilibration times and lack of reproducibility in Normal Phase separations as well as other chromatographic difficulties and is often the main cause of pH hysteresis and lack of reproducibility. This is often the case in RP when using competitive, polar embedded phases.

Strong adsorption of water to type-B silica (shown above) makes using normal phase chromatography very difficult. The silicon-hydride groups (Si-H) found on the surface of TYPE-C Silica™ are not prone to such strong water retention (shown above) as type-B silica making it an excellent choice for Organic-Normal Phase with improvements in speed and range of solvent possibilities. The weaker water adsorption also accounts for the little to no hysteresis observed when changing from Organic-Normal to Aqueous-Normal/Reverse Phase with TYPE-C™ products or when changing pH with TYPE-C Silica™ based HPLC columns such as Cogent UDC-Cholesterol™. This feature makes the column preferred over polar-embedded phases which often exhibit long term memory effects. TYPE-C Silica™ products extend the useful range of Normal phase from Hexane/Ethyl Acetate all the way to Water/Acetonitrile with excellent precision.