Silica Phases Selectivity
Despite its porosity, spherical porous HPLC silica exhibits a high mechanical strength compared with other materials. Additionally, it is readily chemically modified. A wide range of porous silica is available for normal-phase HPLC, characterized by surface area, pore size, and particle size measurements. The use of normal-phase HPLC has not been limited by the silica dissolution or peak tailing problems associated with reversed-phase HPLC.
For analytical work, as the quality and reproducibility of porous silica improve, the use of 3, 3.5 or 4μm particle size materials increases. 10μm particles are less commonly used but remain a key particle size for preparative applications. For economic reasons irregular silicas still share some of this latter market.
The physical characteristics of the newer silica particles have been improved in several ways. However, a number of them are not readily available as they are principally used as a base material for the manufacture of new reversed-phase silicas.
- Surface Activity
A lower level of the unwanted, free and isolated silanol groups is observed. The lower metal ion contaminant level partly contributes to this drop in surface activity. Basic compounds interact less strongly with the silica surface resulting in improved chromatography.
- Physical Properties
Improved control of physical properties such as surface area, pore-volume, mean pore diameter, and particle size have given the new silicas better lot-to-lot reproducibility.
The level of metal ion impurities has in some cases been reduced to cumulative figures < 10ppm. Undesirable chelation of metal ion and solute has been minimised.