How to choose proper silica HPLC columns?

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.

1. Particle Size

2. Pore Size

3. Other Physical Characteristics

Particle Size

For analytical work, as the quality and reproducibility of porous silica improve, the use of 3, 5 or 10μm 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 use a lot in industrial purification.

Pore Size

The common pore size for porous silica is 100A, 120A, 300A. Some suppliers also have silica with 80A, 200A, or even 450A. The selection of the pore size is based on the size of the target samples. Traditional analysis for small molecules uses 100A/120A pole size. Analysis for big molecules uses 300A. For particular samples, the choice of pores in the silica is according to the molecular size.

Other Physical Characteristics

The physical characteristics of the newer silica particles have been improved in several ways. However, many 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 has given the new silicas better lot-to-lot reproducibility.

• Purity

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.