Stationary Phases (Adsorbents)

HPLC separations are based on the surface interactions, and depends on the types of the adsorption sites (surface chemistry). Modern HPLC adsorbents are the small rigid porous particles with high surface area.

Main adsorbent parameters are:
Particle size: 3 to 10 Ám
Particle size distribution: as narrow as possible, usually within 10% of the mean;
Pore size: 70 to 300 ┼;
Surface area: 50 to 250 m2/g
Bonding phase density (number of adsorption sites per surface unit): 1 to 5 per 1 nm2

The last parameter in the list represents an adsorbent surface chemistry. Depending on the type of the ligand attached to the surface, the adsorbent could be normal phase (-OH, -NH2), or reversed-phase (C8, C18, Phenyl), and even anion (NH4+), or cation (-COO-) exchangers.