Define Selectivity - terminologies used in chromatography?
Selectivity is a measure of preference a stationary phase shows for one solute over another and is expressed as:
α = K1/ K2
K1 and K2 are distribution coefficient for two different solutes. For convenience sake, K1 is referred to as slow moving solute.
Selectivity therefore describes relative rates of migration of two solutes and thus is a measurement of zone seperation. The greater the difference between distribution ratios larger is the value of selectivity (α) and hence better is the seperation.