Enthalpic exclusion chromatography

Abstract

Enthalpic exclusion of solutes from a porous matrix arises if the energetic environment within the matrix is unfavourable when compared with that outside. Two forms of enthalpic exclusion are demonstrated: (a) exclusion of non-polar solutes from a matrix with a polar surface (Corning porous glass) when eluted by a polar eluent; (b) exclusion of anionic solutes from a reversed-phase packing material (ODS Hypersil) when eluted by water + ethanol mixtures.

Type (a) is considered to be one of negative adsorption where ΔH_ads is positive rather than negative. In type (b) fixed charges due to residual [graphic omitted]Si–O^– groups are considered to be responsible. These produce a Donnan potential step at the surface of the particle. Exclusion of acids can be explained quantitatively on this basis when allowance is made for the effect of pH on the ratio of ionized to un-ionized forms of the acids.