Chiral chromatography of amino- and hydroxy-acids on surface modified porous graphite

Summary

A new type of chiral stationary phase has been prepared by coating porous graphite with a near-monolayer of an adsorbed enantiomeric modifier which then acts as an adsorbed stationary phase. The most effective modifiers are L- or D-isomers of N-(2-naphthalene-sulphonyl)-phenylalanine (NS-Phe). Conveniently 80% of monolayer coverages, corresponding to 1.2 μmol m⁻², are achieved by adsorption of NS-Phe from methanolic solution. Enantiomeric separations by complexation with cupric ion show base-line resolution of α-amino and α-hydroxy acids with separation factors up to 2. Identical separations are achieved with chiral phases made from the L- and D-isomers of NS-Phe except that the elution orders of enantiomers are inverted. Reduced plate heights are around 5 and the adsorbed chiral phases are extremely stable, retention ratios being unchanged after passage of 7000 column volumes of eluent. A mechanism of retention is proposed, which fits the experimental observations.