Abstract
A comparative characterization of bare silica as well as different reversed phases, SCX and SCX/mixed mode phases is presented. The hydrophobic properties are described through their retention of phenylalkanes and their methylene group selectivity. At similar carbon content, mixed mode phases exhibited less hydrophobic retention than reversed phases. No appropriate inert marker, suitable for all types of phases, could be found. From SCX and bare silica phases, toluene eluted earlier than thiourea and other polar compounds. For all types of phases, electroosmotic flow (EOF) measurements in 20 mM phosphate buffer/acetonitrile 30:70 v/v were carried out between pH 2.3 and 8.5. Phases with sulfonic acid groups showed a weak, but still considerable EOF drop, when the pH was decreased below 5. At pH 2.3, however, they generated more than twice the electroosmotic velocity as the reversed phases. The cation exchange behavior was studied with pharmaceutical compounds of different basicity. A strong focussing effect of tricyclic antidepressants on the SCX material could be encountered, as was previously described in literature.
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