Characterisation of retention in micellar high-performance liquid chromatography, in micellar electrokinetic chromatography and in micellar electrokinetic chromatography with reduced flow

The retention (migration) behaviour of various barbiturates, phenylurea and triazine herbicides in micellar electrokinetic chromatography (MEKC) with uncoated fused-silica capillaries was compared with the behaviour in micell ar electrokinetic chromatography with reduced electroosmotic flow (RF-MEKC) using capillaries modified with linear polyacrylamide. The error in the va lues of the retention factors caused by the neglection of the contribution of the electroosmotic flow in RF-MEKC;C was investigated and a method for c orrecting this error was suggested. The retention was characterised using t he lipophilic and polar indices to characterise and to predict the retentio n as a function of the concentration of the surfactant (sodium dodecylsulph ate) in the running buffer in MEKC and in RF-MEKC. Homologous series of n-a lkylbenzenes and of n-alkan-2-ones were compared as the standard sets for t he calibration of the retention (migration) index scale. The values of the lipophilic indices of a given solute measured in reversed-phase HPLC, MEKC and RF-MEKC are close to each other. Under ideal MEKC conditions, the value s of the polarity indices are close to one for various sample solutes. Howe ver, for partially ionised compounds such as weakly acidic barbiturates, wh ere the contribution of the electrophoretic migration is significant, the v alues of the polarity indices are significantly lower than one. Optimum con ditions for separations of mixtures of triazine and phenylurea herbicides a nd of barbiturates using various techniques tested were compared. (C) 2001 Elsevier Science B.V. All rights reserved.