FIELD-AMPLIFIED SAMPLE STACKING IN MICELLAR ELECTROKINETIC CHROMATOGRAPHY FOR ON-COLUMN SAMPLE CONCENTRATION OF NEUTRAL MOLECULES

On-column concentration of neutral molecules was achieved for the firs t time in micellar electrokinetic chromatography by means of field-amp lified sample stacking. The stacking process was accomplished by disso lving the neutral analytes in a low-concentration micellar solution th at was still above the critical micelle concentration. The lower total ionic strength in the sample buffer compared to the electrophoresis b uffer allowed the negatively charged micelles to migrate rapidly into the boundary between the sample and the running buffer where they slow down. This held-amplified sample stacking was achieved by using norma l or reversed electrode polarity and produced a 75-85-fold increase in sensitivity for 1,2,4,7- and 1,2,4,8-tetrachlorodibenzo-p-dioxins. Th e peak area counts obtained from the sample stacking process were prop ortional to the sample volume injected, and the stacking efficiency wa s dependent on the micellar concentration. The best stacking efficienc y was obtained when the micelle concentration was slightly higher than the critical micelle concentration. When the injection volume was rel atively small, the normal-polarity stacking procedure produced a highe r stacking efficiency. However, when the injection volume was large, r eversed polarity produced a higher stacking efficiency because the non -uniform distribution of the electrical field strength had been elimin ated.