Neutral analyte focusing by micelle collapse in micellar electrokinetic chromatography.

The operating parameters that affect the performance of analyte focusing by micelle collapse (AFMC) to neutral analytes (i.e., dialkyl phthalates) in normal migration micellar electrokinetic chromatography (NM-MEKC) are examined. NM-MEKC is characterized by an electroosmotic flow greater than the electrophoretic velocity of the micellar pseudostationary phase, and was performed using sodium dodecyl sulfate (SDS) with neutral to high pH electrolytes in fused silica capillaries. AFMC is a recently introduced on-line sample preconcentration technique in capillary electrophoresis that can provide hundreds fold improvement in detection sensitivity. The mechanism of AFMC is based on the analyte transport, release, and accumulation by the moving surfactant micelles [e.g., SDS at concentrations closer to the critical micelle concentration (CMC)] in the sample that dilutes below its CMC into a liquid phase zone. The sample is prepared in a matrix that contains SDS micelles and high mobility anions, where the conductivity of the sample matrix is higher compared to that of the separation solution. The sample injection length, sample and separation solution conductivity ratio, and surfactant micelle concentration in the sample were found to affect the AFMC performance, as well as the effective separation length in NM-MEKC. The use of a different electrolyte salt in the sample and separation solution also affected AFMC NM-MEKC results. In particular, sodium chloride in the sample matrix can induce a micelle-mediated neutral analyte isotachophoretic concentration that is detrimental to the technique.