Analysis of nonsteroidal anti-inflammatory drugs in water samples using microemulsion electrokinetic capillary chromatography under pH-suppressed electroosmotic flow with an on-column preconcentration technique

Microemulsion electrokinetic capillary chromatography (MEEKC) in a suppressed electroosmotic flow (EOF) strategy was investigated for analysing a group of nonsteroidal anti-inflammatory drugs (NSAIDs) in water samples. The EOF was effectively suppressed with an acidic buffer as the aqueous phase. Four water immiscible solvents, oils (n-heptone, n-octane, ethyl acetate and di-n-butyl tartrate) and three organic solvents (methanol, 2-propanol and acetonitrile) were tested to optimise the pseuclostationary phase and to obtain efficient separations. The optimum microemulsion background electrolyte (BGE) solution consisted of 0.8% (w/w) n-heptane, 6.6 (w/w) butan-1-ol, 15.0% (w/w) acetonitrile, 3.3% (w/w) sodium dodecyl sulfate (SDS), and 74.3% (w/w) of 25 mM sodium phosphate at pH 2.5. Stacking with reverse migrating pseuclostationary phase (SRMP) was applied to enhance the concentration sensitivity of the NSAIDs. When this preconcentration technique was used, the sample stacking and the separation processes took place successively with the same voltage without an intermediate polarity switching step. Detection limits (LODs) in the order of 5-15 pg L-1 for the NSAIDs were obtained using SRMP for standard solutions. The developed method was validated for the analysis of NSAIDs in tap water samples by combining an off-line solid-phase extraction (SPE) step and the on-column preconcentration technique SRMP. The LODs were in the 100-230 ng L-1 range.