Electrophoresis 2012, 33, 2167–2175 2167 Joselito P. Quirino 1,2 Agnes T. Aranas 1,2,3 1 Australian Centre for Research on Separation Science (ACROSS), School of Chemistry, University of Tasmania, Hobart, Tasmania, Australia 2 Department of Chemistry, School of Science and Engineering, Loyola Schools Ateneo de Manila University, Loyola Heights, Metro Manila, Philippines 3 Department of Chemistry, College of Arts and Science, Ateneo de Davao University, Davao City, Philippines Received January 10, 2012 Revised February 28, 2012 Accepted March 25, 2012 Research Article On-line sample concentration via micelle to solvent stacking of cations prepared with aqueous organic solvents in capillary electrophoresis In this paper, by injecting a SDS micellar plug before the sample prepared in aqueous organic solvents, we show the on-line sample preconcentration of cations via micelle to solvent stacking (MSS) using solvents of as low as 30%. This extends the choice of stacking techniques to include moderate amounts of organic solvent in the sample. The approach is akin to in-line solid phase extraction where the micellar plug acted as a transient micellar phase extractor. Basic studies were conducted (e.g. type and amount of organic solvent in the sample). The calculated sensitivity enhancement factors based on LOD obtained for the six test antipsychotic drugs were from 41 to 68. The peak signals were linear (R 2 > 0.99) from 0.2 to 10.0 g/mL. The intraday and interday reproducibility (n = 10) for migration time, peak height, and corrected peak area were from 0.2 to 13.6%. The technique was also tested on spiked wastewater sample with minimal sample treatment (i.e. dilution and centrifugation). Keywords: Antipsychotic drugs / Capillary electrophoresis / Micelle to solvent stacking / On-line sample concentration / Organic solvent DOI 10.1002/elps.201200023 1 Introduction The CE modes of CZE [1, 2] and MEKC [3–5] are efficient analytical separation techniques for the analysis of small molecules. The poor detection sensitivity using photometric detection is however often referred to as a major limitation of these CE modes. On-line sample concentration or stack- ing techniques were developed to solve this limitation and these techniques allowed for the expansion of CE to real sam- ple applications [6–13]. Stacking relies on the creation of a boundary that was often found between the sample and BGE or background solution (BGS). The composition of the sam- ple solution was normally modified to produce the boundary where focusing of the target analytes occur. For example, in stacking with field amplification/enhancement, the sample Correspondence: Dr. Joselito P. Quirino, Australian Centre for Re- search on Separation Science (ACROSS), School of Chemistry, University of Tasmania, Private Bag 75, Hobart, Tasmania 7001, Australia E-mail: jquirino@utas.edu.au Fax: +61-3-6226-2858 Abbreviations: BGS, background solution; DDW, distilled deionized water; IPA, isopropyl alcohol; MSS, micelle to sol- vent stacking; SEF, sensitivity enhancement factor solution was prepared in a low conductivity matrix (e.g. 10× diluted BGS) [14–16]. In the other known stacking techniques of sweeping [17–20] and dynamic pH junction [21–24], the sample was prepared in a matrix that was void of the additive and with a different pH than the BGS, respectively. The most recent on-line concentration phenomenon in CE is micelle to solvent stacking (MSS) [25, 26]. MSS mecha- nism is based on the reversal of the effective electrophoretic mobility of charged analytes at a boundary. In the original MSS configuration [25], the boundary was formed between the sample solution and BGS (or solvent plug [27]) that con- tained micelles and organic solvent, respectively. The charge of the analyte should be opposite that of the micelle. The direction of the effective electrophoretic mobility in the sam- ple solution zone was dictated by the electrophoretic mobility of the micelle. The micelles transported the analytes to the boundary where the organic solvent reduced the interaction between analytes and micelles. When a sufficient amount of solvent was present, the direction of the effective elec- trophoretic was governed by the electrophoretic mobility of the analyte. Note the opposite direction of electrophoretic mobility between micelles and analytes due to their charges. The analyte transport and mobility reversal at the boundary accumulated or concentrated the analytes injected. MSS was applied to small cationic and anionic analytes with the use of appropriately charged micelles (i.e. SDS, CTAB, and 1- C  2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com