Journal of Chromatography A, 990 (2003) 291–302 www.elsevier.com / locate / chroma Solid-phase extraction and sample stacking–micellar electrokinetic capillary chromatography for the determination of multiresidues of herbicides and metabolites ´ * ´ ´ ´ R. Carabias-Martınez , E. Rodrıguez-Gonzalo, P. Revilla-Ruiz, J. Domınguez-Alvarez ´ ´ ´ ´ ´ Departamento de Quımica Analıtica, Nutricion y Bromatologıa, Facultad de Quımica, Universidad de Salamanca, Plaza de la Merced s / n, 37008 Salamanca, Spain Abstract Micellar electrokinetic capillary chromatography (MEKC) with diode array detection was used for the separation of 13 compounds (eight herbicides widely used in agriculture: metribuzin, lenacil, ethofumesate, atrazine, terbutryn, isoproturon, chlorotoluron and linuron, and five of their principal degradation products; namely, deethylatrazine, 2-hydroxyatrazine, deethyl-2-hydroxyatrazine, deisopropylatrazine and 3-chloro-4-methylphenylurea). Peak separation for the 13 analytes was not successful when a single surfactant system was employed, neither sodium dodecyl sulfate (SDS) nor dioctyl sulfosuccinate (DOSS) sodium salt. However, a mixture of these herbicides was successfully separated using a mixed micellar system involving SDS–DOSS in less than 14 min. An application study of an on-line concentration technique for MEKC was carried out to enhance sensitivity. The optimized on-line stacking procedure consisted simply of the addition of 50 mM of sodium chloride to the injection sample, the stacking effect being more intensive as analyte polarity increased. When this stacking procedure was combined with an off-line sample preconcentration step, based on solid-phase extraction, analytes could be detected in the ppb range. The whole method was applied to ultra-high-quality and natural waters. Linear relationships between the analytical signal and the initial analyte concentration were found to be independent of the type of water, except for the more polar analytes for which small differences were observed.  2002 Elsevier Science B.V. All rights reserved. Keywords: Sample stacking; Micelles, mixed; Pesticides 1. Introduction The pollution of surface and ground waters by pesticides is governed by the physicochemical In recent years, many works have emphasised the characteristics of the compounds, by the properties ecological damage caused by the widespread use of of the medium in which they are applied and by agricultural pesticides. The residue levels of such other external factors. Among the most important compounds may represent an important source of physicochemical properties of pesticides are their pollution, especially if they have been used for long water solubility, their capacity to be retained by the periods of time. organic part of the soil (characterised by the solid organic carbon sorption coefficient, K , which is oc closely related to the octanol / water partition coeffi- *Corresponding author. Tel.: 134-923-294-483; fax: 134-923- cient, K ) and their half-life in soils (DT ). The 294-483. ow 50 ´ E-mail address: rcm@gugu.usal.es (R. Carabias-Martınez). groundwater ubiquity score (GUS) is a simple 0021-9673 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0021-9673(02)01969-6