Determination of urea-derived pesticides in fruits and vegetables by solid-phase preconcentration and capillary electrophoresis.

A multiresidue analytical method based on solid-phase extraction (SPE) enrichment combined with capillary electrophoresis (CE), using micellar electrokinetic capillary chromatography (MEKC), was developed to determine ten substituted urea pesticides in orange and tomato samples. Several factors such as pH, composition and concentration of the buffer, concentration of surfactant, addition of organic solvent, and working voltage were optimized to obtain the best compound separation in the shortest time. Separation can be achieved in 7 min using a micellar aqueous pH 9 buffer composed of 4 mM borate and 35 mM sodium dodecyl sulfate. After an SPE procedure, which provided a 10-fold enrichment, the limit of detection was about 0.05 mg kg(-1), which is in the order of the maximum residue limits (MRLs) established by the European Union (EU) for most of these compounds. Increasing the enrichment factor by using a larger amount of sample is difficult in oranges due to the matrix interferences, but is possible in tomatoes, which gave cleaner extracts and easily reached a 25-fold enrichment factor. The procedure involving SPE and CE provided acceptable recoveries (ranged 42-118%) and relative standard deviations (RSDs; < 19%) at levels between 0.3 and 5 mg kg(-1).