Comparison of silica and porous graphitic carbon as solid-phase extraction materials for the analysis of cationic herbicides in water by liquid chromatography and capillary electrophoresis

Preconcentration methods for the analysis of paraquat (PQ), diquat (DQ) and difenzoquat (DF) in water samples by solid-phase extraction (SPE) with silica and porous graphitic carbon (PGC) were evaluated. The breakthrough volumes of three quaternary ammonium herbicides were calculated in order to compare the loading capacity of both sorbents. They were determined by capillary electrophoresis (CE) and ion-pair reversed-phase liquid chromatography (LC) with UV detection. The recoveries were determined by spiking tap and river waters. Although the data showed that both sorbents were able to preconcentrate the cationic herbicides using CE and LC as separation and quantification techniques, silica was inappropriate for the CE determination of DF because marked distortions in the base line occurred. The best results were obtained for PGC with both separation techniques, which gave recoveries ranging from 65 to 90% and standard deviations between 4 and 10%. Practical detection limits (LODs) with LC were about 0.2 mu g l(-1) and were one order of magnitude higher with CE. (C) 2000 Elsevier Science B.V. All rights reserved.