Characterization of sorption of endosulfan isomers and chlorpyrifos on container walls using mixed solvent systems

The reliability of sorption data for organic contaminants with low water solubility has generated great concern because of the variability in the literature of the soil-water partition coefficient (K(OC)) values for these compounds. In particular, sorption on container walls in aqueous systems when measuring the sorption coefficient, K(D) (used to calculate K(OC) values), for strongly hydrophobic compounds (SHOCs) is a potential source for discrepancies in the K(OC) values. In this study, we eliminated sorption on container walls when measuring sorption of three halogenated compounds (alpha-endosulfan, beta-endosulfan, and chlorpyrifos) using mixed solvents. Various mixtures of methanol and water were used. Sorption experiments were designed using polytetrafluoroethylene (Teflon)-lined centrifuge tubes and a high-performance liquid chromatography (HPLC) syringe. Solution sample analysis was performed using HPLC equipped with a UV diode array detector and C-18 column at a wavelength of 214 nm, with acetonitrile/water (80:20, v/v) as the mobile phase. The solvophobic model was used to calculate the percent recovery (% R(M)) in water of the test compounds. Our results show that there is considerable sorption on container walls for the three chemicals at volume fractions of methanol (f(c) < 0.4). The data show that, in aqueous systems, percent recoveries for alpha-endosulfan, beta-endosulfan, and chlorpyrifos are 48, 45, and 61, respectively. Thus, to generate reliable sorption data for alpha-endosulfan, beta-endosulfan, and chlorpyrifos and other SHOCs, experiments may be conducted using Teflon-lined centrifuge tubes and HPLC syringes at volume fractions of methanol (f(c) >or= 0.5).