HPLC method with on-line SPE preconcentration for quantification of permethric acid sorption to goethite

Pesticide metabolites are often found to be more mobile in soil than their parent compounds. Pyrethroids are bound strongly to soil and therefore sorption of the pyrethroid metabolite permethric acid (PA) to a typical soil sorbent, goethite, was investigated. An on-line solid-phase extraction (SPE)-HPLC-UV procedure was developed for quantification of trans- and cis-permethric acid in aqueous samples. Limits of detection (LOD) were 500 times lower than those obtained with conventional HPLC-UV, resulting in LODs of 1.4 and 0.3 nM for the trans- and cis-isomers, respectively. Sorption of nanomolar concentrations of PA to goethite was found to be specific up to less than 1% surface coverage. In this range the data was described by a Langmuir equation with K_ads = 7.1 × 10⁻⁹ L/mol and Γ_max = 7.1 × 10⁻⁹ mol/m² for total PA (trans + cis) at pH = 3. K_{ads, cis} (1.4 × 10⁶ L/mol) was approximately twice K_{ads, trans} (7.9 × 10⁵ L/mol). At higher PA concentrations the slope of the sorption isotherm increased, which is ascribed to hydrophobic interactions between adsorbed and dissolved PA molecules. Based on comparison with reported K_om values, metal oxides are expected to have a relatively greater significance to the retention of PA than soil organic matter.