Automated on-line in-tube solid-phase microextraction followed by liquid chromatography/electrospray ionization-mass spectrometry for the determination of chlorinated phenoxy acid herbicides in environmental waters

Abstract

A method for the determination of six chlorinated phenoxy acid herbicides in river water was developed using in-tube solid-phase microextraction (SPME) followed by liquid chromatography/electrospray ionization-mass spectrometry (LC/ESI-MS). In-tube SPME is an extraction technique for organic compounds in aqueous samples, in which analytes are extracted from a sample directly into an open tubular capillary by repeated draw/eject cycles of the sample solution. Simple mass spectra with strong signals corresponding to [M–H]⁻ and [M–RCOOH]⁻ were observed for all herbicides tested in this study. The best separation of these compounds was obtained with a C₁₈ column using linear gradient elution with a mobile phase of acetonitrile–water containing 5 mmol l⁻¹ dibutylamine acetate (DBA). To optimize the extraction of herbicides, several in-tube SPME parameters were examined. The optimum extraction conditions were 25 draw/eject cycles of 30 μl of sample in 0.2% formic acid (pH 2) at a flow rate of 200 μl min⁻¹ using a DB-WAX capillary. The herbicides extracted by the capillary were easily desorbed by 10 μl acetonitrile. Using in-tube SPME-LC/ESI-MS with time-scheduled selected ion monitoring, the calibration curves of herbicides were linear in the range 0.05–50 ng ml⁻¹ with correlation coefficients above 0.999. This method was successfully applied to the analysis of river water samples without interference peaks. The limit of quantification was in the range 0.02–0.06 ng ml⁻¹ and the limit of detection (S/N = 3) was in the range 0.005–0.03 ng ml⁻¹. The repeatability and reproducibility were in the range 2.5–4.1% and 6.2–9.1%, respectively.