A multiresidue method with mobile on-site sampling based on SPE for ultratrace analysis of plant protectants in environmental waters

A new multiclass/multiresidue method for monitoring plant protectant residues in raw- and drinking waters with on-site sampling using a mobile, self-contained sampling unit based on SPE was developed and validated. 38 active compounds with widely varying chemical and physicochemical properties (acid-base properties, polarities, vapor pressures, solubilities) are measured from just one sample and work-up. 100 mL water, acidified with acetic acid, are drawn through a cartridge filled with Amberchrom resins by means of a calibrated 100 mL syringe driven by servo motors, control circuit and rechargeable battery. Volumetric flow is high, quantitative extraction of acidic, neutral and weak basic substances is accomplished in a few minutes and loaded cartridges are transported into the laboratory. Further work-up, i.e. elution, drying, concentration and solvent change, is designed strictly non-selectively and produces 250 μL of an “extract”, which is the starting point of two GC measurement lines, GC-NPD and GC-ECD. Chlorinated herbicide acids are derivatised into perfluorinated benzyl esters employing reaction conditions so smooth that the organochlorine hydrocarbons are not destroyed at all and both substance classes can be chromatographed in one GC run. ECD-chromatograms from derivatised solutions are rather complex, therefore the flow is split after injection onto two columns with different polarities and two EC-detectors in order to resolve all analytes and matrix interferents. Short-chain carbonic acids added to the water prior to preconcentration are adsorbed on the hydrophobic surface in a reversible process and impart the Amberchrom resins with a partially hydrophilic character. Method performance was ascertained using tap water fortified at the 0.2 μg L^–1 level: Mean recoveries were between 70 and 115% and method detection limits (MDLs) below 0.08 μg L^–1. Waters from different sources did not affect the recoveries, loaded cartridges are stable for two weeks’ storage when cooled.