Fast screening method for eight phenoxyacid herbicides and bentazone in water Optimization procedures for flow-injection analysis-thermospray tandem mass spectrometry

Abstract

The development of a screening method for the rapid screening of water samples for phenoxyacid herbicides and bentazone with flow-injection analysis-thermospray tandem mass spectrometry is described. A two-step optimization procedure was used to determine the optimum instrumental parameters. First, introducing the analytes of interest continuously, the vaporizer temperature, discharge voltage and repeller voltage were changed stepwise over a wide range under computer control. With different solvent mixtures and source block temperatures, the carrier stream with which the highest molecular (parent) ion intensifies were obtained in the single scan Q3-MS scan mode was selected. This turned out to be 0.1 M aqueous ammonium acetate-acetonitrile (90:10, v/v). For phenoxyacid herbicides and bentazone negative ionization with buffer-assisted ion evaporation (ammonium acetate) gave the most intense parent ions. The discharge voltage had little influence on the signal intensifies and a low repeller voltage turned out to be the best for all analytes. Next, the optimum collision gas pressure and collision offset voltage were determined. The most intense daughter ions were derived with a collision gas pressure (argon) of 3–4 mTorr (1 Torr = 133.322 Pa) and a collision offset voltage of 18 eV for phenoxyacid herbicides and 22 eV for bentazone. In order to obtain maximum sensitivity, a multiple reaction monitoring method was used in which two parent ion-daughter ion pairs were monitored for each phenoxyacid herbicides and one parent ion and three daughter ions for bentazone. Without sample concentration all eight phenoxyacid herbicides and bentazone can be determined at the low μg/1 level. The method is fully automated; with a 5-ml loop injection and the subsequent cleaning runs the analysis time is 10 min. The detection limit is ca. 1 μg/1 with linear calibration graphs up to 50 μ/1. The method was used to confirm the presence of bentazone and the absence of the phenoxyacid herbicides in a surface water sample.