Mid- and near-infrared determination of metribuzin in agrochemicals

Two infrared-based methodologies were developed for metribuzin determination in pesticide formulations after extraction with acetonitrile. Fourier transform mid infrared (MIR) procedure was based on peak area measurements between 1692 and 1670 cm−1 corrected with a baseline fixed at 1877 cm−1. Fourier transform near infrared (NIR) determination was made by measuring the peak area between 6498 and 6332 cm−1 corrected using a two points baseline defined between 6570 and 6212 cm−1. Repeatability, as relative standard deviation, of 5 independent measurements at mg g−1 concentration level was 0.3% and 0.03% for MIR and NIR, respectively, and limit of detection values of 9 and 17 mg kg−1 in the solid sample were obtained for MIR and NIR, respectively. NIR determination provides a measurement sample frequency of 120 h−1, higher than that found by MIR and reference chromatography methods (60 and 10 h−1, respectively). On the other hand, the NIR method reduces the solvent consumption and waste generation, to only 10.5 ml acetonitrile per sample as compared with 16.5 ml required for MIR and 66 ml used in the chromatography reference procedure. However the mean accuracy relative error obtained for the analysis of commercial samples was 0.7 and 2.9% (w/w) for MIR and NIR procedures, respectively as compared with data found by the reference procedure, being thus more accurate the MIR than the NIR methodology. So, vibrational procedures, employing a non-chlorinated solvent, such as acetonitrile, for the extraction of the active principle, can be considered serious alternatives to sensitive but expensive reagents and time consuming chromatography methods usually recommended for quality control of agrochemicals.