Development towards a portable instrument for the determination of pesticide residue in water

Abstract

Pesticide contamination has been a widely publicised topic over the past 30 years and will continue to be a heated discussion point in the future. Given the level of scrutiny under which the water sector operates (namely for water quality, water availability and pricing), a robust technique is needed for the determination of pesticide residues in source waters that is both cost effective and reliable. This research project is aimed at developing such a technique to help water authorities meet this challenge. The method investigated utilised a flow injection system with tris(2,2’-bipyridyl)ruthenium(III) chemiluminescence detection (FICA) applied to the determination of atrazine, simazine, hexazinone, monocrotophos, and dicrotophos in natural waters. The FICA method presented utilises a chemically oxidized chemiluminescent reagent, which was optimised using a combination of multivariate and univariate optimisation procedures. The optimised experimental conditions were: sample and carrier flow rates of 4.6 mL min-1, sample at pH 9 buffered with 50 mM borax, and a reagent concentration of 1 mM tris(2,2’-bipyridyl)ruthenium(III) in 20 mM H2SO4 (pH 1) with a limit of detection of 1.3 ± 0.1 µg L-1 achieved for atrazine in MilliQ water. It was also shown that similar compounds, such as atrazine metabolites and other triazine pesticides, produced a chemiluminescent signal with tris(2,2-bipyridyl)ruthenium(III). When the method was applied to natural waters, the presence of dissolved organic matter (DOM) caused significant interference. This interference was investigated along with various cations and anions at levels commonly found in natural waters. It was observed that Fe3+ and Fe2+ (at concentrations above drinking water guidelines) caused interference. Fluorescence spectra were obtained for these samples to investigate the interaction between DOM, the selected pesticides and the interfering cations and anions. It was concluded that a complex formed between the analyte and DOM, and in the presence of interfering cations and anions the formation of the complex was enhanced. The interference from DOM was removed by solid phase extraction (SPE), and by incorporating an in-line SPE extraction column, the rapid detection of pesticide residues that had previously proven to be difficult due to interfering species was realised. A variety of extraction resins were evaluated for use in the in-line SPE housing, with Nexus© resin being the best of the ones tested. The detection limits achieved were 14, 48 and 32 ng L-1 for atrazine, hexazinone, and simazine, respectively. Lastly, a low-pressure monolithic column was merged with the in-line SPE FICA system in order to create a hybrid FIA system analogous to a low pressure HPLC system. The incorporation of a monolithic column enabled atrazine, simazine and hexazinone to be detected simultaneously with chromatographic differentiation, with method detection limits of 27, 39 and 60 ng L-1, respectively. The FICA system described in this thesis will be very useful as a quick, sensitive screening method for atrazine, simazine, hexazinone in natural waters. The methods developed during the course of this project should be considered by water utilities for inclusion in their ongoing pesticide monitoring programs