Solid-phase extraction coupled with electrochemical detection for the determination of the herbicide bromofenoxim in water samples at low- and sub-μg l^-1 levels

The application of solid-phase extraction (SPE) as a preconcentration and clean-up step with subsequent off-line flow injection amperometric (FI-AD) or batch square-wave voltammetric (SWV) detection of the herbicide bromofenoxim was developed. The selection of an appropriate organic eluent, some parameters influencing the efficiency of the SPE and the electrochemical detection of bromofenoxim in the organic effluent solution were thoroughly investigated. Undiluted acetonitrile with SWV and acetonitrile-water (80 + 20) with FI-AD, both containing 0.1 mol l^-1 LiClO₄, were chosen as the most appropriate SPE eluents. The addition of LiClO₄ as supporting electrolyte to the eluent and acidification of a water sample to 1 x 10^-3 mol l^-1 HClO₄ (pH 3) prior to the SPE procedure were found to improve greatly the current response on mercury drop (SWV) and mercury film (FI-AD) electrodes. Subsequent to the SPE procedure, the effluents were transferred to the voltammetric cell or injected into the flow injection system without any further treatment. The calibration plots obtained for bromofenoxim in pure water samples were linear over the ranges 0.2-12.0 μg l^-1 and 3.0-120 μg l^-1, with calculated detection limits of 0.05 and 1.5 μg l^-1 (100 ml samples), for the SPE-SWV and SPE-FI-AD procedures, respectively. The actual detection capabilities of the proposed methods depend on the water sample volumes applied to the extraction cartridges. The recoveries of the over-all procedures, applying spiked tap water samples, and the corresponding RSDs were 92% and 6% (n = 6) and 121% and 9% (n = 7) for SPE-SWV and SPE-FI-AD, respectively. The practical applicability of the proposed methods for the analysis of ground and tap water samples was confirmed via an inter-laboratory test on a tap water sample containing five common pesticides including bromofenoxim.