Determination of derivatized urea herbicides in water by solid-phase extraction, methylation and gas chromatography with a nitrogen–phosphorus detector

Abstract

Four urea herbicides, isoproturon, chlorotoluron, linuron and diuron, were determined by gas chromatography (GC) with a nitrogen–phosphorus detector (NPD) after derivatization, with detection limits of 0.035, 0.039, 0.041 and 0.036 µg l^–1, respectively. The concentrations of all analytes were linear over the range 0.1–8.0 µg l^–1, with recoveries in excess of 75% from spiked potable waters. In their standard, underivatized form the herbicides were found to be thermally unstable on passage through a GC column. After derivatization, by methylation using iodomethane and a strong base, the resulting compounds were found to be stable at elevated temperatures, and so could be determined by GC. The derivatized herbicides were also analysed by GC–mass spectrometry, in order to elucidate the structures of the derivatized compounds. Each compound yielded a different product with a different retention time. The reaction was of the type typical of nucleophilic displacement, with the methyl group attacking the nitrogen of the amide group, forming a stable tertiary amide and hydrogen iodide gas. This method was found to be more selective than the Standing Committee of Analysts' method owing to the nature of the analysis. Firstly, GC, compared with high-performance liquid chromatography, offers better resolution. There are many ultraviolet absorbers in water which can be detected by the standard method, but use of a specific detector, such as an NPD, offers better selectivity. The method was also applied to other urea herbicides, including monuron, methabenzthiazuron and tebuthiuron, which were also successfully determined, although no quantitative data have been obtained.