Quantification of phenylurea herbicides and their free and humic acid-associated metabolites in natural waters

There is increasing interest in and demand for simultaneously monitoring pesticides as well as related degradation products (DPs) in natural waters, as the latter compounds can be even more toxic than the former ones. A method for determining parts per trillion levels of phenylurea herbicides and their DPs, that is their dealkylated forms and aromatic amines, is described. This method is based on solid-phase extraction with a Carbograph 4 cartridge followed by liquid chromatography (LC) with electrospray (ES) mass spectrometric detection. A study aimed at optimizing the response of the ES-MS detector for very weakly basic chloroanilines was conducted. Results showed that ion signal intensities of the above species were dependent on the composition of the LC mobile phase to an astonishing degree. At concentration levels of a few hundred ng/l, laboratory experiments showed that the aromatic amines considered here were mostly associated to dissolved humic acids (HAs) by both reversible and irreversible bindings. The addition of a reducing agent, i.e., NaBH4, succeeded in liberating that fraction of aromatic amines, which being reversibly bound to quinoidal structures of HAs are bioavailable. Analyte recoveries were better than 85% on extraction from 4 l of drinking water (spike level, 25 ng/l), 21 of ground water (spike level, 50 ng/l) and 0.5 l of river water (spike level, 200 ng/l). Relative standard deviations ranged between 4.6 and 20% for drinking water, 4.3 and 15% for ground water, 5.9 and 13% for river water. Method detection limits calculated for drinking water, groundwater and surface water were between 3 and 11, 6 and 21, 36 and 75 ng/l, respectively. (C) 1999 Elsevier Science B.V. All rights reserved.