Application of liquid chromatography with mass spectrometry combined with photodiode array detection and tandem mass spectrometry for monitoring pesticides in surface waters.

Liquid chromatography with photodiode array detection (LC-DAD) and liquid chromatography with mass spectrometry (LC-MS) are two techniques that have been widely used in monitoring pesticides and their degradation products in the environment. However, the application of liquid chromatography with tandem mass spectrometry (LC-MS-MS) for such purposes, once considered too costly, is now gaining considerable ground. In this study, we compare these methods for the multi-residue analysis of pesticides in surface waters collected from the central and southeastern regions of France, and from the St. Lawrence River in Canada. Forty-eight pesticides belonging to eight different classes (triazine, amide, phenylurea, triazole, triazinone, benzimidazole, morpholine, phenoxyalkanoic), along with some of their degradation products, were monitored on a regular basis in the surface waters. For LC-MS, we used the electrospray ionization (ESI) interface in the negative ionization mode on acidic pesticides (phenoxyalkanoic, sulfonylurea), and the atmospheric pressure chemical ionization (APCI) interface in the positive ionization mode on the remaining chemicals. Different extraction techniques were employed, including liquid-liquid extraction with dichloromethane, and solid-phase extraction using C18-bonded silica and graphitized carbon black cartridges. Eleven of the target chemicals (desethylatrazine, desisopropylatrazine, atrazine, simazine, terbuthylazine, metolachlor, carbendazime, bentazone, penconazole, diuron and isoproturon) were detected by LC-MS at concentrations ranging from 20 to 900 ng/l in the surface waters from France, and six pesticides (atrazine, desethylatrazine, desisopropylatrazine, cyanazine, simazine and metolachlor) were detected by LC-MS and LC-MS-MS at concentrations ranging from 3 to 52 ng/l in the samples drawn from the St. Lawrence River. There was good correlation between the LC-DAD and LC-MS techniques for 60 samples. The slope of the curves expressing the relationship between the results obtained with LC-DAD versus those obtained by LC-MS was near 1, with a correlation coefficient (r) of over 0.93. The identification potential of the LC-MS technique, however, was greater than that of the LC-DAD; its mass spectra, mainly reflecting the pseudomolecular ion resulting from a protonation or a deprotonation of the molecule, was rich in information. The LC-MS-MS technique with ion trap detectors, tested against the LC-MS on 10 surface water samples, gave results that correlated well with the LC-MS results, albeit generating mass spectra that yielded far more information about the structure of unknown substances. The sensitivity of the LC-MS-MS was equivalent to the selected ion monitoring (SIM) acquisition mode in LC-MS. The detection limits of the target pesticides ranged from 20 to 100 ng/l for the LC-MS technique (under full scan acquisition), and from 2 to 6 ng/l for LC-MS-MS. These limits were improved by a factor of almost 10 by increasing the sample volume to 10 l.