High-performance liquid chromatography coupled with mass spectrometry applied to analyses of pesticides in water. Results obtained in HPLC/MS/APCI in positive mode

Abstract
Determinations of pesticides and their degradation products in natural waters, given the great heterogeneity of the substances involved, conventionally demands the use of the following analytical methods:
gas chromatography with different specific detectors such as the electron capture detector, NPD thermo-ionic detector, and mass spectrometry,
high-performance liquid chromatography combined with a diode array ultraviolet detector or a spectrofluorimeter after derivatization,
thin-layer chromatography (TLC) with detection of the isolated substances by UV.
With liquid chromatography, the most widely used detector is the diode-array UV detector. This method, recommended in French standard AFNOR T 90 123 (EN ISO 11369) [1], represents a significant advance in the approach to the identification of polar substances detected in unknown samples, compared with the results obtained by UV detection with a single wavelength. However, this method of detection does not guarantee the identification of the detected substances in absolute terms. Confirmation on a second column or by gas chromatography is therefore necessary.

The atmospheric interfaces equipped with APCI (Atmospheric Pressure Chemical Ionization) or ESI (Electrospray Ionization) ion sources help to broaden the field of application of the couplings of liquid chromatography with mass spectrometry, and have seen substantial developments in recent years [2-4]. They offer performance in agreement with environmental requirements in terms of quantification limits, sensitivity, simplicity of use and reliability [5]. They are accordingly suitable for the identification and quantification of pesticides and their conversion products in waters [6,7].

This article presents a summary of the results obtained on surface waters with the APCI interface in positive ionization mode, after extraction of the pesticides at neutral pH. The analyses were performed as part of a multi-residue approach applied to a priority list of 40 pesticides to be determined in the surface waters of the Centre region of France.

In addition to high-performance liquid chromatography combined with the diode array UV detector and coupled with mass spectrometry, gas chromatography coupled with mass spectrometry was used as a technical complement, because of its suitability for apolar substances such as lindane, organochlorinated pesticides, certain organophosphorus pesticides, trifluraline etc.

For the other substances investigated, such as triazines and some of their degradation products, phenylureas, triazoles, amides and carbamates, high-performance liquid chromatography coupled in series with a UV detector and a mass spectrometer was employed. The results obtained with the diode array UV detector were compared with those obtained with the mass spectrometer in order to validate the results obtained with the latter detector.

Acquisition and quantification methods optimized for HPLC/MS with APCI interface in positive ionization mode are presented in the article.

Results on samples of surface waters with preconcentration following the liquid/liquid or liquid/solid methods are described. The analytical approach is set for: preconcentration methods, HPLC/MS acquisition parameters, quantification methods, practical limits of quantification, and identification.