Use of Chemical Ionization in Multianalysis Gas and Liquid Chromatography Combined With a Single Mass Spectrometer for the Ultra-trace Level Determination of Microcontaminants in Aqueous Samples†

Abstract

An automated system that comprises a single mass spectrometer (MS) in combination with gas (GC) and liquid chromatography [LC; particle beam (PB) interface] has been used for the trace-level detection and identification of contaminants in water samples. The analytes were enriched by on-line solid-phase extraction (SPE). In the present study, the potential of negative chemical ionization (NCI) detection in this so-called multianalysis system was explored. Attention was devoted to the enhancement of selectivity and sensitivity as well as to the additional spectral information obtained for the identification of unknown compounds.

Nine chlorinated pesticides representing three major groups, i.e., triazines, anilides and organophosphorus pesticides, were used as test compounds. Among the three reagent gases used for NCI, isobutane, methane and ammonia, methane gave the best results. For six of the nine pesticides, a 3- to 30-fold increase in sensitivity was observed in the NCI mode as compared with the electron impact (EI) mode. As expected, the NCI mass spectra showed little fragmentation. Electron capture appears to be the dominant ionization mechanism.

In order to study the potential of the total on-line SPE–LC/GC–MS set-up, the pesticides were spiked to tap and surface water samples. The detection limits obtained in the NCI (full-scan) mode ranged from 0.1–3 ng l^–1 for GC–MS and 50 to 200 ng l^–1 for LC–PB–MS for 100-ml tap water samples. The potential of NCI–MS was demonstrated by the identification of several unknown microcontaminants in a river water sample.