Ion-trap multiple mass spectrometry in pesticide analysis

Dossier Structure elucidation by LC-MS ■ Ion-trap multiple mass spectrometry in pesticide analysis B. R. Larsen* Environment Institute, Joint Research Centre, European Commission, I-21020 Ispra (VA), Italy Corresponding author: . these compounds are readily soluble in water their runoff into rivers and lakes pose several problems for the supply of clean drinking water. With the recent development of atmospheric pressure ion sources APCI and ESI and ion-trap multiple mass spectrometers (MSn) it has become technically and economically feasible for many laboratories to analyse unknown polar compounds with LC-MS in a similar way to gas chromatography mass spectrometry run in the full scan mode. Structural information on analytes can be obtained by collision induced dissociation (CID) directly in the trap through resonant excitation followed by collisions with helium buffer gas atoms [6]. The MS-MS process can be repeated a number of times and is thus an ideal tool for the study of fragmentation processes and pathways. ESI and APCI produce even-electron molecular ions (EE) which decomposition processes and pathways are much better understood today than just a few years ago. They include simple bond cleavages, cleavages with hydrogen transfer rearrangement and skeletal rearrangements including ring openings. In our laboratory we routinely use ion-trap LC-MSn for structure elucidation purposes of compounds such as photolysis products of environmental chemicals including pesticides [7-9], photooxidation products of volatile organic compounds in aerosol Liquid chromatography multiple mass spectrometry with atmospheric pressure ionisation and ion-trap instruments is a potentially useful technique for structure elucidation of polar pesticides and their degradation products. The major drawback today is the lack of spectral libraries and our limited knowledge on collision induced dissociation processes. The present paper gives an introduction to this area of research and presents interesting examples of fragmentation pathways for protonated and deprotonated ions obtained by atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI). Introduction Modern pesticides and their degradation products are well suited for liquid chromatography atmospheric pressure ionisation mass spectrometry (LC-API-MS) [1-5]. Since most of ANALUSIS, 2000, 28, N° 10 941 © EDP Sciences, Wiley-VCH 2000 Article available at http://analusis.edpsciences.org or http://dx.doi.org/10.1051/analusis:2000280941 Dossier Structure elucidation by LC-MS Figure 1. Typical proton migration leading to lateral chain cleavage by CID of [M+H]+ ions. [10-12], and metabolites of flavenoids [13]. This paper presents the application of ion-trap LC-MSn for the study of CID fragmentation of pesticides with typical examples of positive ion fragmentation and negative ion fragmentation. Full descriptions of experimental procedures have been given in details in the above mentioned references. In a typical experiment, full scan LC-MS spectra are first recorded and the (de)protonated molecule(s) are identified. Next, LC-MS2 spectra are recorded by isolating the (de)protonated molecule(s) in the ion-trap followed by CID. The energy required in this process varies between 10 and 40 % of the total available collision energy and is selected to preserve a signal of the precursor ion in the order of 5-10 %. This process can be repeated a number of times by successive isolation of one of the generated ions (product ions) as long as the CID process yields products with the charge preserved on a fragment larger than the instrumental lower mass range limit. The obtained information serve as basis for proposing fragmentation pathways. It is important to notice that the pathways still await confirmation e.g. by extensive isotope labelling studies and are drawn mainly to rationalise the observed fragments. Positive ion CID fragmentation schemes A number of pesticides give strong signals as their protonated molecular ions [M+H]+ by soft API ionisation process such as triazines, phenylureas, and carbamates. Other types of ions can also be obtained from the association of the target compound with solvent molecules. Such van der Waals cluster solvent adduct ions are frequently observed and may confuse the analyst e.g. [M+nCH3OH+H]+, [M+nH2O+H]+ and [M+nCH3OH+mH2O+H]+ from methanol or [M+CH3CN+H]+ from acetonitrile [14]. The relative intensities of such clusters are typically not very high, and they can often be differentiated from genuine protonated molecule(s) e.g. by applying a small amount of front-end CID energy. Protonated molecule(s) then appear with unchanged intensity, while adduct ions disappear. Figure 2. CID (MSn) fragmentation scheme for protonated triazine pesticides. tion of the ion at m/z 174 as base peak from triazine. This ion derives by migration of a hydrogen atom from the methyl group in the γ-position to the positively charged amino group, which leads to the loss of a disubstituted ethylene molecule [9]. CID of [M+H]+ ions (MS2) will often yields fragment ions formed by cleavage of lateral chains in the molecular structure. A good example, shown in figure 1, is the forma- Further MSn analysis produces identical results for all investigated triazines and is summarised in figure 2. By MS3 of the precursor ion at m/z 174 two different dissociation 942 ANALUSIS, 2000, 28, N° 10 © EDP Sciences, Wiley-VCH 2000 Dossier Structure elucidation by LC-MS processes are evident. The major pathway is similar to the one described above and leads to formation of an intensive ion at m/z 146 through the loss of CH2=CH2 from the N-ethyl lateral chain. The other pathway results in ring rearrangements and leads to formation of a an ion at m/z 132 and a weak ion at m/z 96. Triazine pesticides contain chlorine in their molecular structure. By MS3 analysis of a precursor ion at m/z 176 containing the chlorine isotope 37Cl it can be confirmed that the ions at m/z 146 and m/z 132 retain the chlorine atom, as opposed to the minor ion at m/z 96. The pathway is in accordance with Stevensons rule [15] stating that when an EE+ ion upon CID fragments into a neutral molecule and another EE+ ion following an internal proton migration. The charge is retained at the atom with the highest proton affinity – in this case the amino-group of the ion at m/z 96. The same account for the pathways with elimination of aminonitrile (Fig. 2). Negative ion CID fragmentation schemes Acidic pesticides such as e.g. phenoxyacids, nitrocresols and halogenated phenols give strong signals as their deprotonated molecular ions [M-H]– by API. Just as for protonated ions adducts can be formed from the association of the target compound with solvent molecules. The need for acidic buffers for chromatographic reasons enhance the formation of van der Waals clusters such as e.g. acetate adducts [M+CH3COO]– or formate adducts [M+CHOO]–, molecular cluster (...truncated)