Negative Ion Chemical Ionization Mass Spectra of Organochlorine Pesticides

Abstract

Negative ion chemical ionization (NCI) mass spectra of 31 organochlorine pesticides and related compounds (Table 1) are discussed in comparison with positive ion chemical ionization (PCI) mass spectra for the purpose of revealing their characteristics. Ion abundances of the specific ions are measured by the selected ion monitoring to estimate ion abundance ratios such as negative base peak/positive base peaks and chloride ion peak/negative base peaks. Methane is used as a reagent gas, and ion source temperature and ionization chamber pressure are maintained at 250°C and 0.20 Torr, respectively. The results obtained can be summarized as follows.
1) In general, simple and characteristic NCI spectra based on less fragmentation are obtained for each compounds (Fig.1). Molecular ions, M are mainly generated in aromatic compounds. Fragment ions are primarily consisted of (M-HmCln)-(m≥0, n≥1) and Cl- for all compounds. Pentachlorocyclopentadienide anion, C₅Cl₅-, resulted from retro Diels-Alder reactions are characteristic for chlorinated dimethanonaphthalenes and methanoindenes.
2) Base peaks (above m/z 100) in NCI and PCI spectra have many varieties (Table 2), and the ion abundance ratios of negative/positive ions ranged from several to several hundred, indicating that highly sensitive detections are possible by the NCI method.
3) Chloride ions, Cl are generated in wide range of magnitude (Table 2), and the ion abundances are frequently more abundant than those of negative base peaks.