2. HIGH-RESOLUTION GAS CHROMATOGRAPHY–LOW-
2.1. Electron-Ionization Mass Spectrometry (EI-MS)
Electron ionization has been currently chosen as a reference ionization method in MS and it has been commonly used in the GC–MS analysis of PCTs and toxaphene. In fact, EI is readily available in all mass spectrometers and, in gen- eral, is stable, reproducible, and easy to operate on a daily basis.
The coupling of HRGC with LRMS (resolving power 500) using selective ion monitoring (SIM) allows one to obtain both structural information and homo- logue composition [13,19]. The EI mass spectra of the PCT congeners show an intense molecular ion M⫹• [28] and relatively intense peaks due to successive loss of two chlorine atoms from the molecular cluster and small fragments due to [M ⫺ Cl]⫹. The homologue profiles for PCT mixtures can be obtained by selecting the most intense ion of the molecular cluster from each homologue group. The traces of Aroclor 5460 are shown in Figure 2 as an example. Ac- cording to this, coelution of PCT congeners containing fewer chlorines did not interfere with the detection and the measurement of the M⫹•ions of a homologue group of congeners, but there was interference by the [M⫺Cl2]⫹fragment ions of homologues containing two additional chlorine atoms [20,37]. Another potential interference may be produced by the [M ⫺ Cl]⫹fragment ions, although this contribution does not seem to be important because the signals of these ions are very weak. In order to avoid these internal interferences, HRMS with resolving
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Analysis of Chlorinated Organic Compounds 127
power of 30,000 is required [20]. Nevertheless, the homologue distribution of PCT mixtures in commercial formulations and in environmental samples can be approximately calculated by eliminating from each SIM trace for each homologue the contribution of the [M ⫺ Cl2]⫹ ions, assuming that virtually no coelution between homologues containing two additional chlorine atoms occurs. This ap- proach was used to determine the approximate homologue composition of the PCT commercial mixtures and for the identification of the source of PCT contam- ination in biota [20].
Electron ionization has also been used for the analysis of toxaphene. How- ever, the mass spectra of CHBs are characterized by complex fragmentation pat- terns. In contrast to PCTs, molecular ions are normally absent in CHBs and major fragment ions in the high-mass range are produced by the sequential loss of a combination of Cl, HCl, CHCl2or CH2Cl from the molecules. In addition, some characteristic fragment ions in the low-mass range are also observed [31]. Stern et al. [79] studied extensively the fragmentation mechanism and reported three major fragmentation pathways: (1) the loss of Cl and HCl, (2) the retro–Diels- Alder reaction, and (3) the loss of CHCl2and CH2Cl.
The ions most commonly chosen for quantitative analysis of CHBs are the [M ⫺Cl]⫹ ions because they offer molecular mass information and have high specificity. However, these ions are usually very weak in EI-MS, resulting in relatively high detection limits. In addition, at low resolving power, the [M⫺ HCl⫺Cl]⫹ions from each homologue group interfere with the [M⫺Cl]⫹ions of homologues with 1 chlorine atom less. Significant contributions in the profile of hepta-CHBs on the hexa-CHBs, or the octa-CHBs on the hepta-CHBs are observed in the HRGC–EI-MS SIM traces of [M⫺Cl]⫹ions for the homologues of toxaphene. This internal interference is named the ‘‘mass leakage’’ or ‘‘cross- over’’ problem by Lau et al. [31]. An additional problem in the use of EI-LRMS is the potential interference of the [M⫺Cl]⫹isotope ions of chlorinated cam- phenes and monounsaturated bornenes (2 nominal mass units lower than fully saturated CHBs) on the signal of the [M⫺HCl⫺Cl]⫹ions from the higher CHB homologues. High-resolution mass spectrometry at a resolving power higher than 20,000 is required in order to remove these specific interferences [31,80].
In general, HRGC–EI-LRMS has been used as a complementary technique to HRGC–ECD or HRGC–ECNI-MS in the analysis of toxaphene in the environ- ment and biota samples [81,82], but its use is limited. Nevertheless, this technique in combination with protium-nuclear magnetic resonance (1H-NMR), linked-field
Figure 2 EI-LRMS-selected ion monitoring for homologues of Aroclor 5460. (a) hex- achloroterphenyls, (b) heptachloroterphenyls, (c) octachloroterphenyls, (d) nonachloroter- phenyls, (e) decachloroterphenyls, and (f) undecachloroterphenyls. (From Ref. 20.)
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scans, EI-HRMS, ECNI-HRMS, and MS–MS have been often used in the identi- fication, structure elucidation, and characterization of major toxaphene congeners from biota samples and technical toxaphene [79,80,82,83].