Chemical ionization (CI) has been used in the selected ion monitoring mode for the analysis of PCB mixtures in environmental sample extracts [34,38]. Chemical ionization is the protonation of a molecule to form a pseudomolecular ion with little accompanying fragmentation. However, CI reagent ion species are formed concurrently in ion/molecule reactions with other ion species; thus, in a given CI system, several reagent ion species can be found. These species usually differ with respect to electron affinity [47] and the propensity with which the proton
Dioxins and PCBs by Quadrupole Ion-Trap 113
(a)
(b)
Figure 7 Selected ion chromatograms for the molecular ion M⫹•, fragment ion [M⫺ Cl•]⫹, and fragment ion [M⫺2Cl•]⫹•: (a) five tetrachlorobiphenyl isomers, (b) three hexa- chlorobiphenyl isomers.
114 Plomley et al.
is held [48]. Both properties influence the thermochemistry of the reactions oc- curring in the ion trap.
Coelution of compounds is problematical to quantitative PCB analysis and, when combined with low concentration of coplanar congeners normally encoun- tered in environmental samples, makes quantitative determination very unrelia- ble. An interesting example is the case of coeluting di-ortho-substituted 110 con- gener (2,3,3′,4′,6-pentachlorobiphenyl) and non-ortho-substituted congener 77 (3,3′,4,4′,-tetrachlorobiphenyl). As the concentration of the highly toxic congener 77 in environmental samples is often⬍1% of that of the 110 congener, quantita- tive determination of congener concentrations is challenging.
Preliminary experiments on congeners 77 and 110 showed almost exact duplication of each peak in the chlorine isotope molecular cluster in methane positive-ion CI mass spectrum of the 77 congener. Thus, almost half of the ion current corresponded to protonated molecules and the remainder to molecular radical cations formed, presumably, by charge exchange. The reagent ion C2H5⫹
as isolated from methane has an electron affinity of 8.13 eV, while C2H4has a relatively low proton affinity of 680 kJ mol⫺1. The values of these properties relative to those of the congeners examined indicated that C2H5⫹had potential analytical application for PCB determination. The dominant ion species observed in the mass spectrum obtained with mass-selected C2H5⫹were the [M ⫹ H]⫹ ions of each congener. The isotopic ratios for each cluster of protonated conge- ners were in good agreement with those expected for pure CI. The mass spectrum was entirely free of fragment ions below [M77⫹H]⫹. If the behavior of each of congeners 77 and 110 is characteristic of that of coplanar and planar PCB mole- cules, respectively, then CI of PCBs using selected C2H5⫹ions from methane offers a promising method for the determination of PCBs.
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5
Gas Chromatography–Mass Spectrometry Analysis of
Chlorinated Organic Compounds
M. T. Galceran and F. J. Santos University of Barcelona, Barcelona, Spain
1. INTRODUCTION
In the last three decades, there has been considerable public concern about the presence of halogenated anthropogenic compounds in the environment because of their persistence, the bioaccumulation potential, and the health risks that they bring about [1–3]. Examples of members of this family of xenobiotics are poly- chlorinated biphenyls (PCBs), polychlorinated terphenyls (PCTs) and toxaphene.
Polychlorinated biphenyls were described as ubiquitous, environmentally harm- ful substances more than 30 years ago [4]. In addition, the presence of PCBs and toxaphene in the environment has been extensively documented [5–7]. In con- trast, little attention has been paid to polychlorinated terphenyls, which are similar to PCBs in chemical characteristics and applications. In this chapter, we discuss different methods described in the literature for the analysis of PCTs and toxa- phene using high-resolution gas chromatography (HRGC) coupled to mass spec- trometry (MS).
Polychlorinated terphenyls have been used extensively in sealants, hydrau- lic fluids, electrical equipment, plasticizers, and paints because of their desirable electrical and flame-retardant properties [8]. The Monsanto Chemical Co. started the production of both PCTs and PCBs in the United States in 1929. Since then, PCTs have also been produced in France, Italy, Germany, and Japan. Information about PCT production is incomplete, but during the period 1955 to 1980, approxi- mately 60,000 metric tons were produced worldwide [8,9], which is 15 to 20 117
118 Galceran and Santos
times lower than the total PCB production during the same period [7]. In 1972, the manufacture of PCTs was voluntarily discontinued in the United States, and later in such countries as Germany, Italy, and France, in 1974, 1975, and 1980, respectively.
Polychlorinated terphenyls have been produced as technical mixtures under different commercial names such as Aroclor, Kanechlor C, Leromoll, Clophen, Cloresil, Electrophenyl T-60, Phenoclor, and Terphenyl Chlore T-60. These tech- nical formulations consisted of mixtures of congeners with variable degrees of chlorination. Commercial formulation of PCTs could be expected to be much more complex than those of PCBs because of the presence of an additional aro- matic ring, which increases the positions available for chlorination. The theoreti- cal number of PCT congeners with C18H14-xClx(1 ⱕ ⫻ ⱕ14) is 8557 [10,11], as shown in Table 1.
Little information is available about the distribution, fate, and effects of PCTs in the environment. However, as highly chlorinated aromatic compounds, PCTs should be highly resistant to biodegradation and photodegradation, to the effect of acids, bases, and oxidizing agents, and should easily accumulate in adi- pose tissues of living organisms. Comparable toxic effects to those of PCBs on animals have been reported for commercial PCT formulations [12]; however, some investigators found that their effects were less severe [8,9].
Table 1 Theoretical Number of PCT Congeners
Terphenyl configuration
Homologue ortho-terphenyl meta-terphenyl para-terphenyl Total
Mono-CTs 5 6 4 15
Di-CTs 28 28 21 77
Tri-CTs 86 90 58 234
Tetra-CTs 217 217 142 576
Penta-CTs 391 400 244 1035
Hexa-CTs 574 574 356 1504
Hepta-CTs 636 648 388 1672
Octa-CTs 574 574 356 1504
Nona-CTs 391 400 244 1035
Deca-CTs 217 217 142 576
Undeca-CTs 86 90 58 234
Dodeca-CTs 28 28 21 77
Trideca-CTs 5 6 4 15
Tetradeca-CTs 1 1 1 3
Total 3239 3279 2039 8557
Source: adapted from Refs. 10 and 11.
Analysis of Chlorinated Organic Compounds 119
From an environmental point of view, polychlorinated terphenyls have been identified in various matrices such as soil and sediments [13–15], water [15,16], waste oils [17], birds [18], aquatic organisms [8,18–23], wolves and pigs [24], human tissue [25,26], as well as in many other samples [27].
Analysis of PCTs has proved to be difficult because of the complexity of the mixtures, the high boiling points of the heavily chlorinated congeners, and the coelution of the lower chlorinated PCTs with some PCBs, but the use of gas chromatographic capillary columns longer than 50 m improves the separation.
Nevertheless, the problem of the chromatographic coelution cannot be solved even using multidimensional gas chromatography (GC). Capillary gas chromato- graphic columns with high thermal stability and relatively nonpolar stationary phases such as 5% fenil–95% methyl siloxane, have been commonly used for PCT separation. Stationary phases of different polarity [10] do not result in better separations. In addition, the high numbers of possible PCT congeners and the unavailability of individual standards hamper their accurate quantification. Re- cently, some individual PCT congeners became commercially available from Pro- mochem (Wesel, Germany), but the number is still very small. This problem can be partly solved using commercial mixtures of PCTs, such as Aroclor 5432, 5442 or 5460, as secondary standards for identification and quantification of total PCTs.
High-resolution gas chromatography (HRGC) combined with an electron- capture detector (ECD) [9,12,20], an electrolytic conductivity detector (ELCD) [13,22], and/or coupled to MS with electron ionization (EI) [9,25,28,29] or nega- tive chemical ionization (NCI) [15,21] are commonly used for the analysis of PCTs. In general, HRGC–ECD provides adequate sensitivity and selectivity in the analysis of PCTs, but the presence of other related compounds, such as PCBs of high chlorinated level or other halogenated compounds of high molecular weight such as polychlorinated naphthalenes and organochlorine pesticides, could potentially interfere with the final determination of these compounds. In addition, only a semiquantitative analysis of PCTs, expressed as total content, can be achieved by HRGC–ECD by assuming equal response factors for all congeners and similar composition between samples and commercial standard mixtures.
High-resolution gas chromatography–mass spectrometry (HRGC-MS) tech- niques could be a good choice to solve interference problems in the analysis of PCTs. Several approaches using low-resolution MS (LRMS), high-resolution MS (HRMS), and tandem mass spectrometry (MS–MS) have been proposed; they are discussed in the respective sections later in this chapter.
Toxaphene is considered to be the most extensively used insecticide in the world. It has been widely used in insect control on cotton and other crops, and in the control of the external insects on livestock [30]. It was banned in the United States in the early 1980s and some time later in Canada and some European countries [31] because of its acute and chronic toxicity to humans, environmental
120 Galceran and Santos
persistence, and bioaccumulating capabilities [32,33]. However, toxaphene and other related compounds are still being manufactured and widely used in Central and South America, African countries, India, the former Soviet Union, and some East European countries [34,35]. Its cumulative global usage averaged 450 thou- sand metric tons, but the total global production between 1950 to 1993 was esti- mated to approach 1.3 megatons [36], which is a higher value than that of poly- chlorinated biphenyls.
Toxaphene was first manufactured in the United States, in 1945, and it is mainly composed by polychlorobornanes (CHBs, 76%), polychlorobornenes (18%), polychlorobornadienes (2%), other chlorinated hydrocarbons (1%), and nonchlorinated hydrocarbons (3%) [37], with a total chlorine content of 67 to 69% [38]. The actual composition of this extremely complex mixture is not fully characterized. Vetter [39] estimated that there are 32,768 theoretically possible CHB congeners. The number of possible congeners decreases considerately after applying some restrictions due to unfavorable chlorine substitutions on the ring and bridge carbon atoms. Oddly enough, the number of toxaphene congeners found in environmental samples is lower than those in technical formulations.
These dissimilarities are due to photodegradation, selective bioaccumulation, and/or the metabolism of toxaphene congeners in aquatic and terrestrial organ- isms, including humans [34,40]. Although the total number of congeners de- creases, the remaining toxaphene mixture is still complex and requires powerful techniques of separation and analysis.
There is an additional problem of nomenclature. In fact, several systematic nomenclature systems for toxaphene congeners have been proposed [21,41–47], but none of them has been universally adopted until now. At the Workshop on Toxaphene, held in Burlington (Ontario, Canada) in 1993, it was agreed that the term chlorobornanes (CHBs) would be used for toxaphene compounds [48]. In addition, difficulties in formulating the correct systematic names for chlorobor- nanes were theoretically solved when the International Union of Pure and Applied Chemistry (IUPAC) presented the definitive numbering of the carbon skeleton (Fig. 1). Inadequate use of the IUPAC nomenclature leads to some confusion [49,50], particularly with the C-8 and C-9 positions of the bornane skeleton. As IUPAC names are very long and not very easy to use, several authors proposed more simple nomenclatures. Initially, some toxaphene congeners were numbered by the research group of Parlar [41,42] using their GC retention time on a given stationary phase, but as each number was assigned to a chromatographic peak, congener coelution could not be excluded. Other nomenclatures were based on binary coding systems, such as those proposed by Tribulovich et al. [43] and Nikiforov et al. [44], Oehme and Kallenborn [45], and Andrews and Vetter [46], which were difficult to handle. Recently, Wester et al. [47,51] have proposed a new system that provides structural information. It is applicable to all the conge- ners and allows discerning enantiomers, and is relatively easy to use. In this
Analysis of Chlorinated Organic Compounds 121
Figure 1 Structure of bornane skeleton with numbering of carbon atoms and endo and exo positions (asterisk indicates chirality carbon atom).
nomenclature, every digit in the code represents the chlorine substitution of the carbon number. The first part of the code reflects the conformation of the six- membered ring (C-2 to C-6) (0, none; 1, endo; 2, exo; 3, both), and the second part gives the number of chlorine atoms attached to C-8, C-9, and C-10, respec- tively. The 8-digit number is preceded by ‘‘B’’ for bornanes, ‘‘E’’ for bornenes, or ‘‘D’’ for bornadienes. An overview of some of these nomenclature systems is presented in Table 2. The various nomenclature systems are still used by different authors, and it would be necessary to adopt a unique nomenclature system in order to end the current confusing situation.
Toxaphene has been described as a ubiquitous contaminant in various envi- ronmental compartments and has a widespread distribution around the world [52–
54], even in very remote areas, such as the polar regions, due to aerial transport [55]. Extensive reviews on the chemistry, biochemistry, toxicity, analysis, and environmental fate of toxaphene have been published [32,38,52]. Toxaphene, similar to other pollutants such as DDT, PCBs, and other organochlorine com- pounds, was found in air [54–56], freshwater [57,58], food [59], soil and sedi- ment [60,61], human milk [53], and even marine biota [41,62–66].
The analysis of toxaphene is currently performed by HRGC [31,52,67]. In general, nonpolar capillary columns with 5% phenyl–95% methyl polysiloxane stationary phases are used. Other columns with semipolar and polar stationary phases, such as cyanopropylphenyl methyl polysiloxane and polyethylene glycol polysiloxane, have also been used [63,68]. However, care is recommended be- cause some compounds may decompose on these polar phases [67]. On the other hand, injector temperatures higher than 240°C produced a substantial decrease in the signal due to decomposition on active sites of the gas chromatograph injec- tor [63]. In general, it is recommended to work with on-column or splitless injec-
122GalceranandSantos Table 2 Correlation of Different Names of Chlorinated Bornane Congeners Appearing in the Literature
Proposed by Proposed by
Parlar’s Nikiforov and Oehme and Proposed by Proposed by Other Names
No. Tribulovich Kallenborn Andrews and Wester et al. (Vetter et al. [49],
[41,42] IUPAC Name (Racemate) [49,50] [43,44] [45] Vetter [46] [47,51] Stern et al. [79]) 21 2-endo,2-exo,5-endo,5-exo,9,10,10- HpCB-6533 99–043 B7-449 B[30030]-(012)
HeptaCHB
32 2-endo,2-exo,5-endo,6-exo,8,9,10- HpCB-6452 195–241 B7-515 B[30012]-(111) Toxicant B HeptaCHB
2-exo,3-endo,5-exo,9,9,10,10-HeptaCHB HpCB-3207 B7-1453 B[21020]-(022) TOX 7 2-exo,3-endo,6-endo,8,9,10,10-HeptaCHB HpCB-3157 134-113 B7-1462 B[21001]-(112)
2-exo,5-endo,5-exo,9,9,10,10-HeptaCHB HpCB-2439 98-033 B7-1715 B[20030]-(022) 39 2-endo,2-exo,3-exo,5-endo,6-exo,8,9,10- OCB-6964 299–421 B8-531 B[32012]-(111)
OctaCHB
25 2-endo,2-exo,3-exo,8,8,9,9,10-OctaCHB OCB-6686 11–631 B8-623 B[32000]-(221) 51 2-endo,2-exo,5-endo,5-exo,8,9,10,10- OCB-6549 99–423 B8-786 B[30030]-(112)
OctaCHB
38 2-endo,2-exo,5-endo,5-exo,9,9,10,10- OCB-6535 99–063 B8-789 B[30030]-(022) OctaCHB
42β 2-endo,2-exo,5-endo,6-exo,8,9,9,10- OCB-6454 291–461 B8-806 B[30012]-(121) Toxicant A2
OctaCHB
42α 2-endo,2-exo,5-endo,6-exo,8,8,9,10- OCB-6460 291–641 B8-809 B[30012]-(211) Toxicant A1
OctaCHB
2-endo,2-exo,5-endo,6-exo,8,9,10,10- OCB-6453 193–243 B8-810 B[30012]-(112) OctaCHB
26 2-endo,3-exo,5-endo,6-exo,8,8,10,10- OCB-4921 297–603 B8-1413 B[12012]-(202) T2, TOX 8 OctaCHB
AnalysisofChlorinatedOrganicCompounds123 40 2-endo,3-exo,5-endo,6-exo,8,9,10,10- OCB-4917 297–243 B8-1414 B[12012]-(112)
OctaCHB
41 2-exo,3-endo,5-exo,8,9,9,10,10-OctaCHB OCB-3223 70–463 B8-1945 B[21020]-(122) 44 2-exo,5-endo,5-exo,8,9,9,10,10-OctaCHB OCB-2455 98–463 B8-2229 B[20030]-(122) 58 2-endo,2-exo,3-exo,5-endo,5-exo,8,9,10, NCB-7061 107–243 B9-715 B[32030]-(112)
10-NonaCHB
2-endo,2-exo,3-exo,5-endo,5-exo,9,9,10, NCB-7047 107–033 B9-718 B[32030]-(022) 10-NonaCHB
2-endo,2-exo,3-exo,5-endo,6-exo,8,9,9,10- NCB-6966 199–043 B9-742 B[32012]-(121) NonaCHB
2-endo,2-exo,5-endo,5-exo,6-exo,8,9,9,10- NCB-6582 227–641 B9-1011 B[30032]-(121) NonaCHB
62 2-endo,2-exo,5-endo,5-exo,8,8,9,10,10- NCB-6551 099–643 B9-1025 B[30030]-(122) NonaCHB
56 2-endo,2-exo,5-endo,6-exo,8,8,9,10,10- NCB-6461 291–645 B9-1046 B[30012]-(212) NonaCHB
59 2-endo,2-exo,5-endo,6-exo,8,9,9,10,10- NCB-6455 291–463 B9-1049 B[30012]-(122) NonaCHB
50 2-endo,3-exo,5-endo,6-exo,8,8,9,10,10- NCB-4925 297–643 B9-1679 B[12012]-(212) T12, Toxicant
NonaCHB Ac, TOX 9
63 2-exo,3-endo,5-exo,6-exo,8,8,9,10,10- NCB-3261 326–643 B9-2206 B[21022]-(212) NonaCHB
2-endo,2-exo,3-exo,5-endo,5-exo,8,9,9,10, DCB-7063 103–643 B10-831 B[32030]-(122) 10-DecaCHB
2-endo,2-exo,3-exo,5-endo,6-exo,8,8,9,10, DCB-6967 199–643 B10-860 B[32012]-(212) 10-DecaCHB
69 2-endo,2-exo,5-endo,5-exo,6-exo,8,9,9,10, NCB-6583 355–463 B10-1110 B[30032]-(122) 10-DecaCHB
124 Galceran and Santos
tion at temperatures below 220°C [50] in order to prevent thermal degradation, and to use the shortest possible column to avoid long retention times and high elution temperatures [67].
High-resolution gas chromatography with chiral capillary columns has also been applied to the analysis of some CHB enantiomers [67,69–71]. Enantiosepar- ation has been achieved using chiral stationary phases (CSPs). Several studies have been performed in order to determine the behavior of enantiomers in the biota and the environment. In fact, differences in the enantiomeric ratios have been observed in seals [52,72], monkey fatty tissue [70], cod liver, and fish- oil samples [73]. This fact suggests that the biological transformation of some enantiomer pairs can change depending on the type of biota. Multidimensional GC has also been used for the analysis of toxaphene congeners. The resolution required for CHB separation can be achieved by selecting an adequate combina- tion of columns. De Boer et al. [68,74] proposed the use of Ultra 2/Rtx 2330 to analyze CHB congeners in biological samples. In general, the main drawback of multidimensional GC is the relatively long analysis time compared with the single-column method.
Two commonly used techniques to analyze CHBs in biological or environ- mental samples at residue levels are HRGC–ECD and HRGC–MS. Although GC–ECD offers high sensitivity at low cost, it is always questionable as a con- firmatory method due to the low specificity in the analysis of complex samples, even if extensive cleanup procedures are applied. Interferences from organochlo- rine pesticides and PCBs and limitations in the quantitative analysis of toxaphene are frequently observed. In contrast, GC–MS is generally recognized as a more powerful analytical tool with adequate sensitivity and specificity. Electron-cap- ture negative-ion MS (ECNI-MS) and EI-MS are the two most popular MS tech- niques. Nevertheless, quantification of toxaphene is difficult, mainly due to the substantial difference in peak profiles of environmental/biological samples and those of industrial formulation, and to the lack of congener-specific standards of the different chlorobornanes. The research group of Parlar [41,75] succeeded in producing the 22 more important single congeners of toxaphene. Most of them are octa- and nonachlorobornanes, which are commercially available from Ehren- storfer (Augsburg, Germany] or Promochem (Wesel, Germany). For an accurate quantification of CHBs, it is absolutely essential to prepare isotopically labeled internal standard congeners.
The aim of this chapter is to present and discuss the different GC–MS techniques and the advances achieved in the characterization and analysis of poly- chlorinated terphenyls and toxaphene. We have paid special attention to the per- formances of the GC–MS techniques and the advantages and drawbacks of its application as a reliable method for the analysis of these compounds in commer- cial mixtures and samples. The use of HRGC–ECNI-MS, HRGC–EI-LRMS, HRGC–EI-HRMS, and HRGC–MS–MS is discussed in the following sections.