Anal. Chem. 1986, 58, 2913-2919
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Selective Detection of Brominated Aromatic Compounds Using Gas Chromatography/Negative Chemical Ionization Mass Spectrometry Hans-Rudolf Buser
Swiss Federal Research Station, CH-8820 Wadenswil, Switzerland
High-resolution gas chromatography/negatIve chemical ionization mass spectrometry (HRGC/NCI-MS, CH, reagent gas) was used for the selectlve detection of brominated and brominated/chlorinated aromatic compounds (benzenes, biphenyls, dibenzodioxins, dibenzofurans, diphenyl ethers, benzofurans, phenols) by monitoring characteristic bromide anions (Br-, m / z 79 and 81) formed under NCI conditions. High sensitivity (0.1 pg or better) with an acceptable linear range (up to several hundred picograms) and high selectivity with little interference from chlorinated analogues were observed. A structural requirement for high sensitivity was the presence of at least two Br substituents In a molecule, or a single Br with additional CI substituents present. NCI mass spectra of the brominated and the brominated/chiorinated compounds investigated showed intense, often dominating Brions. Application of HRGC/NCI-MS is documented with the analysis and detection of bromopolychiorobiphenyis (bromoPCBs) in commercial PCBs, of brominated aromatic compounds in automotive emissions (exhaust), and of brominated dlbenzofurans (PBDFs) in soot from an accidental fire.
Brominated organic compounds are important technical products used as gasoline additives, fumigants, flameproofing agents, medicinals, sanitizers, and others. Among these compounds, polybrominated biphenyls (PBBs) and diphenyl ethers (PBDPEs) are important chemicals used as flame retardants added to plastic and other materials (1,2). In addition, certain brominated aromatic compounds are also thermal reaction products of other brominated compounds. In this way, polybrominated dibenzofurans (PBDFs) and dibenzodioxins (PBDDs) were formed from PBDPE and PBB flame retardants (3-5). Brominated compounds may also be produced in reactions similar to those leading to the chlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs) in combustion processes from suitable precursors (6). Recently, brominated aromatic compounds have also been detected in automotive emissions from leaded gasoline additives (7). Mixed halogenated (brominated/chlorinated) compounds are expected from such processes if both bromine and chlorine sources are present. Assuming a de novo synthesis from Cl', Br', or via small halogenated organic radicals, and with C1 usually more prevalent than Br, bromopolychloro compounds are anticipated, with smaller quantities of the more brominated analogues. Brominated compounds have similar chemical and biological properties and therefore are likely of similar toxicological importance as the chlorinated analogues. High toxicity was shown for some PBDFs and PBDDs as was the case for their chlorinated counterparts (8-10). Although the chlorinated compounds are well-documented environmental contaminants (11-13), comparatively little is known about the environmental occurrence of brominated aromatic compounds apart from an environmental calamity with PBBs in Michigan (14). 0003-2700/86/0358-29 13$01.50/0
Often a large number of isomers and congeners in a group of halogenated chemicals exists. These numbers become very large if mixed halogenated (brominated/chlorinated) compounds are also considered. As an example, there are 1550 and 3050 such mixed halogenated dibenzodioxins and dibenzofurans, as opposed to 75 and 135 analogues containing only one halogen element. Since many of these brominated compounds have similar chemical properties, they are isolated in the same fractions and they often coelute with chlorinated analogues in chromatographic systems. The analysis of brominated and mixed halogenated compounds is further complicated because these compounds are usually accompanied by larger quantities of the chlorinated analogues. The brominated compounds are therefore often difficult to detect and identify. Mass spectrometry (MS) is probably the most useful analytical technique for the detection and identification of halogenated compounds. Generally, MS allows determination of the type of halogenation (number of C1 and Br present) from characteristic isotope distribution patterns. This identification, however, is more difficult for bromo/chloro compounds, especially if only small amounts of a compound are available, such as from environmental and biological samples. The isotope distribution patterns are then often ambiguous, and a method to detect the presence of Br in a compound at trace levels would be desirable. When investigating negative chemical ionization (NCI) mass spectra of a number of brominated aromatic compounds, we observed that most of these compounds yield intense, often dominating bromide anions (Br-, m / t 79 and 81). Although this was previously pointed out for brominated benzenes (15), apparently no analytical use was made of this fact so far. In this paper, we report that high-resolution gas chromatography (HRGC)/NCI-MS and monitoring Br- ions can be used easily and reliably to detect and quantify these brominated compounds at high sensitivity (0.1 pg or better) with minimal interference from other halogenated compounds. Application of the technique is demonstrated with the detection of brominated contaminants in technical PCBs and brominated aromatic compounds in automotive emissions from leaded gasoline additives and the first identification of brominated tricyclic aromatic compounds (PBDFs) in soot from an accidental fire.
EXPERIMENTAL SECTION Materials and Reference Compounds. Mono- and 1,4-dibromobenzene, bromotoluenes, and bromophenols were from Fluka (Buchs, Switzerland). Hexabromobenzene, Firemaster BP-6, various PCB, PCDD, and PCDF isomers, and Aroclor 1254 and 1260 were from C. Rappe (University of Umei, Sweden). 2,3,7,8-Tetrabromodibenzofuran (2,3,7,8-tetra-BDF)was from A. S . Kende (University of Rochester, NY) and 1,2,3,4-tetrabromodibenzodioxin (1,2,3,4-tetra-BDD) from 0. Hutzinger (University of Bayreuth, FRG). Several additional PBDDs and PBDFs were present in pyrolysis samples of PBDPEs (5);these samples also contained brominated benzenes and benzofurans. PBDPEs were present in technical flame retardants (DE 71 and 0 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14,DECEMBER 1986
Figure 1. NCI mass spectra of brominated aromatic compounds: (a)hexabromobenzene, (b) 2,4,5,2',4',5'-hexabromobiphenyl, (c) 2,3,7,8-tetra-BDF, and (d) 1,2,3,4-tetra-BDD.
DE 79) from Great Lakes Chemicals (West Lafayette, IN). Automotive exhaust samples were from an earlier study by Muller and Buser ( 7 ) ; additionally, several brominated/chlorinated phenols and methylphenols were available from that investigation. The extract of a soot sample from an accidental fire in a bowling hall was made available for HRGC/NCI-MS analysis by B. Scholz (Institut Fresenius, Taunusstein, FRG). Bromination of 2,3,4,5-tetra-, 2,4,5,2',4'-penta-, and 2,4,5,2',4',5'-hexachlorobiphenyl(tetra-, penta-, and hexa-CBs) and of 2,3,7,8-tetra-CDDand 2,3,7,8-tetra-CDFin small amounts (lCb-100pg) was carried out as previously described using Br2in CC1, ( 7 ) . This method was not successful for the higher chlorinated PCBs, but gave small yields, sufficient for GC/MS analyses, of the desired mixed halogenated derivatives from the other compounds. GC/NCI-MS Analysis. The analyses were carried out on a 25-m SE 54 high-resolution (HRGC) glass capillary column (0.31 mm i.d.) coupled via a fused silica capillary interface to a Finnigan 4000 quadrupole MS operating in the NCI (methane, 0.31 torr, 140 OC) mode. The ion source potentials were optimized by using ions at m / z 233 and 235 (Reo,-) and m / z 35 (C1-) present in the NCI-MS background. More volatile compounds (up to tribromophenol) were analyzed by temperature programming the column as follows (splitless injection at 50 "C, solvent n-hexane): 50 OC, 2 min isothermal, 20 "C/min to 80 "C, 5 OC/min to 200 "C (program 1). Less volatile compounds were analyzed with the following temperature program (injection at 80 O C , solvent toluene): 80 "C, 2 min isothermal, 20 "C/min to 160 "C, 5 OC/min to 280 "C (program 2). A faster program rate (10 OC/min) and a higher intermediate temperature (180 OC) were used for the sample from the accidental fire (later eluting components). Helium was used as carrier gas (velocity 30-35 cm/s). Mass spectra were recorded with a Finnigan 6115 data system (repetitive scans m / z 30-530 and m / z 30-930 at 2.3 and 3.1 s/scan, respectively). Quantities injected were usually in the range of 0.5-1 ng (complete NCI mass spectra). Selected ion monitoring (SIM) analyses ( m / z 79,81, and other ions) were carried out with injections of 0.1-1000 pg/compound (2-fiLsample volumes). Some care should be exercised not to inject too large a quantity of brominated compounds, for it might cause excessive signals (Br-) in the NCI-MS background. RESULTS AND DISCUSSION NCI-MS of Various Brominated Aromatic Compounds. Negative ion formation in NCI-MS is due to resonance capture of thermalized electrons, dissociative capture of low-energy electrons, and ion/molecule reactions in the ion source (16). A number of specific ion/molecule reactions are known in NCI-MS and involve charge transfer, proton or hydride transfer, oxygen exchange reactions with halogens, and anion-molecule adduct formation. High selectivity for particular classes of compounds and very high sensitivities for these
compounds are attainable, orders of magnitude higher than in EI-MS (12, 17). NCI-MS is thus a relatively common technique for trace-level analysis of electron-capturing compounds. For chlorinated aromatic compounds (PCBs, PCDDs, PCDFs) and the conditions used in our study (CH, reagent gas), molecular anion (M-) formation and dissociative electron capture (e.g., formation of M--C1) are usually observed (12). With most of the brominated aromatic compounds studied, dissociative electron capture and formation of halide (Br-) anion (doublet a t m / z 79 and 81) were the predominant reactions. This changed behavior may arise from the lower bond strength of the C-Br bond (82 kcal/mol) as opposed to that of the C-Cl bond (97 kcal/mol) in aromatic compounds. E1 mass spectra of PBDFs, PBDDs, and PBDPEs have been recently discussed ( 5 ) . The NCI mass spectra of various brominated aromatic compounds are now discussed below. NCI mass spectra of all brominated benzenes, with exception of bromobenzene itself, showed intense, dominating Br(NCI mass spectrum of hexabromobenzene, see Figure la). The higher (tetra- to hexa-) brominated benzenes showed recognizable M- ions with the expected ion clustering due to the Br isotopes and fragmentations leading to M- - 78 (M- Br + H) and M- - 156 (M- - Br2 + 2H) ions. Bromobenzene, bromotoluenes, and bromoalkanes showed very poor electron affinity and were not detectable a t low nanogram quantities (sensitivity at least 100 times lower than for higher brominated benzenes); 1,4-dibromobenzene, however, gave intense Br- but no observable M- ion (
