Electron Impact and Electron Capture Negative Ionization Mass

This review presents the electron impact (EI) and electron capture negative ionization (ECNI) mass spectra of the polybrominated diphenyl ether (PBDE)...
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Environ. Sci. Technol. 2008, 42, 2243–2252

Electron Impact and Electron Capture Negative Ionization Mass Spectra of Polybrominated Diphenyl Ethers and Methoxylated Polybrominated Diphenyl Ethers RONALD A. HITES* School of Public and Environmental Affairs, Indiana University, Bloomington, Indiana 47405

Received August 17, 2007. Revised manuscript received December 20, 2007. Accepted January 10, 2008.

This review presents the electron impact (EI) and electron capture negative ionization (ECNI) mass spectra of the polybrominated diphenyl ether (PBDE) flame retardants and of their methoxy derivatives. Data from the literature are reviewed, and full spectra from our laboratory are reported to correct some of the errors that have crept into some previously published data. The EI spectra of the PBDEs are dominated by molecular ions and by singly and doubly charged ions due to the loss of Br2 from the molecular ion. The ECNI spectra of PBDEs with seven or less bromines are dominated by Br- and by HBr2-; the spectra of those with eight or more bromines are dominated by tetra- or pentabromophenoxide ions due to cleavage of the phenyl-ether linkage. The EI mass spectra of methoxyPBDEs can easily distinguish the position of the methoxy group relative to the phenyl-ether linkage. The ECNI spectra of these compounds are also dominated by Br- and HBr2-. In both ionization modes and for both compound groups, there are some subtle features, which often allow one to rule in or out substitution at one or more of the ortho-ring positions.

usually be achieved, at least for PBDEs and MeO-BDEs with eight or fewer bromine atoms. Quantitation of the nonaand decabrominated congeners is a bit more problematic, but good results are now appearing in the literature. Despite the heavy use of mass spectrometry for the analysis of these compounds, it is ironic that few systematic studies of the full mass spectra of these compounds have appeared. In fact, some of the papers, which cover the partial EI and ECNI mass spectra of these compounds, report incomplete mass spectra (usually the low mass ions are omitted), spectra with no interpretation, ions at the wrong mass numbers, tables with just a few ion abundances, or ECNI spectra taken without attention to the ion source temperature. This paper is a systematic study of the complete EI and ECNI mass spectra of these compounds. Here we report the full spectra of 18 PBDE congeners in both EI and ECNI modes (the latter at two or three ion source temperatures) and of 12 MeO-PBDE congeners, again in EI and ECNI modes. The data are interpreted in terms of the structures of the ions formed in the ion source.

Introduction

Experimental Section

The environmental presence of polybrominated diphenyl ether (PBDE) flame retardants has received considerable attention over the past 10 years because their ambient concentrations have increased substantially as a function of time. For example, PBDE concentrations in people have increased exponentially since the mid-1970s with a doubling time of about 5 years (1). The analysis of PBDEs has focused almost exclusively on gas chromatographic mass spectrometry (GC/MS), usually operating in the electron capture negative ionization (ECNI) mode. Selected ion monitoring of the bromide ions at m/z 79 and 81 usually provides a signal of excellent sensitivity and adequate specificity. Alternately, high-resolution mass spectrometry, operated in the electron impact (EI) mode, is sometimes used (2, 3). For a review of these and other analytical methods, see the papers by de Boer et al. (4) and Stapleton (5). More recently, toxicologists and others have become interested in the hydroxylated metabolites of PBDEs (6, 7). To achieve highquality separations of these compounds, they are usually treated with diazomethane to form their methyl ethers (MeOBDEs), which are analyzed by GC/MS in EI or ECNI modes. Using these techniques and a full range of unlabeled and labeled synthetic standards, excellent quantitative results can

The electron impact and electron capture negative ionization mass spectra of the tri- through octabrominated PBDEs, and all of the methoxy-PBDEs were obtained on an Agilent 5973 gas chromatographic mass spectrometer. The compounds were introduced in a solution of n-hexane. The PBDEs were purchased from AccuStandard (New Haven, CT) and from Cambridge Isotope Laboratories (Andover, MA). The methoxy-PBDEs were prepared from the hydroxylated analogues, which were a gift from G. Marsh (Stockholm University, Sweden); details on the synthesis and characterization of these hydroxylated compounds have been published previously (8). In our laboratory, methylation was done using diazomethane, which was prepared in situ from Diazald (Sigma-Aldrich, St. Louis, MO). About 10–50 ng of each compound was introduced into the gas chromatographic inlet to the mass spectrometer. The injection port and transfer line temperatures were 285 and 280 °C, respectively. In the EI mode, the electron energy was 70 eV. In the ECNI mode, the reagent gas was methane, the ion source pressure was ∼0.5 Torr, and the ion source temperature was either 150, 200, or, in a few cases, 250 °C. All spectra were scanned from m/z 50 to 800. For the nona- and decabrominated PBDEs, the EI and ECNI mass spectra were obtained on a Finnigan MAT-95XP (Thermo Fisher Scientific, Inc., Waltham, MA) high-resolution instrument operating at a resolving power of 5000. In the EI

* Corresponding author fax: 812-855-1076; e-mail: HitesR@ Indiana.edu. 10.1021/es072064g CCC: $40.75

Published on Web 02/29/2008

 2008 American Chemical Society

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mode, the electron energy was 70 eV, and in the ECNI mode, the ion source temperature was 190 °C. On this instrument, all spectra were scanned from m/z 200 to 1000. To obtain the EI spectra of the four nona- and decabrominated PBDEs over the full mass range of m/z 50–1000, ion abundances in the overlapping mass range of the high- and low-resolution spectra (m/z 200–800) were normalized to each other, and that factor was applied to renormalize the abundances in the low-resolution spectrum, thus “stitching together” one spectrum covering the full mass range. The ECNI spectra of these nona- and decabrominated compounds showed almost no ions above m/z 800 in their high-resolution spectra; therefore, the low-resolution ECNI spectra at 150 and 200 °C of these compounds are reported here for consistency. A word on nominal masses is necessary. Bromine has two isotopes, one at 78.9183 and the other at 80.9163, and the natural abundance of these two isotopes is 50.5 and 49.5%, respectively. Thus, decabromodiphenyl ether would have an exact mass of 949.1784, and its mass spectrum would show a cluster of ions separated by 2 mass units centered at 959.1684. By convention, we will refer to ions by the nominal mass of the lightest ion in the cluster; the calculation of nominal masses is based on the masses of all atoms rounded to the nearest integer. For example, in this case, the mass of the molecular ion of decabromodiphenyl ether is 950. The masses given in the figures are the nominal mass of the most abundant ion(s) in the cluster. Rather than present the mass spectra as peaks scaled relative to the most abundant ion (usually given the abundance of 100%), the spectra presented here are based on the percent of total ionization. For example, the bromide ions (at m/z 79 and 81) in the ECNI mass spectrum of 2,2′,4,4′tetrabromodiphenyl ether (BDE-47) represent ∼60% of all the ions observed in this spectrum. This scaling format is used to give the reader better information on ions to select for quantitation. The full mass spectra are given in the Supporting Information. Each page is a set of three spectra for one compound. The top panel is the EI spectrum and compound structure, the middle panel is the ECNI spectrum at an ion source temperature of 150 °C, and the bottom panel is the ECNI spectrum at an ion source temperature of 200 °C. Congeners of major environmental interest are included; the complete list of congeners covered is given in Table 1.

Results and Discussion EI and ECNI Mass Spectra of PBDEs. The complete EI mass spectra of four PBDE congeners are shown in Figure 1, and the complete spectra of all 18 congeners are shown in the Supporting Information. In all cases, the EI spectra are dominated by the molecular ions. With one exception, the abundances of the molecular ions vary from ∼33% of total ionization (for BDE-47) down to ∼14% (for BDE-209), but, in general, it is not possible to identify the substitution pattern of the bromines on the aromatic rings. The exception is BDE77, which has a particularly abundant molecular ion at 59%. Again, with one exception, the other very abundant ion in all of these spectra is due to the loss of two bromines atoms, (M-Br2)+, with abundances varying from ∼39% of total ionization (for BDE-71) down to ∼21% (for BDE-209). Again, the exception is BDE-77, the spectrum of which shows almost no (M-Br2)+ ion. One other ion is significant for the higher brominated PBDEs, and that is the doubly charged M-Br2+ ion. This ion is at m/z 281 in the spectrum of BDE-183 and at m/z 400 in the spectrum of BDE-209 (see Figure 1). It is interesting to note that this ion is more abundant than the doubly charged molecular ion itself. Figure 2 shows the abundances of M+, (M-Br2)+, and (M-Br2)++ for the 18 PBDE congeners in our data set. These data suggest that, for analyses using EI mass spectrometry, selected ion monitoring of the 2244

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TABLE 1. Names and Abbreviations of the Polybrominated Diphenyl Ethers and Methoxylated Polybrominated Diphenyl Ethers for Which Full Mass Spectra Are Presented in the Supporting Information name 2,4,4′-tribromodiphenyl ether 2,2′,4,4′-tetrabromodiphenyl ether 2,3′,4′,6-tetrabromodiphenyl ether 3,3′,4,4′-tetrabromodiphenyl ether 2,2′,3,4,4′-pentabromodiphenyl ether 2,2′,4,4′,5-pentabromodiphenyl ether 2,2′4,4′,6-pentabromodiphenyl ether 2,3′,4,4′,5-pentabromodiphenyl ether 2,2′,4,4′,5,5′-hexabromodiphenyl ether 2,2′,4,4′,5,6′-hexabromodiphenyl ether 2,2′,3,4,4′,5′,6-heptabromodiphenyl ether 2,3,3′,4,4′,5,6-heptabromodiphenyl ether 2,2′,3,3′,4,4′,6,6′-octabromodiphenyl ether 2,2′,3,3′,4,5′,6,6′-octabromodiphenyl ether 2,2′,3,3′,4,4′,5,5′,6-nonabromodiphenyl ether 2,2′,3,3′,4,4′,5,6,6′-nonabromodiphenyl ether 2,2′,3,3′,4,5,5′,6,6′-nonabromodiphenyl ether 2,2′,3,3′,4,4′,5,5′,6,6′-decabromodiphenyl ether 2′-methoxy-2,4,4′-tribromodiphenyl ether 2′-methoxy-2,3′,4,4′-tetrabromodiphenyl ether 2′-methoxy-2,3′,4,5′-tetrabromodiphenyl ether 6-methoxy-2,2′,4,4′-tetrabromodiphenyl ether 6′-methoxy-2,2′,4,4′,5-pentabromodiphenyl ether 5-methoxy-2,2′,4,4′-tetrabromodiphenyl ether 3-methoxy-2,2′,4,4′-tetrabromodiphenyl ether 5-methoxy-2,2′,4,4′,5-pentabromodiphenyl ether 4′-methoxy-2,2′,4-tribromodiphenyl ether 4′-methoxy-2,2′,4,5′-tetrabromodiphenyl ether 4-methoxy-2,2′,3,4′-tetrabromodiphenyl ether 4-methoxy-2,2′,3,4′,5-pentabromodiphenyl ether

congener no. or abbreviation 28 47 71 77 85 99 100 118 153 154 183 190 197 201 206 207 208 209 2′-MeO-BDE-28 2′-MeO-BDE-66 2′-MeO-BDE-68 6-MeO-BDE-47 6′-MeO-BDE-99 5-MeO-BDE-47 3-MeO-BDE-47 5′-MeO-BDE-99 4′-MeO-BDE-17 4′-MeO-BDE-49 4-MeO-BDE-42 4-MeO-BDE-90

(M-Br2)+ ion would be optimum for most of the PBDEs of environmental interest. The lack of an (M-Br2)+ ion in the spectrum of BDE-77 (see Figure 1) suggests that this ion is formed by the elimination, upon electron impact, of Br2 to form a dibenzofuran-like ion. BDE-47, for example, has two bromines ortho to the ether linkage and elimination of these substituents and the formation of a new bond would yield a dibromodibenzofuran ion. On the other hand, BDE-77 has no ortho bromine substituents, and this compound cannot form this stable cation. Isomers with just one ortho bromine substituent (such as BDE-28 and BDE-118) can also form this ion, presumably after a ring rearrangement. The structure of this ion is supported by the similarity of the spectrum of BDE-183 below m/z 570 to that of a pentabromodibenzofuran (9). As shown

FIGURE 1. Electron impact (EI) mass spectra of 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47), 3,3′,4,4′-tetrabromodiphenyl ether (BDE-77), 2,2′,3,4,4′,5′,6-heptabromodiphenyl ether (BDE-183), and 2,2′,3,3′,4,4′,5,5′,6,6′-decabromodiphenyl ether (BDE-209). The y-axis scale is the percent total ionization. Each ion cluster is labeled with the mass of the most abundant ion in that cluster, or if two ions are of similar abundance, both are labeled (as in “562/4” which means m/z 562 and 564). The percentages given near each ion cluster are the fraction of total ionization represented by that entire cluster. The structural source of each ion cluster is given in parentheses, as in “(M-Br2)” for the loss of two bromine atoms from the molecular ion or “(M-Br2)/2” for the doubly charged (M-Br2) ion.

FIGURE 2. Abundances of the molecular ions and the singly and doubly charged (M-Br2) ions in the electron impact mass spectra of 18 polybrominated diphenyl ether congeners. Each set of bars is labeled with the congener number and, in parentheses, the total number of bromines and the number of ortho bromines. in Figure 3, with the exception of the molecular ion, these two spectra are virtually identicalsnote especially the doubly charged ion at m/z 281. A few more or less complete spectra of PBDEs have appeared in the literature. Cooper et al. (10) published the EI spectrum of 2,4′,6-tribromodiphenyl ether (BDE-32), pointed out the abundant (M-Br2)+ ion, and suggested a dibenzofuran-like structure for this ion (see above). Cooper

et al. (10) also published the EI mass spectrum of 2-bromodiphenyl ether (BDE-1) and a truncated spectrum of 2,3,3′,4,5,5′,6-heptabromodiphenyl ether (BDE-192; in the later case, the compound was named incorrectly). Larrazabal et al. (11) published the EI spectra of 2,2′,4-tribromodiphenyl ether (BDE-17) and 3,3′,4,4′,5-pentabromodiphenyl ether (BDE-126) and noted the relatively low abundance of the (M-Br2)+ ion in the spectrum of the later compound, which VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Electron impact mass spectra of BDE-183 (from Figure 1) and of a pentabromodibenzofuran (from ref 9). See Figure 1 caption for details on labeling. The bromine substitution pattern in the dibenzofuran is speculative. localized on the oxygen atom, and thus these ions are brominated phenoxide ions. For example, the ECNI spectrum of BDE-209 (see Figure 4, bottom) shows an abundant phenoxide ion at m/z 483.

lacks bromine atoms ortho to the ether linkage. Only the high mass portions of the EI mass spectra of 2,2′,4,4′tetrabromodiphenyl ether (BDE-47), 2,2′,4,4′,5-pentabromodiphenyl ether (BDE-99), 2,2′,4,4′,5,5′-hexabromodiphenyl ether (BDE-153), 2,2′,3,4,4′,5′,6-heptabromodiphenyl ether (BDE-183), 2,2′,3,3′,4,4′,5,6′-octabromodiphenyl ether (BDE196), and 2,2′,3,3′,4,4′,5,5′,6-nonabromodiphenyl ether (BDE206) were published by Bezares-Cruz et al. (12), but none of these spectra were interpreted in any way. Eljarrat et al. (13, 14) published the EI spectrum of 2,2′,4,4′,5-pentabromodiphenyl ether (BDE-99). As expected, the EI spectrum of this compound showed an abundant (M-Br2)+ ion. Ackerman et al. (15) reported the full EI mass spectrum of 2,2′,3,4,4′,5,6heptabromodiphenyl ether (BDE-181), although the intensities of the ions below m/z 500 may be too high. Partial EI mass spectra have also been published: Marsh et al. (16) reported on the two major ions in the EI mass spectra of 32 PBDE congeners that they had synthesized; Teclechiel et al. (17) reported on the major ions in the EI spectra of six octabrominated diphenyl ethers that they had synthesized; and Christiansson et al. (18) reported on the major ions in the EI spectra of the three nonabrominated diphenyl ethers that they had synthesized. The ECNI mass spectra of PBDEs are dominated by the bromide ions at m/z 79 and 81 and to a lesser extent by the HBr2- ions at m/z 159, 161, and 163. Figure 4 shows some of these spectra taken at an ion source temperature of 150 °C; the Supporting Information shows the full ECNI spectra of all 18 PBDEs at ion source temperatures of 150 and 200 °C. As with EI spectra, with a few exceptions, it is generally not possible to determine the substitution patterns of the bromines on the rings from the ECNI spectra. The dominance of Br- and HBr2- is less for the more highly brominated congeners, ranging from ∼98% of the total ionization for BDE-77 to ∼14% for BDE-209. For the octa- to decabrominated PBDEs, an abundant ion due to cleavage of the C-O bond is observed. In these ions, the negative charge is likely 2246

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Our high-resolution mass spectrum of this compound confirms that the exact mass of this ion is 482.5829, which corresponds to an elemental composition of C6Br5O with an error of 0.0038 mass units. The relatively high abundance of the tetra- and pentabromophenoxide ions suggests that they are more stable under ECNI conditions than bromide. Tribromophenoxide ions are present in some of the hexaand heptabrominated PBDEs, but they are small compared to the bromide ions. The abundances of Br-, HBr2-, and of the phenoxide ions at 150 °C are summarized in Figure 5, which shows that the abundances of these ions are dependent on the total number of bromines and on the number of bromines ortho to the ether linkage. For example, BDE-77, which has no ortho bromines, only has a very abundant Br- ion. We have modeled these data with a simple regression of the form INT ) a0 + a1(no. of Br) + a2(no. of ortho Br) where INT is the Br- ion intensity (in percent of total ionization), no. of Br is the total number of bromine atoms in the molecule, no. of ortho Br is the number of ortho bromines, and ai are the fitted parameters. For the data shown in Figure 5, the fitted values are as follows: a0 ) 105.53, a1 ) -4.72, and a2 ) -12.10. The data and the fitted result are shown in Figure S1; the overall agreement is good (r2 ) 0.815). This equation may be useful in roughly predicting the bromide ion abundance for congeners not in our data set. The ions due to HBr2- at m/z 159–163 are small for those compounds with zero ortho bromine atoms and for six or more total bromines; generally, these ions are not analytically useful for either quantitation (m/z 79 and 81 are preferred) or for identification of those PBDEs with up to seven bromines. The phenoxide ions are analytically useful for those PBDEs with eight or more bromines; indeed, these ions are preferred for selected ion monitoring quantitation of the octa-

FIGURE 4. Electron capture negative ionization (ECNI) mass spectra of 2,2′,4,4′- tetrabromodiphenyl ether (BDE-47), 3,3′,4,4′-tetrabromodiphenyl ether (BDE-77), 2,2′,3,4,4′,5′,6- heptabromodiphenyl ether (BDE-183), and 2,2′,3,3′,4,4′,5,5′,6,6′-decabromodiphenyl ether (BDE-209). These data were all obtained at an ion source temperature of 150 °C. See the Figure 1 caption for details on labeling.

FIGURE 5. Abundances of the bromide ions (sum of m/z 79 + 81), the HBr2- ions (sum of m/z 159–161), and the phenoxide ions in the electron capture negative ionization mass spectra of 18 polybrominated diphenyl ether congeners. The data were obtained at an ion source temperature of 150 °C. Each set of bars is labeled with the congener number and, in parentheses, the total number of bromines and the number of ortho bromines. through the decabrominated congeners. The exception is BDE-206, see below. Other minor ions are present in the ECNI mass spectra of PBDEs. These include (M-Br)-, (M-HBr)-, (M-Br2)-, and (M-HBr2)-. None of these ions are of sufficient abundance or uniqueness to have much analytical value. The exception is BDE-206, which shows a plethora of these ions in addition to ions due to (M-Br3)- and (M-Br4)-. These latter two ions are of relatively low abundance in the spectra of the other

nonabrominated isomers, and thus, they may have some analytical utility. ECNI mass spectra are often strongly influenced by the ion source temperature (19). The ECNI spectra of PBDEs are no exception to this generalization, although the influence of ion source temperature is, in this case, subtle. Figure 6 shows the abundance for the bromide ions (the sum of the percent abundances of m/z 79 and 81) at three different ion source temperatures. Notice that in all cases the abundance VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Abundances of the bromide ions (sum m/z 79 + 81) at three ion source temperatures in the ECNI mass spectra of eight polybrominated diphenyl ether congeners. Each set of bars is labeled with the congener number and, in parentheses, the total number of bromines and the number of ortho bromines. of Br- increases as the ion source temperature increases; in some cases (for example, BDE-183), this increased abundance is substantial. This observation has analytical implications. If one were using selected ion monitoring of Br- to quantitate these compounds, one would obtain higher sensitivities at a higher ion source temperature, such as 250 °C, rather than at 150 °C. A few ECNI mass spectra of PBDEs have appeared in the literature. Eljarrat et al. (13, 14) published the ECNI spectrum of BDE-99, and as expected, it showed very abundant ions due to Br- and HBr2-. They also published the ECNI mass spectrum of 2,2′,4,4′,5,6′-hexabromodiphenyl ether (BDE154), which showed abundant Br- ions and weak ions due to M-HxBry-, where x ) 0 or 1 and y ) 1, 2, or 3 (13, 14). Incidentally, the ion labeled “331” in this spectrum was misinterpreted as (M-HBr4)-; it is more than likely due to a tribromophenoxide structure. Ackerman et al. (15) reported the ECNI mass spectrum of BDE-181, but the abundance of the bromide ion appears to be too small. Bjorklund et al. (20) reported the ECNI spectra of 2,2′,4,4′,6-pentabromodiphenyl ether (BDE-100), and as expected, it showed abundant ions due to Br- and HBr2-. These authors reported the ECNI spectrum of decabromodiphenyl ether (BDE-209), and it showed an abundant ion due to the formation of a pentabromophenoxide structure (see above discussion). This particular spectrum identifies the mass of this ion to be 482.2, which is 0.4 amu too low and which gives an incorrect nominal mass that is 1 amu too low. In addition to tabular data (presented without ion abundances), La Guardia et al. (21) showed the ECNI mass spectra of 2,2′,3,3′,4,4′,6-heptabromodiphenyl ether (BDE171), 2,3,3′,4,4′,5,6-heptabromodiphenyl ether (BDE-190), 2,2′,3,3′,4,4′,6,6′-octabromodiphenyl ether (BDE-197), and 2,2′,3,4,4′,5,6,6′-octabromodiphenyl ether (BDE-204). In three of these spectra, the authors attribute several ions to phenoxide-like structures with three, four, or five bromines. While these assignments are generally correct, the masses given in the figures of this paper are one mass too low. Nevertheless, La Guardia et al. correctly suggest that symmetrically substituted octabrominated diphenyl ethers could be distinguished from asymmetrically substituted PBDEs using these phenoxide ions. For example, 2,2′,3,3′,4,4′,6,6′octabromodiphenyl ether (BDE-197) would give only a tetrabrominated phenoxide ion at m/z 405, but 2,2′,3,4,4′,5,6,6′octabromodiphenyl ether (BDE-204) would give both tri- and pentabrominated phenoxide ions at m/z 327 and 483. Incidentally, other studies of PBDEs using alternate ionization methods or tandem mass spectrometry (MS/MS) have been published. Riu et al. (22) and Cariou et al. (23) have studied the atmospheric pressure photoionization mass spectra of these compounds using MS/MS. Pirard et al. (24) 2248

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developed a quantitative method based on EI and MS/MS, and Ikonomou and Rayne (25) developed a method based on metastable atom bombardment mass spectrometry. None of these alternate methods have been widely adopted for the analysis of PBDEs. EI and ECNI Mass Spectra of Methoxy-PBDEs. PBDEs are metabolized in most animals by hydroxylation (6, 7); in addition, hydroxylated and methoxylated PBDEs have been identified as marine natural products (26–29). In general, the hydroxylated compounds (phenols) give asymmetric peak shapes on most gas chromatographic columns. Methylated phenols are well-transmitted, however, and as a result, the analysis of hydroxylated metabolites first involves their derivatization to form methoxy-PBDEs. Thus, it is important to systematically look at the mass spectra of these methoxy compounds. The complete EI mass spectra of three methoxyPBDE congeners (one each with o-, m-, and p-methoxy substitutions) are shown in Figure 7, and the complete spectra of all 12 congeners we have studied are shown in the Supporting Information. In all cases, these spectra are dominated by the molecular ions, but the abundant (M-Br2)+ ions observed in the spectra of the PBDEs are present only in the meta-substituted congeners. In fact, there are considerable differences in these spectra that allow one to differentiate the substitution position of the methoxy group relative to the aromatic ether linkage. This ortho-meta–para differentiation has been previously discussed by Athanasiadou et al. (30), who reported the EI, ECNI, and positive chemical ionization mass spectra of many of the same compounds presented here but did not speculate on the formation mechanism of these ions. In the EI spectra of the ortho-substituted congeners, ions due to the loss of CH3Br from the molecular ion are prominent (see Figure 7, top). It seems likely that the electron impact reaction produces a stable brominated dibenzo-p-dioxin ion.

This reaction is possible only when both the methoxy group and a bromine atom are ortho to the aromatic ether linkage. This (M-CH3Br)+ ion has been observed before in the spectra of ortho-substituted methoxy polychlorinated and polybrominated biphenyls and polychlorinated diphenyl ethers (31–33), and structures for these ions have been suggested by these authors. This possible electron impact reaction is supported by the observation that the spectrum

FIGURE 7. Electron impact (EI) mass spectra of 2′-methoxy-2,3′,4,4′-tetrabromodiphenyl ether (2′-MeO-BDE-66), 3-methoxy-2,2′,4,4′-tetrabromodiphenyl ether (3-MeO-BDE-47), and 4-methoxy-2,2′,3,4′-tetrabromodiphenyl ether (4-MeO-BDE-42). See Figure 1 caption for details on labeling. of 2′-MeO-BDE-66 below m/z 430 is similar to the spectrum of a tribrominated dibenzo-p-dioxin (not shown) (26). The (M-Br2)+ ion reported by Marsh et al. (34) for 2′-MeO-BDE68 is much bigger than we observe, but our spectra of 6-MeOBDE-47 agree. In ortho-substituted congeners with five or six bromines, the (M-Br2)+ ion tends to dominate over the M-CH3Br ion (27). In the EI spectra of the meta-substituted congeners, ions due to the loss of Br2 from the molecular ion and due to the loss of methyl from the (M-Br2)+ ion are prominent (see Figure 7, middle). It seems likely that the electron impact reaction first produces a stable dibenzofuran ion, which subsequently loses the methyl moiety to form an extended quinone-like ion.

The spectra of meta-substituted MeO-PBDEs are somewhat different than those of MeO-PBBs and MeO-PCBs, both of which give abundant (M-CH3CO) ions (31, 35). In the EI spectra of the para-substituted congeners, ions due to the loss of methyl from the molecular ion are consistently present, and there are few other fragment ions (see Figure 7, bottom). The electron impact reaction in the ion source may well produce a stable extended quinone structure with the charge localized on the central oxygen atom.

PCBs, PBBs, and halogenated diphenyl ethers methoxylated in the para position give M-CH3 ions of similar abundance, and this extended quinone structure for the resulting fragment ion was suggested by Jansson and Sundstrom (31) and Tulp et al. (32, 35). Like the ECNI spectra of PBDEs, the ECNI mass spectra of MeO-PBDEs are also dominated by the bromide ion at m/z 79 and 81 and by HBr2- at m/z 159, 161, and 163. Figure 8 shows three of these spectra for the same compounds shown in Figure 7; these data were taken at an ion source temperature of 150 °C; the Supporting Information shows the full ECNI spectra of all 12 MeO-PBDEs at ion source temperatures of 150 and 200 °C. Unlike the EI spectra of these compounds, with one exception, it is generally not possible to determine the substitution pattern of the methoxy group from their ECNI spectra. The abundances of the Br- and HBr2- ions at 150 and 200 °C are summarized in Figure 9, which shows that most of the abundances of the HBr2- ions are somewhat higher for the meta- and para-substituted congeners as compared to the ortho-substituted congeners. Nevertheless, these data cannot be used to differentiate ring substitution patterns. Higher ion source temperatures tend to favor higher abundances of Br-, but this difference is small. Selected ion monitoring quantitation for these compounds should focus on m/z 79 and 81. In addition to these major ions, the ECNI spectra of these compounds have small ions due to (M-Br)-, (M-HBr)-, (M-Br2)-, (M-HBr2)-, and (M-H2Br2)-. Unlike the higher brominated PBDEs, the phenoxide ions are not predominant for the MeO-PBDEs; one exception is 6′-MeO-BDE-99, where VOL. 42, NO. 7, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 8. Electron capture negative ionization (ECNI) mass spectra of 2′-methoxy-2,3′,4,4′-tetrabromodiphenyl ether (2′-MeO-BDE-66), 3-methoxy-2,2′,4,4′-tetrabromodiphenyl ether (3-MeO-BDE-47), and 4-methoxy-2,2′,3,4′-tetrabromodiphenyl ether (4-MeO-BDE-42). These data were all obtained at an ion source temperature of 150 °C. See Figure 1 caption for details on labeling.

FIGURE 9. Percent of total ionization due to the bromide ions (Br-; at m/z 79 + 81) and of the HBr2- ions (sum of m/z 159–161) at two ion source temperatures in the ECNI mass spectra of 12 methoxylated polybrominated diphenyl ether congeners. Each set of bars is labeled with the congener number and, in parentheses, the total number of bromines and the position of the methoxy group. the tribromophenoxide ion appears at m/z 329/31 (see the spectrum in the Supporting Information). The ortho-substituted MeO-PBDEs have a small pair of ions of varying abundance at m/z 186/8 and 266 (see Figure 8, top). Given that these ions are only observed in congeners with both an o-methoxy group and an o-bromine atom, it seems likely that the mechanism for their formation involves first the elimination of CH3Br to form a brominated dibenzop-dioxin negative ion, which can then cleave on either side of the dioxin to give two o-quinone ions with one or two bromines depending on the exact bromine substitution pattern. The following scheme shows how this might work for, in this example, 2′-MeO-BDE-66. Note that this congener would give both m/z 186/8 and 266, but a related congener, 2′-MeO-BDE-28, would give only m/z 186. By the same 2250

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analogy, 6′-MeO-BDE-99 would give only m/z 266. The measured exact masses of these two ions (185.9319 and 265.8410) agree with elemental compositions of C6H3BrO2 and C6H2Br2O2 with errors of 0.0002 and 0.0008 mass units, respectively. The full mass spectra of some MeO-BDEs with unknown substitution patterns have been published in an attempt to identify some of these compounds as natural products (28, 29, 36). Two full EI and ECNI mass spectra, one of 6-methoxy-2,2′,3,4,4′-pentabromodiphenyl ether (6-MeOBDE-85) and one of 6-methoxy-2,2′,3,4,4′,5-hexabromodiphenyl ether (6-MeO-BDE-137), have been reported by Malmvaern et al. (27). The EI spectra show abundant ions due to (M-CH3Br)+ and (M-Br2)+; and the ECNI spectra show abundant ions due to Br- and HBr2-, and weak ions

due to (M-HBr)-, and (M-Br2)-. Tabular EI spectra, including the six or so major ions, of 16 MeO-BDE congeners have been published as part of their synthetic confirmation (8). Research Needs. While the mass spectra of the most environmentally significant PBDE congeners have been presented here and in the literature, it would be very useful to have a full library of the EI and ECNI mass spectra of all 209 PBDE congeners, most of which are commercially available. A full library of the mass spectra of all the possible methoxy-PBDE congeners is more problematic given that there are several hundred such congeners and that only a few are commercially (or otherwise) available. Nevertheless, a complete reference collection of these mass spectra would be of enormous value for identifying presently unknown congeners and for optimizing their quantitation. This reference collection should also include each compound’s gas chromatographic retention characteristics, preferably given as methylene retention indexes (37).

Acknowledgments Thanks to Marta Venier and Xinghua Qiu for data acquisition and to Göran Marsh for the gift of the hydroxylated PBDEs.

Supporting Information Available Full mass spectra of all cogeners studied. This material is available free of charge via the Internet at http://pubs.acs. org.

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