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Abstract:

Fragmentation of the parent ion/TCE-sulfate moiety
The mass spectrum of compound 5 displays its molecular ion 5a with the typical isotope pattern [18] of a pentachlorinated compound at m/z 448 (relative abundance, 15%) (Figure 2). Scheme 1 illustrates the various fragmentation pathways of this precursor ion. The molecular ion 5a loses a PCB sulfate (ArOSO3-) group, thus yielding TCE group-derived fragment ions. These ions, [CH2CCl3]+ and [CH = CCl2]+, are observed at m/z 131 (6%) and 95 (7%), respectively.
Scheme 1
Principal EI-MS fragmentation pathways of sulfuric acid 2',5'-dichlorobiphenyl-4yl ester 2,2,2-trichloroethyl ester 5.
More complex fragmentation patterns are observed for the fragmentation of the -OSO3-TCE group. The abundant fragment ion 5g (m/z 237, 28%) can be formed by two fragmentation pathways from precursor ion 5a. One pathway involves the release of HCl and Cl from precursor ion 5a, leading to an unstable cyclic fragment ion 5b (m/z 377, 5%). In turn, fragment ion 5b produces the fragment ion 5g after releasing chloroethyne and SO3. The other pathway resulting in the formation of fragment ion 5g involves the removal of two neutral molecules, HCHO and SO2, from the daughter fragment ion 5c (m/z 331, 5%), which is formed by releasing the free radical group CCl3 from fragment ion 5a.
Alternatively, the fragment ion 5c can also produce the daughter fragment ion 5e (m/z 183, 12%) by losing one molecule of SO3. In addition, precursor ion 5a yields the daughter ion 5d by losing a CHCCl3 fragment. The basic fragment ion 5f at m/z 238 is produced from the daughter ion 5d (m/z 318, 20%) by releasing one molecule of SO3. Fragment ions 5f and 5g can produce the important fragment ion 5h at m/z 209 with a relative abundance of 26% by losing a CHO or CO fragment, respectively.
The same fragmentation pathways are observed for the TCE PCB sulfates diesters 1 and 3-10. The only exception is compound 2, which has the TCE sulfate group in ortho position to the other phenyl ring. The mass spectrum of compounds 2 is distinctively different from the mass spectrum of its isomer, compound 3 (Figures 3 and 4). As shown in Scheme 2, the basic fragment ion of compound 2 is the stable dibenzofuran ion 2f. Fragment ion 2f is formed from the parent ion 2a (m/z 414; relative abundance, 21%) by sequential loss of CHCCl3 (2d, m/z 284; relative abundance, 10%), SO3 (2e, m/z 204; relative abundance, 28%) and HCl (2f, m/z 168; relative abundance, 100%). The stable dibenzofuran cation 2f is also formed as the base peak ion in the fragmentation pathways of corresponding methoxylated PCB 11 (Table 1). Similarly, other 2-methoxy PCB derivatives also form a stable dibenzofuran cation after losing chlorine atom and methyl group [19].
Scheme 2
Principal EI-MS fragmentation pathways of sulfuric acid 4'-dichlorobiphenyl-2-yl ester 2,2,2-trichloroethyl ester 2.
The relative abundance of several fragment ions depends on the position of the -OSO3-TCE group and the chlorine substitution pattern. The relative abundance of the [M-HCl2]+ and [M-CCl3]+ fragment ions of compound 2, 9 and 10 are lower compared to all other TCE PCB sulfate diesters (Table 1). This lower relative abundance is due to steric and/or electronic effects resulting from the ortho phenyl substituent in compound 2 or the two ortho chlorine substituents in compounds 9 and 10. In addition, the relative abundance of fragment ion [ArOSO3H]+ of the TCE PCB sulfate diesters 9 and 10 is low compared to the corresponding fragment ion of the other TCE PCB sulfate diesters, with only a trace of the respective ions being observed (Table 1). For example, the relative abundance of [ArOSO3H]+ 5d is 20%, whereas the abundance of the corresponding fragment ions of compounds 9 and 10 is <<1%. Since compounds 9 and 10 have two chlorine substituents in the phenyl ring with the -OSO3TCE group, the low relative abundance of fragment ion [ArOSO3H]+ suggests that the sulfate group of both compounds is less stable, possibly because of the comparatively high pKa value of the Ar-OH group [20]. This observation is in agreement with the decreasing chemical stability of aryl sulfate monoesters with increasing acidity of the phenolic aryl group [21].
{"AnatomicalEntity": []}