1183
Anal. Chem. 1981, 53, 1183-1186
Chemical Ionization Mass Spectrometry of Specific Polychlorinated Biphenyl Isomers A. G. Harrison" Department of Chemlstry, lhiversity of Toronto, Toronto, Ontario, Canada M5S 1A I
F. I. Onuska National Water Research Institute, Burlington, Chtario, Canada L 7R 4A6
C. W. Tsang Department of Applied Science, Hong Kong Polpechnic, Hung Hom, Hong Kong
The H2 and CH4 chemical ionlratlon (CI) mass spectra of selected polychlorinated biphenyl isomers (PCBs) have been studled In detall. The H2 CI mass spectra ,818 shown to provlde more extenslve fragmentatlon whlclh permlts limited differentlation among lsoniers uslng the reletlve Intensities of the [MH' - HCI], [MH' - HCI H2], and [MH' 2HCI H2] fragment Ions. The lattelr two Ions are shown to arlse by reaction of the [MH' HCI] fragment Ion with H2.
+
-
+
-
Polychlorinated biphenyls (PCBs) are among the most widespread and abundant pollutants in the global ecosystem ( I ) . Due to their persistent nature and widespread use, PCBs have been found to bioaccumulate in many plants and animals. Consequently, there has been considerable interest in their analysis by mass spectrometry. Although the complex mixtures found in natural samples can be fieparated by gas chromatography (particularly capillary column gas chromatography) and the components identified by total chlorine content by electron impact mass spectrometry (2, 3), the identification of discrete isomers has not been possible since their electron impact mass spectra usually are very similar (2-9). The exceptions to this rule are those isomers having three ortho chlorines, whilch show characteristically more intense (M - C1)+fragment ion peaks. Despite the similarities in their mass spectra, some distinction among isomers has been achieved through studies of metastable ion abundances (10, 11). Oswald and colleagues (2,12) have reported the methane chemical ionization (CI) mass spectra of selected PCB isomers. In general, the CI mass spectra showed less fragmentation than the electron impact (EI) mass spectra and they concluded that CI was less useful than EI in the identification and characterization of PCB isomern. The negative ion (methane) CI mass spectra of PCBs haw been studied by Dougherty et al. (13) but not to the extent, of examining positional isomers. Previous studies in our laboratory (14-1 7) have shown that there are significant differences in the CH4 and H2 CI mass spectra of substituted chllorobenzene and dichlorobenzene isomers. In brief, the protonated moleculw ion MH+ shows loss of HC1 followed by reaction with CHI or H2 with the extent both of fragmentartion and of further reaction depending strongly on the nature and orientation of the substituent. The present work explores in detail the potential of these reagent gases in differentiating among isomeric PCBs by chemical ionization mass spectrometry. EXPERIMENTAL SECTION The chemical ionization mass spectra were obtained by using a DuPont 21-490 mass spectrometer equipped with a high-pressure chemical ionization source. The source temlperature was approximately 150 "C and the ionizing electron (anergywas 70 eV with the repellers held at cage potential. Samples were introduced
into the source by means of a direct insertion probe. Source pressures were in the 0.3-0.5 torr range as estimated by a calibrated thermocouple gauge probe inserted in place of the direct insertion probe. The approximately linear relation between the thermocouple gauge reading and the pressure behind the reagent gas inlet leak was used to estimate source pressures when the solids probe was used. Individual PCB isomers were obtained from RFR Corp. or kindly donated by S. Safe. Some tetra and more highly chlorinated isomers were found by CHI CI to contain traces of lower chlorinated homologues; these did not seriously affect the interpretation of the present results. Reagent grade CHI and Hz (Matheson and Co.) were used without purification. The Dz (Matheson and Co.) was passed through a heated palladium thimble prior to use, to remove a low-level impurity of mass 28, probably Nz.
RESULTS AND DISCUSSION
H2CI Mass Spectra. The H2 CI mass spectra of the 29 PCB isomers investigated are recorded in Tables I and 11. The mass spectra have been corrected for the natural 13C abundance and the intensities given for each ionic species represent the sums of the relevant chlorine isotopic peaks. For all but the monochloro compounds, the chlorine isotope peaks result in a partial overlap of the [MH+ - HCl] and [MH+ - HC1+ H,] ion signals; the relative contributions were obtained by assuming the appropriate chlorine isotope distribution. For all isomers, significant M+. ion signals are observed arising from charge exchange with H3+;this reaction is observed with most aromatic compounds (14-18). The H2 CI mass spectra of all three monochlorobiphenyls show MH+ as the base peak; however, there are significant differences in the fragment ion intensities. For 2-chlorobiphenyl t h e dominant fragment ion corresponds to [MH+ HCl] (m/z 153). For 3-chlorobiphenyl the m/z 153 intensity is low with the major fragment ion being found at m / z 155 corresponding to [MH+ - HCl + H,]. In the case of 4chlorobiphenyl the intensities of the ions corresponding to [MH+- HCl] and [MH+ - HC1+ H21 are approximately equal. All three isomers show weak ion signals corresponding to loss of C1 from MH+. The m/z 155 [MH+- HCl + H2l could arise either by direct reaction of H3+with the chlorobiphenyl, reaction 1,in competition with simple proton transfer, reaction 2, or, alternatively, by the two-step process reaction 3 plus 4 involving HCl elimination from the MH+ ion formed in eq 2 followed by addition of the substituted phenyl cation to H2. In the D2 CI of 4-chlorobiphenyl the mlz 155 ion is shifted in part to H3+ + C&,C6H4C1+ C,jH&HgH+ HC1 (1)
+
H3+ 4- C&C6H&1----* C G H ~ C ~ H ~ C4- ~HH 2 ' (2)
C6H&&Cl*H+
+
c~H,&jH4+ Hz
0003-2700/81/0353-1183$01.25/0@ 1981 American Chemlcal Society
-
C&,C&+ C6H&&H+
HCI
(3) (4)
ANALYTICAL CHEMISTRY, VOL. 53, NO. 8, JULY 1981
1184
60
I
I
I
I
I
11
I
Table I. H, Chemical Ionization Spectra of Monochlorobiphenyls, Dichlorobiphenyls, and Trichlorobiphenylsa
1
i
I
MHL/ compound
MH+
234-
100 100 100
2,2'3,3'4,4'2,53,4-
100 100 100 100 100
2,5,2'2,5,3'2,5,4'2,3,42,4,52,4,6-
100 100 100 100 100 100
[MH' M+. - Cl]
[MH' [MH+ [MH+HC1 + 2HC1 HCl] H,I + H,I
Monochloro biphenyls 21 22 35
3.0 5.9 0.5
30 2.8 24
1.5 31 21
Dichlorobipheny 1s 13 25 31 23 21
2.7 3.8 4.2 9.3 5.0
40 25 9.8 13 18
1.8 44 24 9.0 8.9
1.2 17
0.1 31 30 1.9 5.3 4.9
2.2 12
8.0 10 5.8
Trichlorobiphenyls
01
,
i
I
40
I
I
I
1
1
I60
I20
80
I Io
200
y PRESSURE ( a r b i t r a r y s c a l e ) Figure 1. Varlation of Ion signals as function of H2 pressure, 3chlorobiphenyl. I
I
I
I
I/ 2
11
i' 0
A
h
I
- ChlotObIphenyl A
I
I
I
1
I
400
800
1200
1600
2000
.
Flgure 2. CeH5CsH5.H+/MH+ ratio as function of H2 pressure.
m / z 157 [MD+ - DC1+ D2] and in part to m / z 158 [MD+ HC1 D2]. The observation of the latter product indicates that the added proton is not always lost with the neutral hydrogen chloride but that there is some involvement of the aromatic ring hydrogens; this has been observed previously (14) for chlorobenzene. The direct reaction (eq 1) in competition with reaction 2 should lead to a C6H5C6H5.H+/MH+ ratio essentially independent of pressure; by contrast the occurrence of the sequence eq 2 to 4 leads to the prediction that the C6H5C6H5.H+/MH+ratio should increase with increasing H2 pressure, at least at relatively low H2 pressures, and that the C6H5C6H5.H+/C6H5C6H4+ ratio should also be a function of Hz pressure. Figure 1 shows a plot of the normalized additive ion intensities in the 3-chlorobiphenyl system as a function of H2 pressure at constant sample pressure. Quantitative interpretation of the results are difficult because of the contribution from electron impact ionization of the chlorobiphenyl at low Hz pressures; however, the results clearly show a linear dependence bf the C6H5C6H5"+/ C6H5C,&+ ration on H2 pressure as predicted by the sequence, reactions 2 to 4. Figure 2 shows a plot of the C6H5C6H5.H+/MH+ intensity ratio as a function of H2 pressure for the three monochlorobiphenyls. The initial approximately linear de-
+
a
17 18 17 12 15 15
3.2 5.1 2.7 7.0 9.0 9.2
15 15 9.9 1.4 8.5 4.3
8.4
0.3 1.8 3.8
Intensities as % of base peak, MH'.
pendence of the ratio on reagent gas pressure and the extrapolation of the ratio to approximately zero at zero H2 pressure also support the sequential mechanism. This is in agreement with earlier studies (15) of the formation of [MH+ - HX + H,] in the Hz CI of substituted halobenzenes. The results in Table I, as well as the pressure variation studies, show that the 3-phenylphenyl cation is more reactive toward H2than the 4-phenylphenyl cation, while the 2-phenylphenyl cation is essentially nonreactive toward H2. Analytically, the occurrence of the two-step reaction sequence means that the relative fragment ion intensities in the H2 CI mass spectra are dependent on the reagent gas pressure and ion source residence time and only spectra obtained under very similar operating conditions should be compared. However, our results indicate that the three isomers are clearly distinguishable over a relatively wide range of H2 pressures. The Hz CI mass spectra of the five dichlorobiphenyl isomers examined, Table I, also show MH+ as the base peak with fragment ions corresponding to [MH+ - Cl], in minor abundance, [MH+ - HCl], and [MH+ - HC1 H,]. Pressure variation studies for the 4,4'-dichlorobiphenyl system showed that the latter ion originated by the two-step mechanism analogous to reactions 3 and 4. An additional fragmentation product observed for the dichloro and more highly chlorinated compounds corresponds to [MH+ - 2HC1+ Hz] and arises by elimination of HCl from the [MH+ - HC1+ H2] adduct ion as has been observed previously for dichlorobenzenes (16). The 2,2'-dichlorobiphenyl isomer is similar to 2-chlorobiphenyl in that the [MH+ - HCl] fragment ion shows only a very low reactivity toward Hz. For the remaining isomers, the [MH+ - HCl] ion is reactive toward H2with appreciable yields of [MH+ - HC1+ H,] and [MH+- 2HC1+ Hz] being observed in all cases. The total extent of fragmentation of the 3,3' isomer is considerably larger than for the other isomers. The two isomers examined which contain the two chlorines on the same phenyl ring exhibit an enhanced tendency for the [MH' - HC1+ H2] ion to fragment further by loss of HC1; thus, the [MH+ - 2HC1 H2]/[MH+ - HC1 H2] ratio is 0.3-0.4 for the 3,3' and 44' isomers but is 0.7-1.1 for the 3,4 and 2,5 isomers. This difference can be rationalized in terms of the likely structures a and b (Figure 3) of the [MH' - HC1+ H2] ions for the two types of compounds. For the symmetrical isomers, a, further loss of HC1 requires transfer of a proton
+
+
+
ANALYTICAL CHEMISTRY, VOL. 53, NO. 8, JULY 1981
C'I
a -
Figure 3. Structures of [MH+ - HCI
Table 11. H, Chemical Ionization Spectra of Tetrachlorobiphenyls, Pentachlorobiphenyls, and Hexachlorobiphenylsa
-b
+ H2] ions. compound
from one phenyl ring to the other, while for the asymmetrical isomers, b, only migration within one phenyl ring is required and might be expected to lead to a more facile elimination of HC1. It appears from the present results that it should be possible to identify the 2,2' isomer from its distinctive spectrum and to distinguish between those isomers with two chlorines on the same ring and those with a chlorine on each phenyl ring, although individual isomer identification may not be possible. The H2 CI mass spectra of six trichlorobiphenyl isomers are summarized in Table I. The CI mass spectra of isomers with the three chlorines on the same ring, e.g., 2,3,4-, 2,4,5-, and 2,4,6-trichlorobiphenyl,are relatively simple with only low abundance [MH+ - HCl], [MH+ - HC1+ H,], and [MH' - 2HC1+ H,] fragment ions being observed. As a result the H2 CI mass spectra of these isomers are practically indistinguishable, although the particularly low extent of fragmentation for the 2,3,4 isomer may be characteristic. By contrast the CI mass spectra of 2,5,3'- and 2,5,4'-trichlorobiphenyl, with chlorines substituted on both rings, show markedly more abundant fragment ion peaks, particularly those corresponding to [MH+- HC1+ H,] and [MH+ - 2HC1 H,]. Pressure variation studies showed that the former originated by the two-step sequence analogous to reactions 3 and 4. 2,5,2'-Trichlorobiphenylis a special case in that it shows a relatively abundant [MH+ - HCl] fragment ion but virtually no products resulting from reaction of this ion with H,. This lack of reactivity is analogous to the lack of reactivity of the similar fragment ion from 2-chlorobiphenyl and 2,2'dichlorobiphenyl. The results suggest that it should be possible to differentiate between trichlorobiphenyl isomers substituted on one ring and those substituted on both rings by Hz CI mass spectrometry. Identification down to the individual isomer level probably is not possible except for a few special cases. In the H2 CI of tetrachlorobiphenyls (Table 11) the stability of the protonated molecule is further enhanced by chlorine substitution; in most cases MH+ accounts for >70% of the total additive ionization. The [MH+ - HCI] and [MH+- 2HC1 + H2] fragment ions are of low abundance, generally less than 20% of the base peak. Low intensity peaks were observed at m / z 257, 259, and 261 corresponding nominally to [MH+ - HC1+ H,]; however, some tetrachlorobiphenyl isomers were found (by CHI CI) to contain traces of trichlorobiphenyl impurities which produce ions, MH+, at the same m / z ratios. The ion intensities corresponding to [MH+ - HC1+ H,] were found, or estimated, to be less than 10% of the base peak, with the one exception noted in the table. Despite the relatively low fragment ion abundances, a trend in the extent of fragmentation of MH+ is clearly observable: fragment ion abundances are greatest for isomers with two chlorines substituted on each phenyl ring, e.g., the 2,5,2',3', 2,5,2',5', and 2,4,2',5' isomers, and least for isomers which contain all chlorine atoms on one phenyl ring, e.g., the 2,3,4,5 and 2,3,5,6 isomers. Isomers with a 3:l chlorine distribution have intermediate fragment ion intensities. Fragment ion abundances also vary with the position of chlorine substitution but not to the extent that individual isomer identification is possible, particularly considering the many isomers possible. It does appear likely that the H2 CI mass spectra will provide information on the distribution of chlorine atoms on the two rings.
+
1185
2,3,4,52,3,5,62,4,6,2'2,3',4',5'2,5,2',5'2,5,2' ,3'3,5,2' ,5'2,4,2',5' -
[ MH' [MH' [MH+- HCl+ 2HC1 MH+ M+. HCl] H,] + H,l Tetrachloro biphenyls
100 100 100 100
100 100 100 100
15 14 13 15 12 21 18 16
1.6 2.1 14 4.8 3.3 9.0 9.8 11
b b b 3.5 6.9 b 18 7.8
2.6 2.1 8.7 12 12 8.4 30 20
Pentachlorobiphenyls 2,3,6,2',5'2,4,6,2',3'2,3,4,2',52,4,6,2',6'-
100 100 100 100
23 23 22 24
4.9 6.2 1.5 3.3
2.4 1.3 1.3 1.3
Hexachloro bip henyls 2,4,5,2',4',5'2,3,4,5,2',3'2,3,4,5,3',4'-
100 100 100
22 20 20
1.0 4.6 5.9
1.2 2. 7 2.1
Intensities are a Intensities as % of base peak, MH'. not accurately measurable due to impurities present but estimated to be less than 10%of base peak. Table 111. CH, Chemical Ionization Spectra of Monochlorobiphenyls, Dichlorobiphenyls, and Trichlorobiphenylsa [MH+[MH' - HC1 t compound MH' HCll CH,] M+ M.C,H,+ M.C,H,+ Monochloro biphenyls 234-
100 100 100
2,2'3,34,4'3,42,5-
100 100
2,5,2'2,5,3'2,5,4'2,3,42,4,6-
100 100 100 100
0.3 5.6 3.4
100
0.7
1.2
0.3 8.9 1.2
29 12 21
30 29 34
4.1 6.3 7.1
30 29 31 35 31
4.3 6.5 2.2 1.2 2.3
39 37 33 40 28
7.6 7.3 6.4 9.2 5.9
Dichlorobiphenyls 100 100 100
0.1 0.4 0.4
0.4 9.6 4.0 0.3 2.2
29 29 31 34 32
Trichlorobiphenyls
a
11 7.5 7.9 8.2 7.0
Intensities as % of base peak, MH+.
There is even less fragmentation in the Hz CI of pentachlorobiphenyls and hexachlorobiphenyls (Table 11). As a result the H2 CI mass spectra are practically indistinguishable for isomers having the same number of chlorine atoms; in this respect CI mass spectrometry offers no advantages over E1 mass spectromery in distinguishing among isomers. CHI CI Mass Spectra. Since the proton transfer reactions of CH5+and C2H5+. are 27 and 58 kcal mol-', respectively, less exothermic than proton transfer reactions of H3+(19),the CHI CI mass spectra are expected to show even less fragmentation than the H2 CI mass spectra. The CHI CI mass spectra of the mono- through trichloro isomers are presented in Table 111. For all compounds MH+ is the base peak and together with the M+. and the M-C2H5+and M-C3H6+cluster ions account for >90% of the total additive ionization. For the
1188
ANALYTICAL CHEMISTRY, VOL. 53, NO. 8, JULY 1981
higher isomers these were the only ions observed in significant abundance. In the CH4 CI mass spectra of monochlorobiphenyls a minor fragment ion is observed at m / z 169 corresponding to [MH+ - HCl + CHl]. This product may arise either from the direct reaction of CH6+with the chlorobiphenyl or by a two-step reaction analogous to that observed in the H2 CI; both reaction routes have been observed (15) with simpler haloaromatics. Despite its low intensity the intensity variation of meta > para > ortho is analogous to the order observed for formation of [MH’ - HC1+ H2] in the H2 CI system and is similar to the enhanced reactivity of other meta-substituted chlorobenzenes under CH4 CI conditions (17). This meta substituent effect is also apparent in the CHI CI of the dichlorobiphenyls where the 3,3’ isomer shows the most abundant [MH+- HCI + CH4] ion. Similarly, the intensity of [MH+ - HCl + CH4] in the series 2,5,2’-, 2,5,3’-, and 2,5,4’-trichlorobiphenyl also shows the order meta > para > ortho. The CHI CI mass spectra are less useful than the H2 CI mass spectra in the differentiation of PCB isomers; however, the very abundant MH+ ion could be used to increase the sensitivity of detection, a matter of great importance in residue analysis. CI Spectra Using Other Reagent Gases. Charge exchange mass spectra of representative PCBs were obtained by using He, Ne, Kr, Ar, and CO as reagent gases. In all cases, the fragmentation routes were similar to those of the E1 spectra, with the extent of fragmentation determined by the recombination energy of the reagent ion. The spectra of positional isomers were essentially identical with the result that no differentiation among isomers was possible. SUMMARY The main features of the results can be summarized as follows. H2 is more useful than CHI as a reagent gas in the chemical ionization of PCBs because it produces more extensive fragmentation of the protonated molecule MH’. The extent of fragmentation and further reaction of the [MH+ HCl] fragment ions with H2 is structure dependent for the lower PCBs permitting some distinction among isomers; however, for penta-substituted and more highly substituted biphenyls the extent of fragmentation becomes too small to be of any diagnostic value. Since most of the PCBs found in nature have more than four chlorines, the usefulness of proton transfer chemical ionization in identifying specific isomers would appear to be limited.
A point of special interest from the present work is the varying reactivity of the substituted phenyl cations toward reaction with H2. In general, the reactivity is greatest when the second phenyl group is meta to the charge site and least when the second phenyl is ortho to the charge site. The origin of this variation in reactivity is not clear and requires further study. Preliminary calculations show that the differing reactivities may correlate with the extent of charge delocalization in the substituted phenyl cations, although energetic effects also play a role.
LITERATURE CITED EHP, Environ. Health Perspect., 1972, 1, 1. Oswald, E. 0.; Levy, L.; Corbett, B. J.; Walker, M. P. J. Chromatogr. 1974, 93, 63-90. Onuska, F. I.; Comba, M. E. I n “Hydrocarbons and Halogenated Hydrocarbons in the Aquatic Environment”; Afghan, B. K., Mackay, D., Eds., Plenum Press: New York, 1980;pp 285-302. Safe, S. Org. Mass Spectrom. 1971, 5, 1221-1226. Safe, S.;Hutzinger, 0. Chem. Commun. 1971, 446-448. Safe, S.; Hutzinger, O., J. Chem. SOC., Perkin Trans. 1 1972,
686-691. Safe, S.; Hutzinger, 0. “Mass Spectrometry of Pesticides and Pollutants”; CRC Press: Cleveland, OH, 1973;Chapter 5. Safe, S.; Hutzinger, 0.; Jamieson, W. D. Org. Mass Spectrom. 1973,
7, 169-176. Levy, L. A.; Oswald, E. 0. Biomed. Mass Spectrom. 1976, 3, 88-93. Hass, J. R.; Bursey, M. M.; Levy, L. A,; Harvan, D. J. Org. Mass Spectrom. 1979, 14, 319-325. Shushan, B.; Bunce, N. J.; Boyd, R. K.; Corke, C. J. Biomed. Mass Spectrom., In press. Oswald, E. 0.; Albro, P. W.; McKlnney, J. D. J. Chromatogr. 1974, 98, 363-448. Dougherty, R . C.; Roberts, J. D.; Tannenbaum, H. P. “Mass Spectrometry and NMR Spectroscopy In Pesticide Chemistry”; Haque, R., Blros, F. J., Eds., Plenum Press: New York, 1974;pp 33-48. Leung, H. W.; Harrison, A. G. Can. J. Chem. 1976, 54, 3439-3452. Leung, H. W.; Ichlkawa, H.; Li Y.-H.; Harrison, A. G. J. Am. Chem. SOC. 1978, 100, 2479-2487. Leuno, H. W.; Harrison, A. G. J. Am. Chem. SOC. 1979, 101,
3168r-3173. Leung, H. W.; Harrison, A. G. J. Am. Chem. SOC. 1960, 102,
1623-1628. Harrison, A. G.;Kallury, R. K. M. R . Org. Mass Spectrom. 1960, 15,
284-288.
Harrison, A. G. In “Hydrocarbons and Halogenated Hydrocarbons In the Aquatic Environment”; Afghan, B. K., Mackay, D., Eds.; Plenum Press: New York, 1980;pp 265-283.
RECEIVED for review February 3,1981. Accepted April 7,1981. This work was supported, in part, by a grant from the Natural Sciences and Engineering Research Council of Canada. C.W.T. gratefully acknowledges the sabbatical leave (summer, 1978) provided by the Hong Kong Polytechnic which made this work possible.
