LITERATURE CITED (1) F. H. Field, Accounts Cbem. Res., 1, 42 (1968) and references therein. (2) F. J. 8iros, R C. Dougherty, and J. Dalton, Org. Mass Specfrom., 6, 1161 (1972). (3) H. M. Fales, F. W. Milne, and T. Axenrod, Anal. Chem., 42, 1432 (1970). (4) H M. Fales, H. A . Lloyd, and G. W. Mi!rie. .J. Amer. Chem. Soc., 92. 1590 (1970). ( 5 ) R. C Dougherty and C . R. Weisenberger, J. Amer. Chem. S O C . 90, 6570 (1968). ( 6 ) R. C. Dougherty. J Chem. fbys.. 50, 1896 (1969). (7) C. Cottrell, R. C. Dougherty, G. Frankel, and E. Peochold, J. Amer. Chem. Soc., 91, 7545 (1969). (8) E. M. Chait, W. B. Askew. and C. 8.Matthews. Org. Mass Specfrom., 2, 1139 (1969). (9) W. T.Naff and C. D. Cooper. J. C k m . Phys., 49. 2784 (1968). (10) J. C. J. Thyme, Chenr. Commun. 1075 (1968). (11) R. S . Gohlke, J. Amer. Chem. Soc., 90, 2713 (1968). (12) J. T.Larkins, J. M. Nicholson, and F. E. Saalfeld. Org Mass Specfrom.. 5, 265 (1971). (13) K. Jaker and A. Henglein, 2. Nafurforsch. A, 22, 700 (1967). (14) S. Tsuda. A . Yokohata, and M. Kawai, Bull. Chem. SOC.Jap., 42, 607 (1969). (15) P. C. Rankin, J. Ass. Offic. Anal. Cbem., 54, 1340j1971). (16) R. C. Dougherty, J Dalton, and F. J. Biros. Org Mass Spectrom., 6 , 1171 (1972).
(17) E . M. Arnett, F. M. Jones 111, M. Tnagepera. W. G. Henderson, D. Holtz, J. L. Beauchamp. and R. W.Taft, J. Amer. Cbem Soc., 94, 4724 (1972). (18) R. Yamidagni and P. Kebarle, J. Amer. Chem. Soc., 94, 2940 (1972). (19) C. Kajoas and R. Tummler, Org, Mass Specfrom., 2, 1049 (1969). (20) D. Beggs, H. M. Fales. G. W.A. Milne, and M. L. Vestal, Rev. Sci. hsfrum,-42, 1578 (1971). (21) J. H. Futrell and L. H. Wojcik, Rev. Sci. lnstrum., 42, 244 (1971). (22) R. C. Dougherty, J. Daiton, and J. D. Roberts, Org. Mass Specfrom., 8, 77 (19741. (23) R. C . Dougherty and J. D. Roberts, Org. Mass Specfrom., 8, 81 (1974). (24) R . N. Compton and R. H. Heubner, Advan. Radiaf. Cbem., 2, 281 (1970). (25) R. S. Berry and C. W. Reimann, J. Chem. fhys., 38, 1540 (1963). (26) R. C. Dougherty, J. D. Roberts and F. J. Biros, Anal. Chem., 47, 54 (1975). (27) R. Yamidagni and P. Kebarle, J. Amer. Cbem. Soc., 93, 7139 (1971). (28) R. C. Dougherty, A. Bergner, P. Levonowich, and J. D. Roberts, to be published.
RECEIVEDfor review .4pril 2, 1974. Accepted September 11. 1974. This work has been supported by a grant from the National Science Foundation. A preliminary account of this work has appeared: R. C. Dougherty, J. Dalton, and J. D. Roberts, 19th Annual Conference on Mass Spectrometry and Applied Topics, Atlanta, Ga., 1971, Paper J. 4.
Positive and Negative Chemical Ionization Mass Spectra of Some Aromatic Chlorinated Pesticides R. C. Dougherty and J. D. Roberts Department of Chemistry, Florida State University, Tallahassee, f l a . 32306
F. J. Biros' Primate and Pesticides E f f e m Laboratory, Environmental Protection Agency, Perrine, f l a . 33 157
The positive (CI) and negative (NCI) isobutane chemical ionization mass spectra of twelve aromatic, chlorinated pesticides, metabolites, and degradation products have been determined. The compounds investigated included six polychlorinated diphenylethanes (DDT-type), three polychlorinated diphenylethylenes (DDE-type), and three diphenylmethanol derivatives ( e.g., Kelthane). The CI spectra are considerably less complex than the corresponding electron impact mass spectra reported previously by other investigators. For the diphenylethanes, the base peak corresponded to elimination of CI from the molecule ion. The CI spectra of the diphenylethylene compounds were dominated by the molecule ion and the quasi-molecule ion, (M -k H)'. This class of pesticides also exhibited ion-molecule attachment peaks corresponding to ( M C3H7)+ and (M C4H9)+, as well as significant ions corresponding to chloride elimination ( M - CI)+. Rearrangement ions were observed in the spectrum of DDMU. The most intense ion in the CI spectra of the diphenylmethanol compounds corresponded to [M - OH]'. The NCI spectra with isobutane as enhancement gas were exceptionally simple. The most abundant ion for all compounds, except DDMU and methoxychlor, CI)-. Low intensity dimers and other attachment was (M ions were also observed in some cases. Fragmentation was noted only for oxygenated molecules including methoxychlor and chlorobenzilate. Potential applications to pesticide residue analysis are discussed.
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i Present addresi I S F:n\ironinental Protection Agency, 401 M Street. S W . LVashlngton, D C 20.160
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In previous publications ( 2 , 21, we have discussed the positive (CI) and negative (NCI) methane chemical ionization mass spectra of a series of polycyclic chlorinated insecticides. The purpose of the present investigation was to employ both chemical ionization techniques, with isobutane as reagent gas, in the study of a series of aromatic chlorinated insecticides and degradation products. The overall objective of this study is the development of rapid and reliable procedures for the qualitative and quantitative measurement of pesticide residues and other toxins in environmental substrates. The range of applicability of the technique should include substrates like air, water, soil, and human or animal tissue. By use of both CI and NCI spectia, the reliability and redundancy of the analysis should be increased to the point that additional purification, e.g., GLC introduction, would not be necessary for minimally cleaned up extracts of environmental substrates. The approach described here is a direct extension of the CI ( 3 ) and GLC-CI ( 4 ) examination of body fluids and other substrates for drugs and related materials. This report discusses the general features of the isobutane CI and NCI spectra of the DDT class of pesticides. When combined with previously published data ( I , 2 1, this information should provide a basis for the analytical procedure suggested above. For the polycyclic chlorinated insecticides ( I 1, the most significant single feature of their CI mass spectra was the high intensity of ( M - Cl)' fragments which are probably formed by chloride abstraction, Equation 1.
A N A L Y T I C A L C H E M I S T R Y , VOL.. 47, NO. 1, J A N U A R Y 1975
RC1
'-C?H,'
----t
R'
+
CZHsC1
(1)
T h e resulting high ion currents and the relative uniformity and simplicity of fragmentation should have substantial analytical value when confirmation of the polycyclic pesticides is desired in an environmental sample. The NCI spectra employing methane as enhancement gas for the same group of compounds were characterized by ion-molecule attachment species, principally (M C1)- (5 ). Disassociative capture of an electron by the molecule produced the chloride in the source, Equation 2, which subsequently attaches to the molecule, Equation 3.
100'
Y
60-
+
RC1
- es-
RC1
f
-
C1-
R-
+
RC1,'
C1-
(2 )
(3)
Ions were also present which corresponded to association of the substrate molecule with H-, 0-, OH-, H.?OCl-, HClp-., and C10-. Fragmentation processes were relatively unimportant and were limited to elimination reactions involving loss of combinations of the neutral species H , C1, and HC1. The NCI technique is truly complementary to CI mass spectrometry and theoretically may be used on the same sample with appropriate field reversals. By obtaining both CI and NCI spectra on the same sample, the reliability of qualitative or quantitative identification of suspected residues in complex mixtures should be considerably enhanced. In examining residues in the submicrogram range, it will be necessary to use methylene chloride or a similar reagent gas as a source of chloride ions for the NCI spectra. Isobutane was selected as reagent gas in the examination of the present group of compounds because of its availability in a purer form than methane and because the major reagent ion under CI conditions is the t - butyl cation (61, a considerably weaker proton acid than the CH;,+ and C?H:+ ions formed when methane is used as the reagent gas ( 7 ) . The lower acidity of CJHg+ as compared to CHn+ will reduce the complexity of the fragmentation because of the lower exothermicity of proton transfer reactions. This will facilkate use of the data in a purely analytical application. Because of the higher purity of isobutane as compared to methane, the formation of additive species other than (IL1 C1)- in the NCI mass spectra should be suppressed because the oxygen-containing anions arising from the reagent gas impurities will be suppressed. Electron impact (EI) mass spectra of most of the present group of compounds have been previously reported by several investigators (8, 9). Fragmentation behavior for this class of compounds is reasonably complex and, for certain groups of nonisonieric compounds, remarkably similar. These characteristics are generally undesirable when applying the mass spectrometric method as a survey tool for confirmation of pesticide residues as well as other materials in environmental media. For example, the E1 mass spectra of Etoxinol, chlorobenzilate, and Kelthane all possess uniformly low intensity molecular ions and dominant peaks a t m / e 251, nile 139, m/e 111,and m/e 75. An abbreviated study of methane CI mass spectra of some of the cornpounds studied in this work has been presented ( 1 0 1. The differences between the present results and this previous report may be accounted for entirely by the increased acidity of CH;+ as compared to tert- C4HS+.
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EXPERIMENTAL T h e aromatic chlorinated pesticides employed in this mass spectrometric investigation were high purity analytical grade crystalline standards used as received from the manufacturer or recrystallized as indicated by gas chromatographic purity checks. Commercial sources and systematic chemical nomenclature for these compounds are given elsewhere ( I 1 ). T h e CI and NCI mass spectra were recorded a t a resolution of M / l M = 1000 on an AEI MS-
1 250
Figure 1. lsobutane CI mass
loo 80
, M/E
350
300
spectrum (4-8k V ) of
p,p'-DDT
I
Figure 2. lsobutane CI mass spectrum (4-8 k V ) of o,p'-DDT
902 double focusing mass spectrometer equipped with a Scientific Research Instruments Corporation dual electron impact,/chemical ionization source (12 ). The spectrometer was operated in the nepative ion mode for determination of the NCI spectra with magnet polarity reversed and an acclerating voltage of -8kV. The data were handled manually and by an AEI DS-30 data system. The IOto 100-pg size samples were examined by direct probe introduction techniques with a source temperature sufficient to produce satisfactory spectrum (200O f 10 "C). Isobutane a t approximately I/; Torr (40 P a ) was the reagent or enhancement gas for all measurements. Primary ionization was obtained by electron impact t'rom a heated rhenium filament a t 470 ','and 0.25 mA emission. The relative intensities of the ions reported in the figures and Tables were substantially constant over a wide range of operating parameters of temperature and pressure and were reprtrducihle from day to day. Low intensity low ( n z / p 450) mass ions, which were both pressure and temperature dependent ( I , 2 ) , were excluded from the discussion since they were not considered to he of analytical utility or ktructurally significant for the purposes of this investigation.
RESULTS AND DISCUSSION The isobutane CI mass spectra of the aromatic pesticidal compounds examined in this study are presented in Figures 1 and 2 and Table I. The major ions in each of the spectra may be accounted for by synthetic and dissociative proton transfer and hydride and chloride abstract ion involving t C J H ~ + In . one case, an apparent rearrangement ion was prominent in the spectrum (see below). In the majority of the spectra, the chloride abstraction ion, ( M - Clj+, was dominant. The NCI spectra recorded for the aromatic chlorinated pesticides are presented in Figures 3 and 4 and Table 11. Low mass ions (
