Study of Electron Capture Behavior of Substituted Aromat ics by PIasma Chromatography Francis W. Karasek, Oswald S. Tatone, and David M. K a n e Department of Chemistry, University of Waterloo. Waterloo, Ontario
The plasma chromatograph can observe experimentally the positive and negative charged species present in the electron capture detector and measure their response and mobility under changing parameters. Experimental evidence of dissociative electron capture is shown for halogenated aromatics with formation of a halogen ion. Aromatics with two different halogen atoms dissociate only the most reactive halogen ion in the order I > Br > CI. Simple electron attachment with formation of a negative molecular ion occurs for nitrobenzene. Chloronitrobenzene and decachlorobiphenyl undergo both associative and dissociative electron capture.
From its conception by Lovelock ( I ) , t h e electron capture (EC) detector has developed into one of the most sensitive and selective devices available for use in gas chromatography. The importance of this detector for analysis of trace compounds of biomedical and environmental significance has led to many studies of such characteristics as its anomalous responses and linearity (2-4), range of relative response factors for different types of compounds (5-7), and t h e effect of temperature on sensitivity (8-10). In 1963 Lovelock (2) described the plane parallel detector design and the pulse-sampling technique of operation and its effect on anomalous responses, errors, and linearity range. This work, together with t h a t reported by
J. E. Lovelock and S. R. Lipsky, J. Amer. Chem. Soc.. 82, 431 (1960). J. E. Lovelock. Ana/. Chem.. 35, 474 (1963) D . C. Fenimore. A. Zlatkis, and W. E. Wentworth, Anal. Chem.. 40, 1594 (1968). D . C. Fenimore and C. M. Davis, J. Chromatogr. Sci.. 8 , 130 (1 970). J. E. Lovelock. Nature (London). 189, 729 (1961). J. E. Lovelock, P. G. Simmonds. and W. J. A. VandenHeuvel, Nature (London). 197, 249, (1963). L. Dehennin, A. Reiffstech. and R. Scholler, J. Chromatogr. Sci.. 10, 224 (1972). 6.C. Pettitt. P. G. Simmonds, and A. Zlatkis, J . Chromatogr. Sci. 6, 85 (1968). W . E. Wentworth and E. Chen, J . Gas Chromatogr., 5 , 170 (1967). P. G. Simmonds, D. C. Fenimore. B. C. Pettitt, J. E. Lovelock. and A . Zlatkis,Anal. Chem.. 39, 1428 (1967).
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 7 , JUNE 1973
Wentworth (9, 11-16) on the mechanism of the electron attachment phenomena occurring, has done much to expand our Understanding of the fundamental physical processes involved in the functions of the EC detector. These early studies and their conclusions were necessarily made from indirect experimental data derived from observations of total currents under different detector parameters of voltage, temperature, and geometry. While these studies provide improved understanding, a greater advance in our knowledge requires detailed observations of the behavior of the charged particles present and the kinetics of the reactions involved. With the recent development of the plasma chromatograph, it is now possible to create conditions found in the E C detector, observe both the positive and negative charged species being formed, and measure their response and mobility under changing parameters. Operating a t atmospheric pressure, the plasma chromatograph involves the generation of charged particles in a carrier gas by a radioactive 63Ni source and a n ion-molecule reactor whose products are subsequently separated and observed in a coupled iondrift spectrometer. Direct experimental observation of the behavior of the charged particles under differing conditions permits one to study many fundamental aspects of the electron attachment phenomena and the E C detector characteristics. Recent work with the plasma chromatograph has verified the existence of dissociative electron capture for monohalogenated benzenes (1 7 ) and has indicated the effect of the ionic species formed by oxygen in carrier gas a t different temperatures (18). This preswork was undertaken to study the stable ionic species
W. E. Wentworth, E. Chen, and J. E. Lovelock. J. Phys. Chem.. 70, 445 (1966). W. E. Wentworth. R. S. Becker, and R. Tung, J. Phys. Chem.. 71, 1652 (1967). W. E. Wentworth and E. Chen, J . Phys. Chem.. 71, 1929 (1967). J. C . Steelhammer and W. E. Wentworth. J. Chem. Phys.. 51, 1802 (1 969). W. E. Wentworth, R . George, and H. Keith, J. Chem. Phys., 51, 1791 (1969). W. E. Wentworth and J. C. Steelhammer, "Radiation Chemistry," American Chemical Society Publication, E. J. Hart, E d . . Washington, D. C., 1968. F. W. Karasek and 0.S. Tatone. Anal. Chem., 4 4 , 1758 (1972). F. W. Karasek and D. M . Kane, Ana/. Chem.. 45, 576 (1973).
GAS EXIT
GATING GRID
SAMPLE INJECTION
ELECTROMETER
Ni-63
P C TUBE HOUSING AND HEATER HEATED CARRIER GAS 10-300 mllMin
Figure
1.
t
HEATED DRIFT GAS 100 - 3 0 0 0 ml/Min
I
Schematic diagram of the BETA-VI plasma chromatograph ~
Table I. Experimental Parameters of the Plasma Chromatograph for Data Obtained Instrument temperature 125 "C
Carrier gas flow Drift gas flow Ion-molecule reactor length Ion-drift space length Electric field Injection pulse Scan pulse Recorded scan Gases
100 crn3/rnin 450 cm3/rnin 6.0cm 6.0 crn 250 V/cm 0.2msec 0.2msec 2 min Nitrogen-Linde high purity grade (99.996%)
formed from electron capture with related substituted aromatic compounds, all of which exhibit large responses in the EC detector.
~~~
Table II. Negative Species Formed by Dissociative Electron Capture Negative species from Compound dissociative capture
Fluorobenzene Chlorobenzene Brornobenzene lodobenzene rn-Dichlorobenzene 0-Dibromobenzene o-Fluorochlorobenzene rn-Fluorochlorobenzene o-Chlorobromobenzene o-Chloroiodobenzene rn-Chloroiodobenzene o-Chlorotoluene rn-Chlorotoluene 4-Chlorobiphenyl 2,2',4,4'-Tetrachlorobiphenyl
CI BrI-
CI BrCI CIBr II-
CI CI CI CI-
EXPERIMENTAL I n s t r u m e n t a t i o n . The basic elements and function of the BETA-VI model plasma chromatograph used in these experiments have been described previously (1 7-19). T h e instrumentation is shown in Figure l. Ions formed by the 63Ni source in the flowing carrier gas are moved by a n electric field through the ionmolecule reactor section toward the drift spectrometer. A pulse of these charged species is injected into t h e drift spectrometer, where separation occurs because of their different mobilities a s they move through a n inert gas. Ions reach the detector in a series of ion peaks recorded as a plasmagram. A synchronized, variable delay gating technique on the scan grid gives a recording of t h e millisecond plasmagram scan in a 2-min time span. The experimental parameters used to obtain plasmagrams are shown in Table I. Gases used were passed through a metal t r a p of 2.25-1. capacity packed with Linde Molecular Sieve 13X. This procedure removes impurities and gives a very low water concentration, estimated at 10 to 100 ppm. The samples were syringe-injected vapors of approximately 10-8 g. FVhen using N2 carrier gas, the positive species are a series of (H,O),H- ions. while only thermal electrons are seen as the negative particles. These electrons appear a s a continuum across t h e plasmagram scan because the grids do not close perfectly t o exclude such high-speed particles. If t h e grids could exclude t h e electrons. the plasmagram would show only a single peak with a drift time of 0.01 millisecond a n d with a magnitude about 15 times greater t h a n t h a t of the continuum. This peak is too fast for the response of the recording system. For t h e negative plasma(19) F. W . Karasek ( 1971 ) .
and M. J. Cohen, J . Chrornatogr, Sci..
9, 390
grams, a n initial drop in electron current with sample admission is seen. T h e rise in negative current t o t h a t of the original electron level by t h e end of the plasmagram occurs because of sample depletion during the 2-min scan interval. Reagents. All t h e substituted benzene compounds were high purity reagents from Analabs, British Drug Houses, J . T . Baker, Eastman Organics, or Fisher Scientific. These compounds were checked by gas chromatography and found to be of better t h a n 99% purity. The P C B compounds were prepared by 0. Hutzinger of t h e NRC Regional Laboratories, Halifax, N. S. Their purity a n d structure were verified by gas chromatography and mass spectrometry.
RESULTS AND DISCUSSION Except for fluorobenzene, which displays no dissociation, the substituted aromatic compounds listed in Table I1 show only a single halogen ion formed by dissociative electron attachment. Where two different halogen atoms are present in the molecule, the halogen t h a t dissociates is determined by the general order of reactivity for electron attachment shown by halogens: I > Br > C1. Since it was previously shown that the monohalogenated compounds produced only their respective halogen ions ( 2 4 , plasmagram data were obtained as appears in Figures 2 and 3 . The coincidence of the I- and Br- peaks for the two compounds involved in each figure serves to identify the dissociated ion in each case. ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
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0 DRIFT
10 riMr M I L L I S E C O N D I
20 DRWT r i M i
Figure 2. Composite negative plasmagrams of bromobenzene and o-chlorobromobenzene
-~irusrco~os
Figure 4. Composite negative and positive plasmagrams of trobenzene
1
N t G A r l V t PLASMAGRAMS
h
' DRIFT r i M i
0
10
DRIFT TIME -MILLISECONDS
20
Figure 3. Composite negative plasmagrams of iodobenzene and o-chloroiodobenzene
71-
Ib - M~LLISECONDS
lk
Figure 5 . Composite negative plasmagrams of 1 -chloro-2-nilro benzene with nitrobenzene and chlorobenzene
I t has been reported by Christophorou (20) t h a t nitrobenzene captures electrons nondissociatively a t electron energies below 0.03 eV, only undergoing dissociative capture to form NOz- a t electron energies above 1.0 eV. The plasmagrams shown in Figure 4 confirm this by showing t h a t only associative electron capture with formation of a negative molecular ion occurs for nitrobenzene. Early exploratory work with plasma chromatography involved instrumentation in which ion identity was obtained with a directly coupled mass spectrometer. Since the BETA-VI model has no coupled mass spectrometer, other means of ion identification must be used. The identity of the C&N02is confirmed by coincidence of its plasmagram peak with t h a t obtained for this compound in the positive plasmagram. It has been well established t h a t a positive quasimolecular ion, (C&N02)HC, is obtained in the plasma chromatograph (27, 21) and could be expected to have a mobility similar to t h a t of its equivalent negative ion. Figure 5 shows the ionic species observed from electron capture of 1-chloro-2-nitrobenzene. This compound exhibits only dissociative electron capture. No negative molecular ion appears, but one does observe both the C&N02-
and C1- dissociative product ions. Identity of these ions is confirmed by superposition of subsequent plasmagrams of nitrobenzene and chlorobenzene on the same plasmagram of the chloronitrobenzene and observation of respective ion peak coincidence. The relative amounts of the ClC&N02-. C6H4N02-. and C1- ions formed by electron capture with the ortho, meta. and para isomers of chloronitrobenzene are shown in Figure 6. These data indicate that the 1-chloro-3-nitrobenzene isomer forms a stable molecular ion by associative electron attachment. with only a slight amount of dissociation occurring to give the C1-. The other two isomers dissociate completely tc the C1- and the C 6 H 4 N 0 2 - .The identity of the molecular ion is confirmed by correspondence of its reduced mobility with that of this same ion formed from electron attachment of dichloro-substituted nitrobenzene compounds (22). The existence of all three ions is predicted by the mechanism previously reported by Wentworth 112. 1.5). Further evidence of multiple ion formation from electron capture is shown in Figure 7 . where decachlorohiphenyl is seen to form the three ions identified as C1 . C12Cla-, and C&110-. Identity of these ions was estahlished by superposition of the plasmagram of octachlorobiphenyl and from previous work on the plasma chromatography of PCB compounds (23). In the previous work.
(20) L. G. Christophorou, R. N. Compton. G. S. Hurst. and P. W. Reinhardt, J. Chem Phys , 45, 536 (1966) (21) G . W. Griffin, I . Dzidic. D. I . Carroll, R . N. Stillwell, and E. C. Horning. A n a / . Chem., 45, in press.
(22) F W Karasek University of Waterloo Waterloo Ontario unpublished work (23) F W Karasek A n a / Chem 43, 1982 ( 1 9 7 1 )
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
~~
YEGAT I V I
PLASMAGRAMS
I
'
/ ! N O 2
I
=---------
0
0
10
DRIFT T I M E
- MILLISECONDS
Figure 6 . Composite plasmagrams obtained for ortho, meta, and para isomers of chloronitrobenzene
Table I l l . Drift Times and Calculated Reduced Mobilities Compound
(KO)for Negative Ions Observed
Ion formed
Drift time. msec
Fluorobenzene Chlorobenzene Bromobenzene I odobenzene Nitrobenzene o-Chloronitrobenzene
Octachlorobiphenyl Decachlorobiphenyl
0
-- 6.55 X TX
P T = degrees Kelvin. X = drift time (seconds). 760
Table I V . Drift Times and Calculated Reduced Mobilities
P = pressure
Reduced mobility K O ,c m 2 / V sec
...
...
5.47 6.13 6.40 8.55 5.45 8.60 9.31 5.47 13.74 5.47 13.74 14.67
2.92 2.61 2.51 1.87 2.92 1.86 1.71 2.92 1.16 2.92 1.16 1.08
(Torr)
(KO)for Positive Ions Observed
Compound
Fluorobenzene Chlorobenzene Bromobenzene I odobenzene Nitrobenzene o-Chloronitrobenzene Octachlorobiphenyl Decachlorobiphenyl
Drift time, msec
Reduced mobility K O , c m 2 / V sec
7.55 7.96 8.28 8.80 8.46 9.40 13.74 14.67
2.1 1 1.99 1.91 1.81 1.88 1.70 1.16 1.08
K O = 655 Tx X 760. !? T = degrees Kelvin. X = drift time (seconds). P = pressure (Torr).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 7, JUNE 1973
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an air carrier gas was used so the reactant negative species was (HzOj,Oz- ions instead of electrons as in this present work. In that case formation of decachlorobiphenyl ions was by a n ion-molecule reaction with the (H2O)nOz- ions. While the negative molecular ion of C12Cllo- was clearly present with a reduced mobility of 1.09 compared to a value of 1.08 for data shown in Figure 7, it was not possible to detect dissociative formation of C1- because the (HzO),Oz- reactant and C1- ion peaks have almost identical mobilities. For decachlorobiphenyl the negative molecular ion peak predominates at all the different concentration conditions observed while the dissociated C1- always remains a very minor one. Mobility data for negative ions of the compounds studied appear in Table 111. All these compounds produce strong positive plasmagrams by ion-molecule reaction k i t h the (H20),H+ species to form the quasi-molecular ions ( M W ) H + . Mobility data contained in Table IV show that the reduced mobility of the quasi-molecular positive ions corresponds to that found in Table I11 for the corresponding negative molecular ions.
CONCLUSIONS Data obtained with the plasma chromatograph show that compounds can undergo simple electron attachment or dissociative capture or both. Those that undergo both are involved in competing reactions t h a t are a function of concentration and temperature. These qualitative studies indicate the differing reactions involved for each compound type after electron attachment. Other studies show t h a t oxygen in the carrier gas will form ( H z 0 j n 0 2 - ions, which in turn react with electron-capturing compounds in competition with electrons (18). The relative amounts of these reactive ions and unreacted thermal electrons are a function of temperature. From a consideration of all these
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ANALYTICAL C H E M I S T R Y , VOL. 4 5 , NO. 7 , JUNE 1973
N I G A I I V E PLASMAGRAMS
jk.
~
-
1,2,3.3,4,4.bL OCIACHLOROBIPWINYL
had-
CI-
1
DICACWLOROIIPHINYL
\
,
r
10 DR!F? IIME
20
- MILLISICONDS
Figure 7. Composite negative plasmagrams for the PCB compounds of octachlorobiphenyl and decachlorobiphenyl
data, one would expect the anomalous responses, linearity limits, temperature effects, and large differences in relative responses so characteristic of the EC detector, particularly in the DC operational mode. Further data of this type may also contribute to understanding of the improved performance found in the pulsed mode of operation. More complete quantitative studies over concentration and temperature ranges will make it possible to determine more exactly the sources of these characteristics and achieve a more complete understanding of the EC detector mechanism. Received for review October 31, 1972. Accepted January 30, 1973. This research was supported by the Defence Research Board of Canada, Grant Number 9530-116.
