Anal. Chem. 1980, 58,2907-2912 (13) Cairns, T.; Sigmund, E. G.; Rodney, L. B. Anal. Chem. 1984, 5 6 , 2547-2552. (14) Straub, K.; Levandowski, P. Biomed. Mass Specfrom., in press. (15) Hunt, D. F.; McEwen, C. N.; Upham, R. A. Tetrahedron Left. 1971, 4 6 , 4539-4542. (16)Buchanan, M. V. Anal. Chem. 1982, 5 4 , 570-574. (17) Lin, Y. Y.; Smith, L. L. Biomed. Mass Spectrom. 1979, I , 15-18. (18) Winkier, F. J.; Stahl, P. J . Am. Chem. SOC. I97B, 101, 3685-3687. (19) Tabet, J. C.; Tondeur, Y.; Hlrano, Y.; Wegmann, A,; Tecon; P.; Djerassi, C. Org. Mass Spectrom. 1984, 473-481. (20) Rudewlcz, P.; Munson, B., paper presented at the 32nd Annual Conference on Mass Spectrometry and Allied Topics, San Antonio TX, May 27-June 1, 1984. (21) Hunt, D. F. I n Advances in Mass Spectrometry; West, A. R., Ed.; Applied Science: London, 1974; Vol. 6. (22) Hunt, D. F. Prog. Anal. Chem. 1973, 6 , 359-376. (23) Keough, T.; DeStefano. A. J. Org. Mass. Spectrom. 1981, 16, 527-533. (24) Horning, E. C.; Stillwell, R. N.; Nowlin, J. G.; Carroll, D. I.Anal. Chem. 1981, 5 3 , 2007-2013. (25) Smith, D. E.; Smith, J. S.;Jerolamon, D.; Weston, A. F.; Richton, D.; Brozowski. E. J.. paper presented at the 26th Annual Conference on Mass Spectrometry and Allied Topics, St. Louis, MO, May 28-June 2, 1978.
2907
(26) Dougherty, R. C.; Roberts, J. D.; Binkley, W. W.; Chizhov, 0. S.;Kadentsev, V. I.; Solov'yov, A. A. J . Org. Chem. 1974, 3 9 , 451-455. (27) Bose, A. K.; Fujlwara, H.; Pramanik, B. N.; Spillert. C. R.; Lazaro, E. Anal. Biochem. 1978, 8 9 , 284-291. (26) Harrison, A. G. I n Chemical Ionization Mass Spectrometry; CRC: Boca Raton, FL, 1983; Chapter 2. (29) Lias, S . G.;Llebman. J. F.; Levin, R. D. J . Phys. Chem. Ref. Data 1984, 13, 659-808. (30) Weinkam, R. J.; Toren, P. C., paper presented at the 31st Annual Conference on Mass Spectrometry and Allied Topics, Boston, MA, May 8-13, 1983. (31) Suzuki, M.; Tatematsu, A.; Takeda, N.; Konishi, H.; Nakata, H. Mass Specfrosc. (Tokyo) 1963, 4 , 275-279. (32) Ferguson, E. E. I n Kinetics of Zon Molecule Reactions; Plenum: New York; 1978; pp 377-403. (33) Meot-Ner, M. J . Am. Chem. SOC. 1984, 106, 1257-1264. (34) Davidson, W. R.; Sunner, J.; Kebarle, P. J . Am. Chem. SOC. 1979, 107. 1675-1680.
RECEIVED for review May 19, 1986. Accepted August 4,1986. This work was supported by a grant from the National Science Foundation. CHE-8312954.
Electron Capture Negative Ion Chemical Ionization Mass Spectrometry of 1,2,3,4-Tetrachlorodibenzo-p -dioxin J. A. Larameg, B. C. Arbogast, and M. L. Deinzer* Department of Agricultural Chemistry and Environmental Health Sciences Center, Oregon State University, Corvallis, Oregon 97331
The conditions necessary for reproducible electron capture negative ion chemlcal lonlzatlon (ECNICI) mass spectrometry of polychlorodlbenzo-p-dloxlnsare dependent on the nature and pressure of the reagent gas In the lonlzatlon source, as well as on other controllable parameters. A comparison of argon, xenon, sulfur hexafluoride, hydrogen, and helium reagent gases for production of molecular Ion Me-,[M a]-, and the CI- Ions from 1,2,3,4-tetrachlorodibenzo-p-dloxln [1,2,3,4-TCDD] shows that Intensities of these ions are hlghly pressure-dependent. These results are rattonallzed on the basis of pressure-dependent electron thermalization as It affects relative cross sectlons for resonance and dlssoclatlve electron capture. The results from this study show that heavier mass gases give best sensitlvltles at lower lonlzatlon source pressures. Helium as reagent gas glves the most intense molecular Ion, and the most llnear Ion abundance over a large pressure range.
-
Electron attachment negative ion chemical ionization mass spectrometry has become an important technique for the analysis of chlorinated aromatic compounds in environmental samples ( 1 ) largely because of its inherent sensitivity and specificity. However, this method is plagued by variable and irreproducible results and the appearance of artifact peaks in the mass spectra (2). A systematic investigation of some variables affecting negative ion mass spectrometry results has now been carried out and we wish to report some of our findings. For compounds with large electron affmities and/or thermal electron capture sites, the technique of negative chemical ionization (NCI) yields detection limits as much as 3 orders of magnitude lower than the positive ion mode. Thus, NCI is the preferred method for polyhalogenated aromatic analytes,
particularly for trace analyses of pesticides, wood preservatives, herbicides, and environmental toxins generated as byproducts from the manufacture of these halogenated compounds. We report investigations using six different reagent gases: methane, hydrogen, helium, sulfur hexafluoride, xenon, and argon. Enhancements of molecular anion or fragment ion currents provide useful a priori data which are of value for organic analytical applications. Negative ion mass spectra are the result of three ion-formation processes: (1) resonance capture, (2) dissociative resonance capture, and (3) ion-pair formation ( 3 ) . The abundance of products from these processes is critically dependent upon, among other factors, sample pressure, reagent gas pressure and nature, ionizing electron energy and current, and ion source temperature ( 4 ) . Of the three negative-ion-formation mechanisms only ionpair production is independent of the ionizing electron's energy although it is dependent upon Frank-Condon factors for excitation. Both resonance and dissociative resonance capture cross sections depend upon, among other considerations, the nature and pressure of the reagent gas and the ionizing electron energy. Resonance capture is the primary mechanism responsible for the formation of molecular radical anions. The cross sections for these modes of ion formation, each with their own dependence on reagent gas pressure, account for the poor reproducibility commonly experienced in negative chemical ionization. EXPERIMENTAL SECTION All measurements were performed on a Finnigan 4500 quadrupole mass spectrometer,equipped with a negative ion detection kit ( 5 ) . The instrumental conditions used were 70 eV electron energy, 0.30 MA electron emission, and an axial ion energy of 5 f 1 eV. The source temperature was 90 "C. All spectra were recorded by a NOVA 3 minicomputer operating under the INCOS software.
0003-2700/86/0358-2907$01.50/0 0 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
l,2.3,4- TC D D
Table I. Calibration for Absolute vs. Relative Pressure Measurements for Various Reagent Gases" reagent gas hydrogen helium methane argon xenon sulfur hexafluoride
calibrationb Pirani Pirani Pirani Pirani Pirani Pirani
reading = 1.42 (mmHg) - (3.91 X reading = 0.697 (mmHg) - (2.63 X reading = 1.56 (mmHg) + (5.04 X reading = 0.578 (mmHg) - (1.48 X reading = 0.364 (mmHg) - (9.33 X IOF3) reading = 1.121 (mmHg) + (2.86 X IO-*)
Absolute pressures were measured with a capacitive manometer, and relative pressures with a Pirani gauge, Granvile-Phillips Model No. 260-009. *Derived from a linear least-squares regression analysis. Typically 40 measurements were taken which yielded a correlation coefficient of 0.998. In order to achieve constant and high sensitivity, the detachable ion volume was cleaned periodically during the run. To maintain stable instrument conditions, each compound was admitted into the instrument through an in-house-constructed batch inlet system. This system was designed to facilitate handling of toxic materials and was disposable. About 1 mg of sample was placed in a 6 mm 0.d. Pyrex tube that was sealed at one end. The sample tube was then connected to a 1 m X 0.20 mm i.d. fused silica capillary via a reducing union. The capillary was inserted through the separator oven to within 1cm of the ion source. Reagent gas pressures were measured by two methods. Initially, a Pirani gauge, connected via a static-pressure line to the ion source, was used. This setup was used for all measurements involving methane reagent gas. An MKS capacitive manometer was used in later experiments. The reagent gas pressure measurements were accomplished with an absolute (C-10 torr) transducer (Model 315) which is bakeable to 200 "C. A signal conditioner (Model 270B) and temperature controller/compensation unit (Model 272) allowed the transducer to be continuously operated at elevated temperatures while maintaining factory calibration a t specific temperatures. Reagent gas pressures were maintained at a constant value (kO.0001mmHg) by a combination pressure/flow controller (Model 244A) and Granville-Phillips needle valve (Model 216). Negative ion mass spectra were recorded in triplicate as a function of reagent gas pressure. At a reagent gas pressure of 0.158 k 0.001 mmHg, a total ion current of 20000 counts was set by adjusting the sample inlet temperature while maintaining a constant potential on the electron-multiplier detector. As the transmission efficiency through a quadrupole is dependent upon the mass of the ion, axial ion energy, and the extraction potentials, a consistent ion focusing scheme was needed to guarantee reproducible spectra. The following procedure was used throughout the work and was found to give month-to-month reproducibility of better than &lo%. The lens closest to the ion source was adjusted to give both a symmetric peak shape and maximum ion current for ion m / e 35. The next lens was set to 130 eV; the final lens potential was adjusted for maximum (M - C1)- ion current. Typically the potential ratios of this symmetric lens were 1:13:5. The quadrupole was scanned from mass m / z 20 to m / z 500 in 1.95 s. Ten scans were acquired a t each reagent gas pressure setting and a pressure range from 0.05 to 1.0 mmHg was sampled independently three times. All graphs were drawn as the average of about 40 measurements, with a standard deviation of &3% (see error bar). All dioxins were purchased from commercial suppliers and were checked for purity with a GC using an OV-I01 column. All dioxins exhibited a purity of better than 9970,and further purification was not required. The reagent gases were of 299% purity. Xenon, sulfur hexafluoride, and argon were used without further purification, and methane and hydrogen were passed through a I-m molecular sieve column. Helium received the most extensive purification, which involved passing the gas through a molecular sieve trap, a heated Pt/Pd getter, and finally an oxy-trap. A minimum of five fill-pump-purge cycles were used to admit a reagent gas into the instrument. Before a new reagent gas was used, the inlet system was pumped for 24 h. The pressure calibration between the capacitive manometer and the Pirani gauge is given in Table I. These data constitute
W
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il:
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02
01
06
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[ M - CI] 08
HELIUM reagert 90s
I 0
ION SOURCE PRESSURE (rnrnHg)
Figure 1. Relative negative ion abundances of 1,2,3,4-TCDD in the presence of hydrogen, argon, and helium at various pressures. 100-
W
2
'
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a
50-
W
-c 2
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.
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Figure 2.
NCI mass spectrum of 1,2,3,4-TCDD recorded in the
presence of helium. a general calibration between absolute and relative pressure measurements, respectively. These relationships are applicable to any Finnigan 4500 GC/MS spectrometer.
RESULTS AND DISCUSSION The mass spectral characteristics of 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TCDD) were sought as a function of the reagent gas and the ionization source pressures. When helium is used as reagent gas, the relative abundances of the molecular anion M'-, (M - Cl)-, and C1- produced from 1,2,3,4-TCDDremain relatively constant over a pressure range of 0.1-0.8 mmHg, with the molecular ion dominating the spectrum (Figure 1). A cluster of peaks beginning at m / z 301 account for about 10% of the ion current. r n / z 301 corresponds to the (M - C1)- anion with an additional oxygen atom. The appearance of m / r 301 cluster suggests that ion/molecule reactions between the analyte of interest and water or air contaminants were occurring more readily as the ion source pressure was increased (Figure 2). In contrast t o helium, hydrogen as a reagent gas (Figure 1) produces a molecular anion that dominated the mass
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986 1.0
/"
70 eV electron energy SO"C ion source t e m p e r a t u r e
/
0
02
04
OF
\
I
\
ION SOURCE PRESSURE
2909
06 XENON (mm Hg)
Figure 3. Relative negative ion abundance of 1,2,3,4-TCDD in the presence of xenon.
spectrum only a t low ion source pressures. The molecular anion contributed 78% to the ion current at the lowest pressure measured, ca. 0.065 mmHg. Chloride ion accounted for the remaining 22% of the ion current. At a pressure of 0.132 mmHg, the species Ma- and C1- were of equal concentration, and a t increased pressures C1- ion became the dominant ion in the spectrum. Between the measured pressure range of 0.0965 to 0.6770 mmHg, 75% and 48%, respectively, of the total detectable ion current was accounted for by the sum of Me-, (M - Cl)-, and C1-. The cluster at m / z 301 contributed the remainder. This is consistent with the extra oxygen-containing species (Oz,HzO) present in the hydrogen as supplied. Argon as a reagent gas produced Me-, (M - Cl)-, and C1ions with relative abundance and pressure-dependent ion intensity changes similar to that produced by hydrogen reagent gas (Figure 1). In the presence of xenon, both the molecular anion and chloride ion abundance exhibited complex behavior with pressure change (Figure 3). The molecular anion's relative abundance decreased over the pressure range from 0.02 to 0.29 mmHg. At all higher xenon pressures, the molecular anion became the dominant ion in the mass spectrum. No other reagent gas studied exhibited this behavior. These effects can be quantitated by considering the fate of the ionizing electron in the presence of a buffer gas. The electrons collide inelastically with the buffer gas. Each collision transfers an average energy which is a fraction of its original mean kinetic energy. This occurs until the electron velocity is thermal with a concomitant increase of the electron energy distribution (6). For a given reagent gas, the mean electron energy decreases with increasing gas pressure, and higher reagent gas pressures cause a wider range of electron energy distributions. A decrease in the average kinetic energy of the ionizing electron occurs by two mechanisms: ionizationlexcitation of the reagent gas (7); kinematic energy loss by collision (8). The initial-mean electron kinetic energy is greater with increasing ionization potential of the reagent gas. Hence, greater reagent gas pressures are necessary in order to thermalize the electrons when helium (ionization potential (IP) = 24.59 eV) is used. In addition, it has been suggested that lower ionizing electron energies favor less endothermic processes (9). Electron affinity (EA) measurements for polychlorodioxins are not available. Presently, we are calculating dioxin electron affinities by a semiempirical SCF approach. A comparison with related polyhaloaromatics suggests that electron affinities become increasingly positive with greater degrees of chlorination. The first three negative ion resonance states of six polyfluorobenzenes are negative ( I O ) . In general, electron-
I
_______~
I ~
Hypothetical Potential Energy Surfaces for Chlorodioxins
(AI Resonance Electron Capture (8) Dissociative Electron Capture
Figure 4. Schematic potential energy diagram.
withdrawing substituents on an aromatic substrate should give more positive EA: a schematic potential energy surface illustrates the EA, vertical detachment energy (VDE) (Figure 4). Resonant electron capture occurs in the ground electronic state of the neutral. The anion is formed over a narrow energy range. The energy spectrum for resonant dissociative capture is given by the Frank-Condon factors. Both the relative abundance and type of negative ions observed will be extremely sensitive to the potential energy available to the neutral molecule. Transition probabilities are influenced by ionizing electron energy, Frank-Condon factors, and ion source temperature ( I O ) . The cross section for dissociative capture (uda) is equal to the product of the electron-capture cross section ( uc) and the dissociation probability at a particular ionizing electron energy, P(E)(eq 1) (11). Dissociative capture cross sections decrease = 6, P(E) (1) with greater electron energies. This may be effected by decreasing the gas pressure for a given reagent gas or increasing the ionization potential as demonstrated most dramatically by helium as reagent gas. Other factors also affect electron capture cross sections (11). Thus, considered from only a statistical point of view, the molecular anion should be most abundant a t low reagent gas pressures. Fewer collisions are needed with more massive reagent gas species in order to obtain a mean electron energy within the 0-15 eV range necessary for ionization ( 2 ) . The total ion current is maximized at 0.47,0.66,0.25,0.15, and 0.15 mmHg for reagent gases, hydrogen, helium, methane, argon, and xenon, respectively (Table 11),demonstrating in general that more massive reagent gas targets yield maximum total ion currents at lower reagent gas pressures. Maximum ion current is experienced when the majority of the ionizing electrons are thermalized. Helium is the most effective reagent gas for producing the molecular ion over a wide pressure range. The molecular anion constitutes approximately 90% of the total ion current. In addition, helium yields a total ion current 3 times greater than argon and 0.7 times that of hydrogen (Figure 5). Hydrogen
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
Table 11. Pressure Dependency of Mass Spectral Ion Currents for 1,2,3,4-TCDD with Various Reagent Gases ion source pressures for indicated conditions max total max molecular [M'-] = ion current anion current [Cl-]
reagent gas mol type wt IP
H2 He CH, Ar Xe
SF,
2 4 16 40 131 146
15.43 24.59 12.62 15.76 12.13 15.35
'Data not obtained.
0.37 0.65
0.47 0.66 0.25 0.15 0.15
-n
0.13 -2 -0
0.12 0.14
0.17 0.25
-i.
-h
-b
* ResDonse not measurable REAGENT GAS PRESSURE (mm Hg)
125K1
Figure 7. Concentration of 1,2,3,4-TCDD molecular anion relative to major dissociative channels as a function of reagent gas pressure.
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+
i
0
0 6-
r
U
+
04-
r 5
0 270 eV electron energy 90-C ton source temperature
A
REAGENT GAS PRESSURE (mm H g )
Figure 5. Total ion current as a function of reagent gas pressure for hydrogen, helium, and argon.
0~ 0
0.2
ION SOURCE PRESSURE
ion source temperature
I 0 , 0
1
02
0 4
,
I
06
OF XENON (mmHg)
Figure 8. Concentration of 1,2,3,4-TCDD molecular anion relative to major dissociative channels as a function of ion source pressure in the presence of xenon
70 eV electron energy 90'C
04
06
ION SOURCE PRESSURE OF XENON (mm Hg)
Figure 6. Total ion current from 1,2,3,4-TCDD in the presence of xenon.
reagent gas yields the absolute maximum total ion current. This occurred at a pressure ca. 0.47 mmHg. Argon (Figure 5 ) and xenon (Figure 6) by contrast show a maximum total ion current at the lowest pressure where the molecular anion intensity was still relatively large, ca. 54% and 64% relative abundance, respectively. Thus, from an experimental point of view, xenon is an effective reagent gas also, since less gas is needed in the ionization source for maximum total ion current. The conditions that favor resonance capture over dissociative capture as a function of reagent gas type and pressure are useful a priori experimental information. The molecular anion is formed by the mechanism of resonance capture. A plot of the concentration of molecular anion [Ma-]divided by the sum of the concentrations of [Me-] + [M - C1-] + [Cl-]
is approximately the fraction resonance capture at any given pressure. The last two terms in the denominator have as their primary contribution dissociative electron capture. The validity of this approximation will be affected by whether the ions Me-, (M - Cl)-, and C1- can account for a majority of the total ion current and by the fraction of molecular anion concentration that is not depleted by metastable decomposition. The effect of reagent gas pressure upon resonance capture as compared to two other dissociative electron capture channels is significant for hydrogen, methane, and argon but apparently much less so for helium (Figure 7 ) . Hydrogen, methane, and argon exhibit a dramatic decrease in resonance capture cross section over the pressure range of 0.05-0.7 mmHg, whereas helium as reagent gas causes a decrease of only 9% resonance capture over the same pressure range. In the presence of xenon, the fraction resonance capture exhibits a bimodal distribution (Figure 8). Initially, increasing xenon pressure causes a decrease in the fraction of molecular ion (Ma-) produced, and at xenon pressures exceeding 0.35 mmHg the relative amount of ions formed by resonance capture begins to increase. A change of ion formation mechanism appears to occur as the ion source pressure varies. In order to determine whether the ionic concentrations of the buffer gas were changing with ion source pressure, a measurement of the relative concentrations of Xe+/Xe2+was made (Figure 9). The Xe2+channel can dispose of 33.3 eV of the ionizing electron's energy. The Xe+/Xe2+ratio changed from 5 to 46 over the pressure range 0.05-0.3 mmHg. At higher pressures, an increase in M'- fraction is observed (Figure 8) and it is concluded that the bimodal behavior of
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986 HELIUM
HYDROGEN
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a E
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70 sV electron energy 9OoC ion source temperature
0
2911
relative error
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0
-
00
02
0.4
ION SOURCE PRESSURE OF XENON (mm Hg)
Figure 9. Xenon ion production as a function of pressure.
M'- production results from an increase in the mean electron energy. This occurs when the Xe2+ channel is no longer available for energy disposal. The prominence of the molecular anion when helium or xenon are present in the ionization source supports the view that greater ionizing electron energies seem to access the molecular anion channel (Figures 1and 8). Sulfur hexafluoride effectively removes all thermal electrons and gave only a weak C1- signal from 0.02 to 0.11 torr. The criterion for choosing an ion source pressure for a particular reagent gas may depend upon the information sought. Lower source pressures give a total ion current predominantly composed of only the molecular anion, and higher source pressures yield greater dissociative capture with less molecular anion current but greater total ion currents. The possibility that a collision-induced dissociation (CID) could account for the decrease in molecular anion concentration is unlikely. Hydrogen and helium, besides having center-of-mass collision energies that differ by only a factor of 2, have similar streaming rates, yet the molecular anion abundance for hydrogen and helium at 0.7 mmHg is 8% and 85%, respectively. For a CID occurring between the acceleration plate and quadrupole entrance, the probability of collision and internal energy deposition should be similar for targets of similar mass. Furthermore, the loss of molecular anion by both unimolecular and bimolecular decomposition was tested by a B/E linked scan on the MS50. Helium was chosen because it is the most efficient target gas for highenergy CID, and thus provided a more stringent test (12). When 1,2,3,4-TCDD was analyzed for unimolecular decomposition, the peak at m / z 285 was all but imperceptible. Only a small signal, ca. 0.1 %, of the m / z 320 current, was observed after torr of helium was admitted into the first-field-free region. Differences in the average internal energy of the molecular anion must, therefore, result from the ionization mechanism rather than CID. Another ion removal mechanism is neutralization by charge exchange
-
M'- + H e Mo + He*M'- + H2 Mo + H- + H'
-
(2)
(3)
The electron affinity for H- is 0.75 eV while that for He'production is only 0.08 eV (13). Assuming there is sufficient internal energy to dissociate the hydrogen molecule, reaction 3 may occur in the ionization source under NCI conditions. Reaction 3 is thus a possible mechanism for the depletion of the molecular anion concentration, and the insensitivity of molecular anion concentration as a function of helium gas pressure may be due in part to the lack of a suitable charge exchange channel.
REAGENT
GAS
P R E S S U R E (mm Hg)
Figure 10. Absolute anion abundances of M'- and CI- as a function of reagent gas pressure for hydrogen, helium, and argon.
An anion will undergo multiple collisions in the ion source before exiting. This excess energy could be transferred to the reagent gas and result in the formation of a metastable neutral reagent gas species. The first metastable state for helium (18 eV) is higher-lying than the metastable state for hydrogen (14) and will therefore absorb more of the anion's internal energy than the latter should the interaction occur. This would yield an intense molecular anion of lower internal energy (Figures 2 and 4). Electrons of the appropriate energy (0-10 eV) to form negative ions by resonance and dissociative capture can also be generated as secondary electrons during the formation of positive ions. Since the nascent molecular anion is formed by resonance capture, its internal energy must be at least equal to the electron affinity of 1,2,3,4-TCDD. The ion may dispose of the excess energy by the process of autodetachment and return to the neutral state (15). For larger molecules this excess energy can be partitioned over many degrees of freedom s (16, 17). Therefore, it is not unwith a lifetime of reasonable to expect that in order for a molecular ion to be observed during the time scale of the experiment, the nascent molecular ions may undergo collisional stabilization with the reagent gas (18). Absolute ion abundances of M'- as a function of reagent gas pressure exhibit the same shape for the various reagent gases. [Cl-] abundances exhibit similar shapes but maximize at pressures distinct from the [M'-] ions (Figures 10 and 11). These data suggest different mechanisms for ion formation. The initial rise in the [M'-] vs. pressure curves is due to a lowering of the ion internal energy by collisional stabilization (19,20) in the resonance electron capture mode. The initial rise in [Cl-] ion production with increasing pressure is rationalized on the basis that larger dissociative cross sections are observed at lower ionizing electron energies (eq 1). The negative slope portions of the absolute ion current plots occur because the ionizing electron energy is lowered to a value less than the energy required to access a given ion-producing channel, beam attenuation by collisions to remove ions from the acceptance cone of the quadrupole, statistically less likelihood of producing ions because thermal electrons are absorbed by other species, and reduced electron penetration a t increasing reagent gas pressures (21-23). The rate of ion depletion is dependent upon the energy distribution of the ionizing electron. Wider energy distributions cause a more gradual ion depletion with respect to reagent gas pressure. Regardless of the reagent gas, the abolute ion current max-
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986 6K 7 0 e V e l e c t r o n energy 9o'C #an W J ~ C B temperature
i 4K
i
i
I
ION SOURCE PRESSURE OF XENON (mm H g )
Figure 11. Absolute anion abundances, using xenon as reagent gas.
imized at consistently higher pressures for C1- than for Ma-. CONCLUSION Ion currents in the electron capture negative ion chemical ionization mode vary as a function of reagent gas pressure. Relative competition between resonance electron capture and dissociative capture is influenced by the reagent gas pressure, mass of the collision gas, and the ionization potential of the reagent gas. Heavier mass gases cause maximum total ion currents to develop at lower ionization source pressures. For similar reagent gas masses, a larger ionization potential will yield greater molecular anion abundance. An, in general, any process that raises the electron energy within the electron capture energy window will yield less dissociative capture. Further study of negative ion formation for a wide variety of systems is likely to reveal an equally wide range of behavior. Applications of this work could be extended to trace analysis of other chlorinated environmental toxins, such as dibenzofurans, biphenyls, and phenoxyphenols.
Registry No. HP, 1333-74-0; He, 7440-59-7; CHI, 74-82-8; Ar, 7440-37-1; Xe,7440-63-3;SFs, 2551-62-4;1,2,3,4-tetrachlorodibenzo-p-dioxin, 30746-58-8.
LITERATURE CITED Dougherty, R. C. Anal. Chem. 1981, 53, 625A. Budzikiewicz, H. Angew. Chem., Int. Ed. Engl. 1981, 2 0 , 624. Melton. C. E. Principles of Mass Spectrometry and Negative Ions; Marcel Dekker: New York, 1970. Stockl, D.; Budzikiewicz, H. Org. Mass Spectrom. 1982, 1 7 , 471. Stafford, G. C. EHP, Environ. Health Perspect. 1980, 35,85. Samaras, D. G. Theory of Ion Flow Dynamics: Dover Publications: New York, 1961. Inokuti, M. Rev. Mod. Phys. 1971, 43, 297. Massey, H. S. W. Trans. Faraday SOC.1935, 31. 556. Greaves, J.; Bekesi, J. G.; Roboz, J. Biomed. Mass Specfrom. 1982, 9 . 406. Frazier, J. R.; Christophorou, L. G.; Carter, J. G.; Schweinler, H. C. J Chem. Phvs. 1978. 69. 3807. Christophdrou, L. G: EHb, Environ. Health Perspect. 1981, 36,3. Hemberger, P. H.; Laramee, J. A.; Hubik, A. R.; Cooks, R. G. J . Phys. Chem. 1981, 85,2335. Rosenstock, H. M.; Draxl, K.; Seiner, B. W.; Herron, J. R. J. Phys. Chem. Ref. Data 1977, 6 . Moore, C. E. Nafl. Stand. Ref. Data Ser. ( U S . Nati. , Bur. Sfand.) 1988. N o . 23. Hue, A.; Cadez, I.; Grestaw, F.; Hall, R. I . ; Vichon, D.; Mazeau, J. Phys. Rev. A 1980, 21, 822. Naff. W. T.; Compton, R. N. J. Chem. Phys. 1971, 54,212. Naff, W. T.; Cooper, C. D.; Compton, R. N. J. Chem. Phys. 1968, 49. 2784. Szulejko. J. E.; Howe, I.; Beynon. J. H.; Schlunegger. U. P. Org. Mass. Spectrom. 1980, 75,263. Hunt, D. F.; Stafford, G. C.; Crow, F. W.: Russell, J. W. A n d . Chem. 1976, 4 8 , 2098. Albers, G.; Knof, H. Org. Mass Spectrom. 1978, 13,258. Hirst, D. M.; Jennings, K. R.; Laramee, J. A.; Shukla, A. K. I n f . J. Mass Spectrom. Ion Processes i985, 63, 119. Hirst, D. M.; Jennings, K. R.; Laramee, J. A. Int. J . Mass Spectrom. Ion Processes 1985, 64,335. Gregor, I. K.; Guilhaus, M. I n f . J. Mass Spectrom. Ion Processes 1984, 56, 167.
RECEIVED for review October 8, 1985. Resubmitted May 1, 1986. Accepted August 1,1986.We gratefully acknowledge support for this work from the National Institutes of Health, NIEHS ES00040 and NIEHS ES00210. This paper is issued as technical paper no. 7681 from the Oregon Agricultural Experiment Station.
