Anal, Chem. 1980, 52, 193-196
193
Comparison between Plasma Chromatography and Electron Capture Detector Sir: Since its earliest conception by Lovelock ( I ) , the electron capture detector (ECD) has developed into one of the leading sensors for gas chromatography. The popularity which it enjoys is related to its mechanical simplicity as well as the sensitivity and selectivity which it enjoys toward compounds of biomedical and environmental significance. In the detector, a small volume is irradiated by short range /3 particles tn create in selected carrier gases dense distributions of positive and negative ions. When placed in a suitable electrostatic potential, the negative ions migrate out of the irradiated zone toward an anode to generate a steady base-line current. By properly pulsing the electrostatic potential, the free electron component of this base-line current can be selectively monitored. For detection, a sample compound is introduced into the volume tc, either dissociatively or nondissociatively capture the free electrons. This results in a drop in current due to changes in ion mobility and recombination rates for the charged particles within the cell. The current satisfies ( 2 )
where Zb is the average detector standing electron current in the absence of an electron capturing species, I , is the average detector current in the presence of an electron capturing species, K is the capture coefficient, and a is the concentration of the capturing species. Pellizari has provided a review for the analytical applications of ECD (3). Atmospheric pressure ionization mass spectrometry (APIMS) has allowed more thorough investigations into EC reactions in recent years ( 4 4 ) . A more recently developed technique for analytical chemistry is plasma chromatography (PC) (7,8). I t addresses the same problems as the ECD except that it provides an additional capability to monitor positive ions as well as negative ions. Although its flexibility may be enhanced when used in conjunction with gas chromatographic separation techniques ( 9 ) , PC is also capable of achieving specific results independently of such techniques. This performance is achieved by an ion-molecule reactor coupled to an ion mobility drift tube spectrometer (10). Analogous to the ECD, the reactor region is a small volume irradiated by short range fl particles to create in selected reagent carrier gases dense distributions of positive and negative ions. When placed in a suitable electrostatic potential, the ions migrate out of the irradiated zone toward a pulsed shutter grid where a portion of the impinging ion cloud is sampled for analysis by the ion mobility spectrometer. The readout is ion current peaks separated by drift time differences as the ions penetrate the drift length of the mobility spectrometer with characteristic drift times. Reviews of PC have been provided by Karasek ( I I ) , Keller and Metro (12),and Revercomb and Mason (13). The purpose of this paper is to discuss problems which may be associated with intercomparisons between ECD and PC. For detection with PC, a sample compound is introduced into the reactor volume to produce product ions from ensuing ion-molecule reactions. The identities of the product ions are not only characteristic of the compound being analyzed, but also of the composition of the reagent carrier gas and the constituents of the mixture which is injected with the sample into the reactor volume. For a pure sample compound with a single product ion response, the average product ion current satisfies (14) 0003-2700/80/0352-0193$01.00/0
I, =
ArYIY!lY&pnRdVieUp
- cHz,r)
P ~ p n + / -+ aspace
(2)
where k , is the rate constant describing the ion-molecule reaction, nRo is the standing axial reactant ion concentration, N , is the sample concentration, e is the electronic charge, up is the product ion velocity, n+,. is the opposing ion concentration, a, is the ion-ion recombination coefficient, /3 is the fraction of the reaction time spent by the product ions in the near radioactive source region, aspace is a correcting space charge term, y1 is the effective transmission of the grids, yz is the proportional amount of time the shutter grid is open, y3 is the effective transmission of the drift region corrected for longitudinal and transverse diffusion, A, is the cross sectional area for the cylindrical drift tube, and +(z,r) contains a functional dependence on the cylindrical orthogonal coordinates z and r. Although the complexity of this expression for P C differs greatly from the simplicity of Equation 1 for the ECD, the two are related under special sets of conditions. Ever since its initial introduction, researchers have compared P C to other technologies. ‘These include chemical ionization mass spectrometry, atmospheric pressure ionization mass spectrometry, and the electron capture detector (11,151. In particular it has been realized that the data contained in the negative mobility spectra of P C are complementary to those obtained with the ECD in that the negative ions formed in PC may be responsible for the decrease in standing current observed in the ECD. Karasek et al. have demonstrated that results obtained with negative ion plasma chromatography (NIPC) using nitrogen drift and carrier gases can supplement ECD results by simultaneously monitoring the electron and product ion currents during sample analysis (16). Studies have also been made which extend the interpretation of data in NIPC to ECD (17, 18). More recently, Spangler and Lawless have reported ion-molecule reaction rates ccdlected under conditions which might be compared to the ECD (14,19). Few users of PC and the ECD understand and/or appreciate the conditions which make one technique more or less sensitive than the other.
THEORETICAL The theoretical basis for an intercomparison between ECD and NIPC has been lacking despite the seeming simplicity with which ion-molecule reactions can be described in PC (8). T o achieve such a comparison, the expression for the PC ion current of Equation 2 can be placed in a format similar to that for the ECD described in Equation 1. This is accomplished by extending Equation 2 to high concentrations and drawing an analogy between the product ion current of NIPC and the difference current, I b - I,, of ECD. Such an analogy leads to a capture coefficient for NIPC which is equivalent to the capture coefficient for the ECD. To modify Equation 2, reference must be made to Maxwell’s laws of electricity and magnetism. A consequence of these laws is that the current flux density J in PC must be conserved and must satisfy the continuity equation C*J= 0
(3)
That is, current flux entering a closed volume must be compensated by a current flux out of the closed volume if the charge density within the volume is to remain finite. This assumes that no recombination can occur outside the imme8 1979 American Chemical Society
194
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
Table I. Summary of Published Rate Constant and Electron Capture Data plasma chromatography' electron _____-__ _____ mass spectrometry,' reaction rate chloroc1n3 molecule-' capture coefficient, benzene swarm beam L mol-' =1 S-' compound s - ' Torr-' 1.2 x l o a 1.o 1.4 x l o 6 7 . 1 x 10." chlorobenzene bromo be nze ne 9.5 x l o h 6.0 390 nitrobenzene *1.3 X 10' 3.3 x l o - ' ' 3.6 x 1 0 7 0-MNT m-MNT 8.3 x lo-" y.1 x 1 0 7 p-MNT 2.1 x lo-" 2 . 3 x 10' 2,4-DNT 2.3 x lo-'" 2.4 x l o x 2,4,6-TNT 8.6 x lo-'" 8.2 x l o K *7.d x l o R 104 SF, 104
CCI, a
(*)
capture
-
L mol-' 8.4 x 2.6 x
lo6 io9
ref
14, 23
3,23-25 25-27 14 14 14 14
8.1x 109
14,28 26, Z 9
3.Y x 1 O ' O
2 7 , 29
Except for ( * ) data, all values are for an E / P of 0.282 V C M - ' Torr-' and are adjusted to a temperature of 200 "C. For data, EIP < 0.1 V cm-' Torr-' with the pressure normalized t o a temperature of 200 "C. _______
diate vicinity of the radioactive source and that losses of ions to the walls are negligible. In terms of ion concentrations in the reactor, this means ~ R N U R= ~ R S ~ npup R
(4)
where n R N is the reactant ion concentration in the presence of no sample, n R S is the reactant ion concentration in the presence of sample, n, is the product ion concentration, uR is the reactant ion drift velocity, and up is the product ion drift velocity. Equation 2 was derived under the assumption that the reactant ion concentration is greater than the product ion concentration (Le., npupsmall compared to nRsf.R' and n s u R ) . T o a first approximation, this assumption can be removed by substituting nRSof Equation 4 for nRo of Equation 2. The result is
I, =
A,Y I Y P Y B ~ ~ ~ R N N F U ~ @a(t,r) (51 Papn+/ + aspace + k p N ~ p / u ~
where n R N is now redefined as the standing axial reactant ion concentration observed without sample. NIPC can be compared to the ECD by drawing a simple analogy between the product ion current I , of NIPC and the standing electron current Ibless the sample reduced electron current I , of the ECD. By direct substitution, the product ion current 1, of Equation 5 for NIPC becomes
I, = Ib
Ka
___
1 + Ka
where
a = Ni and
Equation 9 gives a coefficient for NIPC which is equivalent to the capture coefficient for the ECD. T h e capture coefficient for NIPC is dependent on the forward ion-molecule reaction rate, a reverse recombination reaction rate, space charge, and a compensating velocity ratio required to conserve current. This differs from the thermal capture theory of Wentworth and co-workers (20-22) which is independent of a velocity ratio. A thermal theory does not require a velocity ratio since uR for electrons is nearly equal to up for ions under the assumed conditions. Such conditions, however, are not satisfied in the presence of an electrostatic
drift field. The velocity and the absence of collisional thermalization of the electrons explains why electron capture detectors utilizing continuously applied fields are less sensitive than those using pulsed fields. RESULTS AND DISCUSSIONS Three sources of data are available for comparison purposes. These include: (1)capture coefficient data for the ECD, (2) reaction rate data for NIPC, and ( 3 ) attachment coefficient data from mass spectrometric swarm beam measurements. Summarized in Table I is a noncomprehensive summary of data selected for intercomparison purposes. Since values for n,, uR, and up are known for NIPC, the relative capture coefficients of Table I are calculated via Equation 9. Table I involves several difficulties for interpretation purposes. Most of these difficulties arise from the variety of conditions under which most of the data have been collected. These include: First, only one compound (chlorobenzene) has been measured by all three techniques. However, several compounds are common to two techniques. Second, ECD capture coefficient data are almost always collected under thermal conditions where up is approximately equal to uR. Electron energies near 0.03 eV are therefore utilized. Third, NIPC rate constant data are collected with E / P values near 0.28 V cm-' Torr-' where up is not equal to L'R. Electron energies near 0.49 eV are therefore utilized (30). Fourth, mass spectrometric swarm beam data are collected under variable pressures ranging from 250 to 800 Torr. Data for chlorobenzene and bromobenzene can be adjusted to an E / P of 0.282 V cm-l s? for comparison with PC data (23)while data for sulfur hexafluoride and nitrobenzene collected with an E / P less than 0.1 V cm-' Torr-' are more easily compared with the ECD (26). Fifth, ECD capture coefficient data are known to be temperature dependent. Similar data are not available for NIPC while electron attachment rates in swarm beam data display temperature effects related to total pressure variation (26). Consequently, adjustments required for the intercomparison of data are complicated by a lack of knowledge of temperature relationships. Despite these difficulties which need further study for careful elucidation, a trend has been observed in the data. From careful study of Table I, we make the following observations. First, the reaction rate for NIPC is related to the attachment rate for swarm beam mass spectrometry by a factor of kT (2 X 10l6molecules cm-3 Torr-' for Table I) where k is the Boltzmann constant and T i s the absolute temperature. Under reasonable assumptions about the reaction volume in NIPC
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
at similar E / P conditions, reaction rate data for chlorobenzene in NIPC compare favorably with similar swarm beam attachment rate data ( 1 4 ) . Second, capture coefficient data collected with energetic electrons ( E / P = 0.28 V cm-' Torr-') in NIPC are high for chlorobenzene and low for trinitrotoluene when compared with thermal electron ECD data. The absolute electron capture data gives T N T a relative capture coefficient of about 1000 to 6000 with respect to chlorobenzene, while the NIPC capture coefficients are in the ratio of 6.8. Third, assuming SF6 and C C 4 have similar thermal ( E / P 5 0.1 V cm-' Torr-') swarm beam attachment rates, attachment rates to electron capture coefficient ratios are about 1.3 x s Torr for the electron capture data referenced to chlorobenzene and 50 s Torr L mol-' for the absolute electron capture data. Similar ratios are also observed for nitrobenzene: 3x s Torr for electron capture data referenced to chlorobenzene and 215 s Torr L mol-' for absolute electron capture data. On the other hand, the ratios of the energetic ( E / P = 0.28 V cm-' Torr-') swarm beam attachment rate for chlorobenzene and bromobenzene to the corresponding electron capture coefficient are 6.7 X and 6.3 X Torr, respectively. The ratio for chlorobenzene is two orders of magnitude less than the ratios for SF6,CCl,, and nitrobenzene and the ratio difference appears to be significant. Fourth, the ratio of swarm beam attachment rates for bromobenzene relative to chlorobenzene is 6.8. This result for energetic ( E / P = 0.282 V cm-' Torr-') electron data is similar for thermal electron capture data collected with the electron capture detector. Hence, for these two compounds, consistent results are obtained for relative data in two electron energy ranges. Fifth, the reaction rates for the nitrotoluenes in NIPC increase for the higher members of the homologous series and show variation with isomeric substitution. Christophorou and Grant attribute the first characteristic to the ability of the larger molecules to absorb extra energy (comprised primarily of the electron affinity of the molecule and the kinetic energy of the incident electron) due to their larger number of vibrational degrees of freedom (31, 32). Christophorou, Compton, Hurst, and Reinhardt attribute the second characteristic to a decrease in the dissociation energy due to the proximity of the CH3 and NO2 groups in the ortho and para positions (23). Sixth, the average reaction rate observed under energetic ( E / P = 0.28 V cm-' Torr-') conditions in NIPC for nitrotoluene is 4.6 X lo-" cm3 molecule-' s-'. This reaction rate corresponds to a swarm beam attachment rate of 9.4 x lo5 s-' Torr-' which is low when compared to the thermal ( E / P 5 0.1 V cm-' Torr-') attachment rate of 1.3 X lo7 s-' Torr-' observed for nitrobenzene. Since nitrotoluene is expected to have a faster attachment rate than nitrobenzene because of its ability to more effectively share energy among its larger number of degrees of freedom for vibrational energy, experimental evidence is in apparent contradiction to theory unless the energy of the electron (as indicated by E / P ) influences the results. Seventh, the average capture coefficient of 4.6 X lo7 obtained for MNT under the energetic ( E / P = 0.28 V cm-' Torr-') conditions of NIPC is 6% of the NIPC capture coefficient for trinitrotoluene. Assuming that this relative percentage is significant and can be applied equally as well to thermal ECD data, nitrotoluene should have an ECD capture coefficient of 4.9 x 108 L/mol relative to trinitrotoluene. This proposed value for the ECD capture coefficient for nitrotoluene is an order of magnitude less than that observed for nitrobenzene, and is contrary to considerations based on the effects which vibrational energy has on ion
0 2
0
02
04
36
38
0
195
I2
E L E ~ T R O NEUERGY l t V l
Flgure 1. Comparison of electron-capture cross section as a function of electron energy for nitrobenzene and chlorobenzene (from ref. 23) for electron energies typically employed in NIPC and the ECD. Energy scale calibration is based on SFa- and is not corrected for the swarm beam combination. The peak width for nitrobenzene is instrumental
stability as described above. Hence, conclusions derived from relative data support the conclusions derived from absolute data. To understand more completely these issues, special attention must be given to the mechanisms involved for negative ion formation. Stable negative-ion formation can be viewed as proceeding via a negative-ion intermediate that is itself formed by electron capture in the field of the ground electronic state or by electron capture in the field of an excited state. The intermediate is formed whenever the incoming electron associates with the molecule for a time longer than the transit time of the electron. The time the electron is retained by the molecule can be referred to as the negative-ion (autodetachment) lifetime (32). During the autodetachment lifetime ( T J , the transient molecular ion may decay by one of two different schemes: (1) dissociatively to form ion fragments, or (2) nondissociatively to form molecular negative ions. Dissociative decay follows capture of electrons within a restricted energy range (resonance capture) to form an extremely short lived s IT~ 5 s) intermediate ion which subsequently fragments into a neutral and a negative ion. Nondissociative decay generally follows thermalization or stabilization of the intermediate ion through collision with a neutral so that internal energy (primarily vibrational energy derived from the electron) is removed from the intermediate ion. Under the atmospheric pressure conditions of ECD and NIPC, only negative ions with autoionization lifetimes greater than s can be studied. For these conditions, dissociative capture is observed whenever the electron energy approaches the resonance energy required to initiate the reaction. The capture cross sections for nondissociative capture by nitrobenzene and dissociative capture by chlorobenzene as determined by swarm beam mass spectrometric techniques are displayed in Figure 1 (23). Capture cross sections for nondissociative capture by nitrotoluene are very similar to those for nitrobenzene (23). As indicated in Figure 1,the electron energy of 0.49 eV utilized in NIPC lies closer to the resonance energy required to dissociate chlorobenzene than to the thermal energy required to ionize nitrobenzene. The same is also true for bromobenzene (23). This fact probably accounts for the early success which NIPC displayed in the analysis of halogenated compounds as reported in the open literature. By contrast, the electron energy of 0.03 eV utilized in the ECD lies closer to the thermal energy required to ionize
196
Anal. Chem. 1980, 52, 196-199
nitrobenzene than to the resonance energy required to dissociate chlorobenzene and bromobenzene. Hence differences in electron energies lead to differences in ionization rates between NIPC and the ECD. These differences not only complicate absolute comparisons between the two techniques, but also complicate comparisons based upon responses relative to chlorobenzene. As a result of this observation, future comparisons of NIPC to ECD must include data providing: (1) Knowledge of the product ion velocity up and the reactant ion velocity uR so that corrections for the velocity ratio of up/uR in the capture coefficient of Equation 9 can be made for NIPC. (2) Similar values for the electric field to pressure (E/&') ratio so that electron energies are the same and the effects of resonance capture are minimized between the two techniques. (3) Similar temperatures for the two techniques so that thermal considerations on the capture coefficient do not compromise comparison of the results. While the purpose of this paper is not to provide a recipe for future comparisons, each of the above criteria can be satisfied by pulsing the electric field in the reactor of NIPC at proper temperature before comparing results with ECD. While such a capability is not presently available in PC instrumentation, it has been previously considered (33).
CONCLUSIONS From theoretical considerations in plasma chromatography, it has been demonstrated that the capture coefficient for the electron capture detector depends on the ratio of the product ion to the electron drift velocities. In the case of the pulsed electron capture detector, these velocities cancel since the capture process occurs under thermal conditions during the proportional part of the pulse cycle where the electric field is removed from the detector volume. This is unlike the situation for plasma chromatography which has an extraction field continuously applied to the reaction region. Corrections must be made for this difference before capture data can be compared. Due to differences in electron energies used to collect capture data with the electron capture detector and plasma chromatography, differences are observed in ionization rates between the two techniques. Compounds which undergo resonant capture at elevated electron energies (-0.49 eV) are presently detected more efficiently with plasma chromatography while those which undergo thermal capture are detected more efficiently with the electron capture detector. Because of differences in the electron energies, relative capture data referenced to chlorobenzene obtained with the electron capture detector are not directly comparable to relative capture data obtained with plasma chromatography.
LITERATURE CITED (1) J. E. Lovelock and S. R. Lipsky, J . A m . Cbem. Soc., 82, 431 (1960). (2) W. E. Wentwofth, E. Chen, and J. E. Lovelock, J . Pbys. Cbem., 70, 445 (1966). (3) E. D. Pellizzari, J . Chromatogr., 98, 323 (1974). (4) E. C. Horning, D. I. Carroll, I. Dzidic, S-N Lin, R. N. Stillwell, and J. P. Thenot, J . Cbromatogr.. 142, 481 (1977). (5) M. W. Siege1 and M. C. McKeown, J . Cbromatogr., 122, 397 (1976). (6) E. P. Grimsrud, S.H. Kim, and P. L. Gabby, Anal. Chem., 51, 223 (1979). (7) F. W. Karasek, Res. Dev., 21(3), 34 (1970). (8) A. Zlatkis, Adv. Cbromatogr., 37-50 (1970). (9) F . W. Karasek and S. H. Kim, J . Cbrumatogr., 99, 257 (1974). (IO) E. W. McDaniel and E. A. Mason, "The Mobility and Diffusion of Ions in Gases", Wiley, New York, 1973. (11) F. W. Karasek, Anal. Chem., 45, 710A (1974). (12) R. A. Keller and M. M. Metro, Sep. Purif. Methods, 3(1), 207 (1974). (13) H. E. Revercomb and E. A. Mason, Anal. Chem., 47, 970 (1975). (14) G. E. Spangler and P. A. Lawless, Anal. Cbem., 50, 290 (1978). (15) D. I. Carroll, 1. Dzidic, R. N. Stillwell, M. G. Horning, and E. C. Horning, Anal. Cbem., 48, 706 (1974). (16) F. W. Karasek, S. H. Kim, S. Rokushika, and H. H. Hill, 29th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, 1977. (17) F. W. Karasek and D. M. Kane, Anal. Chem., 45, 576 (1973). (18) F. W. Karasek, 0. S.Tatone, and D. M. Kane, Anal. Chem.. 45, 1210 (1973). (19) G. E. Spangler and P. A. Lawless, Anal. Chem., 50, 884 (1978). (20) W. E. Wentworth, E. Chen, and J. E. Lovelock, J . Phys. Chem., 70, 445 (1966). (21) W. E. Wentworth, R. S. Becker, and R. Tung, J . Phys. Cbem., 71, 1652 (1967). (22) D. E. Durbin, W. E. Wentworth, and A. Zlatkis, J . Am. Cbem. SOC.,92, 5131 (1970). (23) L. G. Christophorou, R. N. Compton, G. S. Hurst. and P. W. Reinhardt, J . Chem. Pbys., 45, 536 (1966). (24) P. Devaux and G. Guiochon, J . Gas Cbromatogr., 5 , 341 (1967). (25) A. Zlatkis and J. E. Lovelock, Clin. Chem.. 11(2), Suppl. 259 (1965). (26) R. N. Compton, L. G. Christophorou, G. S. Hurst, and P. W. Reinhardt, J . Chem. Pbys., 45, 4634 (1966). (27) W. E. Wentworth and E. Chen, J . Gas Cbromatogr., 5 , 170 (1967). (28) G. E. Spangler, Am. Lab., 7(7), 36 (1975). (29) C. A. Clemons and A. P. Altshuller, Anal. Chem., 38, 133 (1966). (30) L. G. H. Huxley and R. W. Crompton, "The Diffusion and Drift of Electrons in Gases", Wiley, New York, 1974. (31) L. G. Christophorou, "Atomic and Molecular Physics", Wiley, New York, 1971, Chapter 6. (32) L. G. Christophorou, M. W. Grant, and D. L. McCorkle, "Advances in Chemical Physics", Volume 36, I. Prigogine and S.A. Rice, Eds., Wiley, New York, 1977, pp 413-520. (33) M. J. Cohen. US. Patent 3593018, 13 July 1971.
'
Present address: Bendix-EPID, 21204.
1400 Taylor Avenue, Baltimore, Md.
Glenn E. Spangler* MERADCOM, Attn: DRDME-US Fort Belvoir, Virginia 22060
Phil A. Lawless Research Triangle Institute Research Triangle Park, North Carolina 27709 RECEIVED for review July 25, 1979. Accepted September 26, 1979. This research was performed under the authority of the Department of Army Project lT161101A91A, Fort Belvoir, Va. The paper has been authorized release as nonsensitive material.
Redox Stability of Inorganic Arsenic(II1) and Arsenic(V) in Aqueous Solution Sir: The development of methods for the species specific determination of As(II1) and As(V) in natural water has received considerable attention (1-3). The differences in toxicities among inorganic As(II1) and As(V) and the various organoarsenicals ( 4 ) has generated much of the impetus for 0003-2700/80/0352-0196$01 .OO/O
the development of speciation techniques, and detection limits of less than 1 ng have been achieved by various analytical methods (1-3). Recently, we have reported that the ratio of As(II1) to As(V) may be useful in gauging the apparent redox condition present in groundwater ( 5 ) . In a "closed" groundC 1979 American Chemical Society
