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Society for Testing and Materials, Philadelphia, Pa., 1976, pp 101-113. (4) D. S. Simons and C. A. Evans, Jr., Anal. Chem., 48, 1341 (1976). (5) J. J. Larsson, A. Lodding, H. Odelius, and L. G. Petersson, "Calibration Methods for Ion Probe Application to Fluorine in Biological Hard Tissue", US-Japan Joint Seminar on SIMS. Honolulu, Hawaii, 1975. (6) H. Tamura, T. Ishitani, and I.Kanamota, "Correction for Ionization Yieid", US.-Japan Joint Seminar on SIMS. Honolulu, Hawaii 1975. (7) G. J. Scilla and G. H. Morrison, Anal. Chem., 49, 1529 (1977). (8) J. D. Fassett, J. R. Roth, and G. H. Morrison, Anal. Chem., 49, 2322 (1977).
(9) A. E. Morgan and H. W. Werner, Anal. Chem., 48, 699 (1976). (10) T. Ishitani, H. Tamura, and T. Kondo, Anal. Chem.. 47, 1294 (1975).
RECEIVED for review August 15, 1977. Accepted October 27, 1977. Financial support was provided by the National Science Foundation under GrantNo. CHE-7608531 and through the Cornel1 Materials Science Center.
Measurement of Electron Capture Rates for Chlorobenzene with Negative Ion Plasma Chromatography Glenn E. Spangler" MERADCOM, Attn: DRDME-0, Fort Belvoir, Virginia 22060
Phll A. Lawless Research Triangle Institute, Research Triangle Park, North Carolina 27709
Theory and experimental procedures are developed to allow measurement of ion-molecule reaction rates with plasma chromatography. The procedures Include the use of the exponential dilution flask. Diagnostically, the dlssociative electron capture of chlorobenzene is shown to procede at a rate of 7.1 f 3.1 X IO-" cm3 molecule-' s-' in agreement with similar results obtained with swarm beam techniques.
Plasma Chromatography is a n analytical technique which has been developed in recent years to detect and identify trace constituents of organic vapors in gaseous mixtures under atmospheric pressure conditions. Review articles describing the technique have been provided by Karasek ( I ) and Metro and Keller ( 2 ) . T h e vapors to be detected are ionized by ion-molecule reactions in the presence of a 63Niradioactive source and are sorted by a coupled ion mobility drift tube spectrometer (3). T h e readout is a n arrival time spectrum of ion current peaks separated by mobility differences through the drift region. Among the parameters affecting the response of the instrument are the nature of the reactant ion and reaction paths available for the ion-molecule reactions ( 4 ) , rate constants affecting the direction and speed of the ionization and recombination processes, efficiency for ion current extraction from the near radioactive source region, masking and/or false identification of ion mobility peaks due to the presence of interfering species ( 5 ) ,peak shape as it is affected by longitudinal diffusion and gate width settings, ( 6 ) ,and ion loss through transverse diffusion and/or charge detachment. Experimentally, changes in gate width, drift temperature, electrostatic drift field, reagent carrier gas composition, sample composition, and sample concentration can all significantly affect results. In particular, when nitrogen is used as drift and reagent carrier gases, electron capture processes are the reactions generally observed producing negative ions. Karasek et al. (7-9) have studied t h e ionization of chlorobenzene and dichlorobenzene in plasma chromatography and have shown that these compounds dissociatively capture free electrons to form detached chloride anions. Resonance capture by these molecules is endothermic by about 0.86 and 0.36 eV, respectively. This is postulated to be due to the 0003-2700/78/0350-0290$0 1.OO/O
formation of an intermediate molecular anion before dissociation into an halide anion and a radical. Thermal electron capture is also possible (10-12). Although the relative attachment coefficients for organic compounds are generally referenced to that for chlorobenzene in the electron capture detector (13-18), the operating conditions utilized in plasma chromatography do not allow direct comparison with ECD data. Rather, E / P values of 0.28 V/cm-Torr, in nitrogen, typically chosen for optimum operation of t h e plasma chromatograph, allow the generation of electrons with a n average energy of 0.3 eV (19). Endothermic reactions are consequently more probable than exothermic or thermal reactions in comparison with thermal electron data. In the present work, dilution flask experiments that measure the efficiency of ionization of chlorobenzene in plasma chromatography are reported. A theoretical treatment is provided to describe the mechanisms of reaction involved in the reaction region of the instrument and to arrive a t values for the dissociative capture rate constant.
EXPERIMENTAL The experimental setup is illustrated in Figure 1. The data were collected on a Franklin GNO Beta/VII Plasma Chromatograph consisting of an ion-molecule reaction region and a coupled ion-mobility drift tube spectrometer. Mobilities of the product ions formed in the reaction region are measured in the drift region by either a one-grid or two-grid pulsing procedure. The one-grid pulsing procedure consists of measuring the ionic drift time spectrum from the pulsed shutter grid (grid 1) to the collector with fast electronics while the two-grid pulsing procedure utilizes a second grid as a variably delayed gate to sample a narrow band of the ion mobility spectrum. For the one-grid pulsing procedure, readout was accomplished with a Nicolet 1072 signal averager, Hewlett-Packard 7035 x-y recorder, and/or Tektronix 5103 oscilloscope. To collect dilution flask data, the two-grid pulsing procedure was used with an automatic baseline correcting circuit whose performance was checked by repetitive scanning and manual integration in the one-grid mode (20). Operating parameters used in the performance of these experiments are displayed in Table I. The dilution flask was of cylindrical design and constructed of Pyrex glass. It was housed in a Thermolyne OV-10600 hot plate oven modified to sit above a Thermolyne Nucerite stirring hot plate. A magnetically driven rotor, with the magnet enclosed in glass, was included in the flask; speed could be adjusted by settings 0 1978 American Chemical Society
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paring new batch samples periodically throughout the experiments, assuring syringe cleanliness by interposing benzene washes between injections, and establishing baselines with blank injections of pure benzene.
THEORETICAL Ionization. T h e theory for the ionization of vapor species in plasma chromatography is derived from the theory developed for ion-mobility drift tube spectrometers (21). T h e transport equations describing the three-dimensional drift and diffusion of the reactant ion concentration nR, neutral sample concentration .Vp and the product ion concentration n p in the presence of ion-molecule reactions are
Table I. Experimental Parameters Plasma chromatograph 0.2 ms Gate 1 width 1.0 ms Gate 2 width 40/s Repetition rate Prepurified nitrogen Carrier gas Prepurified nitrogen Drift gas Drift tube pressure Atmospheric Drift tube temperature 200 "C Drift field 214 V/cm Standing open grid current 5 . 2 5 x lo-'' A Electrons Positive Ions 3.77 x 1 0 - " A Reaction region geometry Cross-sectional area 1.56 cm' Length 6.0 cm Dilution flask Flask temperature 195 "C 469 c m 3 Flask volume 60 rpm Rotor speed 48-101 cm'(NTP)/min Carrier gas flow (Data normalized to 50 cm'imin at 200 "C) available on the stir plate. No significant difference was observed in the exponential decay of the sample concentration with the rotor stirring or not. Since the dilution flask was coupled directly to the plasma chromatograph, the carrier gas from the treatment system of the plasma chromatograph was delivered directly through the dilution flask to the inlet of the instrument. The flask was heated independently of the plasma chromatograph, and the carrier gas inlet line was heated by a wrapped heater tape. Syringe injection of sample into the flask established an initial concentration ."ioof sample within the flask. The gas exit of the plasma chromatograph was connected to a variable speed suction pump in parallel with an oil manometer. This arrangement allowed pressure drops across the plasma chromatograph to be monitored and stabilized. The manometer pressure was adjusted to be less than atmospheric (0.5 cm of oil < AP < 1.0 cm of oil), but not enough less to draw laboratory air through the sample injection hole of the dilution flask. The latter condition was made evident by formation of negative molecular reactant ions (identities unspecified) from the free electrons normally generated in nitrogen. This slight underpressure minimized problems associated with backflushing of the injected sample. Reagent grade chlorobenzene was used as the sample for these experiments. Gas or liquid syringes of various sizes were used to inject vapor or liquid samples, respectively, into the flask. Vapor samples were withdrawn from a closed 551-cm3Erlenmeyer flask which had been flushed with nitrogen flowing at a rate of 450 cm3/min for 30 to 60 min, injected with 1gL of pure chlorobenzene liquid, and then mixed for 20 min with a magnetic stir bar. Liquid samples were withdrawn from solutions of chlorobenzene with pesticide grade benzene (Fisher), prepared by volumetric dilution. Reproducibility was established by using clean glassware, pre-
?
d2Np
-
dnP dt
-= D p n ,
F dNp .
-Popn+np + kpnRNl,
(3)
where h p is the ionizing ion-molecule reaction rate, DLN is the longitudinal molecular diffusion coefficient for the neutrals! A, is the cross-sectional area of the cylindrical drift tube, F is the volumetric flow of carrier gas, and a p is the ion-ion recombination coefficient describing loss of product ions during their exposure to positive ions in the near radioactive source region, which is a fraction 3 of the reaction region volume. D Rand Dp are operators given by
where DT, and DLi are the transverse and longitudinal diffusion coefficients respectively for the various species and u, is the ion drift velocity in the direction of the applied electrostatic drift field, E. A term linear in the gradient of the drift velocity is included in Equation 4 to correct for space charge effects. Since the drift velocities are
v i = piE
i = R,P
(5)
where ~i is the mobility, and since Poisson's equation stipulates
where V is the electrostatic potential, e is the electronic charge, and to is the permittivity of free space, the space charge term becomes
(7) where the ion concentrations are averaged over the observation time so that nonlinearity can be removed from the differential equations. Mathematical simplicity can be achieved in obtaining solutions by working near the limit of detection where nR >> np. T h e steady state solution for the neutral concentration is
where the spacial attenuation coefficient mN is given by
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(9) and Ni is the inlet concentration of sample introduced into the reaction region a t z = 0. T h e coefficient of Equation 8 is independent of F and the expansion of Equation 9 is allowed when B > 1.5 cm3/min > Ar(4DLNkPnR)’/’. This condition is satisfied for the capture of free electrons in nitrogen (Le., k p < IO-’ cm3 molecule-’s-l, n R = 1.2 x io4 ion/cm3. UR = 5 x lo5 cm/s, DLN = 0.10 cm’/s). Np is equivalent to N , when l r n ~ l
