Hydrogenation of Organics. The effect of the palladium surface on organic compounds has been reported by Simmonds et al. (2), who used a palladium alloy tube suspended in air or oxygen as the GC/MS interface. Tests with the cell described in this report yield similar results; i.e., compounds containing conjugated double bonds or triple bonds are hydrogenated, whereas saturated compounds and compounds containing non-conjugated double bonds are not. Life Test. One of the major concerns about the cell was its ability to withstand repeated heating and cooling, and discontinuous exposure to hydrogen. The electrolyte expands at a high rate in its solid form and the effect of this volume change on the stability of the palladium alloy tubes required investigation. In addition, the palladium alloy tubes themselves expand in the presence of hydrogen and contract when it removed (12). In order to determine if these effects would have an adverse effect on separator operation, a life test was devised. The test was based on a 6-hour heating/cooling cycle as shown in Figure 7. After 2000 hours of continuous testing in this mode of operation, the test was terminated. The cell was disassembled and visually inspected for microscopic cracks or fissures in the palladium alloy tubes. None were found. A sample of the electrolyte was analyzed for traces of palladium and silver. Less than 10 ppm (the limit of detection) of each metal was found. From these tests it could be (12) A. S . Darling,Platinum Metals Rec;., 7,126 (1963).
concluded that the concept, design, and construction of the cell were adequate to ensure reliability. Four more cells of this design have been built and tested since the life test: all have yielded equivalent performance. Chromatogram from Total Ion Current Monitor. Figure 8 illustrates separator performance in the GC/MS operating mode. The column used was a 4-ft by 0.030-in. i.d. stainless tube packed with 3 % Dexsil 300 on Chromosorb W, HP 60/80 mesh followed by a 200-ft by 0.020-in. i.d. stainless tube coated with 5x W/V solution of Dexsil 300/Igepal CO 990 (20 :1). The column temperature was programmed from 50 to 200 "Cat 7.5 OC per minute after a 10-minute isothermal hold at 50 "C. Full scale recorder deflection (indicated by the flat top on the solvent peaks) was equivalent to 15 nanograms per second of material flowing into the MS ion source. Of significance is the MS pressure throughout the analysis. Prior to injection, the ion source pressure was 1.8 X 10-8 Torr, and at the end of the analysis, after the phenylundecane peak had passed, the pressure was approximately 8 x 10-8 Torr. The small peaks produced pressures on the order of Torr whereas the phenylundecane peak generated a presTorr, requiring shut-down of the MS ion source. sure of Several hundred analyses of this type have been made with no change in separator performance. RECEIVED for review February 1, 1972. Accepted May 12, 1972. This work was carried out under Jet Propulsion Laboratory Contract NAS 7-100, sponsored by the National Aeronautics and Space Administration.
Plasma Chromatography of the Mono-Halogenated Benzenes Francis W. Karasek and Oswald S. Tatone Department of Chemistry, Unioersity of Waterloo, Waterloo, Ontario
Using thermal electrons and positive reactant ions from nitrogen gas, both positive and negative plasmagram patterns have been obtained for fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene. The plasmagrams give characteristic qualitative data. Positive plasmagrams show protonated molecular ions containing one and two molecules; the negative plasmagrams, except for the fluorobenzene, show only a strong halogen ion peak, which provides experimental evidence for dissociative electron capture by thermal electrons.
THE TECHNIQUE OF PLASMA CHROMATOGRAPHY (PC) permits characterization and analysis of trace constituents in a gaseous mixture at atmospheric pressure. The instrumentation utilizes a 63Niradioactive beta source to create ions for reaction with trace constituents in a gas to produce characteristic positive and negative ion-molecule complexes. The complexes formed in the reactor section are separated in a coupled ion-drift spectrometer and appear as a recorded plasmagram of separated ion-molecule peaks. Basic features of the method and descriptions of the instrumentation have been presented previously (1-3). Some limited work has been (1) F. W. Karasek, Res./Derelop., 21 (3), 34 (1970). (2) M. J. Cohen and F. W. Karasek, J . Chrornatogr. Sci., 8, 330 (1970). (3) F. W. Karasek, Res.JDecelop, 21 (12), 25 (1970).
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reported to demonstrate the qualitative information provided in the positive and negative plasmagrams of such compounds as oxygenated organics ( 4 , 3,polychlorinated biphenyls (6), and the chlorinated dibenzo-p-dioxines (7). Studies by PC are curently being conducted on molecules of biological significance by Griffin, Dzidic, and Carroll (8) and on high molecular weight macro-ions of polymers by Dole (9). The PC technique provides data not only for analytical use but for fundamental studies of ion-molecule reactions and related phenomena such as the mechanism of the gas chromatographic electron capture detector (6) and the stable ionic species observed in mass spectra. The first commercial plasma chromatograph of simplified design was recently installed in the author's laboratory. This BETA-VI instrument has an increased resolution, sensitivity, and stability over that of the prototypes used in previous work and permits (4) F. W. Karasek and M. J. Cohen, J. Chromatogr. Sci., 9, 390
(1971). ( 5 ) F. W. Karasek, W. D. Kilpatrick, and M. J. Cohen, ANAL. CHEM., 43,1441 (1971). (6) F. W. Karasek, ibid.. p 1982. (7) D. I . Carroll, Tech. Rept. No. F-llA, Franklin GNO Corporation, P.O. Box 3250, West Palm Beach, Fla., 33402. (8) G. W. Griffin, Baylor College of Medicine, Houston, Texas 77025, personal communication, 1971. (9) M. C . Dole, Baylor University, Waco, Texas. personal communication, 1972.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 1 1 , SEPTEMBER 1972
~~~
~
MASS-TIME CORRELATION
500
400
Figure 1. Mass-time correlation curve for the mono-halogenated benzene compounds under conditions of Table I
3w
200
100
I
1
2
3
4
4
7
b
b
.
11
DRIFT 11ME -MILLISECONDS
systematic studies to explore the fundamentals and capabilities of the method. This study of the mono-halogenated benzene compounds fluorobenzene, chlorobenzene, bromobenzene, and iodobenzene was undertaken to provide some basic understanding of the analytical and fundamental data obtainable in their plasmagrams. Since these compounds, except for the fluorobenzene, have been studied by Wentworth (IO)to elucidate the mechanism of the electron capture detector, it was anticipated that these data would add to that understanding. EXPERIMENTAL Instrumentation. The basic elements and function of the prototype ALPHA-I1 plasma chromatograph have been described previously ( 5 ) . The BETA-VI model used in this work differs from the ALPHA-I1 in the design changes incorporated in the ion-molecule reactor and ion-drift spectrometer that led to greater speed of response, sensitivity, and resolution. A smaller reactor of 6-cm length coupled to an ion-drift spectrometer with a 6-cm drift region is used; the drift gas flow is axial and countercurrent to the reactant gas flow. Plasmagrams that occur within 0 to 10, 20 or 50 milliseconds are recorded directly on an X-Y recorder in 1- to 10-minute time spans using a gating grid technique which integrates all the millisecond scans occurring during the chosen time span. A grid just prior to the electrometer detector is opened for transmission of ions by a 0.1-millisecond pulse, at times delayed from the ion injection pulse so as to move it at controlled scans of minutes duration across the millisecond plasmagram. If the movable gating grid takes 2 minutes to cover its range, a 2-minute plasmagram will be recorded as the sum of all the 20-millisecond individual plasmagrams occurring during the two minutes. The experimental parameters used during this study are shown in Table I. Procedure. The PC instrument is very sensitive to trace concentrations. A sampling procedure that admits concentrations in the ppm to ppb range is necessary so as not to saturate the instrument. Approximately 0.1 microliter or less of sample vapor was injected directly into the reactant gas stream using a GC liquid sampling syringe. This quan(IO) W. E. Wentworth, R. S . Becker, and R. Tung, J. Phys. Chern., 71, 1652 (1967).
Table I. Experimental Parameters for Plasmagrams Run for Compounds Studied 127 “C Sample temperature: 100 cc of dry N*/minute Reactant gas flow: 450 cc of dry Na/minute Drift gas flow: Ion-molecule reaction space: 6.0 cm 6.0 cm Ion-drift space: 250 V/cm Electric field: 0 . 1 millisecond Injection pulse: 0.1 millisecond Gating pulse: 2 minutes Recorded scan: 10-l2 A full scale Electrometer sensitivity: CP reagent grade with greater Compounds: than 99.9% purity Nitrogen-Linde high purity Gas: grade (99.996 dried by metal trap of 2.25-liter capacity, packed with Linde Molecular Sieve 13X
z),
tity of sample is sufficient to produce plasmagrams for 15 to 30 minutes, depending upon reactivity of the compounds. Concentrations in the reactor section of the PC tube are estimated to range from one ppm to less than one ppb when the plasmagrams were taken. Plasmagrams of comparative concentrations can be taken by recording them at definite time intervals after sample injection. RESULTS AND DISCUSSION Interpretation of Plasmagrams. Early work in PC involved positive mass identification of the charged species in the plasmagram peaks by direct introduction through an interface into a mass spectrometer. However, use of the mass-drift time correlation curve (11, 12) permits one to make (11) W. D. Kilpatrick, “An Experimental Mass-Mobility Relationship for Ions in Air at Atmospheric Pressure,” Proceedings of the 19th Annual Conference on Mass Spectrometry, Atlanta, Ga., May 1971.
(12) D. I. Carroll and E. A . Mason, “The Theoretical Relationship Between Ion Mobility and Mass,” ibid.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
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2
9
L1
; I
Figure 2. Plasmagrams of the positive and negative reactant species using a nitrogen carrier and drift gas
,DlaoI4n +
0
M&NO+
\
--.-ILCCTRONS
5
---
I
DRIFT TIME-MILLISKONDI
Figure 3. Composite negative plasmagrams of the mono-halogenated benzenes
0
5
DRIFT TIME- MILLISKONM
a reasonably adequate assignment of ion-molecule mass associated with each plasmagram peak and permits qualitative studies with the simpler instrumentation of the BETA-VI. The mass assignments are approximate and hold best for a wide mass range of charged species with similar composition and structures. Below 100 mass units, the correlation is more difficult to use, but in this region interpretation is aided most by known ion-molecule reactions and previous work with an interfaced mass spectrometer (7). This is illustrated by the plots shown in Figure 1, where the negative halogen ions follow a curve quite different from the positive ion-molecules. Identification assignments of the ion-molecules observed are indicated in Table 11. Reactant Ions. Reactant ions are formed from the primary ionization of components in the gas used as a carrier for sample introduction. A drying procedure leaves sufficient water vapor, about 10 ppm, in this gas to provide the major source of reactant ions as it passes adjacent to a 12.5 milli1760
curie nickel-63 radioactive beta source. As a first step, positive nitrogen and oxygen ions and secondary electrons are formed. The electrons in the reaction space quickly lose energy by inelastic collisions to reach thermal equilibrium with the gas. With an air carrier gas, electron attachment reaction then takes place with the predominant electronegative oxygen species to form hydrated negative ions of (H20)n02-. When a nitrogen carrier gas is used, only thermal electrons (about 0.5 eV or less) are observed as the reactant particles in the negative plasmagram. Because some of the negative ionic species formed for the compounds being studied occur coincidentally with the (H20)nOz- reactants of air, nitrogen was chosen as the reactant gas for this study. For the positive reactant ions, a sequence of reactions starting with N2+ and 02+ eventually produces predominantly (H20),H+ with some (H20),NO+. The same positive reactant ions appear to be formed whether air or nitrogen is used, since both produce the same plasmagram. The reactant ions shown in Figure 2
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
NEOATIVE CLASMAORAMI
Figure 4. Negative plasmagrams of iodobenzene impurity and iodine
for a nitrogen carrier reveal hydrated Hf and NO+ for the positive plasmagram and only thermal electrons for the negative. Mass spectral data indicate there may be more than one ionic species in each plasmagram peak of the positive reactant ions; the assignments shown in Figure 2 indicate only the most abundant species considered present. The relative concentrations of all these reactant ions will vary considerably with temperature. The total electron current in the plasma is about ampere; the amount observed in a plasmagram scan depends upon the duration of the gating pulses. These electrons appear here as a continuum across the plasmagram because the grids do not close perfectly for such small, high speed particles. If the grids could exclude the electrons, the plasmagram would show a single peak with a drift time of 0.01 millisecond, an event the recording system cannot follow. Plasmagram Results. The plasmagrams obtained for these compounds are quite simple. Taken together, the positive and negative patterns are quite different and characteristic for each compound. For the negative plasmagrams, the fluoride compound exhibits little detectable response; all other compounds show a single strong peak for the halogen ion. The composite plasmagrams shown in Figure 3 reveal the initial disappearance of the thermal electrons followed by the appearance of the halogen ion. Since the sample amount and timing of the 2-minute plasmagram scans after sample injection were held constant for all four scans, the halogen ion peak intensity and rate of rise of the tracing toward the original electron level give an approximate comparison of relative reactivities of these compounds in the electron capture reaction involved. The iodobenzene also shows a weak peak of lower mobility which becomes more pronounced with increased concentration. As shown in Figure 4, further investigation reveals this peak to be coincident with that obtained for molecular iodine. From previous work with the interfaced mass spectrometer this appears to be the Is- ion and apparently arises from a trace of iodine present as an impurity in the iodobenzene. These results are consistent with those obtained in negative ion mass spectrometry. Melton (13) reports that negative (13) C. E. Melton, "Mass Spectrometry of Organic Ions," F. W. McLafferty, Ed., Academic Press, N. Y . , 1963, p 195.
Table 11. Drift Time and Structure Assignments of Charged Species Observed in Plasmagrams of Halogenated Benzenes Drift time (milliseconds) Compound 5.75 Nitrogen (reactant gas) 6.47 7.11
Continuum Fluorobenzene Chlorobenzene Bromobenzene Iodobenzene Carbon tetrachloride n-Butyl chloride
7.39 8.44 7.81 8.60 5.47 8.12 9.00 6.13 8.59 9.94 6.40 5.42 5.42 6.10
halogen ions are observed in the spectra of all halogenated compounds. He gives the appearance potential of the Ffrom CeFBas 3.9 eV, and this value ranges as high as 20 eV for many fluoride compounds. These appearance potentials are far beyond the energy of the thermal electrons, which are less than 0.5 eV, and one would not expect to observe dissociative electron capture for fluorobenzene. However, the appearance potentials of C1-, Br-, and I- from a large number of compounds are reported to be either 0 or 0.1 eV at the most, so the appearance of these ions from thermal electrons is to be expected. In addition, a number of alkyl halides have been run on the BETA-VI plasma chromatograph. In each case the C1-, Br-, and I- ions of the same mobility as the aromatic halides have been observed in the negative plasmagrams (14). The data in Figure 5 reveal the formation of chloride ions from carbon tetrachloride and n-butyl chloride ; these ions (14) F. W. Karasek and 0. S. Tatone, "Plasma Chromatography of Halogenated Alkyl Compounds," unpublished work, Waterloo, Ontario, February 1972.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
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E
Figure 5. Negative plasmagrams of carbon tetrachloride and n-butyl chloride
I?:
!
IODOICNZCNI
Figure 6. Composite positive plasmagrams of the mono-halogenated benzenes
(C&'lH+-&
FLUOROICNZINC
4
io
DRIFT l l M I -MILLISECONDS
have the same mobility as the chloride ion from chlorobenzene. In addition, n-butyl chloride also shows an ion peak that appears to be the intermediate molecular ion. The positive plasmagrams composited in Figure 6 show two ion-molecule peaks for each compound. The major one is interpreted as (C6H5X)H+,and the minor one as (CsH&H+. These interpretations are consistent with previous work with PC and with the phenomena observed in chemical ionization mass spectrometry. One can also note that the most abundant peaks in the electron impact mass spectra of all these compounds are due to the (C6&X)+ molecular ions. Under the concentration conditions used here, the monomer ion-molecule is the most abundant; as in previous PC work, when the concentration increases, the dimer ion-molecule becomes relatively more abundant. Mechanism of the Electron Capture Detector. Karasek (6) has suggested that data obtained in PC could add materially to our understanding of the mechanisms advanced to explain 1762
electron attachment phenomena and the characteristics of the electron capture detector. The PC instrument essentially draws a group of charged particles from a plasma such as that found between the electrodes of the EC detector and subjects these charged species to separation and identification. Both positive and negative particles can be examined separately and individually. The use of nitrogen as a reactant gas, producing only electrons in the negative mode, simulates conditions in the EC detector very closely. Wentworth (10) has studied thermal electron attachment for a number of compounds including the halogenated ones presented here, and he has considered several processes for electron attachment :
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
-
+ e- ABAB + eA + BAB + e--+ A B - - A + BAB
-+
(1)
(2)
(3)
Figure 7. Plasmagrams of the reactant ions using conditions of Table I except that an air carrier is used instead of nitrogen The positive reactant ions are the same as when a nitrogen carrier is used (Figure Z), but the electrons have been replaced with a series of negative reactant ions formed from oxygen in air. A section of the curve was amplified by a factor of 10 to reveal the two small peaks for the OH- and Oz-ions
5
DRlfT TIME-MIILISKONDZ
He distinguishes the occurrence of the dissociative electron capture processes of Equations 2 and 3 from the direct attachment process of Equation 1 by observing the temperature dependency of electron attachment coefficients. The PC data presented here provide direct experimental evidence that the dissociative capture process with thermal electrons does occur for these halogenated aromatic compounds. The data of Figure 5 were shown to indicate the coincidence of chloride ion mobility from different compounds. While it does suggest the formation of an intermediate molecular ion as in Equation 3 for the n-butyl chloride, a definite conclusion must await completion of the more detailed study from which these data were extracted (14). The positive charged species in the plasmagram appear to be simple protonated molecular and di-molecular complexes. These charged particles, both positive and negative, represent the stable species, since they survive a traverse of many milliseconds time through a 6-cm drift region where they are subjected to many millions of collisions with neutral nitrogen atoms. The plasmagrams and drift velocities of the charged species measured also confirm the operation of the EC detector in the pulsed mode. In the EC detector, the application of brief, microsecond pulses at millisecond intervals is sufficient to collect the electrons with velocities about one thousand times greater than the ions, while the millisecond interval between pulses provides time for the slow-moving positive and negative ions to recombine or be removed in the GC effluent. It now appears clear that PC data provide a method of studying details of the EC detector characteristics and fundamentals of electron and ion-molecule reactions under varying
conditions of temperature, pressure, and reaction time. When air is used as a reactant gas, the group of negative reactant ions shown in Figure 7 appears in place of the electrons, while the positive reactant ions remain unchanged. These reactant ions had been previously identified and they do fit the mass-time correlation curve of Figure 2 for the negative species. While the plasmagrams of compounds when using air as a reactant gas are similar to those obtained with nitrogen, the disappearance of the negative reactant ions upon sample addition is often obscured by the appearance of negative product ions that occur coincidentally with these reactants. Using nitrogen as the reactant gas gives an immediate indication of EC response by the disappearance of the electrons with sample addition and the clear appearance of the negative product ions. Further studies are now under way in which many other types of compounds with different responses in the EC detector are being examined using both nitrogen and the argon/methane mixtures as reactant gases. A number of compounds exhibit no response to electron-capture, but respond strongly to the positive ion-molecule reaction, thus providing a method of detection with the plasma chromatograph for these compounds as sensitive as the EC detector. By directly interfacing the PC to a gas chromatograph an understanding of the linearity factors and effects of small trace impurities for the EC detector can be achieved.
RECEIVED for review March 6, 1972. Accepted May 18, 1972. The research for this paper was supported by the Defence Research Board of Canada, Grant Number 9530-116.
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