Products from Electron Capture Reactions Corazon R. Hastings, and Timothy R. Ryan Environmental Trace Substances Research Center, University of Missouri, Columbia, MO. 6520 1
Walter A. Aue Department of Chemistry, Trace Analysis Research Centre, Dalhousie University, Halifax, N.S., Canada
The gas chromatographic trace analysis of a wider variety of compounds-those that readily absorb thermal electrons in the gas phase-became possible with the invention of the electron capture detector by Lovelock and Lipsky ( I ) . The GC/EC technique has been widely used in the residue analysis of agricultural chemicals, drugs, biomedical extracts (usually after derivatization), microbial metabolites, and even metals (Fe, Cu, Cr, Al) in chelated form. These practical applications of the electron capture detector are quite numerous ( 2 ) . Yet, not too much is known about the chemical processes occurring inside the electron capture detector. Wentworth et al. (3-6) described different mechanisms for electron attachment under ECD conditions, using Arrhenius plots as main experimental evidence. Gas phase coulometry with electrons was described by Lovelock et al. (7). Karasek and coworkers (8, 9) compared the electron attachment processes in a plasma chromatograph operated under atmospheric pressure to that of an EC cell. Still further away from conditions typical of the EC detector, considerable information exists in the areas of negative mass spectrometry, electron swarm experiments, and electrical discharges. One of the more important aspects of electron capture is the question what neutral products originate from the detector reactions. In much denser media than an ECD provides, electron-initiated chain reactions lead to a variety of products, some characteristically of the higher molecular weight than the starting material (6). In an EC detector, however, with analytes a t the picogram level, the situation is obviously different. Gas phase coulometry with two detectors in series, as described by Lovelock et al., is, in fact, based on the premise that the products from the reaction
in the first detector do not show appreciable response in the second detector. The authors comment that “There is little chance of a polyatomic molecule surviving. . . [the encounter between oppositely charged species] intact” (7). Even with the odds against success, however, the question of products is an interesting one to pursue. Early experiments in the Columbia laboratory (with the help of Won Kyoo Lee) centered on collecting ECD products in cooled hexane for up to 24 hours. The investigated compounds (lindane, decachlorobiphenyl, etc.) were coated on Chromosorb and filled into a GC column. By selecting an appropriate column bath temperature, controlled concentrations of the test compounds were swept into the EC detector. Carrier gas flow and temperature were chosen such that the standing current was depressed by less than 10%. Products were indeed found on chromatography of the trapping hexane, but the procedure was tedious and the results open to some doubt. We, therefore, tried to develop a method to detect products more directly and to exclude the possibility that thermal decomposition products could be mistaken for products from the EC reaction. It was desirable in this context to turn the EC reaction on and off at will-and we did so by following an approach described by Lovelock et al. ( 7 ) and constructed an EC “Reactor”. This Reactor represents essentially a concentric EC detector lacking the detection capability, and using either zero or a high potential on its polarizing electrode to obtain either maximum or low electron concentrations in the cell, respectively.
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
1169
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Figure 4. Injections of 100 pg lindane at different reactor voltages Chromatographed on Carbowax POM-surface modified Chromosorb G, 60180 mesh. Nitrogen flow rates: Column, 80 mi/min; ECD purge, 80 ml/min. ECD voltage: 35 V. ECD temperature: 350 OC. Column temperature: 140 OC
T o distinguish the original substance from its products, the Reactor was placed between two GC columns and the effluent from the second column monitored by a regular EC detector. Obviously, any product thus detected has to be neutral species with adequate chromatographic and electron-absorbing properties. This type of flow-through arrangement appeared preferable in view of the well-known problems of contamination and decomposition that can plague EC detectors. Obviously, such a set-up can be used for a variety of purposes, including mechanistic (11) and analytical approaches. Our immediate interests in this study involved the latter area, particularly in the context of residue levels of insecticides. EXPERIMENTAL The flow-through system is made of two U-shaped borosilicate glass columns, 4-ft X 2-mm id., each packed with Carbowax 20M surface-modified Chromosorb G, 60/80 (10) in a Microtek 220 gas chromatograph. T h e columns are intercepted, as shown in Figure 1, by a stainless steel cell of 2-ml volume containing a cylindrical “”Ni foil (=“Reactor”). Its co-axial center electrode can be polarized with a Keithley 240A high voltage supply. I t terminates with a P T F E disc to avoid a short on accidental contact with the grounded Reactor body. Figure 2 shows the construction of the cell. The flow-through configuration allows the injected substances to migrate through the first &foot column, to the reactor cell (of variable voltage), to a second identical GC column, and then to a (commercial) EC detector (Microtek 220 with 63Ni and power supply), operating in the DC mode. The detector voltage is optimized from time to time and runs usually 30-35 V. General Experimental Procedure. The compounds tested in most experiments were decachlorobiphenyl (DCB), lindane, and heptachlor epoxide; others, including typical strong and weak electron capturers, were used only in later experiments focusing on products and analytical interferences. A series of calibration curves was run a t various reactor voltages ( R ) , including zero. The column temperature was adjusted such that the substances eluted between 1 and 3 minutes. The other chromatographic conditions were: N2, 80 ml/min; NZ purge through EC, 80 ml/min; EC temperature, 350 “C. The NZwas freed of 0 2 by passing through an indicating oxy-trap (Alltech Associates). The EC detector was made leak-tight and connected to a long lh-in. copper exit tube with terminating valve to prevent back-diffusion of oxygen from the atmosphere. 1170
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
RESULTS AND DISCUSSION Typical gas chromatographic runs for DCB, lindane, and heptachlor epoxide, showing the formation of “products” are shown on Figures 3-5. (Figure 3 shows an unknown peak originating from the solvent hexane). Zero volt represents a condition of maximum electron density in the reactor cell-i.e., electrons are removed only by recombinations and captures, not by an electric field-and many electron capture reactions have high cross-section for thermal electrons. Thus, as expected, low voltage gives more product. On the other hand, a high voltage turns the reactor “off”, in agreement with Lovelock’s finding ( 7 ) . This turn “off” is believed to reflect mainly the fast removal of electrons by the field (their concentration a t any given time in the cell is much lower a t high field strength) and possibly also the variation of electron capture cross section with energyalthough little is known about the latter effect in a system such as the ECD, which operates a t atmospheric pressure and somewhat uncertain electron energy. In our case, with 60 V in the reactor, all electron capture reactions investigated were “turned off” and the response for EC detector approaches that of a conventional GC-EC analysis. This conclusion was checked by removing the “”Ni foil from the otherwise intact flow-through system. All products detected have retention times shorter than the reactant ( = parent substance). Lindane shows two products. I t should be mentioned a t this point that the true number of products may well be larger than what our relatively simple chromatographic system could separate and the EC detector could detect. Calibration curves for reactant/product(s) for the three model compounds are given in Figures 6-11. Short dashes represent the original substances; solid lines or long dashes represent the products. Positive voltages on the coaxial electrode (Figures 6, 8, and 10) appear to be duplicable by somewhat higher negative voltages (Figures 7 , 9 , and 11). Lindane and heptachlor epoxide approach 100% consumption a t zero volt up to 100 pg of the compounds inject-
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