Characterization of the Pulsed Discharge Electron Capture Detector

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Anal. Chem. 1996, 68, 1233-1244

Characterization of the Pulsed Discharge Electron Capture Detector Huamin Cai,*,† Wayne E. Wentworth,‡ and Stanley D. Stearns†

VICI Valco Instruments Co. Inc., P.O. Box 55603, Houston, Texas 77255, and Chemistry Department, University of Houston, Houston, Texas 77204

A new version of the pulsed discharge electron capture detector (PDECD) has been developed and characterized. Changes to the old version include a slightly altered detector geometry, replacement of the polymer insulation with sapphire and quartz, and the use of methane dopant gas instead of nitrogen or hydrogen. Various operating parameters have been investigated and optimized, including discharge current, dopant gas, bias voltage, and sample introduction position. The resulting detector is more inert and more sensitive (a limit of 36 fg for lindane) and capable of operation at temperatures as high as 400 °C. By running 23 halocarbon compounds on the improved PDECD and on a 63Ni-ECD using the same GC system, we find that the PDECD is superior to 63Ni-ECD in terms of sensitivity, linearity, and response time. We attribute the enhanced sensitivity to a lower positive ion concentration, which in turn lowers the electron-positive ion rate of recombination. Pesticides (including some real-world samples) have also been analyzed on the PDECD. The results demonstrate that the PDECD can replace the radioactive ECD typically used in these analyses. The electron capture detector (ECD) has been used as a GC detector for more than three decades. Because it offers the highest sensitivity to electron-capturing compounds, the ECD is the only detector that can detect CFCs and chlorine-containing pesticides at trace levels. Since its introduction by Lovelock in 1960,1 the source of electrons has remained basically unchangeds ECDs currently on the market still use radioactive materials. Since radioactive ECDs have certain drawbacks (safety, surface reactivity, large cell volume), the last 30 years have seen considerable efforts to develop nonradioactive alternatives.2 The other sources of electron formation that have been investigated include the following: (1) an electrical discharge;2 (2) a hydrogen Lyman R emission;3 (3) a thermionic emitter;4 (4) a rare gas resonance lamp source with a MgF2 window;5 (5) ultraviolet irradiation of a metal surface by the photoelectric effect;6 and (6) a microwave-powered helium resonance lamp.7-9 None of these †

VICI Valco Instruments Co. Inc. University of Houston. (1) Lovelock, J. E.; Lipsky, S. R. J. Am. Chem. Soc. 1960, 82, 431. (2) Bochinski, J. H.; Sternberg, J. C. Abstracts of Papers, 148th National Meeting of the American Chemical Society, 1964; American Chemical Society: Washington, DC, Abstr 52, p 22A. (3) Wentworth, W. E.; Tishbee, A.; Batten, C. F.; Zlatkis, A. J. Chromatogr. 1975, 112, 229-246. (4) Neukermans, A.; Kruger, W.; McManigill, D. J. Chromatogr. 1982, 135, 1-20. ‡

0003-2700/96/0368-1233$12.00/0

© 1996 American Chemical Society

nonradioactive sources has successfully replaced the radioactive material. In previous publications,10-12 a new pulsed discharge electron capture detector (PDECD) was presented. This new detector uses a pulsed discharge in helium as the primary source of electron generation instead of the radioactive materials used in conventional ECDs. The original data obtained from a prototype version of the detector indicated that the PDECD has performance and characteristics similar to a radioactive ECD, and its capabilities for analysis of pesticides and CFCs have been demonstrated. Based on these early results, we determined that the PDECD has a high potential for replacing the radioactive ECDs currently on the market. However, in the previous research, H2 and N2 were used as dopant gases, and both H2 and N2 have high ionization potentials (15.4 eV and 15.6, respectively). Therefore, these PDECDs ionized some low electron affinity and low ionization potential compounds such as hydrocarbons, giving ionization peaks for these compounds. In this research, methane is used as the dopant gas. Because the ionization potential of methane (12.6 eV) is lower than that for H2 and N2, interference from extraneous ionization peaks is reduced and sensitivity is increased. Also, the PDECD of prior research used a polymer as the insulating material inside the detector cell. At temperatures above 200 °C, the polymer had problems with bleeding and electrical leakage. In the current version, characterized in this paper, sapphire and quartz replace the polymer and more highly inert electrodes are used. As a result of these improvements, the new version of the PDECD has higher sensitivity, higher operating temperature (up to 400 °C), and response characteristics more closely resembling the radioactive ECD. This paper discusses complete optimization of the PDECD and compares it to a 63Ni-ECD. BASIC WORKING PRINCIPLE AND KINETIC MODEL The PDECD uses a pulsed discharge in pure helium as the primary source of electrons. When an electrical discharge occurs in a flow of helium gas, it generates lots of high-energy particles (5) Kapila, S.; Bornhop, D. J.; Manahan, S. E.; Nickell, G. L. J. Chromatogr. 1983, 259, 205-210. (6) Simmonds, P. G. J. Chromatogr. 1987, 399, 149-164. (7) Freeman, R. Ph.D. Dissertation Thesis, University of Houston, Houston, TX, 1971. (8) Wentworth, W. E.; Limero, T.; Batten, C. F.; Chen, E. C. M. J. Chromatogr. 1988, 441, 215-224. (9) Wentworth, W. E.; Limero, T.; Batten, C. F.; Chen, E. C. M. J. Chromatogr. 1989, 468, 215-224. (10) Wentworth, W. E.; D’sa, E. D.; Cai, H.; Stearns, S. D. J. Chromatogr. Sci. 1992, 30, 478-485. (11) Wentworth, W. E.; Cai, H.; Madabushi, J.; Qin, Y.; Stearns, S. D.; Meyer, C. Process Control Qual. 1993, 5, 193-204. (12) Cai, H. Ph.D. Dissertation Thesis, University of Houston, Houston, TX, 1993.

Analytical Chemistry, Vol. 68, No. 7, April 1, 1996 1233

and photons, such as metastable He, metastable He2, He emissions, He2 emissions, etc. However, the only major energy carrier that is transferred to the ionization region is the continuum emission of He2 in the range of 60-110 nm.12 This corresponds to a range of 11.3-20.7 eV, based on the transition 1 He2 (A1Σ+ u ) f 2He(1 S) + hν

These high-energy photons travel easily in He media and are capable of ionizing almost all the compounds except He itself. If a certain amount of a dopant such as methane is added downstream from the discharge, free electrons are produced by ionization of the dopant. These electrons move toward the collecting electrode along the biased path, forming a constant standing current. The dopant also thermalizes the electrons and lowers the kinetic energy of the free electrons through inelastic collision, making them more readily capturable by analytes. When the analytes flow out of the column and into the detector cell, they capture the thermalized electrons and reduce the standing current, producing a capture signal. The magnitude of the current reduction is a nonlinear function of the concentration of the capture species: the standing current diminishes quickly as the concentration begins to increase and then diminishes at a slower rate as the concentration continues to climb. However, it can be converted to a linear response, or linearized, by applying the following linear function:

Ib - Ie ) K[AB] Ie

(1)

(13) Wentworth, W. E.; Chen, E. C. M.; Lovelock, J. E. J. Phys. Chem. 1966, 70, 445-448.

Analytical Chemistry, Vol. 68, No. 7, April 1, 1996

khν

CH4 + hν (He2 emission) 98 P+ + e-*

(2)

e-* + CH4 f e- (thermal) + CH4

(3)

kD

where AB represents any polyatomic molecule capable of capturing electrons, Ib is the current without the presence of AB, Ie is the current with AB present, K is the capture coefficient of AB, and [AB] is its concentration. This function was first developed and proposed by Wentworth, Chen, and Lovelock in 1966,13 from an EC model with a radioactive source and a pulsed, fixedfrequency collecting mode. Coincidentally, this linear function can be applied to the nonradioactive PDECD with a constant potential collecting mode, since both detectors appear to follow similar basic electron capture mechanisms. However, since the primary source and the collection mode are different, some rate constants used in the radioactive ECD model are not suitable for the PDECD (such as the rate constant of electron formation from β emission), so some modifications of the original assumptions and kinetic scheme are required. The assumptions made for the PDECD cell are as follows: (1) The rate of production of thermal electrons is a constant which is not affected by the presence of analyte. This assumption is true for radioactive ECDs, but only at low analyte concentrations; when analyte concentration goes up, the electron formation rate changes since the analyte itself acts as a dopant. It would seem that this effect would be more serious in the PDECD cell than in the radioactive ECD cell, since the concentration of dopant in the PDECD cell is much lower than in the radioactive cell (0.3% compared to 5-10%) and since the PDECD is more sensitive to changes in dopant concentration. Nevertheless, only compounds with a low capture coefficient affect this assumptionsthe PDECD is so sensitive to high capturing compounds that it becomes saturated before the analyte concentration becomes high enough to disturb the electron production rate.

1234

(2) The rate of production of thermal electrons is a constant which is not affected by the pulsed source. While it is true electron formation in the detector cell changes with the pulse frequency (∼3 kHz in this study), since we are only measuring the average current we can assume that the electron production does not change with time. (3) In the absence of analyte, the concentration of electrons is constant at the point where the analyte is introduced, and the detector current reflects this concentration. In the PDECD cell, the electrons and positive ions are not homogeneousstheir relative concentration changes along the path of bias potential. Positive ions have the highest concentration near E1, decreasing as the distance from the collecting electrode decreases. The electrons have an opposite pattern, present in highest concentration near the collecting electrode and approaching zero near E1. We introduce the analyte near the collecting electrode, where the electron density is the highest. Any change in the electron concentration can be measured through the detector current change. Based on the above assumptions, the kinetic mechanism of the PDECD can be described through the following reactions: (1) Thermal electron formation

e- + P+ 98 neutrals

(4)

where P+ represents the total positive ion species produced during electron production, e-* represents the primary electrons produced from the photoionization of CH4, khν is the rate constant of the photoionization reaction, and kD is the electron neutralization reaction constant. (2) Electron capture and molecular reactions k1

k2

AB + e- {\ } AB- 98 A + Bk

(5)

-1

k12

AB + e- 98 A + B-

(6)

kN

M- + P+ 98 neutral

(7)

where k1 and k-1 are the rate constants of nondissociative electron attachment-detachment, k2 is the rate constant for AB- dissociation, k12 is the rate constant for dissociation electron attachment, and kN is the rate constant for the neutralization of positive and negative ions. Comparing this kinetic model with the radioactive ECD model, we find them similar except for the electron formation step, eq 1. So the differential equations and the steady state solutions from the radioactive ECD model14 can be used in the PDECD model, resulting in eq 1. The capture coefficient K in eq 1 can be expressed in terms of the various rate constants as follows:

K)

(

)

k1(k2 + kN) 1 k12 + kD k-1 + k2 + kN

(8)

Note that kD is the recombination rate constant of e-. Since it is the denominator, if kD is small, K will increase universally. EXPERIMENTAL SECTION A vertical cross section of the PDECD cell is shown in Figure 1. The main body is made from a hollow stainless steel cylinder (14) Wentworth, W. E.; Steelhammer, J. C. In Radiation Chemistry; Hart, E. J., Ed.; Advances in Chemistry 81-82; American Chemistry Society: Washington, DC, 1968; Chapter 4.

Figure 1. Cross section of the PDECD cell.

(95 mm long × 14 mm i.d.) with two end pieces tightened by two nuts. At one end, a stack of spring washers between the nut and the end piece compensates for the difference in thermal expansion between the stainless steel and the insulators. The hollow cavity contains two sectionssthe discharge section and the reaction section, separated by a disklike stainless steel piece which is electrically grounded to the main body. In the discharge section, two platinum-tipped discharge electrodes sit end to end in a piece of quartz (20 mm long × 3 mm i.d.). The gap between these two electrodes is ∼1.6 mm. In the reaction section, four sapphire spacers (8 mm long × 3 mm i.d.) and three electrodes (2 mm thick × 3 mm i.d.) are stacked together. Two of the electrodes are bias electrodes and one is the collecting electrode. Compressed between each of the components is a gold O-ring seal. The helium makeup is brought into the detector through an inlet port at the top. The dopant and column gases flow counter to the makeup flow, brought in through two separate fused-silica capillaries fed through the outlet. The dopant gas inlet ends between the two bias electrodes, while the column gas outlet extends to ∼0.4 cm from the collecting electrode. We used a homemade high-voltage pulse generator with variable pulse width and period, capable of generating a few thousand pulsed volts if the circuit is open. Under normal circumstances, pulse width was set at at 40 µs, with a pulse period of 300 µs. Low constant potentials were applied to the two bias electrodes: E1 is set at -20 VDC and E2 at -0.75 VDC. Grade 6 helium (99.9999% pure; Liquid Air, La Porte, TX) was used as the carrier as well as the discharge gas. The helium flow was split through a T-connection: one portion flowed to the GC as the carrier gas and the other flowed to the detector as discharge gas. The helium going to the detector as discharge gas was purified by a helium purifier (Valco Instruments Co. Inc., Houston, TX),

with a flow rate of 30 mL/min. The dopant gases, 1-5% methane, nitrogen, and xenon, all in helium (Liquid Air), were passed through an Oxytrap before being introduced into the detector. The dopant gas flow rate was about 3-15 mL/min, depending on the concentration of the dopant gas; the total concentration of dopant in the detector cell was adjusted to 0.3%. An HP 5890 system (Hewlett-Packard, Wilmington, DE) was used for this study. When a liquid sample was tested, we used a split manual injector system with a split ratio of 1:100 for the halocarbon mixture and 1:10 for the pesticide analysis. This ensured that only a fraction of the sample injected entered the fused-silica column, avoiding sample tailing. Gas samples were injected with a six-port valve with different sample loops (Valco Instruments Co. Inc.) without a split. The radioactive ECD used in the comparison study was a commercial 63Ni-ECD (Valco Instruments Co. Inc.), which operates in a constant-current/variable-frequency mode. For makeup gas we used 5% CH4 in argon (Liquid Air). Three different columns were used during the experiments: a DB-5 30 m × 0.25 mm with a film thickness of 1.0 µm (J&W Scientific, Folsom, CA) was used for the study of detector parameters; an SPB-1 60 m × 0.25 mm with a film thickness of 1.0 µm (Supelco, Bellefonte, PA) was used for a halocarbon mixture separation; and a 007-608 30 m × 0.53 mm with a film thickness of 0.8 µm (Quadrex, New Haven, CO) was used for the pesticide and herbicide analysis. The GC oven temperature was varied to yield the best separation of samples. For regular evaluation of the PDECD we used a sample of 41 ppb Freon-11 in helium (Liquid Air). The other samples used in this study are a 78 ppm CH3Br in helium (Liquid Air), a 26-halocarbon mixture in methanol solvent (Alltech, Deerfield, IL); a chlorocarbon mixture of CH2Cl2 and CHCl3 (Fisher Scientific), CCl4 (EM Science), and CCl2dCCl2 (Aldrich Chemical Co.) in 2,2,4-trimethylpentane (Aldrich Chemical Co.) solvent, prepared in our laboratory; and, a PM-13 pesticide mixture (Alltech). Liquid samples were diluted with methanol as required. RESULTS AND DISCUSSION Discharge Current. The PDECD uses the radiation from the pulsed discharge in helium for photoionization of the dopant. The advantages of the pulsed discharge source over a dc discharge source are that it is easy to start, highly stable, and quick to recover from disturbances. This particular pulsed discharge source was tested for several weeks continuously, with no significant reduction in intensity. A typical discharge current profile of the PDECD can be roughly divided into three periods: the high discharge current density period (duration, ∼1 µs); the low discharge current density period (duration, ∼40 µs); and the idle, or no discharge current period (duration, ∼260 µs). In the high discharge current density period, the discharge current could go as high as 100-200 mA, but it only lasts ∼1 µs. In the low discharge current period, the discharge current is in the range of 1-5 mA. Since the discharge is off during most of the cycle (87%), the average current of the pulsed discharge is only ∼0.5 mA, which prevents the discharge electrodes from overheating. The average current can be adjusted by changing the discharge current density or the idle period. Since the intensity of the radiation should be a measure of the current passing through the discharge, we studied the effect that the discharge current has on the electron capture process. Specifically, we observed the influence of average discharge Analytical Chemistry, Vol. 68, No. 7, April 1, 1996

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current (in the 0.03-0.65 mA range) on standing current and capture coefficient K of CFCl3. Since the source intensity increases as the average current increases, we would expect a corresponding increase in the standing current. This was exactly the case, with the standing current showing a direct increase from 0.03 to 5.9 nA. However, while the capture coefficient of CFCl3 initially increases with the average discharge current (from 1.2 × 1010 to 4.0 × 1010), it plateaus at an average discharge current current of 0.25 mA. We know that as the average electron energy in the detector cell goes down, the capture efficiency for CFCl3 goes up. Since lowering the average discharge current lowers the intensity of the source, we would expect it to also lower the electron energy level and therefore increase the capture coefficient. However, the results do not confirm this expectation, at least in the lower range. This phenomenon must be related to changes in the energy distribution of the electrons generated in the detector cell. Note that an increase in the discharge current causes an increase in the standing current, with a corresponding increase in the response peak height if the capture coefficient of the analyte remains the same. However, in most cases, increasing the discharge current will also increase the noise level, so that the ratio of signal to noise is usually not measurably improved. Also, if the average discharge current is too high, the discharge electrodes will become overheated and deteriorate. For measurement of CFCl3 in this experiment, an average discharge current of ∼0.5 mA gave the best performance and long-term stability. Bias Voltage and Collecting Mode. The function of the bias potential in the PDECD is to build up an electrical field in the detector cell so that the electrons will move toward the collecting electrode and be collected. Two bias electrodes are used: E1 is closest to the discharge source and E2 is closest to the collecting electrode. Under normal situations, we apply -20 V to E1 and -0.75 V to E2. These two bias voltages build up two electrical fields in the detector cell; one field is between the two bias electrodes and the other is between E2 and the collecting electrode. The electrical field between the two bias electrodes is relative high (25 V/cm), which favors moving electrons and attaining a high standing current. The electrical field between E2 and the collecting electrode is relative low (0.9 V/cm), favoring a reduction in electron energy and a high capture coefficient. We investigated the effects of these two bias voltages on standing current and capture efficiency, with the results shown in Figure 2. The values of V1 (the potential applied to E1) and V2 (the potential applied to E2) are relative to the electrometer reference at ground. Figure 2a is a graph of the standing current vs the bias potential V1 at V2 ) -0.75 V. As the graph indicates, the standing current increases with an increase in the value of negative bias potential V1. There is a rapid linear increase at the beginning, but it tapers off above -10 V. This increase in standing current is due to the increase in the electrical field between E1 and E2. The greater electrical field accelerates positive particles and electrons in opposite directions, reducing their change of recombination and therefore increasing the standing current. Increasing the electrical field between E1 and E2 will also increase the energy level of electrons in the cell, since they accelerate before reaching the collecting electrode. This makes them more difficult to capture, resulting in reduced response. However, varying this field does not affect the response as much as it affects the standing current. As shown in Figure 2a and b, 1236 Analytical Chemistry, Vol. 68, No. 7, April 1, 1996

Figure 2. Effect of bias voltages V1 and V2 on (a, c) standing current and (b, d) response to CFCl3 and CH3Br. Detector temperature, 30 °C.

when V1 increases from -2 to -45 V, the standing current increases by a factor of 10 (from 0.45 to 4.7 nA), while detector response decreases by a factor of only 2. This is the case for high electron capturing compounds (CFCl3) as well as for those with only moderate affinity (CH3Br). For this reason, we selected a voltage of -25 V applied on E1. Figure 2c illustrates the standing current relative to the bias voltage on V2 from -0.2 to -10 V at V1 ) -25 V. We note a similar effect: with V1 at a constant -25 V, the standing current shows a continual increase as V2 changes from -0.2 to -10 V. The dependence of standing current on V2 can be explained by noting that the electrical field between E2 and the collecting electrode increases as V2 is decreased (by increasing the negative value of V2), since the potential of the collecting electrode is always at zero. This electrical field also affects the response of the PDECD, with the results shown in Figure 2d. The response of the PDECD for both CFCl3 and CH3Br is sensitive to changes in V2: in general, an initial increase in V2 negative voltage (increasing the electrical field) results in an increase in detector response. Then the response goes to maximum quickly, followed by a slow roll off. We set V2 at -0.75 V to maximize response. It has been reported that, in conventional ECDs with a radioactive ionization source, a pulsed collecting mode will give a higher sensitivity and a larger linear range than the direct potential mode. It is generally understood that this is because the thermal electron concentration builds up to a maximum value in the time between pulses, when no electrical field is present. During this study, the following pulsed collecting modes on the PDECD were investigated: (1) applying a negative pulse to both E1 and E2, (2) applying a negative pulse to E1 only, and (3) applying a negative pulse to E2 only. We investigated each mode with the collecting pulse and the discharge pulse synchronized, with the

collecting pulse and the discharge pulse not synchronized, and with different pulse voltages. It was found that even at the optimum point in each pulsed collecting mode mentioned above, the response of the PDECD was lower than that obtained using a constant collecting mode, and the standing current was lower as well. The pulse frequency, width, and voltage do not follow the same linear relationship to standing current as a radioactive ECD. For example, a radioactive ECD typically needs a collecting pulse width of only 1-2 µs to achieve maximum standing current, while the PDECD needs 50-100 µs in a 300 µs pulse period, so the PDECD response cannot be linearized by changing the collecting pulse frequency. Moreover, the noise level in the pulsed collecting mode was higher. So there is no advantage in using a pulsed collecting mode in the PDECD. The reason why the pulsed collecting mode does not work in the PDECD is probably because the electrons in the PDECD are generated by a photoionization process, which requires an electrical field to keep electrons flowing and to produce free electrons in the detector cell; otherwise, the electrons are in the form of electron-positive ion pairs, which are difficult to collect and capture. If a pulsed collecting mode is used, there are only a few free electrons in the detector cell during the pulse-off phase, causing the standing current and response to drop. The radioactive ECD can benefit from the pulsed mode because the radioactive source is able to supply electrons continuously through β emission, regardless of whether the collecting pulse is on or off. Dopant Gases. The function of the dopant gas in the PDECD is to supply electrons as it is ionized by the high-energy radiation from He2. It also thermalizes the electrons; i.e., it reduces the average energy of the electrons through inelastic collisions. Therefore the ideal dopant gas for the PDECD is one with a low ionization potential and a large cross-sectional area. Several candidates have been previously investigated: namely, hydrogen, nitrogen, carbon dioxide, methane, ammonia, and trimethylamine.10-12 The results indicated that there were no significant performance advantages to any of these. However, current investigation reveals that carefully purified hydrogen does not get good results. This makes sense, since hydrogen has a higher ionization potential and lower cross-sectional area than what we have described as ideal. In this research, three dopant gases are investigated: nitrogen, methane, and xenon. As reported, in every case the standing current in the PDECD increases with concentration of the dopant, goes through a maximum, and then decreases. A typical relationship between standing current and the concentration of methane is shown in Figure 3a. The standing current starts at 0.02 nA without dopant and reaches a maximum of ∼11 nA at a concentration of ∼0.15% methane. Further increases in the concentration result in decreased standing current until, at ∼0.5%, the standing current is nearly zero. If other experimental conditions remain constant, the standing current changes with different dopant gases. With N2 as dopant, the maximum standing current that can be achieved is 8.7 nA. This can be roughly doubled with CH4 (17.9 nA) and tripled with Xe (29.4 nA). The increase in the maximum standing current from N2 to CH4 is probably due to the difference in their ionization potentials. The photon energy of the He2 emission source is in the range of 13-20 eV. The ionization potential of nitrogen is 15.6 eV, so those photons with energy less than 15.6 eV cannot ionize the nitrogen and generate electrons. CH4 has an ionization

Figure 3. Effect of CH4 dopant concentration on (a) standing current and (b) response to CFCl3 and CH3Br. Detector temperature, 30 °C.

potential of 12.6 eV. Since that is well below the He2 emission range, all the photons from the He2 emission have enough energy to ionize the CH4 and generate electrons. Consequently, the CH4PDECD has a higher maximum standing current than the N2PDECD. It is not understood why Xe produces a higher maximum standing current. Since Xe has an ionization potential close to CH4’s (12.2 eV), we expected similar performance. The response of PDECD to CFCl3 and CH3Br was also measured against the change of CH4 dopant concentration, with the results in Figure 3b. As the CH4 concentration increases, the response to both CFCl3 and CH3Br increases quickly, passes through a maximum response at ∼0.35% CH4, and then slowly decreases. These results can be explained by the fact that as the concentration of CH4 increases, the electrons have more opportunity to be thermalized (thereby becoming more readily captured) through inelastic collisions. The slight response reduction at high CH4 concentration could be the effect of a small amount of uncapturable current originating from the photoelectric effect on the collecting electrode. The effect of this uncapturable current increases as CH4 dopant concentration goes high; the standing current is decreased, with a resulting reduction in the PDECD capture efficiency. Note that the maximum standing current is at ∼0.2% CH4 dopant, while the maximum response is at 0.35% CH4 dopant. To optimize both factors, we usually set the CH4 concentration at 0.3%. For N2 and Xe dopants, the dopant concentration dependence is almost the same as for CH4 dopant. We chose a mixture of 26 halocarbons to evaluate the PDECD because ECDs have selective response to most halocarbon compounds. The results of these analyses are shown in Figure 4a for the CH4-PDECD, Figure 4b for the N2-PDECD, and Figure 4c for the Xe-PDECD. Comparing these chromatograms, we can conclude that there is no significant difference between the N2Analytical Chemistry, Vol. 68, No. 7, April 1, 1996

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Figure 4. Chromatograms of 26 halogenated compounds obtained with (a) CH3-PDECD, (b) N2-PDECD, and (c) Xe-PDECD. Detector temperature, 150 °C; column, DB-1, 60 m × 0.25 mm, 1.0 µm; column temperature, 35 (10 min) to 200 °C at 4 °C/min, hold 2 min; sample, 60 pg each component in detector.

PDECD, the CH4-PDECD, and the Xe-PDECD in terms of relative response to these halocarbons. The exceptions are CH2Cl2 (peak 6), CHCl2CH3 (peak 7), and CHCl2CH2CH3 (peak 12). They are shown as regular capture peaks on the CH4-PDECD and XePDECD and as ionization or bipolar peaks for the N2-PDECD. Table 1 compares the relative responses to 23 halocarbons with the three dopants, along with their capture coefficients K as measured by the CH4-PDECD. (The first two compounds in the 26-halocarbon mix elute with the solvent, so are not considered. Also, peak 8, the trans CHCldCHCl, does not have a response.) We set the response of CCl4 on the CH4-PDECD as 10 000; the 1238 Analytical Chemistry, Vol. 68, No. 7, April 1, 1996

other values in the table are determined relative to that base. As we can see, the response differential of the PDECD to these 23 compounds is on the order of 5 orders of magnitude. The last two columns of Table 1 show the ratios of the relative response between the N2-PDECD and the CH4-PDECD and between the Xe-PDECD and CH4-PDECD for each compound. The ratios of the N2-PDECD to the CH4-PDECD are close to 0.5, which means that the N2-PDECD has less sensitivity than the CH4-PDECD by a factor of 2. Nevertheless, their response characteristics are similar. For most compounds, the Xe-PDECD has a higher response than the CH4-PDECD by a factor of 1.2. For low electron

Table 1. Capture Coefficient K and Relative Response of PDECD with Different Dopant Gases (150 °C) relative response

a

ratio

compounds

K (L/mol)

CH4

N2

Xe

N2/CH4

Xe/CH4

carbon tetrachloride dibromochloromethane dichlorobromomethane trichlorofluoromethane tetrachloroethene bromoform 1,2-dibromoethane 1,1,2,2-tetrachloroethane chloroform trichloroethene cis-1,3-dichloropropene trans-1,3-dichloropropene 1,1,2-trichloroethane 1,3-dichlorobenzene chlorobenzene 1,2-dichlorobenzene 1,1-dichloroethene 1,2-dichloropropane bromomethane 1,4-dichlorobenzene 1,2-dichloroethane methylene chloride 1,1-dichloroethane

8.6 × 6.7 × 1010 5.0 × 1010 4.4 × 1010 4.4 × 1010 3.9 × 1010 1.6 × 1010 8.6 × 109 5.6 × 109 5.0 × 109 1.5 × 109 1.1 × 109 4.2 × 108 1.1 × 108 1.1 × 108 1.1 × 108 6.4 × 107 5.8 × 107 5.8 × 107 4.2 × 107 1.1 × 107 8.3 × 106 2.8 × 106

10000 7600 5900 5300 5000 4500 1900 990 660 570 170 130 50 13 13 12 7.2 6.8 6.8 4.8 1.2 1.0 0.6

5000 3800 3100 2700 2900 2300 1000 560 310 270 89 70 28 7.3 5.7 6.1 5.1 a 3.4 2.0 a a a

12000 9400 7800 6500 5800 5300 2600 1100 710 720 220 170 73 24 20 19 14 5.4 7.2 7.8 5.1 4.0 1.6

0.50 0.50 0.53 0.51 0.58 0.51 0.53 0.57 0.47 0.47 0.52 0.54 0.57 0.57 0.45 0.49 0.71

1.2 1.2 1.3 1.2 1.2 1.2 1.4 1.1 1.1 1.3 1.3 1.3 1.5 1.8 1.6 1.6 1.9 0.8 1.1 1.6 4.1 4.1 4.8

1010

0.50 0.42

A bipolar or W shape peak observed.

capture compounds the sensitivity of the Xe-PDECD is even higher, possibly because the low ionization potential of Xe results in electrons with lower energy. Since the positive responses are affected by the dopant gas and its concentration, in most cases the use of a dopant with a low ionization potential will reduce this positive peak. As shown in Figure 5, with CH4 dopant the methanol solvent peak shows as a “W”, and with N2 dopant it shows as an ionization peak. This indicates that methane reduces the positive solvent peak better than nitrogen, presumably because methane has a lower ionization potential (12.6 eV) than nitrogen (15.6 eV). Of the three dopant gases studied in this work (N2, CH4, Xe), Xe gives the best results in terms of the capture coefficient constant K. Although the performance of Xe is only marginally better than that of CH4 and Xe is more expensive, we consider it the best choice overall since it can ultrapurified in the same manner as the discharge gas. However, we used CH4 as dopant in this study to give a more direct comparison with the radioactive ECD, which usually uses CH4 as the dopant gas. If the PDECD will be coupled to another detector (FID, MS, IR, etc.) N2 is a good choice because of its “cleaner”, smaller molecular weight. Other dopant gases may get even better resultssespecially amine compounds (trimethylamine, triethylamine, etc.), because of their lower ionization potential and larger molecular cross section. However, their irritating odor and the difficulty involved in removing impurities present practical drawbacks to their use. Comparison of the PDECD with a Radioactive ECD. We compared the PDECD halocarbon analyses with the performance of a 63Ni-ECD using the same GC system (Figure 5). Three conclusions can be drawn: (1) The detectors have similar response characteristics; i.e., they have a similar selective response to these 23 halocarbons. (2) Due to the active surface of the 63NiECD, the peaks from the 63Ni-ECD chromatogram are broader than those from the PDECD, especially for earlier eluting narrow peaks such as CFCl3 (4) and polar compound peaks such as water

Figure 5. Chromatograms of 26 halogenated compounds obtained with the CH3-PDECD and the 63Ni-ECD under the same GC conditions; 60 pg each sample in detector.

and methanol solvent. The solvent peak on the 63Ni-ECD chromatogram exhibits significant tailing, whereas the PDECD recovers quickly. (3) The 63Ni-ECD exhibits better response to compounds with a low electron capture coefficient. In Figure 5, water and methanol show higher response with the 63Ni-ECD than with the PDECD. Methanol even shows as a W peak in the PDECD, probably because of the two competing processes in the Analytical Chemistry, Vol. 68, No. 7, April 1, 1996

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Table 2. Comparison of Relative Response and Minimum Detectable Quantity (MDQ) between PDECD and (150 °C) relative response

a

63Ni-ECD

MDQ (pg, s/n ) 2)

compounds

PDECD

63Ni-ECD

ratio

PDECD

63Ni-ECD

ratio

carbon tetrachloride dibromochloromethane dichlorobromomethane trichlorofluoromethane tetrachloroethene bromoform 1,2-dibromoethane 1,1,2,2-tetrachloroethane chloroform trichloroethene cis-1,3-dichloropropene trans-1,3-dichloropropene 1,1,2-trichloroethane 1,3-dichlorobenzene chlorobenzene 1,2-dichlorobenzene 1,1-dichloroethene 1,2-dichloropropane bromomethane 1,4-dichlorobenzene 1,2-dichloroethane methylene chloride 1,1-dichloroethane trans-1,2-dichloroethene

10000 7600 5900 5300 5000 4500 1900 990 660 570 170 130 50 13 13 12 7.2 6.8 6.8 4.8 1.2 1.0 0.3 positivea

10000 5200 4900 5500 4000 2500 1000 590 470 380 120 64 30 24 45 25 6.7 43 35 10 0.8 0.6 0.3 0.7

1.0 1.5 1.2 1.0 1.3 1.8 1.9 1.7 1.4 1.5 1.4 2.0 1.7 0.5 0.3 0.5 1.1 0.2 0.2 0.5 1.6 1.5 1.1

0.006 0.009 0.008 0.007 0.009 0.013 0.021 0.036 0.036 0.037 0.12 0.15 0.47 1.9 2.1 1.9 1.3 0.61 1.5 6.1 14 12 28 positivea

0.019 0.037 0.025 0.017 0.024 0.053 0.091 0.16 0.10 0.11 0.58 0.94 2.2 0.19 0.45 0.11 4.3 0.28 7.1 0.24 46 40 106 37

0.31 0.24 0.32 0.41 0.38 0.25 0.23 0.23 0.36 0.34 0.21 0.16 0.21 10 4.7 17 0.31 2.2 0.21 25 0.30 0.30 0.26

An ionization peak was observed.

PDECD cellselectron capture and ionization. If the ionization process dominates, the response of the detector goes in the direction opposite the capture signal. If the electron capture process dominates, the PDECD shows a normal capture signal. When neither process dominates, the PDECD shows a W peak, as with the methanol in Figure 5. The existence of an ionization process in the PDECD reduces its capture signal, especially for those compounds with a low capture coefficient and low ionization potential. However, some ionization peaks can also be found in the 63Ni-ECD chromatogram in Figure 5, indicating that the ionization process exists in the 63Ni-ECD cell as well, in a weaker form. The PDECD’s lower sensitivity to low electron capturing compounds is due not only to the existence of a stronger ionization process in the PDECD but also to a lower concentration of dopant gas in the detector cell. The 63Ni-ECD has a dopant gas concentration of 5-10% while in the PDECD it is only 0.3-0.35%, so electrons in the 63Ni-ECD cell are more thermalized and therefore easier to capture. While this effect is especially pronounced for compounds with low electron capture coefficients, these differences do not affect the sensitivity of the PDECD to compounds with high electron capture coefficients. By measuring the signal-to-noise ratio on both detectors, we determined that, with a signal-to-noise ratio of 2:1, the minimum detectable quantities (MDQs) of the 23 halocarbons using the PDECD are 3-5 times less than those obtained with the 63Ni-ECD for those compounds with high electron capture coefficients (See Table 2). For compounds with lower capture coefficients and low ionization potential such as chloro- and dichlorobenzene, the 63Ni-ECD has better sensitivity; however, since the capture coefficient is low and the ECD response to these compounds is poor, other detectors, like the FID, the HID, and even the TCD, are more suitable for analyses involving these compounds. 1240

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Table 2 shows a detailed comparison between the relative responses of the PDECD and the 63Ni-ECD, with the detector responses linearized and normalized to CCl4 as 10 000. From Table 2, we can see that the response differential for the 23 halocarbons is close to 5 orders of magnitude for both detectors, while the response ratios of the PDECD to the 63Ni-ECD, which are listed in the fourth column of the table, are ∼1 in the range from 0.2 to 2. This result suggests that the nonradioactive PDECD behaves similarly to the radioactive ECD. It is important in the evaluation of the PDECD to compare the capture coefficients K with those from a radioactive ECD, since K is a fundamental value for determining ECD sensitivity and it is not affected by operation parameters (other than detector temperature). Since the 63Ni-ECD used in our study is designed to operate only in a constant current mode (which cannot be used to determine the capture coefficient K of a compound), and since the 63Ni-ECD has lower sensitivity than a 3H-ECD, we decided to compare our PDECD data to existing 3H-ECD data from the literature. The results of this comparison are shown in Table 3. The third column in the table gives the capture coefficients of the 3H-ECD using argon plus 10% methane as carrier gas. The fourth column in the table shows results from the PDECD using 0.3% methane as the dopant. All the data are obtained at 140 °C. In general, the capture coefficients from the PDECD are greater than those of the 3H-ECD by a factor of 1.5-3, indicating that the PDECD has essentially higher sensitivity than the radioactive ECD for these compounds. We suspect that the higher sensitivity has to do with the geometry of the detector cell. In the radioactive ECD, all the positive ions, electrons, and analytes are mixed together. The positive ions are free to compete for the electrons, reducing the efficiency with which electrons can be captured by the analyte. As Figure 1 shows, the reaction zone of the PDECD can be divided

Table 3. Comparison of Electron Capture Coefficients K (140 °C) electron capture coefficient K (L/mol) compound CCl4 C6H5NO2 CHCl3 CH2Cl2 a

EC characteristics

tritium ECDa

CH4-PDECD

ratio

high capture, dissociative high capture, nondissociative high capture, dissociative moderate capture, dissociative

4.6 × 3.6 × 109 1.7 × 109 1.6 × 106

8.3 × 9.2 × 109 4.4 × 109 3.3 × 106

1.4 2.6 3.1 2.1

1010

1010

Cite from ref 16.

into two sections: the top section (between E1 and E2) is the electron formation section and the bottom section (between E2 and the collecting electrode) is the electron capture section. Since the sample is introduced only into the capture section, the electron formation process and the electron capture process are physically separated and have little chance of interfering with each other. The bias of the electrical field forces the most highly charged of the positive particles formed during the electron formation process to remain at the top of the electron formation section, reducing the possibility of any positive ion-electron recombination reactions and ensuring maximum electron density around the collecting electrode. The small internal volume of the PDECD also contributes to its sensitivity. Even though its standing current is roughly equivalent to that of a radioactive ECD, the smaller volume of the PDECD means that the electron density in the PDECD cell is higher, favoring the electron attachment reaction since this reaction is a electron concentration dependent reaction. Sample Introduction Position. The physical position at which the sample is introduced into the PDECD cell is critical. Results of our experimentation indicate that the maximum response of PDECD for CFCl3 is obtained when the sample enters the detector 0.7 cm from the bottom edge of the collecting electrode (refer to Figure 1). Since the electrode thickness is 0.2 cm, the distance from the upper surface is 0.5 cm. A deviation of only 0.3 cm in either direction will cut the sensitivity to CFCl3 in half. However, this is only true for high electron capture compounds. With a moderate electron capture compound such as CH3Br, the response increases continuously as the distance between the column outlet and the collecting electrode increases, until the column outlet reaches the bias electrode E2. More than 20 chlorinated compounds have been tested with the identical result; that is, the higher the capture coefficient of a compound, the shorter the distance between the column outlet and the collecting electrode required to get the maximum response. It is not clear why the compounds with different capture coefficients show optimum response at different sample introduction positions. It could be associated with the electron capture and positivenegative ion recombination processes, since the concentrations of positive ions and electrons vary at different points within the detector cell. Temperature Dependence. The temperature dependence of the PDECD standing current has been investigated. As the temperature increases from 50 to 400 °C, the standing current increases from 8.7 to 24.9 nA (a factor of three), with a linear relationship. It is likely that the detector temperature does not have a direct effect on standing currentsrather, the observable effect occurs through other temperature-dependent factors such as gas density, discharge source intensity, photoionization process,

Figure 6. Temperature dependence of the PDECD on (a) response to 8 pg of CCl4 and 28 ng of CH2Cl2 and (b) plot of ln KT(3/2) vs 1/T for CCl4, C2Cl4, CHCl3, and CH2Cl2.

migration speed of charged particles, interaction between ions and electrons, and so on. While all of these temperature-dependent factors affect standing current, so far we do not know which one is the major factor. However, since radioactive ECDs exhibit the same phenomenon, the temperature effect on the PDECD and the radioactive ECD may follow the same mechanism. Figure 6a shows the temperature dependence of the PDECD response to CCl4 and CH2Cl2. Note that as the temperature changes from 50 to 400 °C, the response of the PDECD to CCl4 decreases by a factor of 2.4, while the response to CH2Cl2 increases by a factor of 6.5. The different response to temperature changes exhibited by these two compounds is due to different rate constants of the electron attachment reaction. CCl4 has a fast reaction rate, which is less dependent on temperature change, CH2Cl2 has a slow reaction rate, which is more affected by temperature change. The secondary factor contributing to the temperature-dependent response of the PDECD is the gas volume changeswhen the detector temperature increases, the volume of the gas inside the detector increases. This in turn increases the linear velocity of the gas, reducing the sample resident time and, consequently, detector response. However, this factor can be minimized by calibrating the gas flow. Analytical Chemistry, Vol. 68, No. 7, April 1, 1996

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Figure 7. Graph of concentration dependence of the PDECD for (a) chlorinated compounds (detector temperature, 150 °C) and (b) pesticides (detector temperature, 300 °C).

Early research17 has demonstrated that the electron capture or attachment follows different mechanisms, classified as I, II, III and IV, based on the nature of the potential energy curves of the negative ion in relationship to that of the neutral species. The kinetic regions of this potential can be identified by plotting ln K3/2 vs 1/T, in which K is the capture coefficient and T is temperature (in kelvin). Figure 6b shows that plot for four chlorinated compounds: CH2Cl2, CHCl3, CCl4, and C2Cl4, using the PDECD. All these compounds follow mechanisms II and III, and should have a negative or zero slope in the region 28-35. In Figure 6b, all slopes are negative and the intercepts of CH2Cl2, CHCl3, CCl4, and C2Cl4 are in the range of 30.8-34.7. These plots resemble the plots obtained with a radioactive ECD, indicating that electron attachment and chemical reaction may follow the same mechanisms in the PDECD as in the radioactive ECD. Further research on the kinetic mechanism of electron attachment using the PDECD will be published in a later paper. Response Time. The PDECD has a very fast response time. The cylindrical reaction zone of the PDECD is 3 mm i.d. × 8 mm long, with a volume of 56.5 µL. A small portion is occupied by the capillary column (0.8-2.0 µL, depending on the size of the column), yielding an actual active volume of ∼55 µL. With a total detector flow rate of 40 mL/min, the sample residence time in the detector is ∼0.08 ssshort enough for a regular capillary GC analysis, since even the fastest peak in this kind of GC has a >1 s peak width at the base. The PDECD is also satisfactory for GC analysis with a 0.18 mm microbore column; the analysis of three chlorinated compounds (CH2Cl2, CHCl3, CCl4) using this column produced a chromatogram with peak widths of 0.4-0.7 s at the base, with no significant peak tailing or peak broadening by the (15) Wentworth, W. E.; Cai, H.; Stearns, S. D. J. Chromatogr. 1994, 688, 135152. (16) Wentworth, W. E.; Chen, E. C. M. J. Gas Chromatogr. 1967, 5, 170. (17) Wentworth, W. E.; Chen, E. C. M. In Electron Capture: Theory and Practice in Chromatography; Zlatkis, A., Poole, C. F., Eds.; Journal of Chromatography Library 20; Elsevier: New York, 1981; Chapter 3.

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detector. Total run time was only 18 s. Because of its large internal volume and active surface, the radioactive ECD is not suitable for this type of analysis. If an even shorter detector time constant is required, there are two possible ways to achieve it: (1) reduce the internal volume of the detector or (2) increase the flow rate of the discharge gas. However, both methods are detrimental to the sensitivity of the PDECD. If the internal volume is reduced, the standing current will decrease because of the small reaction area; consequently, the absolute output signal will be reduced. However, the percent of electrons captured by the analyte may not change, because the detector flow rate does not change, keeping the analyte concentration in the detector constant. If the detector flow rate is increased, the standing current increases slightly with the increase in detector flow rate. Nevertheless, the percent capture is reduced because of the reduction of analyte concentration in the detector caused by the dilution factor. Basically, the detector time constant is proportional to its sensitivity regardless of which method is used to reduce the detector time constant. A combination of both methods is most effective for reducing the time constant to the 10 ms range. For example, with a detector having a 2 mm i.d. × 4 mm long active volume and a flow rate of 50 mL/min, the time constant is 15 ms. Linear Dynamic Range. It has been reported10 that the PDECD with N2 as dopant gas has a linear dynamic range over 3-4 orders of magnitude for the chloro fluoro compounds. In this research, we studied response linearity in the PDECD with 0.3% CH4 as the dopant. As previously indicated, the PDECD raw signal is nonlinear to analyte concentration and must be converted by the linear function eq 1 to yield a linear signal. Figure 7a shows the plots of peak area using (Ib - Ie)/Ie as response over mass vs concentration for four chlorinated compounds, CH2Cl2, CHCl3, CCl4, and C2Cl4. Straight horizontal lines have been drawn to allow a direct evaluation of the linear relationship with the data. The results indicate a linear relationship over 3-4 orders of magnitude. Since the MDQs of CCl4, CHCl3, and C2Cl4 are in the low femtogram range, this linear range can be extended at least 1 order of magnitude to lower concentrations, giving a linear range over 4 orders of magnitude. The upper limit of the linearity is imposed by the saturation of the detector, which occurs at a percent capture of higher than 90-95%. The PDECD response linearity of seven pesticides is shown in Figure 7b. For these pesticides, the detector response is linear over 3 orders of magnitude. This decreased linear range (compared to the compounds in Figure 7a) is due to the increase in detector noise at the high temperatures required for pesticide analysis. However, 3 orders of linear dynamic range is generally enough for practical application. We found that the 63Ni-ECD working in the pulsed collecting/constant current mode has an S-shaped concentration-dependent curve for certain high capture compounds (CCl4, BHCs). We have not encountered this problem on the PDECD. Pesticide Analysis. The analysis of pesticides is perhaps the most important applications of the ECD, because of their environmental significance and because no other detectors have competitive sensitivity and selectivity. Table 4 lists the MDQ of pesticides obtained with the PDECD, with signal to noise ratio of 2:1. In general, the MDQs of pesticides on the PDECD are in the midfemtogram range, which is equivalent to a radioactive ECD.

Figure 8. Chromatograms of real-world pesticide samples obtained with a PDECD and a 63Ni-ECD under same GC conditions except for columns: PDECD column, 007-608, 30 m × 0.53 mm, df ) 0.8 µm; 63Ni-ECD column, DB-17, 30 m × 0.53 mm, df ) 0.8 µm. Sample: (1) TCMX (surrogate); (2) R-BHC; (3) γ-BHC; (4) β-BHC; (5) heptachlor; (6) δ-BHC; (7) aldrin; (8) heptachlor epoxide; (9) endosulfan I; (10) 4,4′DDE; (11) dieldrin; (12) endrin; (13) 4,4′-DDD; (14) endosulfan II; (15) 4,4′-DDT; (16) endrin aldehyde; (17) endosulfan sulfate; (18) methoxychlor; (19) DCB (surrogate).

Table 4. Minimum Detectable Quantity (MDQ) of PDECD for Pesticides (s/n ) 2) pesticide

MDQ (fg)

pesticide

MDQ (fg)

R-BHC lindane β-BHC heptachlor aldrin heptachlor epoxide p,p′-DDE

34 36 83 37 38 40 50

dieldrin o,p′-DDD endrin o,p′-DDT p,p′-DDD p,p′-DDT

40 77 84 80 70 98

EPA method 608 for pesticide analysis outlines a procedure for monitoring extraction efficiency and ECD performance by adding two surrogate standards to all samples run. Some realworld pesticide samples from a local environmental laboratory have been tested on the PDECD following this method. The laboratory provided their results, obtained with a Hewlett-Packard 63Ni-ECD. Figure 8 shows typical chromatograms of a standard, a waste water extraction, and a matrix spike for the PDECD and the HP 63Ni-ECD. (The elution order differs because of the difference in the polarity of the columns.) A comparison indicates that there is no significant difference between these two detectors

for these samples, except for TCMX and endosulfan sulfat. These compounds show lower responses on the PDECD than on the 63Ni-ECD, probably due to lower capture coefficients. Table 5 details the performance of both detectors in the recovery of the 17 pesticides plus the 2 surrogate standards from the waste water matrix spike and spike duplicate. (Results for the HP 63Ni-ECD were supplied by the outside laboratory.) The recoveries from the PDECD fall in a range of 80-120%, which is close enough to the values obtained with the HP detector that it does not constitute a significant difference. The third column under each detector lists a relative standard deviation (RSD) from the matrix spike and the spike duplicate. These results suggest that the PDECD can replace the radioactive ECD currently used in this field. CONCLUSIONS The pulsed discharge electron capture detector using 0.3% methane as dopant has higher sensitivity than a radioactive ECD for compounds with high and moderate electron capture coefficients. The minimum detectable quantity for CCl4 is 6 fg and for lindane, 36 fg, with a linear dynamic range over 3-4 orders of magnitude. The response characteristics of the PDECD to the 23 halocompounds tested is close to that of the 63Ni-ECD, and Analytical Chemistry, Vol. 68, No. 7, April 1, 1996

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Table 5. Recovery of 17 Pesticides from Waste Water Extraction Matrix Spike and Spike Duplicate Using PDECD and HP 63Ni-ECD HP 63Ni-ECDa

PDECDa

pesticide

rec from MS (%)

rec from MSD (%)

TCMX (surrogate) R-BHC γ-BHC β-BHC heptachlor δ-BHC aldrin heptachlor epoxide endosulfan I 4,4′-DDE dieldrin endrin 4,4′-DDD endosulfan II 4,4′-DDT endrin aldehyde endosulfan sulfate methoxychlor DCB (surrogate)

84.8 91.5 93.4 100.8 97.9 108.7 89.5 98.6 96.9 92.4 90.6 88.9 97.7 97.8 107.7 89.7 90.4 119.7 73.1

78.7 87.0 92.1 97.7 96.6 102.6 88.7 91.6 96.7 91.4 84.8 83.8 96.6 96.3 106.7 85.0 86.0 115.0 69.7

RSD (%)

rec from MS (%)

rec from MSD (%)

RSD (%)

7.5 5.0 1.4 3.1 1.3 5.8 0.9 7.4 0.2 1.1 6.6 5.9 1.1 1.5 0.9 5.4 5.0 4.0 4.8

76.0 91.5 92.5 81.5 110.0 124.0 90.5 86.5 84.0 83.0 89.5 104.0 91.5 96.0 115.5 73.0 105.0 105.5 58.5

75.5 96.5 99.0 85.0 115.5 134.5 95.5 91.5 88.5 87.5 94.5 111.0 97.5 101.5 125.5 76.5 114.5 112.7 58.0

0.7 5.3 6.8 4.2 4.9 8.1 5.4 5.6 5.2 5.3 5.4 6.5 6.3 5.6 8.3 4.7 8.7 6.6 0.9

a MS, matrix spike; MSD, matrix spike duplicate; RSD, relative standard diviation.

ECD. Pesticides were measured using the PDECD, with results indicating that this detector can be used successfully for these environmentally interesting compounds. All this suggests that the basic electron capture mechanism of the nonradioactive PDECD is similar to that of a radioactive ECD. In addition, the detector has proven to be stable, fast (residence time 0.08 s), maintenance-free, and capable of operation at high temperatures (up to 400 °C). It has better resolution, is safer, and costs less than a 63Ni-ECD, leading us to conclude that the PDECD can be considered a viable replacement for the radioactive ECDs in current use.

ACKNOWLEDGMENT The authors thank David Salge of Valco Instruments Co. Inc. for his assistance in preparing and editing the manuscript. We also thank Dr. Wensheng Zhang of Pace Inc. for providing pesticide samples and data, and Dr. E. C. M. Chen of the University of Houston, Clear Lake, for his valuable comments. Part of this paper was presented as Paper 450 at the 1995 Pittsburgh Conference in New Orleans.

Received for review October 19, 1995. Accepted January 10, 1996.X AC951047J

the temperature dependence of four chlorohydrocarbons on the PDECD agrees closely with the values obtained with a radioactive

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X

Abstract published in Advance ACS Abstracts, February 15, 1996.