Anal. Chem. 1998, 70, 3770-3776
Pulsed Discharge Electron Capture Detector Operating in the Constant-Current Mode by Means of Feedback dc Bias Voltage Huamin Cai* and Stanley D. Stearns
VICI Valco Instruments Co. Inc., P.O. Box 55603, Houston, Texas 77255 Wayne E. Wentworth
Chemistry Department, University of Houston, Houston, Texas 77204
A feedback dc bias voltage has been utilized to established a constant-current operation mode (cc-mode) for the pulsed discharge electron capture detector (PDECD). The feedback voltage is directly proportional to the concentration of analyte in the detector cell and can function as output. Working in this mode, the PDECD has a linear dynamic range of 1 × 105 for CCl4 and 1 × 104 for pesticidesalmost 1 order of magnitude better than in the previously documented constant-potential mode (cpmode). The minimum detectable quantity for CCl4 is 1.5 fg (10-15 g) and for lindane, 16 fg. This paper compares the operation of the PDECD in the cc-mode and in the cp-mode. Our findings indicate that a PDECD operating in the cc-mode has relative response, temperature dependence, and dopant gas effects similar to a PDECD in the cp-mode under the same experimental conditions, but has advantages of higher sensitivity, faster response, and simpler operation. The electron capture detector (ECD) is widely used because of its high sensitivity and selectivity to certain compoundss especially to compounds of environmental interest such as chlorofluorocarbons (CFCs), pesticides, and polychlorinated biphenyls (PCBs). Current ECDs still utilize the same primary electron source as the first ECD invented by Lovelock1 three decades agosa radioactive material such as 63Ni and 3H (Lovelock’s detector used 3H). Many nonradioactive sources have been investigated, but none of them have replaced radioactive source because of problems with stability, sensitivity, operational complexity, etc. Recent years have seen the introduction of a new nonradioactive electron capture detector, the pulsed discharge electron capture detector (PDECD).2 This detector uses pulsed high-voltage discharge in pure helium as the primary source of high-energy photons, which in turn ionize the dopant in the detector cell to produce electrons. Current investigation3 indicates that the sensitivity and response characteristics are similar to a radioactive ECD; in fact, the sensitivity of PDECD is higher than (1) Lovelock, J. E.; Lipsky, S. R. J. Am. Chem. Soc. 1960, 82, 431. (2) Wentworth, W. E.; D’sa, E. D.; Cai, H.; Stearns, S. J. Chromatogr. Sci. 1992, 30, 478-485. (3) Cai, H.; Wentworth, W. E.; Stearns, S. Anal. Chem. 1996, 68, 1233-1244.
3770 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
that of most commercial 63Ni ECDs. This detector is now commercially available (Valco Instruments Co. Inc., Houston, TX), and is a good alternative to ECDs in current use. The first commercial PDECD uses a low constant-potential bias voltage; that is, the PDECD works in a constant-potential operational mode (cp-mode). The raw signal output of the PDECD is nonlinear to the concentration of analyte above 5% capture. To get a linear signal, the raw signal must be converted through the Wentworth equation.4
R ) (I0 - Ie)/Ie ) K[A]
(1)
where R is the output signal after conversion, I0 is the standing current (a constant), Ie is the raw signal output (current), [A] is the concentration of analyte A, and K is the capture coefficient. With this conversion, the response of the PDECD has a linear dynamic range of 3-4 orders of magnitude.3 (In commercial PDECD, a microprocessor does this conversion in real time, but the raw signal could be sent to a computer with the proper linearization software.) Though this linear conversion is satisfied in most cases, it has several drawbacks. First, it is difficult to determine the correct I0 for each peak in a GC run because the baseline always drafts to some degree during a whole analysis period. This is especially true when the GC oven temperature is programmed to achieve the best separation. It is possible to trace the baseline and input the correct I0 for each peak, but it takes sophisticated hardware or software to determine when each peak appears. In addition, the output baseline appears to step when the I0 changes. In the commercial PDECD, it is assumed that the I0 dose not change during a GC run, and an initial current taken before the GC run is used as the I0 for every peak. The potential conversion error introduced by such an assumption can be calculated from eq 1, and it is in direct proportion to the relative error of I0 as shown in
h′ ) (I0′/I0)h
(2)
where h stands for the correct peak height obtained from the (4) Wentworth, W. E.; Chen, E. C. M.; Lovelock, J. E. J. Phys. Chem. 1966, 70, 445-448. S0003-2700(97)00635-5 CCC: $15.00
© 1998 American Chemical Society Published on Web 08/13/1998
linear conversion using the correct I0 and h′ stands for the peak height obtained using an incorrect I0′. Another problem with this linear converter has to do with the difficulty of measuring peaks that cannot be separated well. For example, if there is a small peak on the tail of a big peak, the correct I0 for measuring this small peak is the actual tailing current precisely where the small peak appears, which differs from the I0 used for measurement of the large peak. The measurement of this small peak may be associated with a big error if the original I0 is used for this small peak. Other drawbacks of this linearizer are as follows: (a) it has to be rezeroed before every injection (find I0); (b) the random error is not uniform after this linear conversionsa small original measure error will be amplified in the high-concentration region, where Ie is close to 0; and (c) electric circuitry is complicated, requiring either a microprocessor for digital conversion or a linear circuit for analog conversion. Either way, the conversion process slows down detector response time to a point where it not suitable for fast GC analysis. Most radioactive ECDs in currently use employ a constantcurrent pulsed-collecting modulated linearizing method, since it is simple and gives a good linear range (although there have been reports that the responses for some high-capture coefficient compounds deviate from this linearity). This method is based on the fact that (a) the collecting pulse frequency is directly proportional to the detector standing current and (b) the frequency is directly proportional to the analyte concentration, if the collecting frequency is varied to maintain a constant current in the detector cell. So, after a frequency-voltage conversion, the signal output of a detector in this constant-current mode is linear for an analyte concentration range of up to 5 orders of magnitude. This method does not work on the PDECD,3 since the pulsed discharge source produces electrons from a photoionization reaction, unlike a radioactive source, which produces electrons directly from a β emission. This study investigates a constant-current operation mode (ccmode) which uses a variable dc bias potential to maintain a constant detect current. The bias voltage itself is used as the output. In this method, a preselected level of working current (Iw) is demanded by the control circuit, which continually compares it with the current measured from detector cell (Id). When a difference between Iw and Id is detected, the control circuit adjusts the bias voltage until Id equals Iw. In operation, this means that Id drops when an electron-capture compound enters the cell, causing the control circuit to increase the bias voltage to bring Id back to the working level. Thus, the increase in bias voltage is proportional to the concentration of the compound entering the cell.7 This technique was first proposed by Aue, Sui, and coworkers5,6 for use in a radioactive ECD. It did not find widespread acceptance, probably because the resulting sensitivity and linearity were not as good as ECDs operating in the pulsed-collecting mode. Since the pulsed-collecting mode is not an option for the PDECD, this alternative method seems worth pursuing. (5) Aue, W. A.; Siu, K. W. M. Anal. Chem. 1980, 52, 1544-1546. (6) Siu, K. W. M.; Roper, C. M.; Ramaley, L. A.; Walter, A. J. Chromatogr. 1981, 210, 401-407. (7) Stearns, S. D.; Cai, H.; Wentworth, W. E. U.S. Patent 5,767, 683, June 16, 1998.
The purposes of this paper are (1) to establish that the ccmode works for the PDECD, (2) to investigate the effect of different operating conditions, such as working current, dopant gas, temperature, etc., to the performance of the PDECD in ccmode, and (3) to compare the PDECD in the cc-mode to the PDECD in cp-mode. The results shows that, for the PDECD, the cc-mode is superior to the cp-mode in linearity as well as sensitivity. It has additional benefits of easy operation, quick response, and simpler electrical circuitry. EXPERIMENTAL SECTION Figure 1 illustrates the apparatus used for the cc-mode evaluation. A standard commercial PDECD (Valco Instruments Co. Inc., Houston, TX) was used in this study. Dimensions and working principles were described in detail in previous paper.3 In the cc-mode, only two electrodes (E1 and E3) are used, leaving E2 as an electrical float. An electrometer intended for use with the commercial PDECD operating in the cp-mode is used to monitor detector current. The electrometer output goes to homemade compare-and-control circuits, comprising a negative feedback control system. The working current Iw can be adjusted from 0 to 50 nA, and the bias voltage can vary between 0 and 150 V. The bias voltage also functions as detector signal output, giving a 0-1-V full-scale output after going through a 150:1 attenuator. Chromatography was performed on an HP 6890 GC (HewletPackard, Wilmington, DE) with EPC split/splitless inlets. An HP 7673 automatic sampler (Hewlet-Packard) was used for sample injection. HP ChemStation (Hewlet-Packard) was used for GC control and data process. The separations (except for pesticides and PCBs) were accomplished on a DB-5 fused-silica capillary column, 30 m × 0.25 mm with film thickness of 1.0 µm (J&W Scientific, Folsom, CA). Pesticides were separated on a HP-608 fused-silica capillary column 30 m × 0.53 mm with film thickness of 0.5 µm (Hewlet-Packard), and PCBs were separated on an nonpolar 007 series column 15 m × 0.25 mm with 0.25-µm film thickness (Quadrex, New Haven, CT). Research grade pure helium (99.9999%, Air Liquide, La Port, TX) was used as discharge gas and carrier gas, using a T-split. The discharge gas passes through a restrictor and then through a helium purifier (Valco Instrument Co. Inc.) before entering the detector. The flow rate of the discharge gas was adjusted to 30 mL/min. CH4 (5%) in helium (Air Liquide) was used as dopant gas. The flow rate of this dopant was ∼2 mL/min, giving a total concentration of ∼0.3% CH4 in the detector cell. Other dopant gases, 5% N2 and 1% Xe (both from Air Liquide), were used only to study the effect of different dopant gases. A 26-halogen standard mixture with concentration of 100 (µg/mL in methanol (ULTRA Scientific, North Kingstown, RI) was diluted with THM grade methanol (Sigma-Aldrich, St. Louis, MO, Milwaukee, WI) to give a low-level concentration. A chlorocarbon mixture of CHCl3 (Fisher Scientific), CCl4 (EM Science), and CCl2dCCl2 (Aldrich Chemical Co.) was prepared in our laboratory, with THM grade 2,2,4-trimethylpentane (Sigma-Aldrich) as solvent. A Standard pesticide mixture (EPA 625/CLP Pesticides Mix, Alltech, Deerfield, IL) and a standard PCB mixture (525, 5251 PCB Mix, Supelco, Bellefontaine, PA) were diluted with THM grade methanol to the desired concentration. Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
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Figure 1. Block diagram of the working principle of the PDECD operating under the constant-current mode.
RESULTS AND DISCUSSION The constant-current variable dc bias operation mode for PDECD is based on the fact that when an electron capture compound enters the detector cell, detector current is reduced as electrons in the cell are captured. The detector current can be held constant by compensating with an increase in the bias voltage ∆V. There is therefore a certain relationship between the increase in bias voltage ∆V and an increase in the concentration of compound A (∆[A]). Experimental results have shown that, in a certain concentration range, ∆V and ∆[A] have a linear relationship as (refer to Figure 3)
∆V ) Kc∆[A]
(3)
where Kc is a response factor for this constant-current mode (with units of V‚mol-1‚L) determined by analyte A, the geometry of the detector, and the experimental condition (working current Iw, detector temperature, dopant gas, etc.). If initial concentration [A]0 ) 0, we have
∆V ) Kc[A]
(4)
The relationship of detector current Id, bias voltage V, and capture coefficient K has been investigated, with the results shown in Figure 2. As Figure 2A indicates, the detector current Id increases continuously with an increase in the bias voltage. However, it does not increase linearlysit increases faster at low bias voltage than at high bias voltage, according to the equation
Id ) a ln(V) + bV + c
Figure 2. (A) Graphs of a V-A curve; (B) the effect of bias voltage on capture coefficient K in constant-potential mode.
(5)
where a, b, and c are constants. In this particular test, a, b, and 3772 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
c are 14.2, 0.118, and 4.23, respectively (in nA and V units). Since a . b, the Id and V basically follow a logarithmic relationship when bias voltage V is less than 100 V.
Table 1. Effect of Working Current Iw on PDECD Performance Iw (nA)
V0 (V)
∆Va (V)
Kc (V‚mol-1‚ L × 1010)
K′c (V‚mol-1‚ L‚nA-1 × 109)
MDQ (S/N ) 3, fg)
10 20 30 40 50
0.73 1.74 2.99 4.51 6.37
0.15 0.28 0.39 0.54 0.74
4.5 8.2 11.3 16.9 23.5
4.5 4.1 4.0 4.2 4.7
2.66 1.74 1.51 1.82 2.00
a
Peak height for a 0.5-pg CCl4 sample.
Since an increase in detector current ∆I is not linear with an increase in bias voltage ∆V, if the efficiency of electron capture (capture coefficient K in eq 1) is independent of changes in bias voltage, ∆V would not be linear with [A]. However, the efficiency of capturing electrons does change with the bias voltage. As Figure 2B shows, the capture coefficient of CCl4 decreases with an increase in bias voltage, decreasing faster at low bias voltage than at high bias voltage. This follows the same pattern as changes in detector current in (Figure 2A) but in the opposite direction, making ∆V linear to [A]. Working current Iw plays an important role in this constantcurrent operation mode. Table 1 lists the performance of the PDECD with various values of Iw. It is clear that the response factor Kc is a linear function of the working current Iw: when Iw increases by factor 5 (from 10 to 50 nA), the Kc of CCl4 increased by roughly the same factorsfrom 4.5 × 1010 to 23.5 × 1010 V‚mol-1‚L. Thus, eq 4 can be expressed in following way:
∆V ) Kc′Iw[A]
(6)
where Kc′ is defined as the calibrated response factor, which is independent of the working current Iw. The Kc′ of CCl4 in this test is listed in column 5, Table 1 as 4 × 109 V‚mol-1‚L‚nA-1. A high working current can achieve a high response factor, but since it also increases detector noise, the best performance of PDECD in the cc-mode (in terms of minimum detectable quantity (MDQ) or signal-to-noise ratio) is not always attained by increasing the working current. As show in Table 1, the best signal-to-noise ratio is at Iw ) 30 nA. This ratio of 3 corresponds to an MDQ of 1.5 fg of CCl4. With the same detector operating in the cp-mode, the MDQ of CCl4 is only about 5 fg, indicating a higher sensitivity for PDECD in the cc-mode. When a working current is set, a small bias voltage is required to keep this current flowing, even if there is no analyte present in the detector. We define this voltage as the background voltage V0. V0 is affected mainly by working current Iw, detector temperature, and dopant gas. It is also determined by the “cleanliness” of the GC system, with a cleaner system achieving a lower V0. The presence of electron capture compounds such as oxygen in the detector cell will cause a high V0. Since V0 plus maximum peak height cannot excess 150 V (due to the limitation of our electrical circuit and the nonlinear response at high bias voltage when V > 100 V), we expected that the optimum background V0 should be as small as possible so that it does not significantly reduce the detector linear dynamic range. The relationship between V0 and Iw is given in Figure 2A, where the bias voltage
Figure 3. (A) Graphs of the concentration dependence of the PDECD in cc-mode for CCl4 under two different working current Iw, 10 and 20 nA; (B) Graphs of the concentration dependence of the PDECD via cc-mode for C2Cl4, CHCl3, and CH2Cl2, Iw ) 20 nA.
can be considered V0 and detector current can be consider Iw. This curve shows an exponential increase in V0 as Iw increases. Under typical test conditions, with detector temperature set at 150 °C, the V0 is less than 1 V when Iw ) 10 nA and 6.4 V when Iw ) 50 nA; however, with the detector at room temperature, the V0 can be as high as 60 V when Iw ) 50 nA. So, if a high Iw was used, the response factor Kc increased as in Table 1, with a corresponding increase in V0. Therefore, use of a high Iw reduced the linear range of the detector, especially at low temperatures. The effect of dopant gas to background voltage V0 will be discussed later in this paper. Figure 3A shows the dynamic linear range of PDECD in ccmode with two different working currentss10 and 20 nA. The linearity is determined by plotting the response factor in the peak area over mass (V, s, pg-1) against the mass of CCl4 (pg) in the detector cell. This will result in a horizontal line if the detector response is within the linear range. As shown in Figure 3A, when I0 ) 10 nA, the PDECD dynamic linear range covers from 2 fg to 200 pg, which gives a whole dynamic linear range of 5 orders of magnitude; when I0 is increased to 20 nA, the linear dynamic range stays the same, but shifts to the left, reflecting better sensitivity and early saturation. A large working current could reduce the dynamic range, because of the earlier saturation and no further improvement in sensitivity. Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
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Figure 5. Temperature dependence of the PDECD in cc-mode showing (A) background voltage V0 and (B) the response in peak height ∆V for 5 pg of CCl4 and 5 ng of CH2Cl2.
Figure 4. Chromatograms of halogenates compounds obtained with (A) cc-mode, CH4 dopand; (B) cc-mode, N2 dopand; (C) cc-mode, Xe dopand; and (D) cp-mode CH4 dopand. Detector temperature, 150 °C; sample, 75 pg each component in detector; GC condition, DB-5 column, 30 m × 0.25 mm with 1-µm film thickness; oven at 35 (7 min) to 75 °C at 5 °C/min, then to 220 °C at 17 °C/min, hold for 4 min.
Three dopant gases (N2, CH4, Xe) have been examined in this study. Figure 4 shows the chromatograms of a standard mixture of 26-halogen mixture, with each of these dopants. Table 2 listed 19 of the 26, and 7 others either could not be separated from solvent or elicited no significant response. For purposes of comparison, a chromatogram of this mixture with the PDECD in cp-mode is provided in Figure 4D. All four chromatograms show good peak shapes, with no major differences in response. Table 2 lists the response factor Kc of the halogenated compounds with the different dopands. With methane, the response factor in cc-mode varied from 5.9 × 1010 (CCl4) to 5.3 × 106 (CH2Cl2), or 4 orders of magnitude. It is interesting to find that these Kc values are close to the values of the capture coefficient K in cp-mode with methane dopant (shown in column 5 of Table 2). The ratios of Kc to K for the 19 halogenated compounds being tested fall within the range of 0.8-1.3 (Table 3774 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
2, column 6). These results imply an intrinsic relationship between Kc in the cc-mode and K in the cp-mode. If the other experimental conditions remain constant, Kc increases slightly with different dopands in this order, Xe > CH4 > N2, as listed in Table 2. This is the same order and magnitude shown for K in cp-mode with different dopants.3 The last four columns of Table 2 give a detailed comparison of relative response, the response of CCl4 set as 10 000 and the response of other compounds determined relative to that base. As we can see, there is no major difference in relative response with different dopants and modes. Note that, with the N2 dopant, a bipolar or W-shaped peak is observed for those compounds with a low response factor. This is because N2 has a higher ionization potential, causing some high-energy photons or other particles to remain in the detector cell, resulting in a competition between the ionization process and the capture process. The PDECD in cp-mode with N2 dopant exhibits the same phenomenon.3 The dopand concentration also effects the background voltage V0 and response factor Kc of the PDECD in cc-mode. When concentration increases, V0 decrease first, but at about 0.15% it reaches a minimum and begins a slow increase. With the same dopant concentration, the V0 increase with different kinds of dopant in this order: N2 > CH4 > Xe. The response factor Kc increase continuously as dopant concentration increase, with a faster increase at high working current than at low working current. If a high dopant concentration is used, V0 increases, causing not only a reduction in linear range but also an increase noise level. Taking all these factors into account, we determined that 0.3% dopant concentration is optimum and used that concentration in this study.
Table 2. Response Factor Kc and Relative Response of PD-ECD in cc-Mode with Different Dopant Gases (200 °C, Iw ) 20 nA) response factor Kc (V‚ mol-1‚L)
K (mol-1‚L)
Kc/K
relative response
compound
CH4
N2
Xe
CH4
CH4
CH4
N2
Xe
CH4b
carbon tetrachloride dichlorobromomethane dibromochloromethane tetrachloroethene bromoform trichlorofluoromethane 1,2-dibromoethane 1,1,2,2-tetrachloroethane trichloroethene chloroform cis-1,3-dichloropropene trans-1,3-dichloropropene 1,1,2-trichloroethane 1,2-dichlorobenzene 1,3-dichlorobenzene 1,4-dichlorobenzene 1,1-dichloroethane 1,2-dichloroethane methylene chloride
5.9 × 1010 4.7 × 1010 3.3 × 1010 2.9 × 1010 2.3 × 1010 1.6 × 1010 1.1 × 1010 6.8 × 109 5.9 × 109 3.4 × 109 1.5 × 109 1.2 × 109 7.2 × 108 2.5 × 108 9.5 × 107 7.5 × 107 2.3 × 107 1.3 × 107 5.3 × 106
4.0 × 1010 3.1 × 1010 2.3 × 1010 2.2 × 1010 1.7 × 1010 1.1 × 1010 9.5 × 109 5.4 × 109 4.9 × 109 3.2 × 109 8.0 × 108 5.2 × 108 1.7 × 108 4.5 × 107 a a a a a
9.2 × 1010 7.3 × 1010 5.2 × 1010 4.2 × 1010 3.3 × 1010 2.4 × 1010 1.9 × 1010 1.0 × 1010 9.4 × 109 6.2 × 109 2.2 × 109 1.8 × 109 1.0 × 109 2.8 × 108 6.4 × 107 1.2 × 108 4.3 × 107 3.5 × 107 6.3 × 105
5.2 × 1010 4.3 × 1010 3.0 × 1010 2.7 × 1010 2.3 × 1010 1.4 × 1010 1.3 × 1010 7.4 × 109 6.2 × 109 4.2 × 109 1.4 × 109 1.3 × 109 6.5 × 108 2.1 × 108 7.5 × 107 6.8 × 107 1.9 × 107 8.4 × 106 4.3 × 106
1.1 1.1 1.1 1.1 1.0 1.1 0.9 0.9 1.0 0.8 1.0 0.9 1.1 1.2 1.3 1.1 1.2 1.6 1.2
10000 6500 5900 4600 3000 2400 1600 1000 940 740 350 270 140 43 17 13 6.1 4.8 1.6
10000 6600 5700 5000 3000 2500 1900 1200 1100 1000 270 180 48 12 a a a a a
10000 6600 5800 4200 3000 2200 1600 1000 960 860 340 270 130 31 7 14 7.3 5.9 0.1
10000 6800 6000 4900 3000 2600 2000 1300 1100 1000 370 340 140 41 15 14 5.6 2.5 1.5
a
A bipolar or W-shaped peak observed. b Cp-mode.
Figure 6. Chromatograms of (A) standard pesticide mixture and (B) PCB mixture, 25 pg each component in detector. GC conditions: (A) HP-608 column, 30 m × 0.53 mm with 0.5-µm film thickness; 5 mL/min constant column flow; oven at 100 °C (2 min) to 280 °C at 12 °C/min. (B) 007 series column, 15 m × 0.25 mm with 0.25-µm film thickness; 2 mL/min constant column flow; oven at 100 °C (4 min) to 280 °C at 10 °C/min.
Panels A and B of Figure 5 show the temperature dependence of background voltage V0 and detector response, respectively. When the detector temperature increases from 50 to 325 °C, V0 decreases from 1.7 to 1.0 V. Compared with the 150-V dynamic output range, this change is insignificant. However, there are
significant changes to detector response as the temperature changes. As Figure 5B indicates, when the detector temperature increases from 50 to 325 °C, the peak height for 5 pg of CCl4 decreases from 5.2 to 1.9 Vsa factor of 2.7sand the peak height for 5 ng of CH2Cl2 increases from 0.066 to 3.9 Vsa factor of 59. Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
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Table 3. Relative Standard Deviation of Calibration Curve for Pesticides RSD (%) pesticide
cp-mode
cc-mode
R-BHC γ-BHC β-BHC heptachlor aldrin heptachlor epoxide 4,4′-DDE dieldrin endrin 4,4′-DDD 4,4′-DDT
6.8 7.6 5.8 8.7 7.7 9.8 9.3 9.1 10.3 9.5 6.1
7.2 4.9 5.5 3.3 3.4 2.3 3.9 4.9 4.7 3.4 4.2
8.2
4.3
av
Figure 7. Five-level calibration curves of aldrine in (A) cp-mode and (B) cc-mode.
A similar change is also exhibited in the PDECD in cp-mode,3 likely from the same factorssthe type and rate of the electron capture reactions. Panels A and B of Figure 6 are chromatograms of a standard organochloro pesticide mixture and a standard PCB mixture, respectively, with the PDECD in the cc-mode. All pesticides and PCBs are clearly resolved and have good peak shape. Comparing these chromatograms with published chromatograms using the PDECD in cp-mode or with a radioactive ECD, we see that the relative response to these pesticides and PCBs is similar. The PDECD’s sensitivity to various pesticides was tested, with an MDQ for lindane established at 16 fg. This is higher than the MDQ for CCl4 because the detector noise level is higher at the high temperature (300 °C) required for lindane, compared to CCl4 (70 °C). The higher operating temperature also reduces the response factor of most pesticides, contributing to a higher MDQ. The higher MDQs reduce the dynamic linear range for pesticide to 4 orders of magnitude.
3776 Analytical Chemistry, Vol. 70, No. 18, September 15, 1998
EPA method 608 for organochlorine pesticide requires a calibration curve with at least three levels for pesticide analysis. The relative standard deviation (RSD, %) of this calibration curve must be less than 10%. We ran a five-level organochlorine pesticide calibration curve for the PDECD in each mode. A typical calibration curve for aldrin is shown in Figure 7A for the cp-mode and Figure 7B for the cc-mode. As Figure 7A shows, the concentration dependence of the cp-mode causes a slight deviation from linearity, especially at high concentration. In the cc-mode (B), this plot is perfectly straight, indicating that the cc-mode has better linearity than the cp-mode. The other pesticides show similar calibration curves. Table 3 shows RSD of the calibration curve for some of the pesticide in Figure 6. For the cp-mode, the RSD is between 5.8 and 10.3% with an average of 8.2%. For the cc-mode, it is between 2.3 and 7.2% with an average of 4.2%. Both modes meet the QC requirement of EPA method 608 (RSD
