1892
Anal. Chem. 1982, 5 4 , 1892-1893
LITERATURE CITED von Lillin, H. Angew. Chem. 1954, 66, 849. Parrish, J. R. Chem. Ind. (London) 1955, 388-387. Parrish, J. R. Chem. Ind. (London) 1956, 137. Pennington, L. D.; Wllliams. M. B. Ind. Eng. Chem. 1959, 51, 759. Vernon, F.; Eccles, H. Anal. Chlm. Acta 1973, 63, 403-414. Parrlsh, J. R.; Stevenson, R. Anal. Chim. Acta 1974, 70, 189-198. Vernon, F.; Nyo, K. M. Anal. Chlm. Acta 1977, 93, 203-210. SlovBk, 2.; SlovBkovB, S.; Smrz, M. Anal. Chlm. Acta 1975, 7 5 , 127-1 38. Parrish, J. R. Anal. Chem. 1977, 49, 1189-1192. Parrish, J. R. Lab. Pract. 1975, 2 4 , 399-400. Hemmes, M.; Parrlsh, J. R. Anal. Chim. Acta 1977, 9 4 , 307-315.
(12) Vernon, F.; Nyo, K. M. J. Inorg. Nucl. Chem. 1978, 40, 887-891. (13) KL5 Catalogue, Koch-Light Laboratories Ltd., pp 274-278. (14) SlovBk, 2.; Toman, J. Fresenlus' Z . Anal. Chem. 1976, 278, 115-120. (15) SlovBk, 2.; SlovSkovi, S. Fresenius' Z . Anal. Chem. 1976, 292, 2 13-2 15. (18) F. Vernon, Hydrometallurgy, 1979, 4 , 147-157.
RECEIVED for review December 8, 1981. Resubmitted and accepted June 3, 1982. Financial support from the South African Council for Scientific and Industrial Research is gratefully acknowledged.
Circuit for Constant Current Operation of an Electron Capture Detector W. B. Knighton and E. P. Grimsrud" Department of Chemistry, Montana State University, Bozeman, Montana 59717
The constant current (CC) mode of signal processing has become the standard offering today of commercially available gas chromatographs with electron capture detection. First suggested in 1966 by Maggs et al. (I),the CC mode greatly increased the range of linear response of pulsed electron capture detectors (ECD). Whereas the earlier modes of ECD control (the fixed frequency pulse and the direct current modes) provided linear responses only over the initial 10% portion of the dynamic range, the CC mode typically provides linear calibration curves up to 99% of response saturation. [Even with the CC-ECD, nonlinear responses are sometimes observed (2-4). However, these are generally attributed to complications of the electron capture reactions within the cell rather than to the mode of signal processing.] In recent years we have been studying electron capture processes with home-built ECDs of various designs. More recently, we wished to construct a circuit which would allow the operation of one of these in the constant current mode. The function of the circuit required for the CC mode is easily envisioned an electrometer compares the ECD current with a preselected reference current and keeps them equal by initiating feedback which appropriately adjusts the frequency of pulses applied to the cell. The circuitry we found in the literature, however, was either too general (specific identification of circuit element not given) or was too complex for facile construction. By use of the general concept of ECD control as first expressed by the inventors of the technique ( I ) a simple, operationally successful circuit was ultimately found by testing and varying the individual network components. This circuit, which is constructed entirely of readily available and inexpensive components, is described here in the hope that it will be of assistance to anyone who wishes to upgrade a commercial or home-built ECD which is presently limited to the fixed frequency mode of signal processing. EXPERIMENTAL SECTION A complete circuit diagram of the detection system is shown in Figure 1. Circuits A and B show different configurations for pulsing an ECD. Circuit A involves pulsing the anode positively while the cell body is grounded, whereas in circuit B the cell wall is pulsed negatively. In both cases the current is measured at the concentric axial anode. Operational amplifier one (OA l), CA 3140, is operated as an integrator in which the output voltage reflects the time averaged difference between the reference and cell currents. OA 2, CA 3140, inverts the output signal from OA 1making it compatible with the voltage-to-frequency converter (V-to-F),Burr Brown VFC52, which only operates with positive going signals. A noninverting buffer, CD 4050, is used after the 0003-2700/82/0354-1892$01.25/0
V-to-F as a pulse shaper where the buffer improves the rise time of the pulses. The output pulses from the V-to-F are typically of about 2.5 ps in duration. This pulse width is modified to any width desired by the RC coupled NAND gates. The RC couple chosen here yields a pulse width of 1 ps duration. The NAND gates are contained within a quad, two-input NAND gate IC, CD 4011B. If it is desired to pulse the wall as in circuit B, then a third NAND gate on the IC is used to invert the pulse. The pulse train is then amplified by use of a transistor to 50 V. For positive pulses a 2N4402 PNP transistor is used and a 2N4400 NPN transistor for the negative going pulse. The base frequency of pulsing is determined by the current demanded from the reference current source and the cleanliness of the cell. The reference current desired is selected by a 10-turn 5-kQpotentiometer and is variable from 0 to 9 nA. The output signal from OA 2 is sent to a multirange recorder by way of OA 3, LM 741, a voltage follower, and OA 4, CA 3140, which is used to zero the signal. Ideally, an ECD control circuit should also provide a means of measuring standing current and of adjusting the pulse width required to achieve the maximum standing current for a given detector and choice of carrier gas. Since these parameters are most easily measured with a fixed frequency of pulsing, it is desirable to also have this capability built into the electronic package. A circuit diagram for operating an ECD in the fixed frequecy mode is shown in Figure 2. NAND gates 1and 2 control the period of the pulses while NAND gates 3 and 4 control the pulse width as in the constant current circuit. All four NAND gates are contained on a CD 4011B IC. The pulse period can be varied from 10 ps to 2.5 ms using the values of the capacitor and variable resistor shown. The pulse widths can be varied from 1 to 15 ps with the components shown. The pulse amplification and signal output circuitry are the same as shown in Figure 1. The power supplies and batteries used are as follows. All IC's were driven using a 115 V dc, 100-mABoston Tech Model 2.15.100 power supply. The 50-V supply used to amplify the pulse train is supplied by a NJE Model ELS-30-Rregulated dc power supply; however, any 50-V source would be adequate. A 9-V alkaline battery is used for the constant current source. The variable battery used in OA4 to zero the signal consists of a 1.5-V battery and a I-kQ 10-turn precision potentiometer. RESULTS AND DISCUSSION With the circuit shown in Figure 1, the desirable characteristics inherent in the CC mode of signal processing are maintained. For the application described here, the reference current is maintained at 8 nA, where a pulse frequency of 1.0 kHz is observed for a sample-free cell. With increasing sample size the current is maintained constant up to the maximum frequency of this circuit, 100 kHz. This upper limit of response is imposed by the maximum frequency of our V-to-F converter 0 1982 American Chemical Society
Anal. Chem. 1982, 5 4 , 1893-1895
time
--+
Flgure 3. Analysis of 2-mL whole air sample using CC-ECD.
" Flgure 1. Circuit for constant current operation of ECD (A) where positive pulses are applied to the anode and (B) where negatlve pulses are applied to the cathode. Individual circuit elements are described in the Experimental Section. 30
pulse period
background clean air sample is shown in Figure 3. The chromatogram shown was obtained with a home-built GC and ECD where whole air samples are introduced via a 2-mL stainless steel sampling loop operated by a 8030 Carle valve. The column (10 f t X l/s in. stainless steel packed with 10% SF 96 on Chromosorb W) was operated at ambient temperature and the detector at 200 "C. The carrier gas was 10% CH4 in argon with a flow of 60 mL/min. In this 2-mL clean air sample, the concentration of CFC13 can be assumed to be about 200 pptr (v/v) and that of CC14 about 100 pptr (5). We have analyzed numerous clean air samples using the same chromatographic components but with various commercial ECDs and find the quality of the signal-to-noise response shown in Figure 3 compares favorably with those obtained previously. ACKNOWLEDGMENT The authors thank Richard Geer for assistance with circuit designs. LITERATURE CITED
Flgure 2. Circuit for fixed1 frequency operation of ECD.
and could be increased by use of another with higher frequency capability. Little improvement, however, would thereby be gained in our case because at 100 kHz the period between pulses is then only 10 times greater than the width of each pulse and linearity of response becomes progressively lost due to more fundamental causes. In order to demonstra.tethat the sensitivity inherent in ECD detection is maintained with this circuit, an analysis of a
(1) Maggs, R. J.; Jaynes, P.L.; Davies, A. J.; Lovelock, J. E. Anal. Chem. 1971, 43, 1966. (2) Sullivan, J. J.; Burgett, C. A. Chromafographia 1975, 8 ,176. (3) Lovelock, J. E.; Watson, A. J. J . Chromafogr. 1978, 158, 123. (4) Grlmsrud, E. P.;Knighton, W. B., Anal. Chem. 1982, 5 4 , 565. (5) Goidan, Paul National Oceanic and Atmospheric Administration, Bouider, CO. personal communication, March 19, 1982.
RECEIVED for review March 20,1982. Accepted May 21,1982. This work is supported by the National Science Foundation under Grant No. CHE-7824515.
On-Column Injector for Capillary Gas Chromatography T. L. Peters," T. J. Nastrlck, and L. L. Lamparskl Analytical Laboratories, Do w Chemical Company, Midland, Michigan 48640
An on-column injection device for capillary columns was recently described ( I ) and made commercially available on Carlo Erba gas chromatographs. With this injection system, a long (10 cm) 32-gauge needle is inserted through a cooled 0003-2700/82/0354-1893$01.25/0
narrow bore injection valve and into the capillary column. A narrow bore guide and a system of stops for the syringe so the valve can be opened and closed allow injection while maintaining column head pressure. 0 1982 Amerlcan Chemical Society
