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Anal. Chem. 1982, 5 4 , 1888-1889
using a 60 m X 0.32 mm id., DB-1 fused silica capillary column. Once again, excellent peak shape was observed. This demonstrates the feasibility of utilizing on-column injection of gaseous samples with satisfactory results. With on-column injection at a column temperature above the boiling point of the sample solvent, the solvent peak is narrower compared to that obtained below the solvent boiling point. We would expect discrimination effects to be smaller with this procedure than with the use of splitting or the splitless modes of sample introduction (8). It should be possible to apply higher temperature, oncolumn injection for nonbonded stationary phase capillary columns.
I
LITERATURE CITED (1) Schomburg, G. "Capillary Chromatography", Fourth International Symposium, May 3-7, 1981, Hindelang, West Germany: Kaiser, R. E., Ed.; Institute of Chromatography: Bad Durkheim, West Germany, 1981; pp 371-404. (2) Grob, K.; Grob, K., Jr. J . Chromafogr. 1978, 757, 311-320. (3) Grob, K., Jr.; Neukom, H. P. HRC C C , J . High Resolut. Chromatogr. Chromatogr. Commun. 1979, 2 , 15-21. (4) Zlatkls, A,; Walker, J. Q. J . Gas Chromatogr. 1963, 1 (May), 9-11. (5) Schomburg, G.; Behlau, H.; Dielmann, R.: Weeke, F.; Husmann, H. J . Chromatogr. 1977, 142, 87-102. (6) Rinderknecht, F.; Wenger, B. HRC C C , J . High Resolut. Chromatogr. Chromatogr. Commun. 1981, 4 , 295-296. (7) Dawes, E. F., private communication. (8) Zlatkis, A.; Wang, F A . ; Shanfield, H., manuscript in preparation.
I: I 10 pL of natural gas, isothermal at room temperature: attenuation, 26,23 after 7.3 min; chart speed, 0.5 cm/min; column: 60 m X 0.32 mm i.d. DB-1 fused silica capillary.
Flgure 6.
pulse produced by the rapid volatilization and retrograde movement of ether vapor into the inlet portion, which competes easily with a 5 psi condition, but not as well a t a 10 psi condition. Figure 6 shows the chromatogram obtained by injection of 10 MLof natural gas (laboratory gas line) at room temperature
RECEIVED for review March 22,1982. Accepted May 7,1982. This work was supported by the Department of the Army Contract DAAG-29-82-K-0054 and the National Aeronautics and Space Administration Contract NAS 9-16351.
Battery-Powered Apparatus for Chronoamperometric Measurements Greg A. Gerhardt and Ralph N. Adams" Department of Chemistry, University of Kansas, Lawrence, Kansas 66045
In the past few years our laboratory has applied the technique of chronoamperometry to the monitoring of electroactive compounds in biological systems (I). Several commercial (for example, the PAR 174 polarographic analyzer or IBM EC225 voltammetric analyzer) and laboratory-designed (2) instruments exist which are capable of performing these measurements. Here we report the construction and evaluation of a simple, inexpensive battery-powered chronoamperometry apparatus which does not require a time-based recorder for data display. Its versatility enables it to be used for a variety of neurochemical and analytical applications.
EXPERIMENTAL SECTION A schematic diagram of the electrochemical apparatus is seen in Figure 1. The circuit functions as a conventional style potentiostat (3) connected to a sample-and-hold circuit. o p amp one (OAl) is used to set the applied potential; switch two (S2) selects between a two- or three-electrode system. The OA2 is a current-to-voltage converter followed by a variable gain voltage amplifier, OA3. The resistor network of OA3 allows for potentiostat sensitivities of 50, 20, 10,5,2, l, and 0.5 nA/V, as the selected resistance is varied from 2K to 10 Q. Sensitivity of the potentiostat is easily altered by varying the size of the currentto-voltage resistor of OA2. The OA4 is the buffer amplifier of the sample-and-hold circuit. Chronoamperometric measurements are controlled by the two monostables (MS1, MS2) and the two solid-state relays (SR1, SR2). A single push of the start button triggers both monostables.
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The first monostable (MS1) provides a l-s (1 0) pulse closing the first relay (SRl), which allows current to flow through the circuit. MS2 provides a 500-ms (1 0) pulse to SR2, which allows the output of OA3 to be followed by the sample-and-hold circuit. The monostables start l-s current-time measurements in which the current at -500 ms is latched by the sample-and-hold circuit. A voltage proportional t o the current at 500 ms is recorded at output-2 (Out-2) by a digital voltmeter (DVM) or other voltage recording device. Currents at other times during a current-vs.-time measurement can be recorded by varying the duration of the monostable outputs. The present pulse times are optimized for recording currents from 200 to 300 wm carbon epoxy electrodes. For maximum SIN the capacitor on OA2 should not be decreased below the present value. The reset button is used to zero the output of the sampling circuit between measurements. Switch S1 is used to lock on SR1, providing the optional use of this circuit as a standard liquid chromatography potentiostat. The offset circuitry of OA3 and Out-1 are also included for this purpose. The complete apparatus is powered by a pair of rechargeable 12-V gel-type batteries (Power-Sonic,6 A h rating). For maximum battery life the use of low power (LS) integrated circuits is recommended. The complete apparatus including batteries, battery charger, and instrument box costs ca. $200. An inexpensive batterypowered digital multimeter (Micronta, No. 22-197) was used as the data readout device.
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DISCUSSION Tests were performed with a dummy electrochemical cell.
0003-2700/82/0354-1888$01.25/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
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Figure 1. Schematic diagiram of the battery-powered chronoamperometry apparatus. All resistances are given in ohms and capacitances in microfarads. The sample-,and-hold capacitor is Mylar. All resistors are 1 % precision except for the 2 2 4 resistor. MS1 and MS2, 74121 (TI); IC1, 74LS04 (TI); IC2, 74LSOO (TI); O A l and OA4, CA3140S (RCA); OA2, 545KH (Analog Devices);OA3, 517KH (Analog Devices); SR1 and SR2, 640-1 (Teledyne); 7405C (TI); R 1 = 37.4 K, R 2 = 37.4 K, C1 = 38 pF, C2 = 22 pF.
The reproducibility and accuracy of the measured steady-state currents were k l % . This is consistent with the accuracy of the op amp circuits and the reproducibility of the monostable outputs. Calibration curves were prepared for oxidation of dopamine in 0.1 M phosphate buffer, pH 7.4, and p-aminophenol in 2 M sulfuric acid. The concentrations for all solutions varied from 10 t . ~150 pM. The oxidations were carried out at a 1.5 mm diameter carbon paste electrode and the currents were read as voltages from a digital voltmeter. Residual currents were typically 5-10 nA. Both calibration c w e s showed a high degree of linearity, having: correlation factors of 0.9995 and 0.9986 for dopamine and p-aminophenol, respectively, and are representative of the capabilities of the apparatus. The output of the sample-and-hold circuit is fairly stable with time; howevler, for maximum precision we recommend reading the voltage within 10 s of the measurement. The reproducibility of repe,ated measurements at a single concentration is fl-2%. The apparatus was also used as a potentiostat for an electrochemical detectair in liquid chromatography (4).Excellent SIN characteristics were observed even with the small amount of filtering usedl in the present design. We attribute the good signal characteristics to the battery-powered design and its freedom from an ac power source. For routine LCEC measurements, we recommend that the apparatus be operated in a Faraday cage and a larger capacitor be used on OA2. The described apparatus was specifically designed for use in routine analysis schemes such as the determination of ascorbic acid in tissue samples (5). Instruments which record a complete current vs. time measurement, however, require
analysis of each curve to obtain measured currents at specified times. This apparatus allows for immediate readout of the current at a set time, when a digital voltmeter is used for data monitoring. Judging from our experience, this apparatus is also suitable for conducting in vivo measurements of neurotransmitter release in the brains of small laboratory animals. For repeated measurements a commercial biological stimulator can trigger the measurements (Le., Grass Model S48) and they can be recorded on a strip chart recorder. The sample-and-hold circuit need not be reset after each measurement.
ACKNOWLEDGMENT We wish to thank R. M. Dalle-Molle and the University of Kansas Instrument Design Laboratory for helpful discussions.
LITERATURE CITED (1) Cheng, H.-Y.; Schenk, J. 0.;Huff, R . M.; Adams, R. N. J . Electroanal. Chem. 1979, 100,23-31. (2) Cheng, H.-Y.; White, W.; Adams, R. N. Anal. Chem. 1980, 52, 2445. (3) Diefenderfer, J. A. “Principles of Electronic Instrumentation”; Saunders: Philadelphia, PA, 1972. (4) Roston, D. A.; Kissinger, P. T. “Liquid Chromatography/Eiectrochemistry: Principles and Applications”; Kissinger, P. T., Ed.; BAS Press: West Lafayette, IN, 1982; Chapter 7. (5) Schenk, J. 0.;Miller, E.; Adams, R. N. Anal. Chem. 1982, 54, 1452.
RECEIVED for review April 6, 1982. Accepted May 21, 1982. The support of this work by NSF (Neurobiology Section) via Grant BNS 7914226 is gratefully acknowledged. G.A.G. also received partial support from an Analytical Division Fellowship from the American Chemical Society.
