Anal. Chem. 1984, 56, 1747-1749
the pretreated electrode was simply held a t -0.1 V in static solution. I t is apparent that the use of simple electrochemical pretreatment procedures can produce enhanced voltammetric response at carbon paste electrodes in a fashion which is analogous in many respects to that previously seen for glassy carbon electrodes. As has been recently suggested by Adams (II),however, it is possible that the mechanism of surface activation-e.g., oxidative formation of catalytic surface oxides (5, 7)or improved interfacial behavior of the oxidized surface (11)-may be quite different for different carbon substrate materials. Further, the potential applications of electrochemically pretreated carbon paste electrodes in analysis appear to be limited by their relatively poor stability in flowing streams, especially compared to pretreated glassy carbon surfaces whose cyclic voltammetric and LCEC response is sufficiently long-lived to provide reproducible current levels over at least a full day’s continuous usage (12,131. In general, it seems that electrochemical conditioning could probably be used to improve response for virtually any carbon matrix electrode. However, unlike conventional chemically modified electrodes whose electrocatalytic response is often very specific toward selected analyte molecules, this pretreatment appears
1747
to produce a relatively nonspecific enhancement of electrode response for a wide range of irreversibly oxidized compounds. Registry No. NADH, 58-68-4; C, 7440-44-0; hydroquinone, 123-31-9;ascorbic acid, 50-81-7; hydrazine, 302-01-2.
LITERATURE CITED Snell, K. D.;Keenan, A. G. Chem. SOC. Rev. 1979, 8 , 259-282. Murray, R. W. Acc. Chem. Res. 1980, 73,135-141. Zak, J.; Kuwana, T. J . Am. Chem. SOC. 1983, 704, 5514. Stutts, K. J.; Kovach, P. M.; Kuhr, W. G.; Wightman, R. M. Anal. Chem. 1983, 55, 1632-1634. Blaedel, W. J.; Jenkins, R. A. Anal. Chem. 1975, 4 7 , 1337-1343. Engstrom, R. C. Anal. Chem. 1982, 54,2310-2314. Engstrom, R. C. Anal. Chem. 1984, 56, 136-141. Cenas, N.; Rozgaite, T.; Pocius, A,; Kulys, J. J . Electroanal. Chem. 1983, 154, 121-128. Gonon, F. G.; Fombarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1981, 53, 1386-1389. Falat, L.; Cheng, H. Y. J . Electroanal. Chem. 1983, 757,393-397. Rice, M. E.; Galus, 2.; Adams, R. N. J . Electroanal. Chem. 1983, 743,89- 102. Ravichandran, K.; Baldwin, R. P. Anal. Chem. 1983, 55, 1782-1786. Ravichandran, K.; Baldwln, R. P., submitted to J . Liq. Chromatogr.
RECEIVED for review January 30,1984. Accepted April 2,1984. This work was supported by grants from the Commission on Academic Excellence and the College of Arts and Sciences, University of Louisville.
Time and Temperature Controller for Filament Atomizers Joslo P. Oliveira, Eric L. Barish, and Ralph 0. Allen*
Department of Chemistry, university of Virginia, Charlottesville, Virginia 22901 Heated filaments and metal “boats” have been used to introduce samples into several types of analytical devices. The heating rates and temperatures of these electrothermal atomizers often need to be controlled to achieve optimum conditions for vaporizing the sample. The electronic circuit described below allows selection and control of the temperature for a filament (or boat) which is heated by an ac power supply. A microprocessor was used to control the atomization time (as short as 3 X s) and temperature (up to 1400 “C). Our interest in a filament atomizer was a result of our studies on the analytical uses of energy transfer from metastable nitrogen molecules (1, 2). The metastable or active nitrogen is generated in a dielectric discharge operated at 1-10 torr pressure using a 16-kV ac power supply. Since conduction pathways in the dielectric generator have such high resistances, an atomizer filament placed in the system acts as an electric ground. Even though the filament is separated from the dielectric discharge system by 20 cm of low-pressure N2, a filament connected to a grounded power supply becomes the electrical ground for the 15-kV ac spark. For this application it was therefore necessary to float the filament power supply with an isolation transformer and to electrically isolate the controlling electronics from the filament.
EXPERIMENTAL SECTION The block diagram of the electronics used is shown in Figure 1. The filament is heated by an ac voltage from an isolation transformer which is regulated by a feedback system. The regulatory circuit, the feedback loop, and the interface to the computer are all low-voltage dc circuits. A 32-V, 6-A, dc power supply provides power to two parts of the circuit. A low current portion (i < 3 A) provides an electrically isolated power supply for the electronic circuitry. The high current (i = 5 A) portion of the circuit provides the power to the filament at a level controlled via a microcomputer. The voltage for the filament comes from a three-pin regulator (LM338)whose output voltage ( VouJ depends 0003-2700/84/0356-1747$01.50/0
upon the ratio of two resistors, R2 and PR. Vout = (1.25) (1 + R2/PR)V The resistance of R2 is fixed at 8.64 kSE. The resistance of the photoresistor, PR, varies (between 200 SE and 1 MSE) as a function of the intensity of two subminiature incandescent lamps (T-3/4” Sylvania) in the optically isolated feedback circuit (A in Figure 2). The intensities of the lamps are controlled by the application of a voltage to the base of the transistor (2N3904). To isolate the filament, the dc output of the three-pin regulator (LM338) is converted to ac by a 10-kHz oscillator. This ac voltage is applied across the primary windings of a transformer which provides electrical isolation of up to 20 kV for the secondary windings. The ac voltage from the secondary windings powers the filament and is monitored as a dc level by feeding the ac voltage through a full wave rectifier. The ac current through the filament induces an ac voltage in an induction transformer (C in Figure 2) which is proportional to the current. The voltage from this induction transformer is also fed to a full wave rectifier to produce a dc voltage proportional to the filament current. The outputs of both rectifiers are fed into a voltage multiplier (MC1594L) which has an output proportional to the actual power applied to the filament. The multiplier output is compared, in a feedback circuit, to a voltage which is proportional to the desired filament power. The actual power applied to the filament is not measured, but the output from the multiplier is proportional to the applied power and can be calibrated to relate the filament temperature to the multiplier output. Since the filament is heated by passing a 10-kHz alternating current, the effective power We, =Vd-. The two full wave rectifiers (R), which are the inputs to the multiplier, could have been adjusted (using the 5-kQvariable resistors) to give correct RMS values. This, however, required a calibration source of the same wave form as was used in this circuit for supplying power to the filament. Rather than attempting to adjust the full wave rectifiers to give correct RMS values, the multiplier output was compared to the filament temperature as measured by an optical pyrometer. For filament temperatures between 800 and 1400 O C the temperature could 0 1984 American Chemical Society
1748
NO. 9, AUGUST 1984
ANALYTICAL CHEMISTRY, VOL. 56,
-
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Figure 2. Electronic circuit used to provide Isolation and control of filament atomizer (all resistor valves in Q). C-induction transformer to obtain a voltage proportional to the filament'current.
be directly related to the applied power as measured by the multiplier output. The output of the multiplier can be read out to the microcomputer through the ADC. But of greater importance, the multiplier output (which is always a negative voltage) is compared
to a positive reference voltage set via the microcomputer. The reference voltage is established each time the filament is fired. The desired filament temperature is entered at the terminal and the microcomputer converts this to a digital code between 0 and 255 (as discussed later) which is fed to the DAC (Micro Networks
ANALYTICAL CHEMISTRY, VOL. 56,
Corp.). The DAC output represents the desired power level. The computer software controls the length of time that the power is applied to the filament using a time delay loop. After the elapsed time a zero value is output to the DAC. This value reduces the output of the DAC to its minimum value and, through the feedback circuit, reduces the power to the filament. Since the output of the DAC is a positive reference voltage proportional to the desired power, it can be compared to the negative output of the multiplier (which is proportional to the power applied to the filament) by summing the voltages. Since the summing circuit inverts the signal, another op amp is used to reinvert the signal. By use of a large feedback resistor (18 Ma) on the op amp, the circuit is made very sensitive to even small differences between the actual and desired power to the filament. The voltage output from the op amp is applied to the base of the transistor to control the intensity of the incandescent lamps and hence the value of PR. Thus the value of PR is continually adjusted to bring the applied power up to the value set by the DAC and maintain it there until the computer sends the zero to the DAC. Operation of the electronic circuits in the presence of 15-kV ac voltage on the filament requires isolated low-voltage power supplies. Another three-pin regulator (ECG 770) uses fixed value resistors to supply an oscillator which converts dc to ac. An isolation transformer and rectifiers convert the ac back to dc and three-pin regulators are used to produce +5, +15, and -15 V dc. The computer electronics are protected by optical isolation between the computer and both the DAC and the ADC. The emitter-detector pairs (GE H23B1) use Schmitt triggers before the emitters to avoid noise problems.
RESULTS AND DISCUSSION The temperature of an electrically heated tungsten wire of length L and radius r is proportional to the applied power. T o maintain a temperature, T, the total power input ( W) into the wire must equal the heat lost from the wire by conduction (Qk) and radiation (Qr). When the wire is in an evacuated chamber (as in our case) and in the temperature range of 800-1400 "C, most of the heat is lost by radiation so W Qr. The radiative heat loss depends upon the total emissivity (eT) at the temperature (2') of the filament and the Stefan constant of radiation ( a )
-
Qr = 2araeT(T4- T t ) = W
Tois the equilibrium temperature (room temperature) of the filament with no current passing through it. With a current I the power dissipated in a filament with resistivity pT is W = ppT/ar2. Both pT and eT are temperature dependent ( 4 ) p~ =
-4.780
X
+ (3.026 X 10-')T
eT = (0.751(Tp~)~/') - (0.396Tp~) Thus the power (W) can be related to the temperture for a particular filament, or the temperature can be calculated from the power. T o test this model several filaments were tested by measuring temperatures with an optical pyrometer. For each power setting the temperature (7') was measured and the power (Wdcd) was calculated by using the model described above. The output of the multiplier ( WeE)was measured and compared to the calculated power. A least-squares fit for 20 different temperatures (between 800 and 1400 "C) gave a linear relationship between the calculated and effective powers (Weff = 1.83Wcalcd). For temperatures above 1400 "C the lifetime of the filaments was short and below 800 "C the optical pyrometer could not be used to measure the tem-
NO. 9, AUGUST 1984
1749
perature. A total of six filaments made from the same wire were tested and the proportionality constant was within 5% of 1.83. The nonunity value of this constant was a result of not getting the actual values for VRMS and I R M s from the rectifiers. In fact, the relationship between the filament current and the voltage produced by the conversion transformer (C) is not 1 V for 1 A. It was not our goal to actually measure the power, but to set it reproducibly to give the temperature desired. Once measured for a particular type of wire, the proportionality constant can be used to establish that power level to yield the desired temperature. In actual use, the desired temperature is entered on the terminal and used to calculate Qr (using the equations given above). For the tungsten wire filaments used in this work, the heat dissipated by the filament ranged between Qr = 0.068 W for 810 "C and Qr = 0.563 W for 1406 "C. The value of Qr, which is assumed to equal W , is multiplied by the measured proportionality constant (in this case 1.83) and then converted to a digital code (D)to be transmitted to the DAC D = (1.83Qr)a b
+
In our case a = 155 and b = 40. If the power required to heat a particular type of filament is different, the gain of the rectifiers can be changed to keep the output of the multiplier within the range of the reference voltage from the DAC. Since the digital code supplied to the DAC must be an integral number 1255, the value of D calculated from the desired Qr is rounded to the nearest whole number. The digital code of the whole number is transmitted to the DAC, and the computer calculates a new value for Qr. The revised value of Qr is used to calculate the actual filament temperature and this is printed at the terminal along with the time that the filament is turned on. This circuit has proven to provide reproducible power to the filaments and holds the power level (hence temperature) for preset times. The feedback circuit brings the power to the desired level within 1 ms. At temperatures below 1400 "C individual filaments have been reproducibly fired over 80 times. As the filaments age and are oxidized or become thinner, the relationships between applied power and temperature change. This circuit maintains the power level, so the actual temperature eventually differs from that calculated. The circuit described is versatile and can be used to step or ramp the power (hence temperature) by software control. In addition, it provides good isolation of the control circuit from the filament.
ACKNOWLEDGMENT The valuable assistance of M. Grubb and J. Demas in the design and construction of this electrothermal atomizer control is gratefully acknowledged.
LITERATURE CITED (1) Dodge, W. B.; Allen, R. 0.Anal. Chem. 1981, 53, 1279-1286. (2) Kishman, J.; Barlsh, E.; Allen, R. O., Appl. Spectrosc. 1983, 37, 545-552. (3) Jung, W. G. " I C Op-Amp Cookbook"; Howard H. Sams and Co.: Indianapolis, IN, 1974. (4) Slegel, R.; Howell, J. R. "Thermal Radiation Heat Transfer"; McGrawHill: New York. 1972.
RECEIVED for review September 21, 1983. Resubmitted and Accepted April 9, 1984.
