1214
Anal. Chem. 1982, 5 4 , 1214-1215
ACKNOWLEDGMENT The authors gratefully acknowledge financial support by the German Research Foundation (Grant 145/31). LITERATURE CITED Gellmann, W. 2.Anal. Chem. 1956, 760. 410-426. Gellmann, W.; Neeb, K. H. Z . Anal. Chem. 1959, 765, 251-268. Heinrichs, H. Z . Anal. Chem. 1979, 294, 345-351. Erzlnger, J.; Puchelt, H. Geostandards News/. 1960, 4 , 13-16. Meyer, A.; Hofer, Ch.; Tolg. G. Z . Anal. Chem. 1978, 290, 292-298. Tolg, G. Talanta 1974, 27, 327-345. Kaiser, H.; Specker, H. Z . Anal. Chem. 1956, 149, 46-66. Feldmann, C. Anal. Chem. 1977, 49, 825-828. Subramanian, K. S. Z . Anal. Chem. 1981, 305, 382-386. Smith, R. G.; van Loon, J. C.; Knechtel, J. R.; Fraser, J. L.; Pltts, A. E.; Hodges, A. E. Anal. Chlm. Acta 1977, 9 3 , 61-87. (11) Aslin, G. E. M. J . Geochem. Explor. 1976, 6 , 321-330. 112) Abbev. S.: Glllieson. A. H.: Perrault. G. Can. Met. Mineral Enerov Techno/.,MRP-MSL 1975, ' 7 , 75-132. (13) Simon, F. 0.; Brown, F. W.; Greenland, L. P. J . Res. U S . Geol. SUW. 1975, 3, 187-190. (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
. .
(14) Terashlma, S.Anal. Chim. Acta 1976, 86, 43-51. (15) Wangen, L. E.; Gladney, E. S.Anal. Chlm. Acta 1978, 9 6 , 271-277. (16) Morrison, G. H.; Gerard, J. T.; Travesl, A.; Currie, R. L.; Peterson, S. F.; Potter, N. M. Anal. Chem. 1969, 42, 1633-1637. (17) Bowen, H. J. M. "Environmental Chemistry of the Elements"; Academic Press: New York, 1979. (18) Gladney, E. S. Anal. Chim. Acta 1980, 178, 385-396. (19) Brunfelt, A. 0.; Steinnes, E. Geochim. Cosmochlm. Acta 1967, 3 1 , 283-285. (20) Schnepfe, M. M. J . Res. U . S . Geol. Surv. 1974, 2 , 631-636. (21) Golembeskl, T. Talanta 1975, 2 2 , 547-549. (22) Keays, R. R.; Ganapathy, R.; Iaul, J. C.; Krahenbuhl, U.; Morgan, J. W. Anal. Chlm. Acta 1974, 72, 1-29. (23) Laul, J. C.; Ganapathy, R.; Anders, E.; Morgan, J. W. Geochim. Cosmochlm. Acta 1973, 3 7 , 329-357. (24) Nadkarni, R. A.; Haldar, B. C. Radlochem. Radioanal. Lett. 1971. 7 , 305-31 1. 1976, 840, (25) Gregory, J. E.; Lavrakas, V. Geol. Surv. Prof. Pap. (US.) 163.
-. for review December 7, 1981* Accepted March 2,
1982.
Varlable Temperature Controller Scott L. Buell and J. N. Demas" Department of Chemistry, University of Virginia, Charlottesville, Virginia 22903
Spectroscopy often requires a stable temperature in the cell compartment to ensure reproducibility of data from experiment to experiment. For example, our work involves luminescence and kinetic studies on luminescent molecules interacting with micelles. These systems are notoriously sensitive to temperature effects and reasonably reproducible temperature control is essential for reliable results. We describe a variable temperature controller which provides relatively stable temperature control (A-0.5 "C), is inexpensive and simple to construct, and is easily adaptable to a variety of spectrophotometer and fluorimeter configurations. Further, the system is designed to survive a harsh EM1 environment. This latter feature was dictated by the spectrofluorimeter's 40-kV rf xenon arc lamp igniter. The ignition of the lamp regularly destroyed all inadequately protected electronics in its vicinity.
EXPERIMENTAL SECTION The schematic is shown in Figure 1. The system is a simple on-off controller with a DC Wheatstone bridge and a thermistor sensor. Both the thermistor and the flexible Thermofoil heater are attached to the sample cell holder. The trimpot resistor R2 provides for variable temperature control by changing the balance point of the bridge. Bridge imbalance is detected by an operational amplifier. The 8-pin DIP 741 was chosen because it is readily available and operates over a wide voltage range. The variable power supply voltage allows for optimization of the system. When an imbalance in the bridge occurs, the op amp switches on the power transistor which applies power to the heater. The current remains on until the bridge is balanced again. The LED shows when the heater is on. For off-on controllers optimum temperature regulation occurs when the duty cycle is 50%. The duty cycle can be varied by adjusting the power supply which controls the power dissipated in the heater. In our applications, however, we have not found the performance to vary appreciably for duty cycles of -10-90%. The flexible Thermofoil heaters are adaptable to a wide variety of existing sample cell holders and compartments. Figure 2 shows two custom designs used in our work. Figure 2a shows a holder for 2.0 cm diameter cells used in our lifetime apparatus. The design of Figure 2b is for 1.0-cm square cells used in our spec0003-2700/82/0354-12 14$01.2510
trofluorimeter. The Thermofoil heaters are folded to conform to the surface and cemented in place with GE Silastic RTV silicone rubber. The thermistor is either inserted into a hole in the base of the aluminum block or cemented to the outside of the holder with Silastic. The controller circuit is carefully designed to suppress EM1 transients. During the development of this controller, we were plagued by frequent, but random, destruction of the 741 op amp by the rf noise generated by the ignition of our xenon arc lamp. The following features were added to eliminate the failures. D4 and D5 protect the amplifier's inputs. R6, R7, C3, C4, C5, and C6 provide suppression of power supply transients. R8, D6, and D7 protect the output from Q1 switching transients and from EM1 pick up in the heater connections. These features improved the amplifier's life span but did not completely eliminate the problem. The GE MOV transient suppressor was then added t o the ac power line to prevent transients from entering the controller through the ac lines. No failures have occurred since this addition. The layout for the controller printed circuit board is available on request and the printed circuit board can be purchased. For details contact the authors.
RESULTS AND DISCUSSION The system has proved to be robust and flexible. We have three units with accumulated running times of -3 years and have experienced no failures. Figure 3a shows the temperature vs. time profile under actual experimental conditions; a 10-mL sample was used in the holder of Figure 2a. Only minimal insulation was used and precise control of the room temperature was not possible. The exterior of the aluminum holder was covered with a in. sheet of styrofoam and wrapped with plastic tape. The solution temperature was monitored directly with a thermistor. The thermistor's resistance was followed with a Keithley 177 DVM interfaced to an HP-85 microcomputer. During a 12-h period, the temperature varied over a 0.54 "C range with a mean temperature of 25.43 "C. The control can be increased by removing effects of the room air conditioning system. Figure 3b shows the time vs. temperature profile of the controller when the heater and sample cell are well insulated from ambient temperature 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982 * 1215
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Schematic diagram of power supply and controller circuit: power supply, (DI)GE MOV V130LA20A Varistor, (11) 36 VAC 1 19 C.T. transformer, (D2, 03) 1N4004 1 A diode, (CI) 2000 pF electrolytic, (C2) 4.7 pF tantalum, (IC1) LAS1512 1 2 4 three-terminal regulator (Lambda),( R l ) 10 K IO-turn potentiometer; controller circuit, (R2) 50 K 15-turn potentiometer: (R3) 100 K (25 "C) glass probe thermistor GA51P7, Fenwall Electronics, Framingham, MA, (R4) 8.2 K, (R5) 22 K, (R6, R7, R8) 100 a, (R9) 240 0, (R10) 40 (Figure 2a) or 43 Q (Figure 2b) Thermofoil Heater, Minco Products, Inc., Minneapolis, MN; (D4, D5,D6,D7) 1N914 diode, (C4, C6) 0.047 pf, (C3, C5) 1 pF, (OAI) 741 op amp, (Ql) Motorola MJ3001 power Darlington, (LED) light emitting diode.
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The main disadvantages of this controller are that it is not convenient to change temperatures and that it requires about 20 min to warm a 10-mL sample 5 "C to within 0.5 "C of the set temperature. Substitution of a potentiometer with a dial for R2 solved the first problem. The second disadvantage can be minimized by starting with a sample nearer to the set temperature. The low cost and high degree of control far outweigh these minor disadvantages for most applications where a thermostated environment is required. The flexible foil heaters allow the controller to be adapted to new or existing systems. The Thermofoil heaters come in a variety of shapes, sizes, and resistances and may even be custom designed. All parts shown in Figure 1,except the heater and thermistor, are built into one unit with terminals supplied to connect the heater and thermistor. The controller unit is then movable and can be attached to different stationary sensor-heater systems. The absence of pumped fluids and moving parts keeps the heating unit small and Dee from corrosion and leaks. It should also be possible to adapt this same controller design to drive a Peltier effect heat pump to provide above and below room temperature control. Also, for samples well above ambient temperature an auxiliary heater can be used. This will improve temperature control and shorten the initial warmup time. RECEIVED for review January 4,1982. Accepted February 25, 1982. We gratefully acknowledge support by the Air Force Office of Scientific Research (78-3590) and the donors of the Petroleum Research Fund, administered by the American Chemical Society. We also acknowledge partial support of the US. Department of Energy, Grant No. DE-FG02-CS84063; however, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of DOE.
