1I AIDS FOR ANALYTICAL CHEMISTS
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Relay Circuit for Integrating a Gas-Liquid Chromatography Temperature Programmer with an Automatic Sample Injector H. A. McLeod, Ronald Bolohan, and Maarten Van Dyk Food
Research Laboratories, Health Protection Branch, Department of National Health and Welfare. Ottawa, Ontario
GLC temperature programmers integrated with automatic sample injection systems are not readily available to the analytical laboratory and are expensive when purchased on a custom basis. During the development of an automated GLC system in our laboratory, a need arose to synchronize the triggering of the automatic injection unit with the restarting of the temperature program cycle. This was accomplished by devising a simple relay circuit, integrated with the oven cool-down cycle. The system has proved reliable in operation, and its concept can be adapted to various models of equipment. The temperature program circuits of different gas chromatographs have the same cool-down sequence in many respects. Voltage under the control of a logic circuit is supplied to a motor or solenoid to operate a n oven-venting mechanism and turn on lights indicating the program status. A re-cycle is initiated by closing a start switch(es), momentarily. Depending upon the model of temperature programmer, a voltage can be selected that will trigger a relay or series of relays, whose contacts are appropriately wired across the contacts of the manually operated start switches. Figure 1 outlines the relay and other component circuitry installed in our GLC. Component description is given in Table I. All parts are standard electronic components that are available from an electronic supply house. The values used here may not be satisfactory for some models, but will be for others. Choice will be based on the value of voltages available to trigger the re-cycling circuitry.
Table I. Component Specifications for Circuit in Figure 1 Number required
Circuit No.
Relay
2
K1 8 K 3
Relay
1
K2
Capacitor
1
C1
100 MFD. minimum 150 VDC.
Switch
1
s1
Rectifier
1
10D8
Toggle. single pole, double throw, 5-ampere rating. Diode, 1 ampere, PIV 800 V.
Component
Description
110-volt ac double pole, double throw, 5-ampere rating. 1 10-volt dc double pole, double throw, 5-ampere rating.
The sequence of operation (Figure 1) in the automatic mode for our machine is as follows: The oven is equilibrated to the initial temperature, the various program parameters are set, and continuous operation is initiated by closing the start switches of the automatic injector and temperature programmer. When the oven vent opening motor is energized a t the beginning of the cool-down cycle, the same voltage. energizes relays K1 and K2, but 110 VAC does not reach K3 because the con-
TERMINAL BOLRO
-1 TRlGGERiNG VOLTAGE.
WMMONC-
-
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T ! TO START BUTTON CONTACTS OF TEMP P R O G
4 4 I 1
1 I
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*TRIGGERING VOLTAGE WILL D E P E N D ON CIRCUIT C H A R I C T E R I S T I C S TEMPERATURE PROORAMHER TO 8E MODlFiED
Figure 1. Relay circuit for repetitive temperature programming and sample injection 320
ANALYTICAL CHEMISTRY, VOL. 46, NO. 2, FEBRUARY 1974
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TO s w r BUTTON OF S ACONTbCTS MPLE
INJECTOR
t a c k a t K1 open. As the vent fully opens, the power to the motor is interrupted by a cam-switch arrangement a t the motor. However, K1 and K2 remain energized as they are supplied by the voltage leading to the cam-switch circuitry. When the door is closed, this voltage is also removed and K1 is de-energized. The 110 VAC contacts close a t K1 and remain closed at K2 because of the charge on C1; thus, K3 is energized. The pair of contacts a t K3, paralleling the manually operated start switches, are closed for a time period determined by C1 (approximately 1second), and a new cycle is initiated.
Switch S1 in the circuit allows selection of the automatic repetitive temperature programming, or, when in the manual position, disconnects K3 so that programming is restricted t o one cycle only.
ACKNOWLEDGMENT The authors thank Mrs. Janet Chapman for preparing the circuit drawing. Received for review January 12, 1973. Accepted July 27, 1973.
Simple Method for Continuous Monitoring of Electrode Rotation Rate Ira B. Goldberg, Richard S. Carpenter I I , and W. F. Goeppinger Science Center, Rockwell Internationd, Thousand Oaks, Calif. 97360
Rotating electrodes are a valuable tool for the study of electrode processes ( I , 2 ) . A rotating ring-disk electrode assembly, similar to those discussed in the literature ( 3 ) , was set up in this laboratory. In these cases, rotation rates were calibrated separately from the actual experiment. The experiments were done using a motor-tachometergenerator, which feeds into a speed control unit that regulates the voltage to the motor. A ten-turn potentiometer is used to regulate the rotation rate. After several calibration runs on various days, we have found that for a given potentiometer setting on the speed controller, the rotation rate may vary by as much as 5%, even though the rotation rate is constant to within 0.2% over a period of an hour or more. This suggests the need to monitor the rotation rate continuously or at least to calibrate the rotation rate prior to each experiment. Another alternative would be to use a precise frequency generator and power amplifier to control the rotation rate. Little attention has been given to the continuous measurement of rotation rate. Sonner et al. ( 4 ) use a photoelectric pick off on a reflective surface on the shaft of the electrode to measure the rotation rate. The time interval is monitored by a frequency counter. Hintermann and Suter ( 5 ) mounted a disk with holes on the shaft of the electrode. A light was placed under the disk and a phototransistor was placed over the disk so that a number of light pulses proportional to the rotation rate were detected and counted with a frequency counter. Other mechanical devices were also devised to given an electrical impulse per revolution (6). The goals which were kept in mind for the electrode assembly used in this laboratory were that the rotation rate could be monitored on an inexpensive frequency counter or timer, the electrochemical system could be easily interfaced with a computer for interactive control, the electrodes could be easily interchanged, and the entire assembly should be compact enough to be easily transported. Adams, "Electrochemistry at Solid Elecrrodes." Marcel Dekker. New York. N . Y . . 1969. (2) W. J. Albery and M . L. Hitchman, "Ring-Disc Electrodes." Clarendon Press, Oxford, England, 1971 (3) V. J. Puglisi and A. J. Bard, J. Electrochem SOC..119. 829 (1972). (4) R. H . Sonner, 8. Miller, and R . E. Visco, Anal. Chem.. 41, 1498 (1) R . N.
(1969).
(5) H. E. Hintermann and E. Suter, Rev. Sci. Instrum., 36,1610 (1964). (6) J. Woitowicz and 8 . E. Conway, J. Electroanal. Chem., 13, 333 (1967).
n
I
Figure 1. Schematic diagram of synchronous for the measurement of rotation speeds
pulse generator
EXPERIMENTAL A Pine Instrument Co. (Grove City, Pa.) rotating ring-disk electrode was connected to an Electrocraft (Hopkins, Minn.) speed controlled Model 550 motor, using a stainless steel collet. Ten nonreflective strips were painted on a 5-mm strip of reflective tape which was equal in length to the circumference of the collet. The tape was carefully placed around the collet so that the light of a Skan-a-matic Corp. (Elbridge, N . Y . ) Model S-351 subminiature reflective scanner illuminated the tape. Ten light pulses are obtained a t the phototransistor for each revolution of the rotating electrode. The small scanner actually permits the use of more than ten stripes to be painted on the reflective tape if desired. Although the transistor of the scanner is rated at up to 30 V, it was convenient to use a 5-V input. The low voltage side of the phototransistor was connected to a resistor R I N(Figure 1) and to the negative edge trigger imput of a Motorola SN74121 monostable multivibrator. R I Nis adjusted so that the voltage maximum and minimum at this point are about 2 to 0.8 V to enable the multivibrator to trigger. Since these voltages vary among these components, R I Nmust be determined empirically. In our case, 1 K Q was sufficient for the device to trigger even when the photodetector was used in room light with no particular shielding. The pulse duration was set for 90% of the minimum pulse rate (about 1 msec) by adjusting Rr and CT given by the tables in the manufacturer's literature. The monostable multivibrator provides a
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