Use of a Multichannel Analyzer as a Data Acquisition System in Electrochemistry GEORGE LAUER and R. A. OSTERYOUNG North American Aviation Science Center, Thousand Oaks, Calif.
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b The use of a multichannel analyzer as a data acquisition system in electrochemical studies is described with particular reference to chronocoulometric studies. Interface equipment used to co-ordinate chronocoulometric potential pulses with the analyzer's timed sequential operations is described. Use of a voltage-to-frequency converter and the internal analyzer analog-to-digital converter is described. With a fixed voltage input into the voltage-tofrequency converter, the number of counts recorded decreases as the sampling rate increases. With the internal converter, the number of counts recorded with a constant voltage input i s independent of the sampling rate up to a limit depending on the conversion time. Use of this system increases both the rapidity and accuracy with which data may be acquired compared to photographing an oscilloscope face.
I
ri many
electrochemical experimental measurements, a variable (voltage, current, charge) is examined as a function of time. If the rate of change of the variable with time is rapid, an oscilloscope is generally employed to observe the variable, with the Y-axis conventionally being used for the variable, and the X-axis being a time axis as supplied by the oscilloscope time base. A photograph of the transient is obtained and the data are taken from the picture (either Polaroid or an enlargement of 35 mm. film). Lack of precision and tedium are obvious disadvantages to this method. The use of equipment possessing digital readout capability offers numerous advantages in accuracy and ease of data handling. We will discuss here the use of a multichannel analyzer as a data-acquisition system with particular reference to its use in chronocoulometric studies. It will not be the purpose of this paper to go into the detail of multichannel analyzer operation, but sufficient material will be given to facilitate appreciation and application to other than the specific analyzer used here. Consideration is given only to operation of the instrument in a time mode (rather than pulse height analyzer mode), in which case it is serving essentially as a memory oscilloscope with digital readout capability.
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Figure 1. E-f and Q twave forms in double potential step chronocoulometry
To initiate the discussion, consider how a chronopotentiometric curve could be recorded on an analyzer. A switch is closed, which results in application of a constant current to an electrochemical cell, and generation of a triggering pulse which is applied to a time-base generator. The output of this time-base generator is a series of pulses at regularly timed intervals, say 0.01 second. The chronopotentiometric potential change is measured with a high input impedance amplifier and applied to the input of a voltage-to-frequency converter (V-F-C). This instrument has a sinusoidal output of fixed amplitude whose frequency is directly proportional to the input potential. The output of the V-F-C is input to the analyzer, which may be regarded initially as a group of data scalers counting the sine wave input pulses from the TI-F-C. Thus, for a fixed time interval, determined by the time-base generator, the analyzer counts a sine-wave whose frequency is determined by the referenceindicator electrode potential variation. An output pulse from the time-base generator, however, causes the analyzer to switch from one scaler or channel to another after the predetermined time period, in this case 0.01 second. The input from the V-F-C is thus averaged over the channel time period. However, before switching to another channel, the iiiformation contained in the initial channel (consisting of the number
of sine-wave pulses received from the V-F-C in 0.01 second) is stored in the memory of the analyzer. I n the case of a 400-channel analyzer and the 0.01second time interval referred to above, 400 data points covering a 4-second time interval can be obtained. When the last channel has data written into it, the experiment is terminated and the data contained in the memory may be read out as digital information on suitable readout devices such as a typewriter or punched tape. With most analyzers, provision exists to observe the data stored in the memory as an oscilloscope trace, which is synthesized by rapid and repetitive interrogation of the analyzer memory. APPLICATION TO DOUBLE POTENTIAL-STEP CHRONOCOULOMETRY
The main portion of this paper is devoted to a discussion of the use of a Radiation Instrument Development Laboratory (Melrose Park, Ill.) Model 34-12B multichannel analyzer as a recording device in chronocoulometric studies ( 1 , 2 , 6 ,6). Detail of peripheral equipment which takes advantage of the capabilities inherent in the analyzer will be given. In chronocoulometry, a potential step is applied to the electrode and the coulomb-time behavior is followed (3, 6, 6). Starting a t a potential El in a solution containing only Ox, a potential step is applied to the electrode, where the reaction a t ES Ox
+ ne e Red (Hg)
E = E*, t < 7 (1) takes place, then a t a time 7 stepped back to E1 where the oxidation of Red from the amalgam takes place. Figure 1 shows the E-t and &-t behavior expected (1, .9,.4,6). Fieure 2 is a block diaeram of the appGatus used. Referring-to the block diagram of Figure 2, the experiment is initiated by a push button on the control box. The push button starts a sequence which : a) provides a pulse to initiate the oscillator of the time base generator, b) unshorts the capacitor on the operational amplifier used as an integrator, c) receives and blocks the time-base generator pulses until after the integrator is unshorted, then d) simultaneously supplies a voltage step into the potentiostat and permits VOL. 38, NO. 9, AUGUST 1966
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Push button
Timer start pulse from control box.
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RI.DL. MODEL 34-128 TRANSISTORIZU 4OOCHANNEL ANALYZER
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Output pulses from time base generator to control box
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Amplifier 1 is a Wenking fast-rise potenticstat, amplifler 2 is a Philbrick P75, and amplifiers 3 and 4 are Philbrick SP656's. The R1.D.L. 99-24 Punch type Matrix and Model 54-6 Time Base Generator are contained in a Model 29-1 B instrument case and power supply. An inverting amplifler may b e placed at point (A) if the voltage-to-frequency converter is unipolar and input would have wrong polarity. The Control Box detail is given in Figure 7 and is discussed in detail in the Appendix
+4) the
contents of the memory in channel N 1 are then read into the data scaler, 5) the blocking pulse is removed from the Detector Gate, permitting the ac1 138
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ANALYTICAL CHEMISTRY
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Figure 2. Block diagram of apparatus used in double potential step chronocoulometric studies
the channel advance pulses from the time base generator to pass into the Oscillator Input of the analyzer. There is no channel advance until the first pulse from the time base generator has passed through the control box and then to the Oscillator Input of the analyzer. Channel zero will contain a number of counts corresponding to random noise generated by the system. The control box then counts the pulses from the time base generator and, after a given number of pulses, removes the potential step, bringing the applied potential from Ez to E,. The sequence of events is indicated in Figure 3. The output of the V-F-C is counted in the interval between the time base pulses. The V-F-C output is fed into the internal amplifier of the analyzer, and the output of this amplifier is then fed into the Detector Input of the analyzer. The pulses from the V-F-C at the Detector Input will pass into the analyzer and be counted unless a blocking pulse is applied at the Detector Gate Input. When a pulse is delivered to the Oscillator Input of the analyzer, providing for channel advance, the signal a t the Detector Input is blocked. The following sequence of events then occurs in the analyzer: 1) a signal is generated which causes the contents of the data scaler to be written into the part of the core-memory which corresponds to a given address or channel, say channel N , 2) the data scaler is then reset to clear it of the accumulated data, 3) an internally generated address advance pulse causes the address scaler to select that portion of the corememory corresponding to channel N
.
Pulses to Analyzer Oscillator input from control box Address overflow pulse to time base generator ond control box
cumulation of data into the data scaler 1. a t channel N
+
(In the present case, the core was reset to zero in all channels in which data were to be recorded prior to running an experiment.) The time during which the analyzer is unable to accept input is its dead time and is generally a fixed interval of 12.5 psec. controlled by the time-base generator disabling pulse at the Detector Gate Input. (The dead time has no effect on the accuracy of the experiment with either analog-todigital converter used, since with the V-F-C the time interval during which data is being input is known precisely and with the internal analog-to-digital converter (see below) the time when the input is sampled, as well as the interval between samplings, is precisely known.) Figure 4 indicates, in block diagram form, the circuits involved, while the events taking place are indicated in Figures 5.4, B, and C. In the R.I.D.L. unit employed, storage may be carried out in 100, 200, or all 400 channels. At the end of the selected storage interval, after data have been input into the last channel, a completion or address overflow pulse is generated by the analyzer. This pulse cuts off the time-base generator and is applied to the control box, where it causes the integrator to be shorted, resets the circuit which counts the time-base generator pulses, and deactivates the box. A detailed schematic of the box is given in Figure 7, and operation is discussed in the appendix. The use of the analyzer in this manner has certain advantages and disadvantages. On the plus side is the fact that no modification of the analyzer is required. While impossible to generalize to other units, it is likely that most analyzers can be used in this manner.
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However, different designs (logic and interfacing) may present problems of vastly differing magnitude. Although the R.I.D.L. unit with typewriter and tape readout employed here costs about $15,000, other less sophisticated units are available a t a cost not too much greater than a good oscilloscope. Converting the unit from a data acquisition mode of operation back to its pulse height analyzer mode is rapidly and easily accomplished. One of the major advantages of using a voltage-to-frequency converter, rather than the internal analog-to-digital conversion procedure discussed below, is the great increase in precision obtainable when the signal is a slowly varying potential. .4ny random noise is averaged out over the time period sampled; a minor drawback previously mentioned is that in effect one averages over the time period between time-base generator pulses. However, a more serious problem involves the sensitivity of the V-F-C. Typically, a current measuring resistor, R,, of 100 ohms was used, and the integrator had an RC constant of, say, sec. The V-F-C used had 1 megacycle output a t 10 volts input. Thus, a current, I , of one microampere flowing across the 100-ohm resistor for one second, a microcoulomb, resulted in a final voltage output from the integrator of
1 x 10-6 x 102 x 1 = 1.0 volt 10-4 This resulted in a frequency output of 1.o - X lo6 = lo5 cycles/sec. from the 10.0 V-F-C. If the time-base generator was set a t 400 microseconds-Le., it gen-
Pulse from V - F - C (via internal analyzer amplifier)
Detector Reset
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Figure 4. Block diagram of analyzer circuitry in Time-Mode of operation
erated a channel advance pulse every 400 microseconds-then a microcoulomb corresponded to 400 x 10-6 x 105 = 40 counts/microcoulomb since each cycle input is recorded as one count by the analyzer. If the timebase generator was operating a t 800 microseconds per channel advance pulse, then 80 counts equalled one microcoulomb. In a chronocoulometric experiment with an electrode area of about 0.03 cm.2, with the time-base generator set a t 400 microseconds and a r of about 25 milliseconds, 1-2 microcoulombs of charge from diffusing material will have passed at r for a solution 1 millimolar in electroactive species. This means that, with the values for R,, R , and C given above, one would obtain 40-80 counts at t = r. At 25 milliseconds after the potential step is applied, the faradaic current is of the order 25-50 microamperes, which means a change of only 1-2 counts every 2-3 channels. Clearly, sufficient counts must be obtained in the interval 0 < t < r to permit analyzing the data. If the timebase generator were operating a t 200 microseconds and r were maintained a t about 25 milliseconds, only 20-40 counts would be obtained a t t = r. Although some increase in sensitivity could be obtained by varying the measuring resistor, R,, and the RC time constant of the integrator, it was not possible to obtain sufficient counts with the time-base generator operating faster than 200 microseconds. In the step experiments, R , is limited because the initial current which flows, largely double layer charging current, may be of the order 20-60 milliamperes and the maximum voltage output of the currentmeasuring amplifier is + 10-12 volts. USE OF INTERNAL ANALOG-TO-DIGITAL CONVERTER
An alternate method of analog-todigital conversion was also used. The analyzer contains an internal Analogto-Digital Converter (A-D-C) which may be used to convert an analog voltage input into pulses which can be counted by the data scaler. In the R.I.D.L. unit, conversion is accomplished by feeding the voltage into a control or
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Linear Gate of the A-D-C. This gate permits generation of an 8-megacycle pulse train for a time directly proportional to the level of the i n p u t voltage. This 8-megacycle signal is generated and divided down from a 16-megacycle crystal oscillator and is extremely stable. In operation, the output from the integrator (or any other voltage source) is applied to what is called the Base Line input. This input point is directly in front of the Linear Gate which controls the time during which the 8-megacycle signal is output from the A-D-C. The pulses from the timebase generator are fed through the control box into the analyzer Oscillator Input, as before, providing channel advance pulses, and are also fed into the Short Time Constant input of the analog-todigital converter. The pulse at this input provides an enabling or strobe pulse, causing the linear gate to examine the voltage a t the Base Line input and subsequently controls the 8-megacycle signal a t the Detector Input which is fed into the data scaler for a time proportional to the voltage a t the Base Line input; this is the conversion cycle. The sequence of events is indicated in Figure 5A, B, and D. Linearity of input voltage to output counts is shown in Figure 6. With our instrument, for example, voltage into the Base Line input resulted in a straight line of the form Counts
=
73
DISCUSSION
As shown in Figure 2 suitable readout equipment such as punched tape or typewriter may be employed. The advantages of obtaining digital readout in chronocoulometric adsorption or kinetic studies cannot be overstressed. Accuracy is greater than can be attained by photographic methods; ease of obtaining data is increased to such an extent that experiments are possible which were heretofore prevented largely by the length of time spent in obtaining photographs and taking data from them. The data acquisition system described here has been used in chronocoulometric studies of adsorption and kinetics (9, 6-8); it is doubtful if the detail of these studies would have been possible without this equipment. This paper is intended as an introduction to the use of multichannel anaalyzers to data acquisition in electrochemical studies. Applications to other work where a signal may be converted to a voltage are obvious.
+ 474 x voltage input
That is, 0 volts resulted in 73 counts. Correction for this could easily be made. This sensitivity corresponds to about i2 millivolt resolution. Provided the conversion time, voltage input X (474 8 X lo6
+ 73)
is less than the time-base generator period, the counts/volt is independent of the time-base generator setting. This was not the case when the voltageto-frequency converter input was used. In short, the internal A-D-C provides an order of magnitude, or better, increase in sensitivity as compared to using the V-F-C.
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Figure 6. Counts per channel vs. input voltage-internal analog-to-digital converter with 8-megacycle pulse train VOL. 38, NO. 9, AUGUST 1966
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Other more or less sophisticated acquisition systems may be devised, depending only on the finances and ingenuity of the experimentalist. The use of computer-type systems, which include not only acquisitional but computational capability, may be predicted. Xevertheless, the analyzer system used in this work possesses a larger degree of versatility than one might guess at first glance. While a certain amount of “interfacing,” as exemplified by the control-box, is required, it is the authors’ contention that a multichannel analyzer offers a large return in capability and flexibility per dollar of investment. ACKNOWLEDGMENT
The aid of Helmar Schlein in designing portions of the control box circuitry is appreciated. The helpfulness of John Brandt and Don Frank of R.I.D.L. is appreciated. Helpful and occasionally argumentative discussions with F. C. Anson also contributed to this work. APPENDIX
Control Box Operation. The control box is constructed largely from Engineered Electronics Co. (Santa Ana, Calif.) units. The push-button a t the extreme left of Figure 7 initiates the experiment, supplying a pulse to start the time-base generator and operating the relay which unshorts the capacitor across the integrator, and removes a -15-volt blocking input at plug-in 21, an “and” gate. The output from the time-base oscillator is applied at the second input of the “and” gate. This procedure prevents passage of these pulses until the integrator is unshorted. The time-base generator pulses are then delivered a) via pin 7 of plug-in 5 to the analyzer Oscillator Input, where it supplies channel advance pulses, b) via pin 8 of plug-in 5, to pin 5 of plug-in 8, and c) pin 4 of the binary counting circuit, comprised of plug-ins 11-19. (Note that this circuit is pre-set by depressing the push-button which starts the experiment.) The output from pin 7 of plug-in 8 supplies the pulse to trigger the potential step, via plug-ins 9 and 10 and the diode network at their right. The diode network permits obtaining positive or negative going potential pulses and further clamps the base line of the pulses to about 0 volt. This is necessary because the potential pulse is applied about the pre-set bias voltage obtained from the potentiometer a t top right. The pulse at pin 5 of plugin 9 results in a level shift from 0 to 8 volts across the 100K potentiometer a t
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ANALYTICAL CHEMISTRY
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Figure 7. Control unit for application of double potential step and channel control of R.I.D.L. Analyzer Plug-In No. 1,20 2, 8, 9
T-166, One shot (single pulse generator) T-103, Set-Reset Fllp Flop 3 T-1 102, Relay Driver 4, 5, 6, 10 T-1 103, Shoper 7 T-109, Reset Generator 11-19 Modifled T-lOlB, triggered reset Flip Flop (pin 5 shorted to junction of C 6 and R 1 0 internal of units). 2 T-641, DC logic-”and” gote Above units are plug-ins rnanufoctured b y Engineering Electronics Co., Santa Ana, Calif. Relay Is Clare H G 2A1003. Philbrick P75 arnplifler serves as follower ond i s located on control box. Horrison l a b Power Supplies used to supply regulated f15 volts. @) Indicates B.N.C. connectors
the output of the diode network; the level of the pulse may be adjusted by the potentiometer control. An input pulse at pin 3 of plug-in 9 will return the output of the diode network to 0 volt. This output will be supplied via the binary counting circuit and its output, plug-ins 11-20. The counting circuit counts the time-base generator pulses, which are fed to it from pin 8 of plug-in 5. The number of counts which will result in an output from the logic circuit, removing the potential step, may be set by positioning the toggle switches shown. In the chronocoulometric work discussed elsewhere (2, 6), reversal was held a t channel 64, regardless of the time-base generator setting. The address overflow pulse from the analyzer, which signals that all the channels have had data input, terminated the experiment, Via plug-in 6, the overflow pulse serves to inactivate the relay, short the integrator, and the input a t pin 3 of plug-in 8 resets it. The result of all this is that a) the integrator is opened before the potential step is applied, b) no channel advance pulse is supplied to the analyzer until the integrator is opened,
c) the potential step rise is exactly synchronized to the first channel advance pulse reaching the analyzer and its removal is synchronized to any given channel advance pulse from the timebase generator as set in the logic circuit. LITERATURE CITED
(1) Anson, F. C., ANAL. CHEM.38, 54
(1965).
(2) Anson, F. C., Christie, J. H., Oster-
voung, R. A., J. Electroanal. Chem., in press (1966j. (3) Anson, F. C., Payne, D. A., J . Electroanal. Chem., in press (1966). (4) Christie, J. H., J . Electroanal. Chem., in ress ( 1966). (5) ristie, J. H., Lauer, G., Osteryoung, R. A., J . Electroanal. Chem. 7, 60 (1564). (6) Christie, J. H., Osteryoung, R. A., Anson. F. C.. J . Electroanal. Chem.. in press (1966j. (7) Lauer, G., Ostegoung, R. A., paper presented at the Winter Meeting of the American Chemical Society, Phoenix, Arizona, January 1966. (8) Lauer, G., Osteryoung, R. A., ANAL. CHEM.,39, 1106 (1966). ’
RECEIVEDfor review March 7, 1966. Accepted April 26, 1966: Division of Analytical Chemistry, Winter Meeting, ACS, Phoenix, Ariz., January 1966.
