Anal. Chem. 1980, 52, 205~207
The influence of the rotation speed, flow rate, cell dimensions, etc. on the detector response is currently under investigation. Preliminary experiments show deviations with the ideal behavior of the RDE in which the limiting current is proportional to the square root of the angular velocity. These deviations are probably due to wall effects in the limited volume of the flow cell.
20 r . p s
na da
205
I
ACKNOWLEDGMENT
0 r.p.s.
T h e authors thank P. M. J. Coenegracht for his valuable discussions concerning the theory and practice of rotating disk electrodes. The authors are also indebted to J. F. C. Nienhuis of the instrumental workshop of our laboratory (supervisor A. Oosterhoff) for constructing the electrochemical flow cell.
I
jnnA
LITERATURE CITED
a
4 MIN.
0
8
4
0
MIN.
Chromatograms of a test mixture composed of 2.5 ng noradrenaline (na),2.5 ng dopamine (da), and 2.5 ng 3,4-dihydroxyphenylacetic acid (dopac),without (0 rps) and with (20 rps) rotation of the electrode Figure 4.
analysis, a linear relationship exists between the peak height in nanoamperes and the sample concentration. When trapped air bubbles are to be removed or when repacking of the electrode is needed, the flow cell can be dismantled within some seconds by lifting the RDE holder.
(1) Brunt, K. fharm. Weekbl. 1978, 113, 689-698. (2) Kissinaer, P. T.: Refshauae. C.: Dreilina, R.; Adams, R. N. Anal. Len. 1973,-6, 465-477. (3) Fleet, B.; Little, C. J. J . Chromatogr. Sci. 1974, 12, 747-752. (4) Bollet, C.; Oliva, P.; Caude, M. J . Chromatogr. 1978, 149, 625-645. (5) Blank, C. L. J . Chromatogr. 1976, 117, 35-46. (6) Brunt, K.; Bruins, C. H. P. J . Chromatogr. 1978, 161, 310-314 (71 I , Brunt. K.: Bruins. C. H. P. J . Chromatoor. 1979. 172. 37-47. (8) Wang, J.: Ariel,-M. Anal. Chim. Ac& 1578, 99; 89-98. (9) Weber, S. G.; Purdy, W . C. Anal. Chim. Acta 1978, 100, 531-544. (10) Yamada, H.; Matsuda, H. J . Nectroanal. Chem. 1973, 44, 189-198. (11) Coenegracht, P. M. J. fharm. Weekbl. 1972, 107, 769-782. (12) Sternson, A. W.; McCreery, R.; Feinberg. B.; Adams, R. N. J . flectroanal. Chem. 1973, 46, 313-321. (13) Strohl, A. N.; Curran, D. J. Anal. Chem. 1979, 51. 1045-1049. ~~
RECEIVED for review June 18,1979. Accepted September 10, 1979.
Microcomputer Interfaced Spectrophotometer for Kinetic Studies C. S. Nichols, J. N. Demas," and T. H. Cromartie" Department of Chemistry, University of Virginia, Charlottesville, Virginia 2290 1
Detailed kinetic studies routinely require collection and reduction of great amounts of data which is generally a manpower-intensive process. With the advent of the minicomputer, computerized data acquisition and reduction has reduced these manpower requirements. Initial interfaces between recording spectrophotometers and computers were inconvenient because of the need for direct connections to the internal circuits of the spectrophotometer ( 1 - 4 ) . These interfaces were not widely adopted because of expense, complexity, and difficulty of fabrication. Modern spectrometers are generally equipped with T T L compatible digital communication lines which can greatly simplify interfacing. For example, for signal averaging, an infrared spectrometer has been interfaced to a minicomputer using its standard spectrophotometer interface plug ( 5 ) . We describe here a n inexpensive microcomputer interface for the common Beckman 25 absorption instrument and a Processor Technology SOL-20 microcomputer. The same interface would, however, work with any microcomputer having one 8-bit input and one 8-bit output port. This system permits data acquisition a t any rate from 1 s per point to >6500 s per point. Data acquisition may be terminated a t any time by the operator, and the acquired data may then be reduced by the method of initial rate for kinetics done under zeroth-order conditions or by a linear least-squares fit of the semilogarithmic data plots for first-order kinetics. Data reduction is complete within seconds of termination of the 0003-2700/80/0352-0205$01.OO/O
kinetics run. The interface does not interfere with operation of the spectrophotometer recorder so that graphs may be obtained concomitant with computer-controlled data acquisition.
INSTRUMENTATION AND SOFTWARE The Beckman 25 is one of the many absorption instruments currently available which has a digital voltmeter display of the absorbance; the binary coded decimal (BCD) data for the digital readout is brought out to a rear connector along with several control lines. These TTL compatible output and control lines were intended for interfacing with the manufacturer's data logging printer but, with a minimal interface, they can be used for transferring data into a dedicated 8-bit microcomputer for logging and later reduction. The format and pin basing for the Beckman interface plug is shown in Figure 1 (6). The output of the analog-to-digital converter (ADC) is four digits. The range on the absorbance scale is 0.ooOto 2.8 absorbance units, but the spectrophotometer is only rated t o 2.000 absorbance units. Underranges are indicated by a 9 in the left-most digit. The decimal point is always fixed with three digits to its right. A concentration range mode is also available, but this cannot be used conveniently with the interface described here. The remaining spectrophotometer pins have the following functions. When the Convert Input (CI) pin is high, the spectrophotometer is free running; but when this input goes low, the current conversion is stopped and no new ones ace initiated. After 1 transition on the CI CI has been 0 for at least 150 ws, a 0
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1979 American Chemical Society
206
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980 BECKMAN 25
I
OUTPUTS
INPUTS I ‘S
C
DH
G
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7-SEGMENT
...............
......... _ _ ? ’
__.. TES+
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4
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7-SEGMENT DISPLAY
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..--
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‘‘I:
Sample kinetic run of pig liver carboxylesterase. The solid line is the strip chart recording displaced by 0.01 absorbance unit for clarity. The “s are the computer collected data with the least-squares fit (- - -). The recorder was set on the 0- 1 absorbance range to yield a wide dynamic range for the kinetics. Time is in seconds Figure 2.
102
I N P U T PORT
,
.
OUTPUT PORT
SOL 20 Figure 1. Schematic diagram of interfacing between Beckman 25 spectrophotometer and SOL-20 microcomputer. The L’s are 74LS373 octal three-state latches. The open collector inverters are from a 7406. The oscillator for the DR clock continually strobes the SOL data ready line and is not required for the software described here. The displays used DL707 seven-segment displays and 7447 drivers
pin initiates a new conversion and restores the spectrophotometer to the free running mode. The Display Hold (DH) line controls the ADC output latches. The DH pin is wired to 5 V which causes the absorbances to be latched into the output registers after each conversion. Completion of analog-to-digital conversion and the availability of good data on the BCD output registers and displays are signified by the Data Ready (DR) pin going high -590 ms after initiation of conversion. The datum point is then good for 250 ms. The computer available for interfacing was a Processor Technology SOL-20 Intel 8080 based microcomputer equipped with 20 K of read/write memory. The SOL-20 computer contains one 8-bit input port and one 8-bit latched output port. Software available included a Processor Technology extended BASIC interpreter and an assembler. The first problem was how to route 16 BCD data lines and the DR line into the single 8-bit parallel input port. The number of necessary lines was reduced to 16 by noting that the most significant absorbance digit could assume only four values (0, 1, 2, and 9). By monitoring only the two least significant and the most significant bits of this BCD digit, a unique and readily identifiable combination was always detectible (0x00, 0x01, 0x10, 1x01). The DR line was then routed to the unused bit. All data input lines then fit into two bytes. The two resulting 8-bit words were then multiplexed into the 8-bit input port using one bit of the output port to control which 8-bits were multiplexed on. The multiplexing was accomplished with minimal IC’s by using a pair of 8-bit three-state octal latches (74LS373) and an inverter function. The latches were held continuously transparent by a high a t their enable inputs. Both latches were connected directly to the input port, but only one at a time was three-stated on with the other one being in the high impedance off state. The complete circuit is shown in Figure 1. The TEST function and seven segment displays are not essential, but they are useful for verifying proper interface operation. Normally, the two least significant digits are output to the two TEST LED displays; these values should correspond to the two least significant digits on the Beckman display. On pressing the TEST button, the two most significant digits are gated to the LED display. The normal data acquisition software will not work properly during the TEST sequence. The only remaining difficulty concerned the method of clocking the data acquisition. This was accomplished by means of a software clock written in machine code.
We now describe the complete software package. This consists of a main BASIC program and a machine language subroutine. The BASIC program handles all data reduction, display functions, and sets up the clock parameter used in the subroutine. On entering the BASIC control program, the operator specifies the number of seconds between data points (21s in 0.1-s intervals) which the BASIC program will pass to the subroutine. After preparing the spectrophotometer and sample, the operator initiates data acquisition by pressing a “2”. The BASIC program then makes a subroutine call to the machine language program. The machine language subroutine (
