Versatile Reciprocal Time Computer for Reaction Rate Methods S . R. Crouch Department of Chemistry, Michigan State University, East Lansing, Mich. 48823 A new method is describ.ed for computing the reciprocal of time for use in reaction rate methods. The instrument is all-electronic and can be used to measure the rates of both fast and slow reactions. The computer can be directly calibrated in reciprocal time or concentration units. Results are presented for synthetic signals which vary in rate from 5 mV/sec to 20 V/sec. Standard deviations and relative errors are less than 1% in most cases. Data for the determination of phosphate by a spectrophotometric rate method are presented to demonstrate the application of the reciprocal time computer to routine analysis.
SEVERALRECENT PUBLICATIONS have dealt with automated measurement systems for reaction rate methods ( I , 2). Two general approaches have proved most reliable (3) : the fixed time method in which the concentration change occurring in a preselected time interval is measured; and the variable time method in which the time required for the reaction to proceed by a fixed amount is measured. As Pardue has observed (4), this latter approach has several advantages for many reactions. The major disadvantage of the variable time method is the inverse relationship between the measured time interval and concentration. To overcome this disadvantage, Pardue and coworkers ( I , 5, 6) have developed systems for computing the reciprocal of the time interval and have obtained a direct proportionality between readout and concentration. In the most elegant of these systems ( I ) , a voltage is generated proportional to the time interval required for the reaction to proceed by a preset amount. The logarithm of the voltage is computed and summed with a reference voltage. The final step computes the antilogarithm of the inverted sum, giving a voltage readout proportional to the reciprocal of the time interval. The system described here computes the reciprocal of time by measuring the period of a pulse train whose frequency is directly proportional to the desired time interval. This approach eliminates the critical logarithmic elements which are necessary in other all electronic systems ( I ) and is extremely versatile. The instrument is easily calibrated in reciprocal time or concentration units and can be used for reaction time intervals from 5 msec to several minutes. The instrument requires a voltage input and can measure the rates of both positive- and negative-going signals. In addition, a simple modification allows rates to be monitored continuously during a single reaction. Although the system described here does not possess the noise-averaging characteristics of the fixedtime integration method (2), the fixed concentration interval can be easily adjusted to minimize the effects of noisy input
(1) G. E. James and H. L. Pardue, ANAL.CHEM., 40, 797 (1968). (2) E. M. Cordos, S. R. Crouch, and H. V. Malmstadt, ibid., p 1812. (3) W. J. Blaedel and G. P. Hicks, “Advances in Analytical Chemistry and Instrumentation,” Vol. 3, Wiley, New York, 1964, pp 105-42. (4) H. L. Pardue, Rec. Chem. Progress, 27, 151 (1966). (5) H. L. Pardue, C. S.Frings, and C . J. Delaney, ANAL.CHEM., 37, 1426 (1965). (6) H. L. Pardue and R. L. Habig, Anal. Chim. Acra., 35, 383 (1966). 880
ANALYTICAL CHEMISTRY
I
I
Figure 1. Block diagram of reaction-rate measurement system Reciprocal time computer shown in dotted lines
signals and experimental results on synthetic signals are comparable in precision and accuracy. The circuit is readily constructed from a few commercially available circuit cards and digital modules, and a digital instrument. INSTRUMENT DESCRIPTION
A block diagram of a rate measuring system using the reciprocal time computer is shown in Figure 1. The chemical reaction is monitored in a reaction cell by a suitable transducer whose signal is modified to obtain a voltage input for the reciprocal time system. The reciprocal time computer is shown in dotted lines in Figure 1. The voltage interval sensor has two set comparison voltages VI and V2, corresponding to two concentrations of the monitored reactant or product. When the input voltage reaches VI, a trigger signal begins a linear sweep generator which serves as a time interval to voltage converter. When the input voltage reaches VZ,the linear sweep generator is stopped. Its output voltage is then proportional to the time required for the input signal to change by AV = (VZ- VI). The voltage output of the time interval to voltage converter is the input to a voltage to frequency converter whose output frequency is thus proportional to the time interval. A digital period meter, triggered when level VZis reached, measures the reciprocal of the frequency and hence the reciprocal of the time interval. Voltage Interval Sensor. The voltage interval sensor is shown in Figure 2. Switch SIand relay R1 comprise an on-off switch for the reciprocal time computer. During periods of sample and reagent introduction, this switch is placed in the off position to prevent false triggering by noise. The voltage interval sensor consists of two high speed voltage comparators and two stable reference voltages, which provide the comparison voltages VI and VZ. Comparison voltage VI is obtained directly from reference voltage source VI. Comparison voltage Vz is obtained from the sum of a second voltage source AV and VI. Thus VZalways differs from V Iby AV. This configuration allows the first comparator trigger level VI to be varied without varying AV. In addition, this configuration allows multiple measurements to be made on the same reaction, If a stair case generator is substituted for VI, the absolute magnitudes of VI and V2can be advanced in steps while AV remains constant. Hence the comparators can be programmed to move along the reaction curve, allowing the reciprocal of time to be computed at various concentration intervals throughout the reaction. For high versatility, voltage reference sources VI and A V are continuously variable and reversible in polarity. Logic driver outputs of z 0 V and + 5 V from the comparators are used to trigger the time interval to voltage con-
'I5V
7
IOK
FET I
6+5v " 0 ; L
RDI
---I
+sv2.F
S,
i
!
Figure 2. Voltage interval sensor Comparators-Heath EU-800-HB comparator cards GI, Ga-NAND gates, Heath EU-800-JC NAND gate card RD1, 2, 3 and R1,R2, &-Relay drivers and relays, Heath EUSOO-JD Relay Card Vl, AV-Voltage reference sources, Heath EUW-16
verter and the digital period readout. For positive-going input signals, the comparator logic driver outputs are used directly to trigger these subsequent circuits. For negative input slopes, however, the slope control switch SZactivates relays Rz and R 3 which introduce NAND gates G1 and Gz. This inverts the logic levels so that the same switching circuits can be used to start and stop the time interval to voltage converter. The actual polarities of reference voltages V I and AV will depend on the polarity of the input signal from the signal modifier and the polarity of its slope. For an input signal which begins at zero V or some positive voltage with respect to ground and continues to move positive, the polarities of Vl and AV will be as shown in Figure 2. Other cases can be accommodated by reversing Vl and/or AV and adjusting switch Sz. Time Interval to Voltage Converter. The linear sweep generator, used to produce a voltage proportional to the time interval, is an operational amplifier integrator whose integration time is controlled by the outputs of comparators l and 2. Figure 3 shows the integrator and associated solid-state switches. Transistors Ql and FET 1 make up a fast analog gate which is closed, shorting integrating capacitor C , until the input signal reaches comparator level VI. When VI is reached, a +5-V signal from the output of comparator 1 is applied to the base of Ql, which turns Ql and FET 1 OFF, allowing C, to charge. Likewise, QZand FET 2 switch off the integrator input voltage Ei, when level VZ is reached. Thus the output voltage of OA1 increases linearly for a time interval (At) when the input signal is between Vl and V Z . The integrator output voltage is thus given by Equation 1
Solid-state switches are used to control the time interval to voltage converter because of their rapid switching times when compared to mechanical switches. The switches shown in Figure 3 are capable of switching in about 20 psec. The input voltage of OA,, Ei,, is variable from 0-10 V so that a wide variety of input rates can be handled with high accuracy. For rapid reactions, At is short, and Ei, should be large enough to make EOin Equation 1 several mV for accurate voltage to frequency conversion. Conversely, when At is large, Ei, should be small enough to avoid limiting OA1 before the end of the time interval. Usually, the sweep rate of the integrator was 1 V/sec for rapidly changing input signals and 100 mV/sec for slow signals. Potentiometer PZ at the output of OA1 allows a selected fraction of EOto be fed to the voltage to frequency converter. This adjustment can
+5v
Figure 3. Time interval to voltage converter Ql, Qz-2N1274 D1, Dz-lN4149 FET 1, FET 2-MPF 105 OA1-Philbrick PP25AU PI,P2-100K on Heath EU-801A Analog Digital Designer Ri,, Ct-typically 1 meg, lbf
be used for direct concentration or rate readout as explained in a later section. Voltage to Frequency Converter and Period Meter. The voltage to frequency converter operates on the output voltage of the integrator Eo, and produces pulses at a frequency f, which is proportional to Eo as shown by Equation 2 f
=
kEo
(2)
where k is the conversion rate in Hz/V. If a period meter is used to read the reciprocal of the frequency, the resulting period Tis directly proportional to the reciprocal of the time interval At, as shown by combining Equations 1 and 2 (3)
where the minus sign in Equation 1 has been dropped because Ei, is always a negative voltage. Any accurate and stable voltage to frequency (V-F) converter can be used to provide input pulses to the period meter. In this work, however, it was convenient to use the internal V-F converter in the Heath Universal Digital Instrument (UDI, Heath EU-805A). This V-F converter is normally used only when the instrument is in the digital voltmeter (DVM) mode. However, the output of the V-F converter is available at the rear panel even when the instrument is used in another mode, such as the period mode. Hence, the same instrument can provide V-F conversion and period measurement. For high accuracy, the instrument was usually operated in the multiple period averaging mode as explained in the next paragraph. The V-F converter in the Heath UDI has a conversion rate of 100 KHz/V. Because of this high conversion rate, it i s not practical to obtain high accuracy by measuring single periods. For example, a 1-V signal at the output of OAl would result in a V-F output period of only 10 p e c . Thus, to obtain higher resolution, multiple periods were taken. In this mode, the internal 1-MHz oscillator of the UDI is counted for a scaled number of input periods (1, 10, 100, etc.), permitting 1 psec resolution. To accomplish this, the V-F converter output at the rear panel was connected to UDI input B and the rear panel clock output was connected to input A . The frequency meter push button was pushed (7). The front panel scaling factor switch selects the number (7) Heath Instruction Manual EU-805A, Universal Digital Instrument (1968). VOL. 41, NO. 7,JUNE 1969
881
of periods to be averaged. The necessary connections for multiple period averaging of the V-F output are shown in Figure 4. Relay Rq is used to keep the readout off until comparator 2 (Figure 2) has switched. This switch can operate leisurely and hence an electromechanical relay was used. In the multiple period averaging mode, Equation 3 is altered somewhat. If F periods are averaged by selecting a scaling factor of F, the UDI counts the number of psec in F periods and Equation 3 becomes Readout (psec) =
106FRinC/ kEinAt
(4)
Substituting for the fixed constants in Equation 4 ( k = 100 KHz/V, R i X f = 1 sec), gives
: (i)
Readout (psec) = -
Thus the scaling factor F and the integrator input voltage Ed, can be adjusted to accommodate a wide variety of input slopes with high accuracy. If Ei, is a multiple of 10 (100 mV, 1 V, etc.), the readout will indicate the reciprocal time directly with only a decimal conversion factor. PROCEDURES Circuit Layout and Construction. The circuit cards listed in Figures 2 and 3 were inserted into a Heath “Analog-Digital Designer,” EU-801A, which supplied the necessary power to operate the system. A special printed circuit card containing the two analog gates shown in Figure 3 was prepared. The
Table I. Automatic Reciprocal Time Measurements Input rate, mV/sec
Digital readout,”
1,000 5,ooo 7,000
497 1,004 2,998 5,007 7,004 9,979 29,780 49,740 1,001 5,020 7,041
10,000
10,100
20,000
19,840
+5
10 30 50
70 100 300 500
Re1 std dev,
z
MSec
0.15 0.25 0.22 0.21 0.13 0.06 0.34 0.27 0.38 0.59 0.87 1.32 1.82
Re1 error: -0.6 +0.4 -0.1
...
+o. 1
-0.2 -0.7 -0.6 $0.1 +0.4
+0.6
+1.0 -0.8
Averages of 10 results; comparators A V = 100 mV; for rates to 500 mV/sec, F = lo2,Ei, = 100 mV; for rates 21 V/sec, F = 10, Ei, = 1 V. * Based on calibration with 50 mV/sec input rate. 5
Table 11. Automatic Phosphate Determinations Digital concentration readouta 2.98 5.02 8.03 9.96
Phosphorus concentration, ppm Re1 std Taken Re1 error2 dev,
z
3.0
-0.7
5.0
...
8.0
+0.4 -0.4
10.0
Averages of 5 results; VI
=
0.975 V, AV
50 mV, F = 10.
* Based on calibration with 5.0 ppm standard,
882
ANALYTICAL CHEMISTRY
z
2.8 3.7 2.3 1.2 =
25 mV, E,n
=
FROU OAl FlWRE 3
Figure 4. Connections for V-F conversion and multiple period averaging UDI-Heath EU-805A Universal Digital Instrument driver and relay, Heath EU-800-JD relay
RD4, &-Relay
card
Philbrick operational amplifier was mounted on a Heath circuit board (Heath part no. 85-247-1). Connections within and between cards were made with No. 22 patch wires. Once the initial instrument testing had been completed, a motherboard containing all necessary card connections was prepared by mounting several 32 pin connector strips (Heath part No. 432-79) on 2 Heath EU-50-MD multiple connector/blank pc cards. The necessary connections were then made by etching the circuit board. Preparation of Instrument. To prepare the reciprocal time computer for measurements, it is necessary to choose comparison voltages Vl and Vz. This is most easily done by recording the signal modifier output voltage on a strip chart recorder and picking the appropriate levels from the chart paper. Exact trigger levels are then set as described in the comparator card instruction manual (8). For direct concentration or rate readout, potentiometer Pz (Figure 3) provides a convenient adjustment. Because OA1 has a low bias current, the output of OAl stays constant for some time, allowing PZto be adjusted to give the desired readout. Alternate adjustments such as AV or Ei,can be made only once for any given reaction. Spectrophotometric Measurements. A Heath single beam spectrophotometer EU-701 was used for all spectrophotometric measurements. The photomultiplier current was converted to a voltage using a standard operational amplifier circuit. The spectrophotometer slit width and photomultiplier power supply voltage were adjusted to give a 1.000-V output of the operational amplifier for 100% transmittance. The voltage output of the operational amplifier was the input to the reciprocal time computer (input to relay Rl, Figure 2). The reaction cell was a standard 1.00-cm spectrophotometric cell surrounded by a brass jacket through which water at 25.0 “C was circulated. A magnetic stirrer was positioned below the cell to allow rapid mixing. Phosphate Determinations. For the determination of phosphate, all reagents except the acid-molybdate solution were prepared as previously described (9). The acid-molybdate reagent was prepared by dissolving 1.089 grams of NazMoOl. 2H20 in water in a 250-ml volumetric flask. A total of 9.4 ml of concentrated H2S04was then added and the solution was diluted to volume. The reaction was followed at 650 mp. Two milliliters of the acid-molybdate reagent and one milliliter of the phosphate sample were added to the cell. The reaction was begun by injecting 0.25 ml of 0.10M ascorbic acid with a hypodermic syringe. At the end of the measurement time, the result appears automatically on the Universal Digital Instrument.
(8) Heath Instruction Manual EU-BOO-HB, V to F/Comparator Card, (1968). (9) S. R. Crouch and H. V. Malmstadt, ANAL.CHEM.39, 1090 (1967).
RESULTS
The reciprocal time computer was tested by applying voltage signals of known input slopes to the voltage interval sensor (Figure 1) to simulate outputs of typical reaction monitorsignal modifier systems. Table I shows the results of measurements of input rates from 5 mV/sec to 20 V/sec. The instrument was calibrated with a 50 mV/sec input rate by adjusting Pz(Figure 3) to give the desired readout. This adjustment is necessary because of component tolerances in the linear sweep generator. The entire range of input rates shown in Table I was measured with only one instrument change. For rates > lV/sec, the scaling factor F and integrator input voltage Em were changed by factors of ten. At the highest rate measured, the actual time interval (At) was 5 msec. With this short Ar, the time interval to voltage converter output was only 5 mV. Faster rates could be measured by increasing Ei,. However, the analog gates used to switch Et, (Figure 3) are only capable of switching up to 5 V. Table I1 shows results for the determination of phosphate. For this procedure, the input voltage to the reciprocal time
computer had a negative slope, and the NAND gates shown in Figure 2 were used. With the 100% transmittance set at 1.000 V, recorded curves enabled the selection of VI and AV. Vl was set at 0.975 volt and AV was adjusted to be 25 mV. Because of the slowness of the reaction, Et, was reduced to 50 mV to avoid limiting OA1. Potentiometer PZ(Figure 3) was adjusted to give a direct concentration readout using a phosphate standard of 5 ppm P. At the lowest concentration used, the measurement time At was about 120 sec. The results shown in Tables I and I1 are typical of many which have been obtained. The reciprocal time computer was constructed using the same digital modules as used in the fixed time readout system described earlier (2). By a simple change of circuit cards and intercard connections, the same basic modules can be used for both variable time and fixed time methods. This allows the user of reaction rate methods to have both automated readout systems readily available for use with different reactions.
RECEIVED for review March 14, 1969. Accepted April 11, 1969.
Cornputer-Assisted Gas-Liq uid Chromatography H. M. Gladney, B. F. Dowden, and J. D. Swalen International Business Machine Corp., Research Division, San Jose, CaIq. 95114 The use of computers for data acquisition and analysis in gas-liquid chromatography is becoming increasingly widespread. We describe an implementation of computer-assisted chromatography within a generalpurpose time-shared laboratory automation system. Particular attention is paid to a method of avoiding timing conflicts at the computer, to an inexpensive method of providing a computer-to-instrument interface, and to an economical method of curve-fitting to resolve overlapping skewed peaks. It is anticipated that many of the methods and some of the computer programs will be directly applicable to various spectroscopic experiments.
THERECENT IMPLEMENTATION in this laboratory of a timeshared laboratory automation system for spectroscopic-type instruments ( I ) has made possible on-line data collection and computation for a high-sensitivity gas chromatograph. We wish to describe our system which includes two novel features: a simple and inexpensive interface for time-shared communications between an instrument and a remote computer, and a method for the resolution of overlapping skewed peaks, involving a least-squares curve fitting procedure. In Figure 1 is shown a simple calculated curve compared with the observed chromatogram. In general, the methods used to schedule the communications between a process-control computer and several instruments are largely selected by considerations of economy and of the relative requirements of the instruments being connected. The choice of method becomes particularly difficult if the full requirements of the laboratory cannot be anticipated-which is usually the case. It is also conceded that an extremely desirable, if not absolutely necessary, feature be that each experiment can be programmed and run inde(1) H. M. Gladney, J. Cornp. Physics, 2, 255 (1968).
Figure 1. A poorly resolved chromatogram of O C petroleum ether showing computer fitted peaks
30-60
pendently. From the point of view of the experiment and experimenter, it is perhaps conceptually simpler if the computer is enslaved to the laboratory instrument. However, at the computer, this arrangement implies not one master, but several, usually with conflicting requirements. There are at least three possible resolutions of the problem: To dedicate a computer or a sub-computer (commonly called a data channel) either permanently or temporarily to each experiment (2). To employ a computer which is so fast relative to the time-sensitive experiments that the unavoidable inter(2) T. R. Lusebrink and C.H. Sederholm, ZBM J. Res. Develop., 13,65 (1969). VOL. 41, NO. 7, JUNE 1969
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