Table I. Relative SO2 Response for Evaluated Substrates Cell 1
AF*so2
Tridodecyl amine Melamine Diallyl Melamine lgepal CO-880 (Appl. Sci. Lab.) Cellulose nitrate Diallyl amine
160. 4. 24.
69. 35. 70.
Cell 2
Phenyldiethanolamine UC-W98 (Appl. Sci. Lab.) Versamid 900 (Appl. Sci. Lab.) PP-2040 (lonac Chem. Go.) PE-100 (lonac Chem. Go.) Tripropyl amine
150. 45. 50. 190. 68. 225.
2,2-(m-tolylimino)diethanol S D M polymer (UniRoyal)
195.
3.
“bleed” from the crystal, thus causing a sensitivity change in the detector. In addition they tend to show a “partial retention” of SOz, producing a serious detector fatigue. The SDM polymer proves to be an excellent choice as a coating for the “sorption” detector. Phenyldiethanolamine
and PP-2040 would also seem to be good choices as substrates. Air, nitrogen, carbon monoxide, and carbon dioxide produced no response above background noise levels when compared to that of SOz. The only common interference found was nitrogen dioxide. NO2 produced a response about half that of an equal amount of SO2 but was in many cases more irreversible. Further study with the SDM polymer has shown that 100 ppm of SO2 can be easily detected using the described experimental set up. The design in Figure 1 has a total retail cost of $1000. A similar device with greater utility could be fabricated from two electronic oscillators and a mixer circuit which would compare the frequency of the sample crystal to that of a reference crystal. Such a design would cost about $500. These devices can be battery-powered for portability. The quartz crystals are brittle, but with proper mounting they are immune to physical shocks. The detection limit of SO2 with the cell 2 is 5 ppm. More work is being done to lower the detection limit and perfect the system. Received for review January 17, 1973. Accepted March 1, 1973.
Ratemeter Interface for a Minicomputer-Controlled ReactionRate Instrument E. S. Iracki’ and H. V. Malmstadt
Department of Chemistry, School of Chemical Sciences, University of lllinois at Urbana-Champaign, 111. 67801 The past few years have seen an increased interest in the utilization of reaction-rate methods of analysis. Recent reports (1-3) have shown that the increased popularity of these methods is largely a result of the development of a new generation of elegant, completely automated, and computer-controlled chemical instrumentation which makes it just as easy to obtain sensitive quantitative chemical results with reaction-rate methods as with conventional equilibrium or end-point methods (I). The use of digital computers in automated systems of this type has led to the development of several approaches to the acquisition of data and the ultimate determination of the rate of the reaction. The techniques may be generally grouped into two categories. The first category is that in which a domain conversion from the physical or chemical domain to an electrical signal is performed and the resulting signal is digitized and stored in the computer memory followed by reduction to the desired rate result ( 4 , 5 ) . The second approach involves the use of an addi‘Present address, Environmental Systems Engineering, Clemson University, Clemson, S.C. 29631.
tional device to compute the rate of signal change followed by the digitization of the computed rate result and storage in the computer. Both of these approaches have demonstrated good precision and accuracy. The hardware-software approach has been successfully implemented by Hicks and his coworkers (6). The device used to compute the rate of the reaction was an analog integrating ratemeter based upon the original design of Cordos e t al. (7). In addition to the ratemeter itself, circuitry for scaling the analog output of the ratemeter is required and additional software is necessary for calibrating the ratemeter and controlling the device. Ingle and Crouch have developed a digital integrating ratemeter which was also based upon the integration-subtraction technique (8). The instrument provided direct digital readout of rate measurements and was capable of operating in two modes-single measurement or continuous measurement modes. The ratemeter described in this paper is similar in principle and design to those described earlier (7, 8). It is based upon the conversion of the transducer signal to a pulse train by means of a voltage-to-frequency converter
(1) H. V. Malmstadt, E. A. Cordos, and C. J. Delaney. Ana/. Chem.. 44 (12), 26A (1972). ( 2 ) H. V . Malrnstadt, C. J. Delaney, and E. A. Cordos, A n a / . Chem.. 44 (12), 79A (1972). (3) H. V . Malmstadt, C. J. Delaney. and E. A. Cordos, Crit. Rev. Anal. Chem.. 2, 559 (1972). (4) G. E. James and H. L. Pardue, Anal. Chem.. 41,1618 (1969).
(5) B. G. Willis, J. A. Bittikofer, H. L. Pardue, and D. W. Margerum, Anal. Chem., 42, 1340 (1970). (6) A. A. Eggert, G. D. Hicks, and J. E. Davis, Anal. Chem., 43, 736 (1971). (7) E. A. Cordos. S. R. Crouch, and H. V. Malrnstadt, Ana/. Chem.. 40, 1813 (1968). (8) J. D. Ingle and S . R. Crouch, Anal. Chem.. 42, 1055 (1970).
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with subsequent upfdown counting of the pulse train to achieve the integration-subtraction operations. Consequently, the relationship between the instrumental readout and the rate of signal change as well as the range and limitations of the instrument are similar to those reported. The digital ratemeter lends itself directly to computer automation because of its inherent binary output. The digital integrating ratemeter and associated software described here are designed to eliminate some of the shortcomings found in other systems and so incorporate the following features: (1) The output of the ratemeter is a 24-bit word. This feature eliminates the need for scaling circuitry. (2) Drifts and nonlinearities of the computational circuitry are eliminated. (3) The control circuitry starts the second integration period a t the same instant that it terminates the first integration period, thereby eliminating the “idle” time characteristic of some ratemeters (6, 7). (4) The rather inconveniently long delay times of a similar instrument (8) are avoided. In making continuous measurements, the delay between measurements amounts to 5% of the total computation time; e . g . , for a 20 second measurement, only 1 second elapses between measurements. ( 5 ) If the desired number of rate measurements have not yet been made on a signal, the computer retriggers the ratemeter immediately after acquiring the 24 ’ bits of information. The 24 bits are then processed and printed on the teletype. In this fashion, the two seconds required for the teletype to print the rate are not added to the total time required for a series of measurements; rather, the rate information is printed while another rate measurement is being made. This results in a significant saving in time and more efficient operation if the system is to operate over extended periods of time. The ratemeter described here may also be used as an ordinary analog-to-digital converter. This is accomplished through the control logic of the ratemeter so that the input to the instrument is directed only to the UP counter rather than to both UP and DOWN counters as is normally done in performing a rate measurement. The desired operating mode of the instrument is computer controlled as are all other aspects of the instrument. In the Ratemeter Mode, the start pulses generated by the computer cause the control logic to gate the pulse train from the voltage-to-frequency converter to the DOWN counter for exactly ten clock periods of the time base. For the next ten clock periods, the pulse train is gated to the U P counter. At the end of the measurement interval, the instrument flag is set indicating that a measurement is complete and the information may be transferred to the computer memory and printed on the teletype. The twenty-four bits of information are then transferred to the computer in two twelve-bit words by means of the shift register circuits. In the A/D Mode, the pulse train from the voltage-to-frequency converter is directed only to the UP counter for ten clock periods. The result is transferred to the computer in the manner described above. Although this method of digitization is rather slow compared to commercially available A/D converters, a large number of laboratory measurements do not require high speed, e . g . , measurements that are performed with a digital voltmeter are generally made in 0.1 to 10 seconds; it is this range of conversion times that is usually used in the present instrument.
INSTRUMENTATION A block diagram of the system used to acquire rate data is shown in Figure 1. Signal modifiers are used to convert the signal from the rate monitor to a voltage range that is compatible with the voltage-to-frequency converter. The
Monitor ond Signal Modifiers
Up- Down
Frequency Converter
Counters I I
Gated Driver Circuits
-
Device Timing
I
Computer
Figure 1. Block diagram of the system for automated rate mea-
surements pulse train output of the V-to-F converter is then directed to the counter circuits in the proper sequence by means of the control logic. The twenty-four bits of information are then transferred to the computer in two twelve-bit words by means of the shift register circuits. The interface module contains the necessary circuits for device decoding, generating timing pulses, and gating information to the computer. The control logic is shown in Figure 2 and the counter-shift register circuits are shown in Figure 3. The computer is a PDP-8/E (Digital Equipment Corporation, Maynard, Mass.). Ratemeter Mode. This mode is selected when the software executes a Device Select 42 (DS 42) and an Input/ Output Pulse 4 (IOP 4). These two signals are gated through gate nineteen (G19) which sets flip-flop five (FF5). This effectively enables G11, G14, and G18, and the control logic generates the waveforms shown in Figure 4. FF4 serves as the device flag for the ratemeter. The software service routine begins by NANDING DS 43 and IOP 2 to clear the flag. A measurement cycle is initiated when the computer’s DS 43 and IOP 4 are XAKDED through G5 producing pulse T. This effectively clears FF1, FF2, the up-down counters and the shift registers, sets FF3, and also resets the decade counting unit (DCU) and the Time Base. After a 1-period delay, FF2 directs the pulses from the V-F converter t o the DOWN input of the counting circuitry for 10 clock periods. On the trailing edge of the 11th clock pulse, Qz goes high, the DOWN input is closed, and for the next 10 clock periods, the UP input is open. A complete measurement cycle then requires 21 clock periods of which one period is a delay period. The 24 bits of rate information are then transferred in two 12-bit words to the computer memory via the accumulator register. At the end of the measurement cycle, Q4, the device flag, and 8 3 are set, and this results in setting the shift registers in the “parellel load” mode. The computer responds by supplying a single pulse (a SANDED DS 43 and IOP 1) to parallel load the shift registers with the contents of the up-down counters. The 12 most significant bits are then read into the computer via the
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
1767
3543 lop2
52
.
I V-F
UP
I
I s5 4 4
IOP 1
FF 5 DS 4 2 10P4
Figure 2. Diagram of the counter-shift register control logic
11109 8
- Sel
DS 42 IOP 2
IOP 1
.STR
7 6 5 4 GATED DRIVER CARD
3 2 i o
(ACCUMULATOR INWT)
Figure 3. Counter-shiftregister circuits for digital rate measurements
n
C
Q1
U
n
D
Q3
r
Q4 ~
Figure 4. Waveforms generated by counter-shift register control logic-ratemeter mode
gated driver and 1/0 patch cards by supplying a DS 42 and IOP 2 to the gated driver. The ratemeter device flag is then cleared so that the SOand SI inputs to the shift registers are 0 and 1, respectively, thus putting the shift registers in the "right shift" mode. The computer then pulses the shift registers 12 times, and the 12 least significant bits are read into the computer when a DS 42 and an IOP 2 are executed. 1768
ANALYTICAL CHEMISTRY, VOL. 45,
NO. 9,
Immediately after storing the 24 bits of information, the program checks a software counter to see if more rate measurements are to be made. If another measurement is to be made, the computer initiates another hardware rate measurement and then proceeds to convert the two 12-bit words into floating point format (9) and print the result on the teletype. A/D Mode. This mode is entered by the generation of a DS 44 and an IOP 1. As a result, G13 and G16 are enabled with G18 being disabled. The control waveforms shown in Figure 5 are generated when the start command is issued. The modifications in the control logic now cause the of FF2 to toggle FF3 and the pulse train is routed to the count-up input after a one-period delay. After eleven clock periods have elapsed, the instrument flag is set and a conversion is complete. The contents of the counters are transferred to the computer in the manner described above. A total of eleven clock periods is required for each digitization period. The delay time in both operating modes could be shortened as desired by further division of the DCU output with appropriate multiplication of the clock frequency. The delay time, for example, could be reduced to 0.5% of the measurement interval by increasing the clock frequency tenfold and by dividing the output (9) "Programming Languages," Digital Equipment Corporation, Maynard, Mass.. 1972.
AUGUST 1973
Table I. Least Squares Analysis of Rate Measurements of Synthetic Slopes-Analysis Number Onea Reiative input
Rateb
Fit
Deviation
1 3 5 10 30 50
8600 28307 48041 97750 294667 492082
8657.46 28388.7 481 19.9 97448.1 294760 492073
+57.4551 +81.7110 4-78.9766 -301.875 f93.6875 -96.8750
-1.00000; RMS deviation in rate, f139.173; % error -0.0154404. 20-Second integration time, single result.
C Q1
I
in siope,
Table 1 1 . Least Squares Analysis of Rate Measurements of
75
100 200 300 400
741 0 9928 19755 29635 39487
7433.58 9899.62 19763.7 29627.9 39492.1
i-98.6415; intercept, 4-35.4620; correlation coefficient, -0.999999: R M S deviation in rate, f17.5872; % error in slope, -0.0653. 2.0-Second integration time, single resuit.
Table I l l . Least Squares Analysis of Rate Measurements of Synthetic Slopes-Analysis Number Threea Rateb
Fit
Deviation
1000 2000 3000 4000
9886 19769 29627 39354
9919.68 19745.8 29572.0 39398.3
f33.6777 -23.1 172 -54.9141 4-44.289 1
Table IV. Rate Measurements of 100 mV/sec Synthetic Slopes with Superimposed Sine Wave
0 20 60 60 60 1000 100000
Noise amplitude, volts p-p
0 0.1 0.01 0.1 1 .o
0.1 0.1
Digital readout, countsa
Rei std dev, %
9885
0.01 7
9861 9881 9889 10008 9881 9882
0.25 0.025 0.057 0.1 1 0.030 0.053
Average of 5 results. integration time, 2 seconds. having no added noise. a
I
Q,
Waveforms generated by counter-shift register control mode
the signal-time profile in volts/sec, At = integration period in seconds for counting UP or for counting DOWN, and K is the conversion rate of the voltage to frequency in Hz/volt. The voltage-to-frequency converter should have the highest possible conversion factor (Hz/V) and linearity, since both of these factors will affect the attained accuracy. The maximum counting rate is determined by the tracking rate of the voltage-to-frequency converter. In the normal A/D mode, the readout is given by
R = VAtK
+9826.2; intercept, +93.475; correlation coefficient, a Slope, -.999994; RMS deviation in rate, f40.76; % error in siope, -0.18. 2-Second integration time, single result.
Added noise frequency, Hz
Q2
logic-A/D
a Slope,
I
-
Figure 5.
+23.5791 -28.3808 +a. 77344 -9.07032 4-5.08594
Relative input
n
D
Rei error, % b
... -0.25 -0.04 +0.04 +0.23 -0.04 -0.03 Based on slope
of the DCU by a factor of ten with another DCU. Regardless of the operating mode of the instrument, the software subroutine that services the device remains unchanged. Before entering the routine, however, it is necessary to execute the appropriate 1/0 instruction to select the proper operating mode. Characteristics of the Digital Integrating Ratemeter. It has been demonstrated (8) that the following relationship is true for the type of ratemeter used in this work:
R = SK(At)2 (1) where R = the readout of the ratemeter, S = the slope of
where R and K are as previously defined, V is the voltage input and A t is the conversion time in seconds. As is the case in the Ratemeter Mode, At is equal to ten clock periods. Construction of the Ratemeter. The voltage-to-frequency converter in the Heath Universal Digital Instrument (Model EU-805) is used to convert the analog signal to a pulse train. The scaled output of the Heath 1 MHz time base card (Model EU-800-KC) is used as the time base for the ratemeter. The control logic circuitry is hard wired on a blank, 32 connection Heath card (Model EU50-MD). The integrated circuits are products of Texas Instruments, Inc. (Dallas, Texas). Gates 3, 4, 8, and 10 are type SN7410; all others are type SN7400. All flip-flops are type SN7476; and the DCU is a decade counting unit type SN7490. The counters (Cl-C6) are type SN74193 and the shift registers (SR1-SR3) are type SN74198. The circuits for the counter-shift register section of the ratemeter (Figure 5) are mounted on a printed circuit board which has been cut and fitted with a plastic 32-pin head to mimic a standard Heath card. The 12 most significant bits of the shift registers are connected to pins 1 to 12 of the card top to facilitate transfer of the 12 bit word to the accumulator driver card (Heath Model EU-800-JL) which gates information to the computer accumulator by means of the 1/0patch card (Heath Model EU-801-21). The ratemeter interface was connected to the computer by means of a Computer Interface Analog/Digital Designer, Model EU-801E (Heath Company, Benton Harbor, Mich.). An ASR-33 teletype was used in conjunction with the computer. Evaluation of the Ratemeter. The ratemeter of Ingle and Crouch has demonstrated good rate measuring characteristics with typical standard deviations and relative errors of less than 0.3% for synthetic signals with and
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, A U G U S T 1973
1769
without noise (8). Test results for the instrument used in this work are present in Tables I to IV. The results are in good agreement with those reported earlier. The results presented in Tables I, 11, and 111 are for various ranges of synthetic slopes that were measured by the ratemeter. Each listed rate measurement is a single ratemeter readout of counted pulses. The synthetic slopes were generated by a n operational amplifier wired as an integration circuit. The time constant was approximately one second. No attempt was made to calibrate the slope generator, but rather, the input voltage to the slope generator was varied over a wide range of voltages with the slope generated by a 1-millivolt input to the integrator serving as unity for the relative inputs to the ratemeter. Computer evaluation of linear least squares fits to data in various ranges shows good linearity. Table I11 demonstrates decreased linearity. This may be attributed to the tracking speed of the voltage-to-frequency converter imposing its limitation on the accuracy of the rate determination.
To verify the noise immunity of the ratemeter, sine waves of various frequencies and amplitudes were summed with a synthetic slope of approximately 100 millivolts per second. Table IV presents the test results for these measurements. Good noise immunity is indicated with relative errors and standard deviations generally well below the 0.25% maximum. These results are in good agreement with those reported earlier. The successful application of the ratemeter to chemical analyses in a completely automated reaction-rate system will be described in a later report.
ACKNOWLEDGMENT The authors acknowledge the technical assistance of Gary Speas, Environmental Systems Engineering, Clemson University. Received for review December 13, 1972. Accepted March 20, 1973.
Composite Interference Optics for the Analytical Ultracentrifuge Charles H. Chervenka and Lee Gropper SDinco Division of Beckman Instruments, lnc., PaIo Alto, Calif. 94304
The Rayleigh interferometer of the analytical ultracentrifuge ( I ) forms an optical pattern of fringes by constructive and destructive interference of light passing through two channels of the spinning centrifuge cell. Differences in refractive index in the solutions contained in the cell cause shifts or bending of the fringes in the pattern, and measurement of these changes allows the quantitative analysis of refractive index changes in the cell. In typical physicochemical studies of the properties of a macromolecule in the ultracentrifuge, a dilute solution of the sample is placed in one channel of the cell and solvent is placed in the other. Changes in the distribution of the sample in the cell under the influence of high centrifugal forces can be followed during the experiment by measuring the resulting refractive index changes from the interference patterns formed. The interference patterns formed represent the total of refractive index changes in the centrifuge cell, which reflects changes in concentration of the sample solution plus a contribution due to any inhomogeneity in refractive index of the transparent optical windows used to enclose the cell. In order to make a correction for the contribution due to the windows, it is common practice to repeat a part of the experiment with only water in the cell; the fringes formed then represent a base line of zero refractive index change for the cell contents. We have been seeking ways to include information relative to the base-line fringes directly on photographs of the sample patterns recorded during an experiment. A procedure is described here in which sets of interference fringes
are superimposed on the photographic plate. One useful and direct way to accomplish this is to use a centrifuge cell with three sectorially oriented channels instead of the usual two. If two adjacent channels are filled with solvent and the third with sample solution, for example, then the conditions for interference are met twice during each revolution of the rotor-first when the two channels containing solvent are aligned in the optical system, and second when one solvent channel and the solution channel are aligned. The resulting photographic record is then a composite of the two sets of fringes, containing information about both the base-line conditions and the distribution of sample in the cell.
(1) E G Richards and H K Schachrnan, J Phys C h e m , 63, 1578
( 2 ) A. T. Ansevin, D. E. Roark, and D. A. Yphantis, Anal. Biochem., 34, 237 (1970).
(1959) 1770
APPARATUS AND PROCEDURES The Beckman/Spinco analytical ultracentrifuge was used in this work. The triple-sector concept in its simplest form can be used with no modification of the interferometric optical system of the ultracentrifuge. Only a special cell is required. One configuration of the cell-the unsymmetrical type-uses the standard double-sector cell housing and windows, but requires extra-wide aperture window holders and a special centerpiece as illustrated in Figure 1A. The clarity of the resulting fringes can be improved considerably if a three-slit limiting mask is used in the cell between the lower window and its holder. This mask can be machined from phosphor-bronze stock of 0.025-cm thickness, as shown in Figure l A , with 0.40-cm spacing between slits. Window distortion a t high speeds can be reduced by using polyvinyl chloride window liners, in place of the usual linen-bakelite liners (2). The cell is assembled so that one slit of the mask is over the narrower of the channels, and the other two are over the wider channel. Sample solution is placed in the narrower channel and
ANALYTICAL CHEMISTRY, VOL. 45, NO. 9, AUGUST 1973
