uids of low volatility but with highly volatile ether, greasing the stopper greatly enhanced evaporation, presumably by preventing the stopper from reseating properly after rising due to internal pressure. Storage of samples in a desiccator containing the same solvent greatly minimized evaporation and/or transpiration losses, especially in the case of the volatile organic solvents. Some gained weight. For example, in polyethylene, 1211 H N 0 3 showed a 1.6% gain. Time was allowed for evaporation of solvent condensed on the exterior of the container, but part of the gain could still be attributed to specimens condensate. The weight gain on the 12M "03 undoubtedly includes gain due to absorption of moisture. It was noted earlier that the 12M H N 0 3 specimens in open air also gained weight at the outset. Because of the small changes on most of the containerwater and container-"03 combinations, the acid and cerium(II1) titration data did not add materially to the interpretation of the weight data. In general, when a signifi-
cant weight loss was observed as in the case of water and 12M H N 0 3 in glass bottles with standard glass stoppers, a corresponding increase in the concentration of cerium(II1) or HNOJ was observed. The results of this study provide some valuable guidelines for the storage of liquid reagents. Evaporation and transpiration should be anticipated and when necessary, provisions should be made to minimize their effects. When small quantities of liquids are involved, the relative loss and corresponding concentration change can be highly significant. Containment in sealed glass ampoules has been found to be the most satisfactory packaging procedure at this laboratory. When this is not feasible, packaging in high density polyethylene or polypropylene or glass bottles fitted with tight sealing caps is usually satisfactory for periods up to a t least 1year. Received for review July 28, 1972. Accepted November 29, 1972.
Low-Cost integrator with Digital Character Stephen R. Pareles' Analytical Services, Aarlab, lnc.. Jersey City, N.J. 07307 The advantages of digital electronic integrators in the analytical laboratory are well established. Their most frequent application is quantifying gas chromatographic effluents. Unfortunately, their cost continues to be prohibitive for most small laboratories. Less versatile, but relatively inexpensive short-term analog integrators using one or more operational amplifiers have been described in detail for several years ( 2 - 4 ) . A typical circuit is shown in Figure 1. A chopper-stabilized operational amplifier integrator capable of accurate and precise long-term integration was described by Harrar and Behrin ( 5 ) . Recently, low-drift chopper-stabilized operational amplifiers have become available in hybrid form for less than $100. Most recent still are FET-input, low-drift "chopperless" operational amplifiers available for less than $50. Manufacturers are Burr-Brown, Analog Devices, Teledyne-Philbrick, Bell and Howell, Analogic, Datel, and others. A long-term integrator of excellent precision and accuracy built from an FET-input operational amplifier was recently described in detail by Kendall(6). Kendall used a Burr-Brown Model 3420K operational amplifier with a 1000 ohm input resistor and a 1 mF (200V polystyrene) capacitor in the feedback loop. The sensitivity of the integrator so built was thus
L"Figure 1. Typical operational amplifier integrator Null provision varies with manufacturer
R E S E T RELAY
Eo(V)/Eln(V-sec)= 1/RC = 1000 Linearity and precision of better than 1% were achieved I
I
l P r e s e n t address, D e p a r t m e n t of F o o d Science, Rutgers-The S t a t e U n i v e r s i t y , N e w B r u n s w i c k , N.J., 08903. (1) C. N. Reiiiey, J. Chem. Educ., 39, (11) A 8 5 3 ; (12) A 9 3 3 (1962). (2) H. V. Maimstadt and C. G. Enke. "Electronics for Scientists," W. A. Benjamin, New York, N. Y., 1963, p 356. (3) J. R. Barnes and H. L. Pardue, Anal. Chem., 38, 156 (1966). (4) "Operational Amplifiers, Design and Applications," Burr-Brown Publications, Tucson, Ariz.. 1972. ( 5 ) J . E. Harrarand E. Behrin,Ana/. Chem., 39,1230 (1967). (6) D.R. Kendall, Anal. Chem., 44, 1109 (1972).
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A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. u, M A Y 1973
ANALOG OUTPUT " % over last reset''
DIGITAL OUTPUT
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Figure 2. Functional diagram of automatically resetting operational amplifier integrator
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for the integration of 1-2 mV peaks of several seconds duration. Although the linearity and precision of this circuit are sufficient for gas chromatographic work, a drawback of the arrangement is the integrator will not function after its maximum output voltage Emax is reached. This is typically 10-15 V, depending on the supply voltage. The operator must reset the device by short circuiting the capacitor before Eo,max is reached to avoid discontinuity of integration for larger than expected peaks.' For example, if Eo,maxwere 10 V and Ein were 10 mV, the integrator would have to be reset before the end of 1 sec and Eo a t reset recorded. One obvious way around this annoyance is to reduce the integration constant 1IRC with larger values of C and/or R. Difficulty is now encountered in discerning the small voltage changes associated with the integration of small peaks. An expensive digital voltmeter would supply greater visual resolution than would an inexpensive panel meter, but we avoid this solution in the circuit described in the present paper. PRELIMINARY APPROACH A solution of the duration and resolution difficulties should be obtained by a circuit that automatically triggers a device short circuiting the capacitor and resetting the integrator whenever a preset value of Eo is reached. One such method uses an electromechanical relay (7). The relay, closed to discharge the capacitor, reopens as soon as E A drops below the preset voltage. The number of resets can be counted electrically. The integrator thus takes on limited digital character. The readout device is still used, but only to interpolate between counts or digits. Figure 2 is a block diagram of this arrangement. The resetting capability must be frequent enough and, consequently, the digital count for a peak great enough, that the inexpensive panel meter used to read the excess over the last reset or digit is only reading a fraction of the total value. In the process of arriving at the circuit design described forthwith, we evaluated a relay resetting device in our laboratory. Using a digital voltmeter to monitor the analog output Eo a frequency counter to monitor the number of resets, we found an accuracy and precision equal to the unmodified circuit already described for reset frequencies less than 1 Hz but found serious nonlinearity for 1 Hz and ( 7 ) L. P Morgenthaler, MPI Notes, 2, 15 (1967).
above which may have been caused by a combination of the following: relay activation time lag, contact bounce, capacitor hysteresis, power surge associated with relay operation, change in temperature within the cabinet due to heating of the relay coil at higher frequencies. These factors caused resetting to occur a t inconsistent output voltages Eo and nonreproducible Eo after reset a t frequencies 1 Hz or greater. We could not accept this low reset rate and still avoid the requirement of a digital voltmeter. SOLID STATE SOLUTION The present paper describes a simple and novel solidstate revision of this automatic reset concept through the use of a silicon controlled rectifier (SCR) and peripheral components to obviate all of the difficulties previously described (Figure 3). The modified integrator can reset a t frequencies as high as 100 Hz. No measurable imprecision or inaccuracy is introduced to the basic analog integrator by the extra circuitry described. The arrangement is compatible with TTL logic, in particular, a digital display described in Figure 4. Although this device has a great deal of digital character, it is still fundamentally an analog device, and the inexpensive panel voltmeter can now be used conveniently to interpolate between counts when desirable. Moreover, with this analog capability still intact, the high counting frequencies (1 to 10 kHz) and subsequent pulse-scaling employed in expensive, purely digital integrators is avoided. CIRCUIT DESCRIPTION In principle, the circuit operates as indicated in Figure 2. Integration of Ein (negative) proceeds until the threshold Eo (positive) is reached. Comparator 1, sensing that the reference value has been equalled, swings into positive saturation, biasing transistor Q into conduction. Q powers the gate of the SCR. The SCR conducts in one direction, short-circuiting capacitor C without hysteresis. Comparator 1 swings into negative saturation, shutting transistor Q off as soon as Eo falls below the reference Eo and in fact before the SCR shuts off. The SCR independently shuts itself off when the capacitor reaches a reproducible ground state close to a noncharged condition (8). The speed or slew rate of the 709 comparator is rated at (8) "SCR Manual," 4th ed, General Electric Co., Chicago, Ill., 1967 A N A L Y T I C A L C H E M I S T R Y , VOL. 45, N O . 6, M A Y 1973
999
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0.25 V/psec. The speed of transistor Q and the SCR are rated in nanoseconds. The discharge time of the capacitor, or speed of reset, can be calculated from the value of C and the resistance between the anode and cathode of the SCR. This falls in the sub-microsecond range. These considerations lend theoretical reinforcement to the lack of detectable error introduced to the basic integrating operational amplifier by this modification for frequencies less than 100 Hz. The device was not tested a t higher frequencies. Comparator 2 functions to produce 1 pulse per reset cycle of the integrator. It is set a t approximately 0.5Eo,rer of comparator 1 so that a pulse is produced of sufficient duration to be counted by the subsequent TTL logic circuitry. This second comparator is necessary because the output pulse of comparator 1 was of insufficient duration to drive the logic. Both comparators were preceded by an isolation amplifier as shown to increase impedance seen by the output of the integrating stage and to drive the meter. Theoretically, a series resistor to each input of the operational amplifier comparators should reduce the need for the isolation amplifier that precedes them, but this was not tried. Some applications of the integrator might require the high input impedance offered by a good quality isolation amplifier placed before the integrator as well or even a stable preamplifier. A low-drift operational amplifier equivalent to BurrBrown Model 3420K, as used by Kendall, is desirable for the integrator although an inexpensive 741 operational amplifier, available for less than $1, will suit some less precise applications of this circuit, such as low-speed voltage-to-frequency conversion. We recommend trial assembly with an inexpensive operational amplifier in any case. Comparator 1 was referenced a t 10 V and comparator 2 a t 6 V. Values of C and R of 1 mF and 1 kohm were found
1000
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 6. MAY 1973
convenient for signals from 1 to 1000 mV produced by the thermal conductivity detector of a Beckman GC-5 gas chromatograph. It should be noted that the circuit described is set up to integrate signals that are negative with respect to the instrument ground. All the operational amplifiers use a well regulated A15 vdc supply. The TTL display requires +5 vdc. These supplies are easily built or purchased. The modification of the basic Operational amplifier integrator circuit cost about $50. All IC's peripheral to the integrator are available from Texas Instruments, Inc. (9) and hobbyist sections of electronic shops, as well. SUMMARY
A low-cost modified operational amplifier integrator with large digital capability has been described. Using a low-drift operational amplifier a t the integration stage, and values of C and R indicated, a precision and accuracy better than 1% is obtainable for peaks ranging between 1 and 1000 mV and up to 10 sec duration. This surpasses the performance of the typical ball-disk integrator (6) which costs several times as much. Commercially available purely digital integrators report precision and accuracy of 0.2% but cost well over $1000. ACKNOWLEDGMENT.
The author would like to thank Roy I. Edenson for his technical suggestions. Received for review October 5, 1972. Accepted February 2, 1973.
(9) "Integrated Circuits Catalog for Design Engineers," 1st ed, Texas instruments, Springfield, N. J., 1971.
