Digital Control of Sample Introduction and Data Readout in Gas Chromatography James E. Oberholtzer Chemistry Department, Lawrence Radiation Laboratory, University of California, Livermore, Calif. 94550,and Department of Chemistry, Purdue University, Lufayette, Ind. 47907l A digital programmer capable of automatically performing a series of injections and controlling data readout has been designed. Digital timing, employing integrated circuitry and driven by a 0.01% tuning fork, accurately controls the time between injections. Triggering the readout system from the same time base permits accurate control of the time of each injection with respect to the start of the readout. In addition, initiation of data readlout can be delayed to reduce the amount of unnecessary base line data recorded. Solidstate switching permits control of electrical drive power with a precision of :kl psecond. When used with a high-speed, pneumatically-operated gas sampling valve, the system permits injections with a precision of &1.1 msecond.
RECENTSTUDIES(1-4)ha.ve indicated that increased separation efficiency as well as reduced analysis time can be achieved with high-speed gas ‘chromatography. Using small-bore Aerogel columns, Hala.’z (1)was able to separate a mixture of 14 hydrocarbons in 60 seconds. He noted a 10-fold increase in theoretical plates per unit time for his column over conventional packed columns. His work was performed at an average linear velocity of about 20 cm/second, well within the region of laminar gas flow. In a theoretical study of chromatography under conditions of turbulent flow, Pretorius and Smuts (2) c:oncluded that gas chromatographic analysis times about one tenth of those obtained under laminar flow conditions should be possible. Recently, Giddings and coworkers ( 3 ) experimentally achieved fullydeveloped turbulence in gas chromatography. After the onset of turbulence, both separation speed and resolution increased simultaneously as flow rate increased. These developments should serve to increase greatly the utility of gas chromatography as an analytical tool. Good resolution at high speed is also a desired feature in processcontrol applications, especially in feed-forward control where analysis lag cannot be tolerated. Full realization of the advantages of high-speed techniques will require more sophis5cated control of sample introduction than is now readily ava Jable. One important consideration is the fact that as retention times decrease, the accuracy and precision with which thi: time of sample introduction can be determined and related to the time base of a readout device become increasingly important. A second consideration is the speed with which the sample can be introduced into the carrier stream. To produce injected slugs of minimum effective volume, approximating a square pulse, it is necessary for the switching transition ‘:o be as short as possible. However, the higher inlet pressures associated with increased flow rates Present address. (1) I. Halasz and 0. Gerlach, ANAL.CHEM., 38, 281 (1966). (2) V. Pretorius and T. W . Smuts, Ibid.,p. 215. (3) J. C. Giddings, W. A. Manwaring, and M. N. Myers, Science, 154, 146 (1966). (4) M. N. Myers and J. C Giddings, ANAL.CHEM., 38, 294 (1966).
make high-speed switching increasingly difficult. This problem has been noted by Giddings and coworkers in both high-speed laminar flow ( 4 ) and turbulent flow (3) studies. The necessity for precise, accurate sample injection in gas chromatography extends beyond the application of highspeed techniques. The broad potential of gas chromatography as a tool for basic physicochemical measurements, most of which are of direct interest to the analytical chemist, has been reviewed by Purnell(5). In such studies, even lowspeed chromatography is capable of producing data in a fraction of the time required by classical physical methods. Once the variations of temperature and carrier flow rate have been minimized by proper thermostating and flow regulation, sample injection and retention time measurements remain as major factors that limit the precision and accuracy of the data. These sampling requirements necessitate the use of a fast, reproducible valve, as described by Kieselbach (6), or some other power-driven device capable of millisecond switching precision. A programming system capable of equally precise operation would permit not only greater accuracy in measurements of retention times but also automation of sample injection. The precision and accuracy of timing inherent in digital counting circuitry has been discussed by Booman (7). The recent availability of low-cost, well-characterized integrated circuit modules permits a highly accurate digital timer to be constructed at a cost not prohibitively far above that of a conventional timing unit. The purpose of this paper is to describe the design and evaluation of a digital control system, employing integrated circuitry, capable of millisecond injection precision. INSTRUMENT DESIGN
The control system consists of a digital counting circuit which produces an initial pulse synchronized with the start of the counting system followed by a variable, preset number of pulses at a preset time interval. The output pulses are then amplified through a drive system which controls the injection valve(s). Digital Logic Description. A block diagram of the programmer timing logic is shown in Figure 1. Detailed descriptions of control-gate circuitry and preset decade counter circuitry are presented in the Appendix. The programmer is driven by the 400 f 0.04 Hz output of a tuning-fork-controlled oscillator which provides the timing control for a commercially-available digital data acquisition system (Model CRS-30D, Infotronics Corp., Houston, Texas). The flow of timing pulses to the subsequent counting circuitry is controlled by a two-input gate driven by the output of the “run” flip-flop. The voltage-level change which effects opening of the control gate is differentiated by a simple R-C ( 5 ) J. H. Purnell, Endeacour, 23, 142 (1964). 35, 1342 (1963). (6) R. Kieselbach, ANAL.CHEM., (7) G. L. Booman, Zbid.,38, 1141 (1966). VOL. 39, NO. 8, JULY 1967
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~ K X ) Hc ~
CONTROL GATE
1
START
RUN FLIP-FLOP
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2 FLIP-FLOPS
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+4
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DECAM
DECADE
COUNTER
COUNTER
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3 DECADE DELAY COUNTER
DELAYED PULSES
I DECADE READOUT DELAY
START READOUT PULSE
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2 DECADE CYCLE COUNTER
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L
logic
To PULSE
Figure 2. Schematic diagram of drive circuitry
B1- silicon bridge rectifier, GB AlOllBBlADl
Cl- timing capacitor (see text) Di,DO,D8 1N914 diode Zl dual two-input gate, Fairchild pL-914 ZO - J-K flip-flop, Fairchild pL-926 la- buffer, Fairchild pL-900 L1- air solenoid coil, Skinner V53ADA2100 R1- timing resistor (see text) Ro,Rs mercury-wetted relay, SPDT, Clare HGSMlOlO S1, SO,S3- SPDT toggle switch SI- pushbutton switch, momentary SPST, normally open SCRl silicon controlled rectifier, GE C20C T I ,T3,T, - 2N2219 transistor T2 - 2N3823 field-effect transistor
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-
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-
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ANALYTICAL CHEMISTRY
network, and, after buffering, the resulting pulse, TO, is available to the valve dr ve circuitry. The clocked set-reset facility of the “run” flip-flop permits synchronization of the program start with the time base without additional gating. Two flip-flops divided the time base down to 100 Hz; the two decade counters provide frequencies of 10 Hz and 1 Hz, respectively. A front-panel range switch selects one of the above three frequencies for further division by a three-decade, preset coimter. The range switch permits full scale delays with respect to Toof 9.99, 99.9, and 999 seconds. The three digital thumh switches which monitor the binarycoded-decimal outputs from the decade counters permit resolution of 0.1 full scale. After shaping and buffering, the delayed pulse is available to trigger the injector drive circuitry. In addition, the delayed pulses are fed to a two-decade preset counter. After the preset number of delayed injections has been performed, the output of the cycle counter clears the “run” flip-flop, and stops the flow of pulses to the counting circuitry, thus ending the program. A one-decade preset counter delays the start of data readout until after a preset number of delayed pulses have occurred. This feature partially eliminates the large amount of base line data produced by a high-resolution readout device while preserving the overall ti me accuracy of the data. Valve Drive Circuitry. In digital programming systems, consideration must be given to the drive circuitry which interfaces the digital logic output to the physical device which, in turn, effects the actual control. In order to take advantage of the speed arid timing precision of digital control, the actuated valve must be switched very rapidly, thereby demanding multiwatt power levels in most cases. Since the power output available from integrated circuitry is in the low milliwatt region, the drive circuitry must provide amplification of the order of one thousand. As in linear amplification a reciprocal gain-bandwidth relationship must be recognized when designing drive systems. The power requirements for switching a given valve in 1 msecond may demand drive circuitry having a transition time of several milliseconds. Clearly, overall bandwidth matching is necessary. Likewise, the noise inherent in high-speed, high-power electrical switching must be recognized. The relatively low (300 mV) noise immunity of integrated circuitry makes it quite susceptible to both radiated and conducted radio-frequency interference (RFI). Other sensitive instruments, such as the elestrometer of a gas chromatographic ionization detector may make additional R F I suppression necessary. A schematic diagram of the drive circuitry for one gas sampling valve appears in Figure 2. The drive circuit for the second one is identical. Either the initial TOpulse or the delayed pulses, or both, may be selected by closing front panel switches Sz and S1, respectively. The two pulse lines are combined in one gate of Zl; subsequent inversion produces the desired “or” function. Transistors Tl and Tzare connected in the familiar “delay one-shot” configuration. The width of the output pulse from the “one-shot” is determined by the R1-Cltime constant, and the width controls the length of time the sampling valve is held in the energized, inject position. Depending on the size of the gas-sampling loop and carrier gas flow rate, injection times approaching one second may be necessary for complete purging of the sample loop. The low input impedance of a convenl ional transistor in position Tz would put an upper limit on the usable value of R1 and make it difficult to achieve the long reproducible delays necessary. However, the high inpul impedance of a field effect transistor (FET) raises the usable value of R1 into the megohm region and makes delays for milliseconds to minutes easily obtain-
Figure 3.
Injection precision of pneumatic gas sampling valve
Response of flame-ionization detector connected directly to valve. Sweep delayed 0.4 second after triggering of air solenoid. Base line displaced 1 div. between each of 5 replicate injections. Horizontal scale, 10 msec/div.
-Ll-
eration caused by the large amount of RFI produced by contact arcing, However, the use of a silicon controlled rectifier (SCR) effects this switching with minimal noise. In order to isolate the high- and low-power switching components, the SCR gate current is switched by a mercury relay driven by transistor T3. Since the SCR gate current is only a few milliamperes, no noise problems were encountered in the use of relay for this purpose. The SCR is incorporated in a silicon rectifier bridge circuit to permit solenoid pull-in regardless of power-line polarity. The bridge circuit also assures reliable SCR turn-off, a problem often encountered when switching inductive loads with simpler SCR configurations (8). The choice of pneumatic power, rather than electrical, for operating the sampling valve was dictated mainly by availability of the former. However, the noiseless characteristics of pneumatic drive would make it the logical choice even if both types had been available. Electrical switching circuitry to control the pneumatic power can be shielded and located well away from sensitive ionization detector circuitry. The pneumatically actuated valve may then be located where desired. The possibility of detector noise due to vibration of high impedance cables caused by the valve actuation is the only factor that determines the necessary valve-detector separation. Data Readout Control. The circuitry for control of data readout is included in Figure 2. Switch SQselects either the Topulse for readout synchronized with the start of a program cycle or the start readout pulse for delayed initiation of readout. The selected pulse causes pin 7 of flip-flop Zzto fall to a logical “0”. The buffer element, Z3, inverts the flip-flop output and provides enough current gain to drive the base of relay driver, T4. Once initiated, readout continues even after the program has ended. Momentarily closing the stop readout switch, S4, causes the flip-flop output to rise to a logical “1.” Buffer drive of T4 is removed and relay Rz opens, turning off the controlled device.
Direct relay switching of the power to L1,the Coil Of an air solenoid, was impractical due to spurious programmer op-
(8) General Electric CO.,Auburn, N. Y., “Silicon Controlled Rectifier Manual,” 3rd ed. (1964).
au1e.
VOL. 39, NO. 8, JULY 1967
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INSTRUMENT CONSTRUCTION
All circuitry, with the exception of the 115-volt valve drive components, was contained in a universal rack mount designed for 8.5- X 4.5-inch printed circuit boards and equipped with hinged front and rear panels (Elco Corp., Willow Grove, Pa.). In the prototype programmer, no difficulty was experienced in mounting integrated circuits and discrete components on the same circuit board. However, crosstalk problems between high- and low-level logic were experienced in the wires connecting the printed-circuitboard connectors with the front-panel controls. By lacing the low-level wires together and using a multi-conductor shileded cable for the high-level signals, those problems were eliminated. The isolation problem would have been easier if all high-level circuitry had been confined to a separate circuit board. Power for both high- and low-level logic was provided by a Redule Model UPM-1 dual regulated power supply (Power Designs, Inc., Westbury, N. Y.). Because the supply is actually two separate power supplies on one chassis, no problems of interference between the two levels of logic were experienced. No power supply decoupling was necessary in the relatively small digital circuit of the programmer. The air solenoids, bridge rectifiers, and SCR’s with their gate-control relays were mounted in a small steel utility box for maximum RFI suppression. INSTRUMENT EVALUATION
The high switching speed of integrated digital circuitry makes an evaluation of switching delays necessary only in relatively complex circuits operating at very fast repetition rates. However, delays in drive-circuit switching can become significant if high drive amplification is involved. With proper engineering, drive delays are relatively constant, and, with appropriate corrections of the data, their effect can be made negligible. Electrical Drive Precision. The mercury relays used in firing the SCR’s have a pull-in time of several milliseconds. However, because the readout time base is established by a similar relay, much of the pull-in delay should be cancelled. The remaining delay of importance is made up of any difference between relay pull-in time plus the time required to fire the SCR. This delay was evaluated by using a Tektronix Model 536 oscilloscope equipped with a Type D differential vertical input amplifier plug-in (Tektronix, Inc., Portland, Ore.). The oscilloscope sweep was initiated by a 6-volt battery connected through the readout relay contacts to the oscilloscope trigger input. The differential vertical inputs were connected across the air solenoid. The delay between start of data readout and firing of the SCR was found to be 83 =t1 psecond for six measurements. The magnitude and precision of the delay remains constant, regardless of whether the TOor the delayed pulses are used to trigger the drive circuitry. For chromatographic operation, that delay is insignificant and may be neglected. However, because it is quite precise, a reliable correction can be made if other applications warrant it. Sample Introduction Precision. A factor of vital concern to the chromatographer is the precision with which he can put a slug of sample onto the chromatographic column. In order to evaluate that precision as realistically as possible, a Seiscor Model VI11 gas sampling valve (Seismograph Service Corp., Tulsa, Okla.) was connected directly to the flame ionization detector of an Aerograph 660 chromatograph (Varian-Aerograph, Walnut Creek, Calif.) equipped with a single-channel electrometer. The Seiscor valve was chosen because of its fast, 10-msecond-rated, switching time. The valve sample inlet was connected directly to the laboratory gas line. Because the gas-line pressure was only a few psi, 962
ANALYTICAL CHEMISTRY
the valve diaphragm was operated against a vacuum, according to the manufacturer’s recommendations. The nitrogen carrier gas pressure was regulated by a Negretti and Zambra Model R/182 NC precision pressure regulator (Negretti and Zambra, Ltd., Stockdale, Aylesbury, Bucks, U. K.). The pneumatic drive was nitrogen at 32 psi from an ordinary cylinder regulator. A 7-micron sintered filter was placed between the air solenoid and sampling valve to remove particulate matter from the drive gas. Similar filters were placed in the sample and carrier streams. Detector response was read out on the oscilloscope, triggered in the same manner as described above. The sampling valve was actuated by the Topulse. The oscilloscope sweep was delayed by use of the delayed data readout facility of the programmer so as to provide increased time resolution. The timing precision of the injection system is illustrated in Figure 3 by the detector response curves for five replicate injections. For clarity, the base line was changed one vertical division between each injection. The plateau at the end of each rising trace is due to saturation of the detector by the large (approximately 50-pl) samples. Because of the long rise time of the curves, an arbitrary point of reference three divisions above base line was chosen for measurements of timing precision. The five curves reached that arbitrary point with a standard deviation of 1.1 msecond. A similar precision was obtained regardless of the point of reference, indicating that the curve shape was quite reproducible. Even though it was reproducible, the slow rise time of the curve was undesirable because it indicated a non-square sample slug that would result in broadened eluted peaks. The slow rise time is assumed to result from the slow response of a plastic diaphragm in the valve for the following reasons. An analysis of detector time constant and longitudinal diffusion in the valve-detector tubing showed that these factors could not have produced the 60-msecond rise time. Evaluation of the pneumatic drive system indicates that the valve should indeed switch in the rated 10-msecond period. However, in the Seiscor valve, the actuated piston does not actually effect the switching; it merely moves away from a plastic diaphragm which blocks various inter-port connections following which the diaphragm moves away from its blocking position and actually effects the switching. This last step appears to be the slow one which causes the diffuse slug front. A cursory examination of the effect of pneumatic drive pressure on the injection delay was made. Over the range of 20 to 40 psi, a decrease in injection delay with increasing drive pressure of about 1.5 msecond/lb was noted. With only modest regulation of drive pressure, the use of a sampling valve with positive openings of ports should permit injection precision better than the 1.1 msecond obtained in this work. Positive switching would result in a sample slug shape much closer to that of the ideal square pulse. DISCUSSION
In its simplest mode of operation, the programmer permits accurate synchronization between a single injection and data readout, In that mode, various combinations of delay time and readout delay may be used for maximum flexibility of the readout delay facility. By operating a single valve triggered by both Toand delayed pulses, a present number of unattended injections can be performed. By choosing the injection delay so that the eluted peaks are no farther apart than necessary to establish a base line, a series of injections can be performed in minimum time. Such spacing also reduces the amount of useless base line data produced. In that mode, the flexibility in delaying the start of data readout is limited, of course, to multiples of the injection delay. By making multiple injections from an exponentially decreasing sample concentration from a gas
-L 4V I
Cycle Counte r
Figure 4.
Zl,
I
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To Drive, Circuitry
Schematic diagram of digital timing circuitry
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1 2 , 13, 113 dual two#-inputgate, Fairchild pL-914 la, Z5, le J-K flip-flop, Fairchild pL-926 Z,, Z8, Zlo decade couizter, Fairchild CpL-958
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ZIl four-input gate, Fairchild pL-907 ZI2 buffer, Fairchild pL-900 P D 2 ,PD3 - preset decades identical to IiO-s3 (see text) SI pushbutton switch, momentary SPST, normally open Sz 3-position, single-pole rotary switch, shorting S3 10-position digital thumbswitch, 1-2-4-8 BCD terminals, EECoSWITCH Model 228 (Engineered Electrons Co., Santa Ana, Calif.)
dilution flask (9), retention time as a function of sample size can be quickly determined. Because overall timing accuracy for a series of peaks can be maintained to about 1 msecond, coherent addition of replicate chromatograms for signal/noise enhancement should be possible. The advantages of this technique have been demonstrated in spectrometry and may permit the gas chroniatographic determination of even smaller sample concentriitions than now possible. The use of two gas sampling valves and a dilution flask as a means of regulating sample concentrations has been discussed (9). Because the, programmer is capable of regulating two sampling valves, control of sample size would be quite simple. One valve, actuated by the Topulse, would inject a slug of pure sample into the dilution flask. The second valve would be programmed to make one injection from the dilution flask effluent at a preset, later time. Sample size could then be varied by simply varying the programmer delay. Thus we see that despite its relatively simple logic circuitry and small number of operating controls, the present digital programmer is surprisingly flexible. Regardless of the mode (9) C. H. Hartmann and K. P. Dimick, J . Gas Chromatog., 4, 163 (1966).
of operation, the accuracy of determining the time coordinate of a chromatogram is limited only by the accuracy and resolution of the readout device down to the 1.l-msecond injection limitation. Because of the ease of interconnection of the various integrated circuit modules, construction of digital control for various applications is fast and simple. With modest attention to noise reduction and circuit loading, reliable operation can be achieved. In the resistor-transistor logic (RTL) used, loading calculations are simplified by using “units of drive” ratings. (Outputs are rated in integral units of drive supplied, and inputs in units of drive required.) A quick glance can determine if a given output has sufficient drive for the various inputs into which it is connected. The low cost of integrated circuitry, especially the plastic encapsulated lines recently introduced, permits duplication of components for flexibility without inordinately large amounts of complex switching. For flexibility, the use of patchboard programming, as described by Booman (7), is feasible, but the possibility of crosstalk must be recognized if high-level logic is to be controlled on the same patchboard. Digital timers are not the only source of pulses for chromatographic control. In column, detector, and fractionVOL. 39, NO. 8, JULY 1967
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collector switching applications, timed control demands constant peak retention times. In actual laboratory systems, that criterion may be difficult to meet. However, the digital chromatographic integrator, currently in use in many laboratories, includes sophisticated, fast-acting, digital peak-detection circuitry to control the digital integration process. Use of that circuitry to control fraction collection, as well, would permit collection of very closely eluted peaks with no interference from slight changes in retention time. Addition of simple integrated counting circuitry would permit accurate switching of desired peaks or a series of peaks to either a second column for futher separation or a second detector for optimum response. Hupe and coworkers (IO), using a specially-designed peak detector, have demonstrated the feasibility of such an approach in an automatic fraction collector for preparative gas chromatography. The fast operation inherent in digital circuitry permits application of most column and detector switching schemes currently in use to the high-speed chromatographic analyses desired in process control. ACKNOWLEDGMENT
The informative discussions of digital circuitry with Eugene Fisher are appreciated very much. The support and encouragement of L. B. Rogers and Jack Frazer contributed greatly to this work. APPENDIX
The functions of pulse gating and counting, common to all digital control systems, can be illustrated with the rather simple circuitry of the chromatographic programmer shown in Figure 4. Typical logic levels for the silicon resistortransistor logic (RTL) used are 0.2 volt and 1.8 volts. In the following discussion, these levels will be designated “low” and “high,” respectively. The 70-psecond-wide, 0- to - 8-volt time-base pulses are adapted to the positive voltage levels of the integrated circuitry by the input R-C differentiating network. Because high frequency noise on the input is enhanced by the differentiation, the R-C time constant must be long enough to prevent spurious triggering of the counting circuitry. Gate ZIA,like all other RTL gates, regardless of number of inputs, produces the “NAND/NOR” function (output “high,” if, and only if, all inputs are “low”) of the input signals. This gate squares and inverts the differentiated pulse to produce the positive-going pulse shown. After a second inversion, the pulse arrives at the control gate, &A. A “high” level on pin 5 of this gate blocks the pulse from entering the counting stages when the programmer is in a standby condition. Opening of the control gate is synchronized with the time base by use of the “clocked” inputs of the “run” flip-flop, 1,. When pins 2 and 4 of the flip-flop are “high,” the clock pulses arriving at pin 3 have no effect on the output at pin 9. However, after SIis closed, the next negative-going voltage change causes the flip-flop to change state, and the resulting “low” output opens the control gate, ZzA. The complimentary output (pin 7) of l a , which goes “high” when the gate is opened, is differentiated to produce the TO pulse.
(10)K . P. Hupe, V. Busch, and W. Kuhn, J. Gas Chromatog., 3, 92 (1965).
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ANALYTICAL CHEMISTRY
Succeeding clock pulses have no effect on the flip-flop regardless of the condition of SI. For correct timing, the clock pulse which triggers 1 4 must not reach the counting stages. Because the propagation delay through the flip-flop is at least 10 nseconds greater than through gate ZIB,triggering the flipflop at the end of the clock pulse prevents this error. By grounding both pin 2 and pin 4 of flip-flop Z5, each negative-going voltage change causes the flip-flop to change state. This “toggling” mode effects division of the 400-Hz input pulse train by two. Further division by 16 produces a 100-Hz output. The decade counter module, 17, contains the four flip-flops and associated gating necessary to divide an input frequency by ten. Unlike the flip-flop, the decade counter is triggered by a positive-going voltage. By using the complimentary output of 16 (pin 7), which is “high” when the counting units are reset, the first count is registered in Z, after the first four clock pulses have occurred. The 10-Hz output of Z7 is again divided by ten by Is. The major source of flexibility in the programmer is the 3-decade preset counter which divides the frequency selected by SZby any number from 1 to 999. The ZIO-S~ combmation determines the least significant decimal digit in this divisor. Identical circuits, PDz and PD3, determine the other two digits. The contents of the decade counter, h o , is continuously available in 1-2-4-8 binary-coded-decimal (BCD) notation, with a “low” output indicating presence of a given BCD bit. A rotary thumbswitch, Sa, connects the proper BCD combination for a given decimal digit to its common output, C. The diodes in the BCD lines provide isolation between the various outputs of Zlo, while the resistors are necessary for impedance-matching between ZIOand RTL gates. Terminal C remains “high” until Zlocontains the number of counts specified by Sa, at which time it goes “low”. The outputs from the three decimal stages are combined in GI. For reliable operation, the preset counter must be allowed to settle before the decision is made as to whether the preset count has been reached. Since the counting circuitry is pulsed by the front edge of the clock pulse, applying the output of ZaA to Zlldelays the decision until the end of the clock pulse. Because the Topulse was produced at the end of a clock pulse, synchronizing the output of ZIIwith the end of the pulse is necessary for accurate timing as well as for reliable counting. Applying the output of 14 to Ill prevents spurious outputs when the preset counter switches are being set prior to a program cycle. The Z i T Z 1 3 combination illustrates construction of a “oneshot” with integrated circuitry. The output of buffer 2 2 provides sufficientcurrent to reliably reset all previous counting stages as well as to triger the cycle counter and drive circuitry. In “one-shot” applications requiring less drive, the buffer may be replaced by a conventional gate. Use of a dual two-input gate permits the entire “one-shot” to be contained in a single module.
RECEIVED for review March 6, 1967. Accepted April 21, 1967. Presented in part at the Eighteenth Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1967. Work performed as part of Contract W-7405eng-48 with the Lawrence Radiation Laboratory under the auspices of the U. S. Atomic Energy Commission. The support of the U. S. Atomic Energy Commission under Contract AT(11-1)-1222 is also acknowledged.
