cell; however, the absolute limit of detection is quite good with the novel front surface cell because of the need of only about 150 MI of solution as opposed to several milliliters of solution in the standard 1-cmz cells. The major advantages of this new front surface cell d o not rest with detectability but rather with simplicity of filling, durability, adaptability to existing instruments, usefulness with turbid and highly colored solutions, and usefulness with pastes (in this case, the front surface screen is removed to allow easy filling and filling of the sample cell cavity).
ACKNOWLEDGMENT
The authors acknowledge the assistance of Arthur Grant and D. J. Burch in helping to design and construct the front surface cell. RECEIVED for review April 7 , 1972. Accepted May 25, 1972. Research carried out as a part of a study on the phosphorimetric analysis of blood and urine, supported by U S . Public Health Service Grant G M 11373-09.
Triac Switching Circuitry for Eliminating Interfering Transients in Digital Logic Automated Systems E. S. Iracki,’ M. B. Denton,Z and H. V. Malmstadt School of’CiieinicalSciences, Department of Chemistry, Unicersity of’lllinois at Urbanri-Ciiat,ipc~i~n, Urbana, Ill. 61801
THEUTILIZATION OF DIGITAL LOGIC (1-3) and computers (4-6) for automation of analytical instrumentation is continuously increasing. Quite often, it is necessary for the associated circuitry to switch various 120-volt ac line operated devices, such as solenoid valves, motors, etc. When logic driven relays and motor-driven cam operated switches are employed, transients are often encountered which interfere with both digital and analog electronic systems. These transients are the result of arcs between opening switch contacts caused by energy stored within inductive loads. A triac is useful for switching ac loads; it ceases conduction at zero current, causing the circuit to be opened at the zero voltage crossing point. This results in no energy being stored and no transients are generated (7). Circuit. Figure 1 gives a circuit of four triacs and associated circuitry which can be driven by the popular TTL (zero, and +5 volt) current sinking logic levels. Any number of these switching circuits can be added together as long as the isolation transformer is capable of handling the combined load. This form of isolation was most convenient for the authors because of the “spare parts” availability of the transformer; however, one might wish to utilize photoconductors ( I ) or other devices in alternative isolation techniques. The basic simplicity of the transformer isolation technique renders its use rather appealing; however, the cost of devices with higher volt-ampere ratings (e.g., several kilovolt-amperes) might preclude their use in a designer’s circuit. Operation. When the input is a t ground level (zero logic level) the 2N5138 transistor is forward-biased, causing its collector to be positive; and the triac gate diode is reversebiased. Therefore, the triac does not conduct. When a +5 volt logic level is applied, the 2N.5138 is no longer forwardbiased and the collector approaches -15 volts. The gate 1 Present address, Environmental Systems Engineering, Clemson University, Clemson. S.C. * Present address, Department of Chemistry, University of Arizona, Tucson. Ariz.
(1) H. V. Malmstadt and C. G. Enke, “Digital Electronics for Scientists,” W. A. Benjamin, New York, N.Y., 1969. (2) H . V. Malmstadt and C. G. Enke, “Computer Logic,” W. A. Benjamin, New York, N.Y., 1969. (3) J. S. Springer, AXAL.CHEM., 42 (8), 22A (1970). (4) J. W. Frazer. ;bid.. 40 (8), 26A (1968). ( 5 ) G. Lauer and R. A. Osteryoung, ;bid., 40 (101, 30A (1968). (6) S. P. Perone, ihitl., 43, 1288 (1971). (7) “Silicon-Controlled Rectifier Manual,” 4th ed., General Electric, Syracuse, N. Y . , 1967. 1924
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Figure 1. Schematic of triac switching circuit diode is forward-biased and the triac conducts. The SC45B triacs are rated a t 10 anipercs, and loads LIP10 this valuc may be applied on any individual circuit. However, total power consumption should not exceed the rating of the isolation transformer. Transformer TI is an isolation transformer to prevent the ground level of the logic system from becoming 120 volts ac. This device should not be operated without some form of isolation as severe electrical shock could easily result. Construction. The unit was constructed on a 7- X 9- X 2-inch aluminum chassis. A small front panel holds ChinchJones 300 series plugs for connecting the solenoids. Special plugs are recommended to prevent someone from rashly plugging in a large load. Banana plugs are used for the inputs. For general use, it is suggested that a cover of some type be placed over the triacs and output plugs to prevent any possible shock hazard. Application. Any source of TTL logic levels can be used. The authors have found two types of driving circuits particularly useful. When a load is going to bc turned on for a fixed time up to approximately 40 seconds, a monostable such as the SN74121 (Texas Instruments, Inc., Dallas, Texas 75222) or the Heath dual monostable multivibrator card EU-800LA (Benton Harbor, Mich. 49022) can be triggered by a computer or controlling logic (see Figure 2 4 . In both cases, the monostable is triggered by the trailing edge of the
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
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trigger pulse* a flip-flop, (FF), (e.g., SN7472, Texas Instruments, Inc.) may be “toggled” by the trigger pulse to the as in Figure 2B, Or separate pulses may be “set,” (S), and “reset,” (R), inputs as in Figure 2C ( I ) . In Figure 2C the device select, (DS), and input/output pulses (8), (ZOP and ZOP’), from a Digital Equipment Corporation PDP 8/E computer were gated to provide the turn ON and O F F Pulses. Computer execution of the inPut/outPut in(8) “Small Computer Handbook,” Digital Equipment Corporation,
Maynard, Mass., 1912.
Typical driving circuits for the triac switch
A . Monostable drive. tl is determined by R1and C1 B. Flip-flop (FF) drive. f 2 is determined by the length of time between turn on and turn off pulses C. Computer controlled flip-flop driver. f a is determined by length of time between turn on and turn off pulses. DS = Device select pulse from computer, IOP = Input/output pulse from computer
struction containing the selected device code and input/output pulse results in the switching of the line voltage. Similar gating circuits may be employed with other types of computers. Automated analytical instrumentation in the authors’ laboratory frequently requires the switching of ac line voltage. The circuitry described in this article has been successfully applied in instrumentation for computer assisted, automated reaction rate analyses and studies, and also in automated instrumentation for atomic absorption/emission/fluorescence analyses (9). It has proved to be a reliable, trouble-free method of switching line voltage and does not generate transients that interfere with associated digital electronic systems. Commercially available units with similar characteristics can be assembled from available circuit cards ( I o ) .
RECEIVED ._-_ for review December 13, 1971. Accepted May 18, 1y72*
(9) M. B. Denton, Ph.D. Thesis, University of Illinois at UrbanaChampaign, 1972. (10) ‘‘Logic Handbook,” Digital Equipment Corporation, Maynard, Mass., 1970.
Technique for Chemical Ionization Operation of a CEC 21-110B Mass Spectrometer T. J Odiorne and D. M. Desiderio Institute for Lipid Research and Department of Biochemistry, Bnylor College of Medicine, Houston, Texas 77025
THE POPULARITY of chemical ionization (CI) mass spectrometry in recent years has led to the field modification of several types of mass spectrometers for chemical ionization work. A rather elaborate system has been described by Futrell et al. ( I ) for the modification of a CEC 21-llOB high resolution mass spectrometer for obtaining chemical ionization mass spectra. In an earlier communication ( 2 ) , we reported the conversion of a CEC 21-1 10B (lot 9) to CI operation based on the method of Futrell ( I ) . A particularly troublesome aspect of the initial conversion has been the introduction of the C I gas into the ion source high voltage (-8 kV). This has been complicated by the need to also frequently run normal E1 high resolution spectra of molecules with relatively high molecular weights (m/e -800). In this latter aspect particularly, our conversion technique proved to be inadequate. T o alleviate this problem, we have returned to usage of the original heated coverplate and added a small Vespel ferrule and a ‘is-in. monel tube as shown in Figure 1. Vespel is the trademark of E.I. du Pont de Nemours and Co. for their polyimide resin. The spring and washers are added simply to keep the hemispherical nose of the tube well seated in the hole of the ion source. (1) J. H. Futrell and L. H. Wojcik, Rec. Sci. I/isfnim., 42, 244
(1971). (2) Jsmet Dzidic, D. M. Desiderio, M. S. Wilson, P. F. Crain, and James A. McCloskey, ANAL.CHEM.,43, 1877 (1971).
When operating the source at low pressures in the E1 mode, the tube is used to simply conduct the gas from the coverplate to the source and all seals are more than adequate. At the higher pressures used for CI a similar tube is used, except with a crimp placed immediately behind the hemispherical nose and having a conductance of approximately 1. sec-l. This provides a source pressure of 0.5 Torr with a pressure behind the crimp of 250 Torr, the higher pressure preventing high voltage breakdown of the gas. The ferrule is designed to use this pressure differential to the best possible advantage. The sealing points are machined to a thin edge allowing some distortion with mechanical force. This helps to ensure that the ferrule will conform to the size and curvature of the recess on the inside of the coverplate. It should be noted here that this recess already existed in the two coverplates we examined and that no modification of the coverplate was necessary. Furthermore, if the ferrule is properly fitted, it will seal just as well in a straight portion of the coverplate without the recess. No dimensions are given in Figure 1 , as it is necessary to custom-fit each ferrule to the individual coverplate. Our experience indicates that the 0.d. of the ferrule should be less than ca. 0.005 inch plus the i.d. of the indentation. The i.d. of the ferrule should be 0.002 inch less than the 0.d. of the tube. The length of the ferrule is cn. 0.15 inch. Monel was chosen because of its chemical inertness and high heat conductance. As the coverplate and source were both normally
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
1925
