liquid flow rates for toluene and methyl iodide are shown as examples. The dashed lines indicate the theoretical delivery rate predicted from measurements of syringe capacity and rate of plunger advance. Note that for toluene, using the unmodified system of tubing clamps on silicone rubber, the actual output flow builds up gradually but never exceeds 80% of the theoretical output. The balance is lost by diffusion of the liquid through the tubing walls. The large periodic negative spikes are caused when the infusion cycle changes from one syringe to the other; the system pressure first drops but is slowly built up as the syringe moves forward. However, if the inert, air-activated valve switching system is substituted, the actual and theoretical outputs are practically coincident, and the actual output is almost pulse-free during the infusion syringe changeover. The case of methyl iodide is more dramatic and illustrates the problems encountered with gross incompatibility between solvent and tubing. Using the unmodified system of tubing clamps on surgical gum latex, the output flow rate again approaches about 80% theoretical output, but as time progresses liquid not only is lost by diffusion but actually begins to leak through fissures in the tubing wall. The air-activated valves, however, again provide dependable, pulse-free injection rates.
Chlorinated hydrocarbons, alcohols, ketones, and aromatic materials have all been pumped with the air-activated valve system, and the amount of solvent actually delivered has been consistently within *0.6% of the theoretical injection rate. By actually sweeping out the valve volume during syringe changeover, rather than depressing the tubing with solenoid-activated clamps, an accurate and practically pulse-free liquid displacement can be achieved for almost any solvent or solution. ACKNOWLEDGMENT
The authors gratefully acknowledge the efforts of Bernell J. Bequette for his work in valve machining and assembly and James E. Dixon for his skillful electronic modifications.
RECEIVED for review February 8, 1971. Accepted May 3, 1971. Reference to a company or product name does not imply approval or recommendation of the product by the University of California or the U.S. Atomic Energy Commission to the exclusion of others that may be suitable. Work performed under the auspices of the U S . Atomic Energy Commission.
Reliable Low Voltage Signal Generator for Cyclic Voletammetry Russell H. Bull Department of Chemistry, St. Louis University, St. Louis Mo. 63156
Geoffrey C . Bull Department of Electrical Engineering, Christian Brothers College, Memphis, Tenn. 38104
IN ELECTROCHEMICAL investigations conducted with laboratory constructed devices capable of generating voltage or current programs over a wide range of frequencies and amplitudes, each instrument must incorporate the dual features of potentiostatic or amperostatic control of the electrolysis cell and a reliable signal generator for flexible signal shaping at the working electrode. Described here is a functionally reliable, compact, low voltage signal generator easily adapted to any potentiostat design the experimenter may require to fit the needs of a particular cell configuration
(0. GENERAL REQUIREMENTS
Whether single sweep, repetitive scan, or cyclic experiments are to be conducted, certain controls must be present in the signal generator for ease in interpreting the results. It is necessary that the signal produce a truly linear relationship between voltage and time and that the frequency of the sweep [i.e., scan rate = frequency X (anodic voltage limitcathodic voltage limit)] be independent of the amplitude, i.e., limits, of the sweep. It is often necessary to vary the initial voltage level of the working electrode, i.e., the electrode potential may need to be at some nonzero (either positive or negative) potential to prevent the formation of some spurious material. Further, it may be necessary to stop (1) W. M. Schwarz and I. Shain, ANAL.CHEM., 35, 1770 (1963).
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the experiment at either of its voltage limits for a period of time long enough to reset frequency and voltage limits and then resume cyclic operation, without disturbing the “holding” program. Additional flexibility may be found by the incorporation of a method for shifting a fixed voltage amplitude over a wide voltage range. Thus a “reference level” voltage should be present and adjustable without changing either the amplitude or frequency of the resultant triangular output. As an experiment requires the frequency to increase beyond the limits of manual control over the number of cycles generated, a device should be used that allows the user to generate a single cycle at any preset frequency and amplitude. All of these capabilities have been incorporated into the design of the following signal generator. CIRCUIT DESCRIPTION
Circuits for the generation of nonsinusoidal or discontinuous signals invariably employ a multivibrator (either electronic or mechanical) circuit to produce a linear function of various frequencies and amplitudes. The present signal generator is concerned with the formation of a symmetric triangle which may be easily produced by the electronic integration of a square wave. The following instrument uses an operational amplifier as a sensing device which essentially compares the instantaneous output voltage of an operational amplifier/integrator circuit with a reference
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
Figure 1. Triangular wave generator Operational Amplifiers 1, 2, 3, and 4 are Analog Devices Model 405. Power supply is an Analog Devices Model 904. All resistors are is%, ‘/z watt, unless otherwise noted in the text
voltage established by a transistor switching circuit. The sensing amplifier output controls an operational amplifier which delivers a constant charging current to the integrator, producing a linear sweep output, and controls the switching circuit, producing reversal of the direction of sweep when the sweep output voltage equals that applied by the switching network. The provision of a separate control loop for each of these functions permits the sweep rate to be variable yet independent of sweep limits. The Heathkit Operational Amplifier system of Weir and Enke ( 2 ) has been modified and expanded to exploit the reliability and greater sensitivity of solid state devices presently available. If the reader is not familiar with the concept of operational amplifiers, ready reference to available literature will be of valuable assistance (3-5). In normal sweep operation (Figure l), the amplitudecontrolled square wave signal from the transistor switching network is inverted by amplifier 1 and compared with the midpotential bias and the sweep output voltage at the input of the sensing amplifier, amplifier 2. The output of this amplifier controls the transistor switching network and establishes, after inversion by amplifier 3, the constant charging current to the integrator circuit. With this control scheme, the sensing amplifier is not “in control,” i.e., its summing point is not maintained at common, though its output, through the feedback capacitor loop, is such as to move the amplifier toward control. At the instant that control is attained (at the sweep limit), the amplifier input inverts, the reference voltage changes, and the sweep reverses, (2) W. C. Weir and C. G. Enke, Rec. Sci. Instrum., 35, 833 (1964). (3) Philbrick/Nexus Research, “Applications Manual for Operational Amplifiers,” Dedham, Mass., 1968. (4) E. R. Brown, D. E. Smith, and G. L. Booman, ANAL.CHEM., 40, 1411 (1968). (5) H. V. Malmstadt, C. G. Enke, and E. C. Toren, Jr., “Electronics for Scientists,” W. A. Benjamin, Inc., New York, 1963, Chapter 8.
always moving in such a direction as to cause the amplifier to approach control. The peak-to-peak amplitude of the square wave is fixed by the feedback resistor of amplifier 1, but the variable resistor in the feedback loop of amplifier 3 allows some amplitude control at the integrator, amplifier 4. The frequency of the square wave is regulated by the sweep rate controls and is thus subject to the same ranges. Once the sweep is begun, it may be halted at either limit by applying to the transistor bases on overriding holding voltage, the polarity of which is chosen to correspond to the sweep limit at which holding is desired. This causes the switching network to become a source of constant reference voltage at a level corresponding to the sweep limit set by R1 or R2, whichever is in the conducting path of the transistor held “on” by the holding voltage. This “hold” switch brings into play a silicon controlled switch (3N81) network (6) that expands the instrumental capability to include a “one-shot” triangular wave at a preset frequency and amplitude. This transistor is based on a pair of P-N junctions as the common diode. In this circuit, it is made to operate as a switch by constructing the bias circuit such that the transistor is either in the cutoff state (switch open) or in the saturated state (switch closed). The switch is controlled by the current applied at the base. Because of the four accessible semiconductor regions of the SCS, triangular signals may be generated in either a positive or negative going sense, with the frequency limited only by the switching capabilities of the device. In the present configuration, when the limit-hold switch is in the negative position, the SCS circuit which is needed to override the signal from amplifier 2 is activated, and the necessary override voltage is applied to the bases of the transistor switching network. At this point the SCS is on and needs only a negative pulse at the cathode gate to turn it off, allowing one cycle (6) “General Electric Transistor Manual,” 7th ed., General Electric Co., Syracuse, N. Y . , 1964, p 391.
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-15V
I
This system is designed to work with unstabilized operational amplifiers since little overall advantage is to be gained by use of more expensive chopper-stabilized types. Calibration of the frequency is easily accomplished with the aid of an oscilloscope and monitoring of the sweep limits may be accomplished with a vacuum tube volt meter. ELECTRONIC CHARACTERISTICS
POS IN
n
Figure 2. Monostable multivibrator provides the negative output pulse necessary to initiate a single cycle through the SCS network in Figure 1 to occur before it returns to its stable state at the preset holding voltage. This pulse is supplied by a monostable multivibrator whose R C network is set to ensure an adequate pulse to trigger the SCS in the control loop. If the positive limit is to be the starting point for a cycle, the polarity of the hold switch is reversed and control is passed through the NPN transistor and a “one-shot” triangle is generated by another negative pulse from the monostable multivibrator. The amplitude and frequency for a single triangle are set in the same way as the free-running case. Because of the inherent sensitivity of the SCS, the switching times (or latching time) are on the order of 50 microseconds or less. The monostable multivibrator (Figure 2) was redesigned from circuits of well known performance (6, 7). (7) J. Markus, “Sourcebook of Electronic Circuits,” McGrawHill Book Co., New York, N. Y., 1968, Chapter 54.
The above circuit will allow any voltage between +3 and -3 volts to be chosen (this is within the limits of useful voltammetry) for the working electrode and frequencies up to 225 volts per second (depending on the value of R3). Any A15 volt power supply of 50 milliamperes current capability will adequately drive the signal generator. If a three operational amplifier potentiostat configuration is attached to this signal generator, 100 milliamperes of current will drive both sections of the instrument. The values of each component are given in the appropriate circuit diagram with the following exceptions. R4 through R14 in Figure 1 have the values of 10, 5, 4, 3, 2, and 1 Megohm, 750, 500, 250, and 100 Kilohms, respectively. Capacitors are in microfarads rated at 25 working volts direct current or larger.
RECEIVED for review March 19, 1971. Accepted May 3,1971. We wish to thank the Department of Chemistry, St. Louis University, for its financial support of this project. One of us (RHB) is grateful to the National Science Foundation for a Traineeship during 1970.
Gas Chromatographic Injection System for Light Liquid Hydrocarbons Encapsulated in Indium Tubing A. S. Dunlop and S. A. Pollard PoIymer Corporation Limited, Sarnia, Ontario, Canada
IT HAS LONG BEEN recognized in gas chromatography that the analysis of light hydrocarbons in the Czto CSrange presents a problem in that it is difficult to introduce a small aliquot which is truly representative of the bulk material. Some of the more common methods which have been used are: expansion of a liquid aliquot (in the order of 1 to 10 ml) into a gas container, and introduction of an aliquot of the gas into a gas chromatograph; direct introduction of a liquid aliquot by high pressure syringe; and direct introduction of a liquid aliquot by high pressure liquid sampling valve. In our experience, the last named method has been the most reliable in terms of repeatability of sample size and reproducibility of analysis. However, it does involve a safety hazard in that pressurized sample containers must be connected directly to the sample valve which is close-coupled to the gas chromatograph. Any leakage from the container or connections could form an explosive mixture which would find a ready source of ignition in the instrument. Earlier work described by Ehrhardt, Grubb, and Moeller ( I ) , and by Nerheim ( 2 ) had shown that indium tubing could (1) C. H. Ehrhardt, H. M. Grubb, and W. H. Moeller, U. S. Patent 3,103,277, September 10, 1963. (2) A. G. Nerheim, ANAL.CHEM., 36, 1686-88 (1964).
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be used for encapsulating and introducing liquified light hydrocarbons for instrumental analyses. Indium metal is unreactive, nontoxic, has a melting point of 156.4 “C, and capillary bore tubing extruded from indium can be self-sealed to contain pressures up to 300 psig by simply cutting with a pair of pliers. Thus, a length of indium tubing can be filled with liquified hydrocarbon and sections of the tubing containing sample cut and sealed in one operation. When the capsules are introduced into a heated chamber, the indium melts and the released sample vaporizes. By locating the heated chamber in the carrier gas flow path of a gas chromatograph, a convenient method is offered for introducing liquified light hydrocarbon samples. An evaluation of a commercially available system for introducing samples encapsulated in indium tubing into a gas chromatograph demonstrated that the technique for encapsulating and introducing samples was relatively simple; system efficiency as measured by the degree of valleying between overlapping peaks was slightly better than that obtained with a gas sampling valve on the same instrument; and sample aliquots were representative of the material in the sample container which consisted of Ce to Cg hydrocarbons.
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