-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.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
k"STAINLESS STEEL
G
T
G YE STAINLESS T
O COLUMN
YCOPPER COOLING
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t
JPLE
I +INDIUM
TUBING
Figure 2. Adapter for sampling with indium tubing 41LEY
ENT PLUG
I
HEATER
. .4-L
Figure 1. Injection system for indium tubing
Because of certain design features, e.g., the use of an O-ring gland seal in the entry port, the system evaluated did not appear rugged enough for long-term use by routine laboratory personnel. Since no other commercially available system was identified, it was decided to develop a system to meet our requirements. EXPERIMENTAL Apparatus. A sectional view of the system which we have designed is shown in Figure 1. For the entry port for indium capsules, a Whitey ball valve, series No. 43, connected to the block by a vertical section of 1/4-in. stainless steel tubing, provides a smooth passage in. in diameter. The stainless steel block, 4 in. x 15/8in. x 3/4 in., is drilled for the inlet, outlet, and drain ports, and for the heater and thermocouple cavities. All connections in the carrier gas flow path are silver-soldered to eliminate leakage which tends to occur at threaded joints at high temperatures. The cooling coil silver-soldered onto the inlet connector is required to protect the Teflon (Du Pont) seat in the Whitey valve from conducted heat. For draining molten indium, a Bailey steam vent plug has proved satisfactory. This is located directly below the expansion chamber ( A ) so that, in use, the threaded joint is covered with molten indium (B). A 90-watt cartridge heater, such as the G.E. W-90, powered from a 110-volt outlet through a voltage controller, is adequate for maintaining the block temperature at approximately 300 "C. The thermocouple in the block may be connected to a simple pyrometer with a range of 0-400 "C. The assembly may be mounted on either the front or side of the gas chromatograph in such a position that the outlet port is as close to the column inlet as possible, while still leaving the Whitey valve and the vent plug readily accessible. The exposed portion of the block should be insulated, or a guard installed, to protect the operator from burns. Procedure. Sample containers are allowed to come to room temperature, purged to remove any water or sediment, then pressured to at least 100 psig (up to 150 psig may be required if significant amounts of C-2's and C-3's are present.) This step is necessary to minimize preferential flashing of lighter components in the sample container and in the indium
tubing as it is being filled. The use of carrier gas for pressuring will eliminate the possibility of an undesirable peak from dissolved pressuring gas. A length of indium tubing of not less than 3 in., with both ends open, is connected to the sample container through a silicone rubber plug backed by a retaining nut. See Figure 2. The container valve is opened, and as soon as a steady flow of liquid sample is established, the outer end of the tubing is pinched down until a very fine spray of sample is obtained. After purging at this rate for 10 seconds to eliminate as far as possible vaporized sample in the tubing, a pair of pliers is used to seal the outer end of the tubing, then to cut off two sample pieces. The first piece is discarded to eliminate end effects, and the other is taken to the gas chromatograph in a small box or tray. Handling of sample pieces should be avoided as body heat is sufficient to increase the hydrostatic pressure of the sample to the point of bursting the seal. The sample piece is dropped into the upper port of the Whitey valve; then, with a finger held over the upper valve port to retain carrier gas pressure, the valve is opened to allow the sample piece to drop through, then closed immediately. The molten indium must be drained often enough to ensure that it is does not build up to the level of the outlet passage. Using 0.06-in. 0.d. x 0.03-in. i.d. tubing, with sample pieces approximately 6 / / R in. in length, we have found that draining after each six samples is adequate. We are currently using Indalloy No. 5 tubing (Indium Corp. of America). The pure indium can be used but is difficult to manipulate because it is very soft and malleable. RESULTS AND DISCUSSION
Table I shows the results of a series of analyses performed by five technicians on a plant sample, using both the indium introduction system and introduction by gas tight syringe. Gas samples were drawn from the liquid phase in the sample container at a constant controlled rate of vaporization. As shown the reproducibility with indium tubing sampling was much better than that with syringe sampling. It is reasoned that the chief cause of the difference in reproducibility between methods is that, with indium sample introduction, vaporization takes place inside the instrument, giving essentially no opportunity for sample distortion, while with syringe sampling, vaporization takes place at a needle valve, allowing nonreproducible fractionation of the sample. At one stage in the development, 22 encapsulated samples were held at room temperature for 4 to 6 hours before analysis.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
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Table I. Comparison of Reproducibility of Results between Indium Tubing Sampling and Gas Syringe Sampling on One Sample by 5 Operators on 5 Different Days Indium tubing Gas syringe Operator/ Total C-4‘s Total C-5’s Total C-3’s Total C-3’s Total C-4’s Total C-5’s day No. 1 0.69 Wt. 97.27 Wt. 2.04 Wt. % 97.10 Wt. 0.77 Wt. % 2.13 Wt. No. 1 0.71 97.18 2.11 0.75 97.22 2.03 No. 2 0.66 97.26 2.08 95.63 0.79 2.68 No. 2 0.65 97.29 2.06 97.19 0.63 2.18 No. 3 0.62 97.21 2.17 0.71 97.03 2.26 No. 3 0.63 97.29 2.08 0.67 96.92 2.41 No. 4 97.55 0.60 97.24 2.16 0.54 1.91 No. 4 0.58 97.26 2.16 0.57 97.72 1.71 No. 5 0.61 97.31 2.08 0.63 96.85 2.53 No. 5 0.54 97.41 2.05 0.60 97.77 1.62 Mean 0.63 97.27 2.10 0.67 97.10 2.15 U 0.05 0.06 0.05 0.61 0.34 0.09
z
Three of these were found to be empty. It is known that, with zero vapor space in the capsule, a small rise in temperature will increase the hydrostatic pressure of the liquid enough to burst the seal. Keeping the capsules below room temperature will reduce the incidence of failure from this cause. Studies on repeatability of sample size showed that the first capsule cut off from the length of filled tubing gave unacceptable repeatability. Repeatability of sample size for subsequent capsules was found to be in the order of 3.5% numerical, which is quite adequate for normalization or internal standard methods, and for all but the most rigorous external standard methods. A number of indium tubing introduction systems have been installed on routine gas chromatographs in our laboratory,
z
and have been in regular use for periods up to six months with no major problems. Applications include the analysis of C-5 and heavier hydrocarbons and of oxygenated compounds in C-4 hydrocarbons. The effect has been an improvement in safety, housekeeping, and accuracy of analyses. ACKNOWLEDGMENT We thank our machine shop personnel for their assistance.
RECEIVED for review February 8, 1971. Accepted May 4, 1971. We express our thanks to Polymer Corporation Limited, Sarnia, for permission to publish.
Photometric Device for Determination of Trace Amounts of Selenium Brick B. Mesman and Harold A. Doppelmayr USAF Environmental Health Laboratory, McClellan Air Force Base, Calif. 95652 WITH THE RECENT emphasis on environmental control and pollution monitoring, it has become evident that analytical techniques rapid enough to handle the increasing volume of work, yet sufficiently accurate to determine submicrogram quantities of the substance sought, are urgent necessities. An example is the determination of selenium in water. At present, the method of broadest currency is probably the diaminobenzidine reaction ( I , 2), which, while sensitive, is lengthy and critically dependent on pH. A rapid, sensitive method based on selenium catalysis of sulfide reduction of methylene blue has been described (3-5). A simple photometric device has been developed in this laboratory to increase the efficiency of the technique. The (1) “Standard Methods for the Examination of Water and Waste Water,” 12th Edition, American Public Health Association, Inc., New York, N. Y., 1965. (2) “FWPCA Methods for Chemical Analysis of Water and Wastes,” U. S. Department of the Interior, Cincinnati, Ohio,
1969. (3) F. Feigl and P. W. West, ANAL.CHEM. 19, 351 (1947). (4) F. Feigl, “Spot Tests, Inorganic Applications, Volume I,” 4th ed., Elsevier Press, New York, N. Y . , 1954. (5) P. W. West and T. V. Ramakrishna, ANAL. CHEM.40, 966 (1968). 1346
visual end point has been replaced by photoelectric detection, reduction time being documented on a recorder. A description of the equipment and method follows. EXPERIMENTAL Reagents. All reagents are ACS grade, prepared according to West and Ramakrishna (5). In preparing the sodium sulfide solution, any grayish powder should be excluded: freshly washed crystals are used, thus avoiding a spurious rapid color discharge. Similarly the Triethanolamine used must be water-white; yellowish material has caused false positive results. Instrument Design. The photometer consists of a type 90 lamp as a light source, an International rectifier, type B2M photocell, and an optical slit 20 mm long by 2 mm wide, positioned between the sample and light source. The slit is constructed from opposing edges of stainless-steel razor blades. A blower, Dayton, Model 2C782 1/250 HP is installed to dissipate heat within the photometer enclosure (Figure 1). Figure 2 shows the inside arrangement of the above mentioned components. A cuvette holder of sufficient size to accomodate the reaction vessel is used. Stray light is prevented from entering the cell compartment, because of the snug fit of the compartment and the black screw cap which completely blocks the well in which the reaction vessel is lowered.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 10, AUGUST 1971
