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(Gelman Instrument Co., Ann Arbor, MI). The filtering apparatus was attached to a vacuum system. The transfer and rinse of the sample were made with a minimum of 4% hydrofluoric acid wash solution. The uranium filtrate was caught in a 50-mL roundbottomed polycarbonate centriguge tube and saved for the uranium determination. The filter containing the precipitated plutonium, americium, thorium, and curium was placed in a 50-mL c glass beaker. Four milliliters of 12 M perchloric acid and 10 drops of 1% potassium dichromate solution were added to the filter and precipitate containing plutonium, americium, thorium, and curium. The contents were heated until the filter was charred and then treated with (1:l)16 M nitric acid-12 M perchloric acid to completely oxidize the filter paper. The sample was fumed for 15 min. The final volume was 2-3 mL and the solution was dichromate yellow throughout the 15 min. The solution was cooled in a cold water bath, diluted with 10 mL of 1% perchloric acid0.01 % potassium dichromate wash solution, and transferred to a polycarbonate centrifuge tube containing 5 mL of 30 M hydrofluoricacid and 10 drops of 1% potassium dichromate solution. The transfer was made with a minimum of 1% perchloric acid0.01% potassium dichromate wash solution. The sample was allowed to stand for 30 min. A 25-mm, HT-100, filter was mounted as previously described for the 25-mm DM-450. With vacuum applied, 1-2 mL of 8 2 95% ethanol: water was drawn through the filter. As the filter went dry the following solutions were added, in order, to the center of the filter: 5 mL of substrate suspension (6) (after vigorous shaking), the sample (after vigorous swirling), a 2-3 mL 4% hydrofluoric acid4.01% potassium dichromate rinse of the centrifuge tube, and 1-2 mL of the 8:2 95% ethanokwater rinse of the filter. The ethanokwater rinse of the filter was not added to the filtrate containing plutonium. Only the plutonium filtrate and the rinse of the centrifuge tube which followed was caught in a 50-mL polycarbonate centrifuge tube and saved for the determination of plutonium. The filter was dried for 5 min under a heat lamp at a distance of 12-16 in. and sumitted to a spectrometry for the determination of americium. Thorium and curium would be here also if present in the sample. To the uranium filtrate 1drop of 0.1% safranine 0 , l drop of 20% titanium trichloride, and 0.1 mL of neodymium nitrate solution were added, with swirling. To the plutonium filtrate solid stannous chloride was added, with swirling, until the yellow chromium(V1) was completely reduced to green chromium(III), then 0.1 mL of neodymium nitrate solution was added with swirling. The uranium and plutonium filtrates were allowed to stand for 30 min and filtered onto 25-mm HT-100, filters as described for americium. However, the sample solutions were rinsed from the centrifuge tubes with 4% hydrofluoric acid wash solution in
place of the 4% hydrofluoric acid wash solution containing 0.01% potassium dichromate. The filters were dried and submitted to a spectrometry for uranium and plutonium determination.
RESULTS AND DISCUSSION A precipitation of all of the nuclides in the sample had been made by reducing uranium with titanium trichloride before the precipitation with hydrofluoric acid and before the filtration on to an HT-100 filter. After the sample was examined by a spectrometry, the separations of uranium and plutonium from the rest were made when desired. Samples have been mounted for a spectrometry with this procedure in place of electrodeposition routinely for the past 4 years, with excellent results. Furthermore, uranium and plutonium can now be separated, one from the other, after, rather than before the extraction (3). Thus, the extraction time can be decreased by half because only one extraction separation is needed for both nuclides rather than an extraction separation for each. Hundreds of routine bioassay samples have been analyzed this way over the past 4 years with little if any trouble. Future studies will show where the fractions of protactinium, neptunium, and radium actually remain. A more rigorous paper is contemplated to evaluate the remaining unknowns. ACKNOWLEDGMENT The author wishes to thank K. W. Puphal for useful suggestions in the development of this procedure. The author also thanks R. L. Williams for the many a spectrometry measurements needed to complete this work. Registry No. Neodymium fluoride, 13709-42-7;uranium, 7440-61-1;plutonium, 7440-07-5; americium, 7440-35-9. LITERATURE CITED (1) Puphai, K. W.; Oisen, D. R. Anal. Chem. 1972, 4 4 , 284-289. (2) Lieberman, R.; Moghissi, A. A. Health Phys. 1968, 75,359-362. (3) Sill, C. W.; Puphal, K. W.; Hindman, F. D. Anal. Chem. 1974, 46, 1725-1737. ~. . (4) Bernabee, R. P.; Percival, D. R.; Hindman, F. D. Anal. Chem. 1980, 52. 2351-2353. . ~_.. , (5) Filer, T. D. Anal. Chem. 1974, 46, 608-610. (6) Stili, C. W.; Williams, R. L. Anal. Chem. 1981, 53, 412-415. (7) Sill, C. W. Anal. Cbem. 1974, 46, 1426-1431.
RECEIVEDfor review July 11,1983. Accepted August 18,1983. Use of commercial product names is for accuracy in technical reporting and does not constitute endorsement of the product by the United States Government.
Comparison of Sample Injection Systems for Flow Injection Analysis Jeffrey J. Harrow and Jiti Janata*
Department of Bioengineering, University of Utah, Salt Lake City, Utah 84112 In low-pressure analytical systems such as flow injection analysis, the traditional method of sample injection has been some sort of a commutating valve, either rotary or sliding, actuated manually or mechanically. Such valves are usually the "weakest link in flow injection analysis technology" ( l ) , as they require expensive precision manufacturing and, in our hands and others (2),usually wear and leak. Efforts to improve the method of injection have resulted in several alternative solutions; these have been recently reviewed (3). The commercially available FIAtron SHS-200 system (FIAtron,
Milwaukee, WI) uses four three-way magnetic solenoid valves. The commercially available FIA 5020 (Tecator, Sweden) in its "hydrodynamic mode" uses two precision two-channel peristaltic pumps, which are alternately pumped (I). A British group uses a precision pump for controlled aspiration, followed by subsequent return of the sample aspirating probe to a carrier reservoir (21,calling this method "controlled dispersion analysis". The purpose of this paper is to show a new method for injection that is extremely inexpensive and reliable. Appli-
0003-2700/83/0355-2461$01.50/00 1983 American Chemical Society
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cation-dependent selection factors will be developed for building a FIA system, based on criteria such as precision, reliability, cost, and flexibility.
EXPERIMENTAL SECTION Experiments were done to assess precision in the case of medium dispersion using a conductivity cell as the sensor. Both a manual valve and various configurations of solenoid valves (v.i.) were used. Precision was also assessed for the case of low dispersion, using a chemically sensitive field effect transistor (CHEMFET) sensitive to pH as a detector. Manifold. All experiments were done on a homemade Leg0 breadboard similar to the FIASTAR (Tecator), using Freon gas as the solution propellant. The overall size of our system was width 28 cm, depth 28 cm, and height 25 cm. The tubing was 0.5 mm i.d. Teflon or Tygon. Manual Valve. The manual rotary injection valve which switches both sample and bypass loops was manufactured inhouse. It is mounted with its axis of rotation horizontal,permitting straight-through holes which completely eliminate any dead volume of turbulence. The seal is a Teflon washer, fixed to the rotary half of the valve. The valve is equipped with a microswitch in order to send an initiation pulse to the computer. Magnetic Valves. Two types of on-off valves were investigated. In the early experiments magnetic reed valves (kindly supplied by Reedex Inc., Waterbury, MA) were evaluated. These are extremely small valves which use a flexing magnetic reed in the fluid path. The valve has an internal volume of 100 pL, switches in 2 ms, requires only 350 mW of coil driving power, and costs $17. However, the reeds are in contact with the solution. The other valve evaluated was a subminiaturesolenoid pinch valve (Brunswick Technetics, Cedar Knolls, NJ). These work by pinching a 0.7 mm i.d. silicone tube. They switch in 5 ms, require 750 mW of coil drive, and cost $27. Because these valves do not suffer from the materials problem, they were used for all experiments after the original configuration studies. The valves were controlled by an Apple I1 Plus computer using a Sprague ULN-2003A relay driver chip. This chip incorporates flyback diodes to dissipate the energy stored in the coil. This eliminated switching transients which were disturbing the detector electronics. Conductivity System. The conductivity cell consisted of two bright platinum wires inserted 1cm apart through the wall of a 2 cm length of silicone tubing, 1mm i.d. These were excited with a 15-mV 1000-Hz sine wave. The resulting ac current was measured and its RMS value was sent both to a chart recorder and to a 12-bitA-D converter (AI-13, Interactive Structures, Inc., Bala Cynwyd, PA) controlled by the Apple I1 computer. CHEMFET System. The principles of chemically sensitive field effect transistors and their use in FIA have been described previously (4). Flow through cells containing a pH-sensitive field effect transistor and a downstream reference electrode were kindly supplied by Critikon, Inc. (Salt Lake City, UT). They feature a cell volume of
