Purification of Radioactive Tracers for Use in High Sensitivity Alpha Spectrometry Claude W. Sill Health Services Laboratory, U.S. Atomic Energy Commission, ldaho falls, ldaho 8340 1
When several alpha-emitting radionuclides are determined simultaneously in a single sample by alpha spectrometry, the several tracers used to determine the chemical yields must each be relatively free of all the other radionuclides being determined to avoid increaslng the uncertainty of the measurements due to the contaminants added with the tracers. Selected procedures for purification of tracers of all elements from thorium through californium are described that have been demonstrated to provide the necessary decontamination for use in high-sensitivity alpha spectrometry. Except for protactinium, all final tracer solutions can be evaporated directly on stainless steel plates to simplify standardization.
The large area, high resolution, and low background of modern surface-barrier detectors have made alpha spectrometry an extremely sensitive technique for the detection and measurement of alpha-emitting nuclides a t extremely low levels. Counting times of lo3 minutes have become routine and even longer times up to several days are used frequently. As little as 10% of one nuclide in another can be detected, particularly when the nuclide in lower concentration gives alpha particles having the higher energy. During development of a unified procedure for the simultaneous determination of virtually all alpha emitters in a single sample of soil ( I ) , radioactive tracers were required that could be used in combination to determine the chemical yields. Contaminants in each tracer solution must be kept very small indeed to keep the extraneous nuclides added with the tracer from decreasing significantly the sensitivity and/or precision of the determination of the same nuclide from the sample. The alpha-emitting parents and daughters of each tracer must be particularly pr_ovided for, as well as extraneous impurities. Previous directions for the preparation of 234Th (2), 239NP ( 3 ) , and 233Pa ( 4 ) gave tracers of adequate radiochemical purity for the applications intended a t the time, which involved gross-gamma counting only, but require considerable modification to obtain sufficient decontamination from the alpha emitters present. Several alphaemitting tracers also require special purification for use in the determination of several nuclides simultaneously. Plutonium-236 is widely used in the determination of 238Pu and 239Pu but decays to 232U, 22sTh, and subsequent daughters in the natural thorium chain which interfere with determination of radionuclides having alpha emission in the same energy region. Uranium-232 is perhaps the best tracer for the other uranium isotopes and has similar problems with interference from 228Thand its daughters. Also, purified solutions of 23iNp, 239Pu, 241Am, etc., were required for use in preparation of standard soils containing
(1) (2) (3) (4)
C.W. Sill, K. W. Puphal, and F. D. Hindman, Ana/. Chem. in press C. W. Sill, Anal. Chem.,36, 675 (1964). C. W. Sill, Anal. Chem., 38, 802 (1966). C. W. Sill, Anal. Chem., 38, 1458 (1966).
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exactly known quantities of alpha emitters in several combinations ( 5 ) . The present paper presents selected procedures for purification of radionuclides of thorium through californium that have been modified to meet the exacting requirements of high-sensitivity alpha spectrometry and to simplify standardization of the resulting solutions. With the exception of protactinium, the final tracers are obtained in pure nitric acid solutions so that aliquots can be evaporated directly on stainless steel plates for standardization. Electrodeposition yields plates giving much better resolution of alpha spectra but is considerably more time-consuming and less accurate and reliable for quantitative standardization (5). Losses of several percent during electrodeposition are commonplace, some elements being deposited more quantitatively and reliably than others (6,7), and corrections are nearly always required for the losses in the electrolyte. In many cases, direct evaporation gives adequate resolution when the solutions are properly prepared. When the higher resolution provided only by electrodeposition is necessary, direct evaporation still provides a simple and rapid check on the recovery obtained during electrodeposition. Instrumentation and techniques used in standardization are given elsewhere ( 5 ) . Perchloric acid, sulfates, chlorides, or other substances that corrode stainless steel or introduce nonvolatile, absorbing materials must not be present in significant quantities. Ammonium salts cause significant loss of activity during sublimation as well as corrosion of stainless steel and must be removed chemically. Particularly, ammonium perchlorate ignites on strong heating and gives very large losses. This is especially important because any ammonium salts present will be converted to the perchlorates after fuming with perchloric acid during wet oxidation of organic matter. Fortunately, ammonium salts can be oxidized to elemental nitrogen rapidly and completely by boiling for a few minutes with aqua regia. Particular care must be taken not to overheat residues obtained on evaporating solutions of quadrivalent elements to dryness for removal of perchloric acid. Otherwise, the tracers will not redissolve completely in nitric acid.
EXPERIMENTAL Reagents. Aliquat-336 ( N o d ,30%. Dissolve 900 ml of Aliquat336 (Cl) (General Mills, Inc., Kankakee, Ill.) in 2100 ml of xylene in a &liter separatory funnel. Shake vigorously for 4 minutes with each of two successive 500-ml portions of 4M nitric acid to convert the amine to the nitrate form. Wash the organic solution three successive times with 1 liter of water, again shaking for 4 minutes each time. Draw the organic phase into a suitable bottle for storage. Protactinium Wash Solution. Dissolve 9 grams of ammonium sulfate in 35 ml of water and 15 ml of concentrated sulfuric acid. Add 100 ml of concentrated hydrochloric acid and cool to room temperature. Aluminum Nitrate, 0.75M. Dissolve 280 grams of dry Al(NOd3. 9H20 in 720 ml of water with slight warming as necessary. Add 128 ml of concentrated nitric acid and cool to room temperature to give (5) C. W. Sill and F. D. Hindrnan, Anal. Chem., 46, 113 (1974). (6) K. W. Puphal and D . R. Olsen, Anal. Chem., 44, 284 (1972). (7) K. W. Puphal and D. R. Olsen, Anal. Chem., 44, 1301 (1972).
SEPTEMBER 1974
1 liter of solution. Filter through a DM-450 membrane filter (Gelman Instrument Co., Ann Arbor, Mich.) in a filtering chimney. The density a t 25 "C should be 1.170 grams/ml. Aluminum Nitrate, Acid-Deficient, 2.5M. Heat 938 grams of solid AI(NO&.SHzO in a graduated 1-liter beaker on an uncovered hot plate until the salt melts and the resultant solution boils. Leaving the beaker uncovered, boil the solution vigorously until the initial volume of 600 ml has been reduced t o about 380 ml to volatilize as much nitric acid as possible. Remove the beaker from the hot plate and rinse the walls with several 10-ml portions of water, stirring each into the solution gently to moderate the vigorous evolution of water vapor that occurs. As soon as the vigor of the reaction will permit, add an additional 200 ml of water and boil the solution for a few minutes to redissolve the small quantity of aluminum salts that will have dried on the bottom and sides of the beaker. When the solution is clear, dilute to about 950 ml, cool to room temperature, and dilute t o 1 liter. Filter the solution through a DM-450 membrane filter. T h e density of the solution a t 25 "C should be 1.288 grams/ml, and the p H as measured with a glass electrode is 1.6. Procedures. Thorium-234. Convert 1 pound of uranyl nitrate hexahydrate to the chloride form, filter the 4-to-1 (9.6M) hydrochloric acid solution through a well-washed double glass-fiber filter in a Buchner funnel to eliminate insoluble impurities, and load the uranium on a large column of Dowex 1 as described previously (2).Wash the freshly prepared column with an additional 2 or 3 liters of 4-to-1 (9.6M) hydrochloric acid to remove natural thorium and other nonexchangeable impurities in the uranium and, particularly, to eliminate 2'30Ththat will have grown in since the uranyl nitrate was purified. After a suitable period of regrowth of the 24.1-day 234Thisotope, elute the column with 2 liters of 9.6M hydrochloric acid as described previously (2). Evaporate the eluate in a 4-liter beaker to ahout 50 ml. Transfer i,he solution to a 250-ml beaker and evaporate to 2 or 3 ml. Do not allow the solution to bake, even in local spots, or a refractory oxide will be formed that will not redissolve completely. Rinse the 4-liter beaker with 25 ml of concentrated nitric acid, add the rinse t o the 250-ml beaker, and repeat the evaporation to about 5 ml to remove chlorides. Add 25 ml of 8 M nitric acid and heat to boiling. If significant silica or other insoluble material is present (do not use silicone grease on the stopcock of the column), transfer the solution to a 40-ml conical centrifuge tube, centrifuge, and decant into a 250-ml separatory funnel. Rinse the beaker and/or centrifuge tube with another 15 ml of 8M nitric acid, centrifuge, and add the rinse to the separatory funnel. Shake the solution vigorously for 2 minutes with 50 ml of 30% Aliquat-336 ( N o s ) in xylene to separate thorium from nonextractable impurities. Discard the aqueous phase. Shake the organic phase with 25 ml of 8M nitric acid for 1 minute and discard the aqueous phase. Strip the thorium by shaking the organic phase three consecutive times with 50 ml of 10M hydrochloric acid for 2 minutes each time, leaving all other extractable alpha emitters, particularly uranium, in the organic phase. Extract the combined hydrochloric acid strips with 25 ml of 30% Aliquat-336, preferably but not necessarily in the chloride form, and discard the organic phase to remove the small fraction of uranium that will have distributed into the aqueous phase. Place the combined strips in a 250-ml Erlenmeyer flask, preferably one made of Vycor or quartz, add 5 ml of 72% perchloric acid and two 8-mesh silicon carbide boiling chips, and evaporate the solution to fumes. Flasks made of borosilicate glass can be used, but will result in larger quantities of nonvolatile materials leached from the glass during evaporation. When the perchloric acid begins to fume and the solution turns brown, add a few drops of nitric acid to moderate the oxidation of the waxy flocs of Aliquat-336 or other organic matter remaining from the extraction. Add several drops of nitric acid as necessary throughout the evaporation of the perchloric acid to ensure complete oxidation of all organic matter. Near the end of the evaporation, swirl the flask continously over a small flame from a blast lamp so that the solution does not dry on the hottom, even in local spots, until virtually all the perchloric acid has been volatilized. Heat the walls of the flask just enough to prevent condensation of the perchloric acid fumes. When it appears that the remaining perchloric acid will volatilize from the heat in the glass itself, remove the flask from the flame so that the residue is not heated any hotter or longer than necessary. Otherwise, the residue might not redissolve completely on subsequent treatment with nitric acid. Add 5 ml each of concentrated hydrochloric and nitric acids to the flask and reevaporate to about 2 ml to oxidize ammonium salts
and to eliminate most of the hydrochloric acid. Add 15 ml of concentrated nitric acid and boil down to about 5 ml to ensure complete dissolution of thorium and complete oxidation of chlorides as indicated by the absence of color or fumes of chlorine and/or nitrogen oxides. Cool, add 25 ml of water and filter the solution through a 1-inch DM-450 membrane filter in a filtering chimney to remove a small additional quantity of silica that apparently comes through the extraction. Wash the flask and filter with enough water to give a final volume of 50 ml. Dilute aliquots of the stock solution further with 10%nitric acid to give the concentrations desired for use. This procedure is virtually specific for thorium, separating it from all other alpha emitters. Protactinium-231. Add 3 ml of concentrated sulfuric acid and 2 grams of anhydrous sodium sulfate to the protactinium tracer solution in a 50-ml platinum dish and evaporate to fumes of sulfuric acid to remove silica and the hydrofluoric acid that is generally used to keep the protactinium in solution. Cool, dissolve the cake in 15 ml of water, and transfer to a 250-ml Erlenmeyer flask to avoid dissolution of traces of platinum. Reevaporate the solution until all excess sulfuric acid has been volatilized and a clear pyrosulfate fusion is obtained to ensure complete dissolution of hydrolyzed or refractory forms of protactinium. Cool the melt, add 20 ml of water and 100 mg of sodium metabisulfite, and warm until the cake has dissolved. Cool and transfer the solution to a 250-ml separatory funnel with 40 ml of concentrated hydrochloric acid. A copious precipitate of sodium chloride forms but does not interfere with either complete phase separation or extraction of protactinium. Extract with 50 ml of 50% diisobutylcarbinol (Union Carbide Chemicals Co., New York, N.Y.) in xylene, scrub the organic phase twice with 50 ml of 8M hydrochloric acid, and finish as described previously for the separation of 233Pafrom 2:i7Np( 3 ) . This procedure is virtually specific for protactinium, separating it from most other alpha emitters. Protactinium-233. Prepare the solution of 28SNpin hydrochloric and hydriodic acids, separate both 23iNp and ':):'Pa from plutonium to decrease the hazard from possibly large quantities of ":IRPu, and extract the 233Pa from its 23iNp parent in hydrochloric and sulfuric acids with diisobutylcarbinol as described previously ( 4 ) . Add 0.5 gram of hydroxylamine hydrochloride to the ',''Wp solution prepared for milking instead of the sodium acid sulfite called for. Scrub the diisobutylcarbinol extract with 50 ml of 8 M hydrochloric acid for 5 minutes on a mechanical shaker and reserve the aqueous phase for subsequent evaporation to recover the approximately 0.4% of the 23iNp that it contains. During the extractions, place the separatory funnel in a 4- by 2- by ll:y&inch polyethylene bag and place another bag down over the funnel from the top, overlapping the two bags, at least through the first scrub. This simple precaution virtually eliminates the hazard of handling the relatively high-level alpha activity and prevents contamination of the laboratory during the extraction without need for glove boxes. Scrub the organic phase vigorously three additional times with 50 ml of the protactinium wash solution for 5 minutes each time on a mechanical shaker, adding 10 drops of fresh 2096 titanium trichloride (Fisher Scientific Co., Fair Lawn, N.J., SO-T-43) to the first scrub only. If the titanium trichloride is not completely clear, centrifuge or filter the reagent before use or a significant quantity of hydrolyzed titanium will become entrained in the organic phase and will strip out in the final tracer solution. Rinse the stopper of the separatory funnel thoroughly after each scrub. Also, lomen the stopcock retaining nut and a!low part of the last scrub to drain out between the stopcock plug and housing directly to the sink to remove any traces of the original high-activity solution of 2,i7Npthat might have seeped into that space. Strip the 2:13Pafrom the organic phase with 50 ml of water containing 3 grams of sodium sulfate and 5 ml of sulfuric acid and finish as described previously ( 3 ) . A yellow color in the pyrosulfate melt indicates the presence of a significant quantity of titanium. Filter the final tracer solution through a DM-450 membrane filter to remove traces of silica or other insoluble material that might be present. Otherwise, protactinium will adsorb strongly on the silica and a highly inhomogeneous solution will result. Cranium-232, 1000, 100, a n d 20 dpmlml. Fuse 5 X lo4 dpm of 232U with 3 grams of anhydrous potassium sulfate and 3 mi of concentrated sulfuric acid and precipitate barium sulfate by dropwise addition of two 1-ml portions of 0.4590 barium chloride as described previously (8)to separate all other elements precipitable with barium sulfate from the uranium, using 1 drop of 30% hydrogen peroxide to keep the uranium in the hexavalent state. Centri(8)
c. W. Sill and R. L. Williams,
Anal. Chern., 41, 1624 (1969)
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fuge and decant the solution back into the Erlenmeyer flask, discarding the barium sulfate. Repeat the precipitation of barium sulfate with two more 1-ml portions of barium chloride to scavenge the last traces of impurities, centrifuge, and decant the supernate back into a clean 250-ml Erlenmeyer flask. Add enough potassium metabisulfite to give an odor of sulfur dioxide to ensure reduction of the hydrogen peroxide present and evaporate the supernate to 30 ml. Add 3 drops of fresh 20% titanium trichloride to reduce uranium to the quadrivalent state, and again precipitate barium sulfate with two more I-ml portions of barium chloride to precipitate the uranium. If more than about 1 mg of uranium is present, the number of portions of barium chloride solution must be increased proportionately. Centrifuge and wash the barium sulfate with 5 ml of 0.5% sulfuric acid, discarding the supernate and wash. Dissolve the barium sulfate in 1 ml of boiling 72% perchloric acid and complete as described below for 236Puexcept substitute 10M hydrochloric acid for the 8M nitric acid and omit the sodium nitrite. This procedure is virtually specific for uranium, separating it from all other alpha emitters, particularly from 230Th and/or ?‘Pa from which 232Uis produced, and from the thorium and radium daughters into which 232Udecays. Obviously, the same procedure works equally well for the purification of 233Uor for natural, depleted, or enriched uranium, provided not more than about 1 mg of uranium is present. If separation from 231Pais not necessary, the procedure can be simplified by eliminating the barium sulfate separations and extracting the uranium directly from 10M HC1 with Aliquat-336, which will separate the uranium from the thorium and radium daughters and the tervalent actinides but not from protactinium, neptunium, or plutonium, and incompletely from lead. When small quantities of sulfates can be tolerated, a perpetually self-cleaning tracer solution can be prepared in which all of the radioactive daughters of 232U are continuously and automatically removed from the solution without changing the concentration of the 232U.Protactinium and the transuranium elements are also removed under the same conditions. Add 10 ml of concentrated sulfuric acid to 5 X lo3 dpm of 232Uin a 250-ml Erlenmeyer flask and evaporate the solution to fumes of sulfuric acid. Add 25 grams of anhydrous potassium sulfate and 2 drops of 72% perchloric acid and heat gently over a blast lamp while swirling the flask continuously until the perchloric acid has been decomposed completely and a pyrosulfate fusion is obtained. Cool the melt, add 150 ml of water and heat the solution to boiling to dissolve the cake. Cool the solution to room temperature, add 1 ml of 30% hydrogen peroxide and transfer to a 250-ml volumetric flask. Add 20 ml of 0.45% barium chloride dihydrate while swirling the solution vigorously and dilute to 250 ml. Mix thoroughly and pour enough of the suspension for a few days work into a screw-cap vial that will fit the cup of the centrifuge a t hand. Each day that tracer is to be used, shake the solution vigorously to resuspend the barium sulfate and centrifuge the solution for about 5 minutes. Remove suitable aliquots of the supernatant solution without disturbing the barium sulfate. Discard the residual barium sulfate in the vial before refilling from the stock suspension. Neptunium-237. Plutonium-238 is one of the most common impurities to be encountered in the much longer-lived transuranium elements. Because the half-life of 238Pu is only 87.4 years compared to 2.14 X IO6 years for 237Np,commercial preparations of neptunium oxide frequently contain much more activity from the 238Pu present as an impurity than that of the 237Npitself. Consequently, extremely high decontamination factors might be required to reduce the 238Put o acceptable levels for high-sensitivity alpha spectrometry when 237Npis used to prepare standard samples on which L38Puis also to be determined simultaneously. Extract the 237Np into 50 ml of 25% triisoctylamine (Adogen 381, Archer-Daniels-Midland Co., Minneapolis, Minn.) from hydrochloric-hyriodic acid solution and scrub as described previously ( 4 ) .If pure unstabilized hydriodic acid is not available, use 5 grams of ammonium iodide in its place. Strip the 237Npfrom the organic phase with two consecutive 50-ml portions of water and evaporate the combined strips to 1 or 2 ml. Add 50 ml of concentrated hydrochloric acid and 5 ml of 48% hydriodic acid, boil, and repeat the extraction and scrubs. With severely contaminated 237Np,a third cycle might be necessary. The separatory funnel must be cleaned thoroughly, including the stopcock and housing, between extractions. Strip the final extract with water and finish as described beginning with the next to the last paragraph under “Thorium-234.” This procedure separates neptunium from radium, thorium, plutonium, and the tervalent actinides but not from uranium or protactinium. 1428
Neptunium-239. A solution of 239Npprepared according to the previous directions ( 3 ) contained several thousand dpm of alpha activity per milliliter of solution, most of which was 23sPu present as an impurity in the 243Am.Because plutonium is removed efficiently under the same conditions used to extract neptunium, it is generally easier and safer to purify the 243Amthan to determine whether or not purification is necessary. Consequently, during the original preparation of the 243Amsolution, boil the 72% perchloric acid extensively to help dissolve any refractory or polymeric forms of plutonium that might be present before evaporating to a few drops. Dissolve the residual perchloric acid in 20 ml of water and 60 ml of hydrochloric acid instead of the 50 ml of concentrated hydrochloric acid previously recommended. When the 243Amsolution is first prepared, extract its solution in 9M hydrochloric acid for at least 1 minute with each of three consecutive 50-ml portions of 10% triisooctylamine in xylene, each of which has been preequilibrated with 15 ml of 9M hydrochloric acid before use, to remove most of the plutonium and neptunium. Discard the organic extracts. After a suitable period of time for regrowth of the 2.35-day 239Np daughter, shake the solution for 1 to 2 minutes with 50 ml of 10% triisooctylamine in xylene, preequilibrated with 15 ml of 9M hydrochloric acid, instead of the 5% reagent previously recommended (3). Allow the layers to separate for at least 10 minutes and draw the lower aqueous phase into a 250-ml borosilicate glass bottle with a screw cap with a polyethylene liner for storage until the next milking. If the aqueous phase does not separate almost completely clear, shake again less vigorously and/or let stand longer for more complete phase separation before drawing off into the storage bottle. The organic phase decomposes on standing in the strong acid for a few weeks, and the decomposition products decrease the extractability of the neptunium. Shake the organic phase vigorously on a mechanical shaker for 5 minutes with 20 ml of water and 60 ml of concentrated hydrochloric acid, and reserve the aqueous phase for subsequent evaporation to recover the approximately 0.3% of the 243Amthat it contains. Keep the separatory funnel in polyethylene bags through the first scrub as described for 233Pato decrease the hazard. Add 20 ml of water and 20 drops of fresh 20% titanium trichloride to the separatory funnel containing the organic phase and shake vigorously for 5 minutes to strip most of the 24JAm,219Np, and 239Pufrom the organic phase and to reduce plutonium to the tervalent state. Without removing the aqueous phase, add 60 ml of concentrated hydrochloric acid and shake again vigorously for 1 minute to reextract the 239Np,leaving most of the 243Amand 2JgPu in the aqueous phase. Repeat the water strip and hydrochloric acid reextraction twice more for the same times, adding 20 drops of 20% titanium trichloride to the first one but not to the second. One scrub after the last use of titanium is necessary to remove detectable titanium from the final tracer. Rinse the stopper of the separatory funnel thoroughly after each scrub. Also, loosen the stopcock retaining nut and allow part of the last scrub to wash the stopcock and housing as described for 233Pa,Strip the organic phase for 1 or 2 minutes with 50 ml of water and treat as described beginning with the next to the last paragraph under “Thorium-234.” This procedure separates neptunium from radium, thorium, plutonium, and the tervalent actinides but not from protactinium or uranium. Plutonium-236, 500, 50, and 10 dpmlml. Fuse 2.5 X lo4 dpm of 236Pu with 3 grams of anhydrous potassium sulfate and 3 ml of concentrated sulfuric acid, precipitate, and wash barium sulfate to eliminate uranium as described previously (8), using 1 drop of 30% hydrogen peroxide to ensure complete oxidation of the uranium. Add 1 ml of 72% perchloric acid to the centrifuge tube and heat carefully with continuous swirling to dissolve the barium sulfate without letting the solution bump. Cool the solution for 1 minute and dilute quickly with 25 ml of 8M nitric acid containing 1 ml of 25% sodium nitrite. Transfer the solution to a 250-ml separatory funnel with another 25 ml of 8M nitric acid and extract for 2 minutes with 50 ml of 30% Aliquat-336 (NO3) in xylene. Scrub the organic extract for 1 minute each with two successive 50-ml portions of 10M hydrochloric acid, discarding the scrubs. Strip the organic extract for 1 minute, first with 50 ml of solution containing 5 ml of 7290 perchloric acid and 2 grams of oxalic acid and then with 25 ml of water. Treat the combined strips as described beginning with the next to the last paragraph under “Thorium-234,” omitting the extra perchloric acid. The oxalic acid decarboxylates smoothly without special attention. This procedure separates plutonium from uranium, thorium, radium, and all tervalent actinides but not from protactinium or neptunium. Dilute aliquots of the stock solution tenfold and/or
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 11, SEPTEMBER 1974
fiftyfold with 10% nitric acid to obtain additional concentrations of 50 dpm/ml and/or 10 dpm/ml, respectively, depending on the concentations of plutonium to be traced. Use 1 ml of the stock solution for standardization and prepare the others by exact dilution for the most accurate results over the widest range of activities. Plutonium-239. If the tracer might require purification from uranium isotopes, use the full procedure described for 236Pu. Neptunium-237 can be eliminated simultaneously with uranium by boiling the solution of the pyrosulfate cake for 2 minutes with 10 mg of potassium dichromate instead of hydrogen peroxide immediately before the precipitation of barium sulfate. However, most commercial 239Pu tracer usually requires purification only from the 241Amdaughter of 241Pu,and the procedure can be simplified considerably. Obtain the plutonium tracer in 50 ml of 10M hydrochloric acid, add a few drops of 30% hydrogen peroxide, and extract with an equal volume of 30%Aliquat-336 (C1) in xylene to separate plutonium from thorium and transplutonium elements but not from protactinium, uranium, or neptunium. Scrub the organic phase with an equal volume of 10M hydrochloric acid and finish as described for 236Pu. Americium-241 andlor Americium-243. Add 3 grams of anhydrous potassium sulfate and 3 ml of concentrated sulfuric acid to the americium tracer in a 250-ml Erlenmeyer flask and evaporate the solution to fumes. Heat the solution over a Fisher high temperature blast burner until a clear pyrosulfate fusion is obtained to ensure complete dissolution of refractory compounds, particularly of plutonium. Cool the melt, add 0.5 ml of concentrated sulfuric acid and 30 ml of water, and heat to boiling. Add 5 drops of 1%silver nitrate and 0.5 gram of potassium peroxydisulfate and boil the solution for 2 minutes to oxidize the elements from uranium through americium to the sexivalent state (8).Add 1 ml of a 0.48% solution of anhydrous barium nitrate to the boiling solution at about 1 drop every 2 seconds while swirling the flask continuously to precipitate large ter- and quadrivalent elements as described previously (8). Boil the solution for 1 minute and repeat the addition of 1 ml of the barium nitrate solution and 1-minute boiling two more times. Cool the solution for 10 minutes in a cold water bath and transfer to a 40-ml conical centrifuge tube. Centrifuge a t 2000 rpm for 5 minutes and decant the supernate through a 1-inch GA-6 membrane filter (Gelman) in a filtering chimney into a clean 250-ml Erlenmeyer flask to remove a small scum of barium sulfate that clings to the surface of the solution. Discard the barium sulfate which will contain any radium, thorium, protactinium, and the transamericium elements, particularly 244Cm,that might be present. If decontamination from any of these elements greater than IO4 might be necessary, omit the filtration, add an additional 0.25 gram of potassium peroxydisulfate to the supernate, heat to boiling, and repeat the precipitation with three more 1-ml portions of barium nitrate. Centrifuge and decant the supernate back into the flask. Evaporate the supernate in the Erlenmeyer flask back to a pyrosulfate fusion to destroy all excess peroxydisulfate and to decompose sexivalent americium, plutonium, and neptunium to the terand quadrivalent states. Add 0.5 ml of concentrated sulfuric acid and 30 ml of water to the pyrosulfate cake and heat to boiling. Add 2 drops of 30% hydrogen peroxide to ensure complete oxidation of uranium and reduction of americium. Repeat the precipitation of barium sulfate with three separate portions of barium nitrate as described above to precipitate neptunium, plutonium, and americium. Cool and filter the solution through a 1-inch GA-6 membrane filter in a filtering chimney. Wash the flask and filter paper with a few milliliters of 0.5% sulfuric acid. Discard the supernate and washes which will contain any uranium present. Place the membrane filter containing the barium sulfate into a 150-ml beaker and add 1 ml each of concentrated nitric and 72% perchloric acids. Evaporate to fumes of perchloric acid to oxidize the filter and to volatilize as much excess perchloric acid as possible but do not allow the solution to dry even in local spots on the bottom of the beaker. Add 3 ml of concentrated nitric acid and 50 grams of dry aluminum nitrate nonahydrate, cover the beaker with a cover glass, and heat until the solid melts and then just boils. As soon as boiling begins, place the beaker on a clay triangle a t the edge of the hot plate so that very little loss of nitric acid occurs and hold a t that temperature for 2 or 3 minutes until the solution is completely clear. While swirling the solution continuously, add 30 ml of 0.75M acidic aluminum nitrate solution to which 1 ml of 25% sodium nitrite was added immediately before use. Cool the solution to room temperature and transfer to a 250-ml separatory funnel. Rinse the walls of the beaker with another 20 ml of 0.75M aluminum nitrate, cool thoroughly, and add 5 ml of 25% sodium ni-
trite. Swirl just once and add the rinse quickly to the separatory funnel to minimize loss of nitrous acid. Extract the solution for 2 minutes wth 50 ml of 30% Aliquat-336 (Nos) in xylene. Discard the organic phase which contains nearly all of the plutonium and neptunium, about 3% of the americium, and any traces of uranium, thorium, and protactinium that might have come through the previous separations. Add 50 ml of 2.5M acid-deficient aluminum nitrate to the aqueous phase and repeat the extraction with 50 ml of fresh 30% Aliquat-336 (N03) in xylene for 2 minutes to extract americium. Scrub the organic phase four consecutive times for 1 minute with 15 ml each of 10.8M ammonium nitrate to remove entrained aluminum and barium. Strip the organic phase with successive 30-ml and 10-ml portions of 8M nitric acid and discard the organic phase which contains the small quantities of neptunium and plutonium that were not removed in the first extraction. Add 5 ml of concentrated hydrochloric acid to the combined strips and treat as described beginning with the next to the last paragraph under “Thorium-234.’’ The double evaporation with nitric and hydrochloric acids is necessary to remove the relatively large quantity of ammonium salts that will be present. This procedure separates americium from all other alpha-emitting nuclides, particularly 238Pu and 244Cm which have been encountered most frequently in commercial americium tracers. Americium-241 solutions should be purified even though an alpha spectrum shows the presence of only a single peak a t 5.48 MeV, unless 238Pu is specifically known to be absent. Curium-244 a n d Californium-252. Follow the procedure described for americium tracers above through the first paragraph, except do not discard the main barium sulfate containing the transamericium elements. Discard the barium sulfate scavenge if used. Suspend the precipitate in the centrifuge tube in 10 ml of 0.5% sulfuric acid and filter through a 1-inch GA-6 membrane filter in a filtering chimney. Wash filter and tube with a few milliliters of. 0.5% sulfuric acid. Treat the barium sulfate as described beginning with the third paragraph of the americium section. This procedure separates curium and californium from all other alpha emitters except the transcurium elements. Most of the thorium and protactinium, and any trances of uranium, neptunium, and plutonium precipitated in the first barium sulfate precipitation, are removed in the first extraction. The last traces remain in the second extraction after stripping the tervalent actinides.
RESULTS AND DISCUSSION W i t h the g a m m a - e m i t t i n g tracers, at least lo4 d p m was desired p e r r u n so that the relative standard deviation in the yield d e t e r m i n a t i o n could be k e p t less than a b o u t 1%i n a c o u n t i n g t i m e of 10 m i n u t e at 10% counting efficiency t o accommodate the most precise work likely to be e n c o u n tered. With a l p h a - e m i t t i n g tracers, only 10 to 100 dpm are r e q u i r e d , and the purification r e q u i r e m e n t s a r e correspondingly less stringent. The q u a n t i t y of tracer t o be used s h o u l d be e q u a l to o r g r e a t e r than that of the isotopic activi t y being d e t e r m i n e d so that the statistical u n c e r t a i n t y in the yield d e t e r m i n a t i o n will n o t b e larger than that of the nuclide being d e t e r m i n e d . To p e r m i t t w o o r m o r e tracers to be used simultaneously i n the same s a m p l e , the alphae m i t t i n g parents a n d / o r d a u g h t e r s of the tracers, or a n y other i n a d v e r t e n t alpha-emitting impurities, s h o u l d be virtually u n d e t e c t a b l e i n the q u a n t i t y of t r a c e r used i n a 103m i n u t e c o u n t , o r less than a b o u t 0.01 d p m . L a r g e b l a n k corrections raise t h e detection l i m i t and decrease the precision and accuracy of the d e t e r m i n a t i o n . E v e n a f t e r the init i a l purification has been carried o u t successfully, ingrowth of n e w radioactive d a u g h t e r s m u s t also be considered in det e r m i n i n g how long a purified solution can be used before repurification becomes necessary. Thorium-234 Tracer. No difficulty is e n c o u n t e r e d i n s e p a r a t i n g the 234Thf r o m the original 23sU and 234U so that n e i t h e r can be d e t e c t e d in a 103-minute a l p h a spect r u m . Also, the 234Uthat will have regrown i n t o the p u r i fied 234Tht r a c e r a f t e r 48.2 d a y s or t w o half-lives, is only a b o u t 3 counts i n lo3 m i n u t e s at 25% c o u n t i n g efficiency for lo4 dpm of 234Thtracer. By f a r the m o s t serious and limiting consideration is the
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ingrowth of,230Thfrom the 234Uon the ion exchange column since the last elution. At a counting efficiency of 25%, lo4 dpm of 234Th tracer will contain 2.1, 2.9, and 21.7 counts of 230Thin lo3 minutes after growth times of 1 day, 24.1 days, and 1 year, respectively, after the last elution. If several months or years have elapsed since the last milking, which would result in an undesirably large blank correction, the column should be eluted and the tracer discarded at least a month before the tracer is to be needed, to permit ingrowth of fresh 234Threlatively free of 230Th. Uranium is available commercially (National Lead Co., Albany, N.Y.) that is greatly depleted in 2a4U during production of enriched uranium and will give a reduction of about eightfold in the relative quantity of 230Th produced in a given time compared to that obtained from natural uranium. Unfortunately, much of the depleted uranium currently available is a mixture of pooled material from several sources, some of which have been irradiated in a reactor and contain a small quantity of 232Uwhich gives rise to 228Thand daughters in the final 234Thtracer. However, after the initial purification, regrowth of the 1.9-year zz8Th will be relatively slow, and the slight interference from 228Th and its daughters is more than offset by the much greater reduction in the 230Th activity resulting from use of depleted material. Protactinium-233 Tracer. One of the most significant findings in the present investigation on 233Pa is the confirmation of the stability of protactinium in highly-acidic sulfate solutions. Neptunium-237 solutions over 8 years old gave the same efficient extractability of the 233Padaughter into diisobutylcarbinol as they did when first prepared in the original work ( 4 ) , over 99% of the protactinium still being extractable in 1 minute of vigorous shaking. Not the slightest indication of the hydrolytic or capricious behavior so characteristic of protactinium in other media has been observed. The main difficulty encountered in purification of the 233Pa tracer was in obtaining adequate decontamination from the 237Npparent. The ratio of 237Np found in each scrub to that in the subsequent one decreased progressively from as much as 300 for the original extraction and first scrub to less than 2 for the fourth and fifth scrubs. This effect suggests either that the extractable species are changing or that small residual quantities of neptunium are being removed very slowly from the organic phase or from some other source such as ions sorbed on the container walls, precipitates, etc. Because quadrivalent neptunium is relatively easily oxidized to the pentavalent state, which is extractable, a strong reducing agent must be present. Titanium trichloride seems to be more effective than either hydroxylamine or sulfite in increasing the decontamination of the organic phase from neptunium, suggesting that the oxidation mechanism is at least partially correct. Tervalent titanium does not reduce protactinium to an inextractable oxidation state. However, tervalent titanium is extracted slightly into diisobutylcarbinol so that the quantity used must be kept small and its use should be followed by two scrubs to keep contamination of the final product by titanium at an acceptable level. The procedure described gives an overall decontamination of about 108 from 237Np. As might be expected from its half-life of 1.62 X lo5 years, ingrowth of 233U is not very rapid. After 54.0 days, or two half-lives of 233Pa,104 dpm of 233Pawill contain only 3 counts of 233Uin lo3 minutes at 25% counting efficiency. Because 75% of the 233Pa tracer will have decayed by this time, new tracer should be prepared. The 233U grown into the 237Npsolution is extracted by diisobutylcarbinol to the extent of only a few percent, and that which is extracted is easily removed in the scrubs. 1430
232UTracer. Because z28Thgrows back into the 232Usolution at a rate of 0.10% of the equilibrium activity per day, the 232Umust be purified frequently for certain applications such as the simultaneous determination of both uranium and thorium on the same sample, necessitating restandardization after each repurification, or for standardization by 2-a counting. The self-cleaning solution described above is very efficient and convenient and can be prepared by exact dilution of the purified solution which has been standardized by 2-a counting. Only about 0.4% of the 232Uis precipitated from the hexavalent state with the barium sulfate. Alternatively, an aliquot of the self-cleaning solution can be electrodeposited and standardized directly by 2-a counting, taking the necessary precautions to correct for incomplete electrodeposition. Neptunium-239 Tracer. As with 237Np,the small quantity of 243Amdistributing into the organic phase during the initial extraction from the highly active solution is very difficult to remove completely. The separation factor for 243Am also decreased progressively with each successive scrub. Because americium does not have an oxidation state that can be achieved under the conditions used from which significant extraction can occur, as does neptunium in the diisobutylcarbinol system, poor reaction kinetics are particularly indicated in this case. Also, unless reduced to the tervalent state, the 239Pudaughter extracts quantitatively with the 239Nptracer. If iodide is used to reduce plutonium to a nonextractable state without reducing neptunium, as is generally recommended, removal of both the tervalent americium and plutonium from the organic phase is further retarded by the iodide extracted into the amine. Originally, it was necessary to strip the tracer and residual impurities from the organic phase with water, evaporate with hydrochloric and hydriodic acids and reextract into fresh amine to achieve the necessary purification. However, both the 239Np tracer and the 243Am and 239Pu impurities can be stripped from the organic phase with water and the plutonium reduced with titanium trichloride in the aqueous phase, thus markedly facilitating their removal from the organic phase. After addition of hydrochloric acid, the neptunium is reextracted rapidly and quantitatively into the same organic phase without significant reextraction of the tervalent impurities. This reversion technique gives essentially the effect of recycling but without having to change either separatory funnel or extractant. The significant reduction in time is a distinct advantage in preparation of a tracer with a half-life of only 2.35 days. Titanium trichloride reduces plutonium quantitatively to the nonextractable tervalent state in either dilute or concentrated hydrochloric acid without interfering in any way either with extraction of the quadrivalent neptunium or with scrubbing of the tervalent americium and plutonium from the organic phase. Neither ter- nor quadrivalent titanium is extracted significantly under the conditions used. The decontamination factors for the procedure described are about 2 X 108 for 243Amand about 8 X lo3 for 239Pu. For most work, the second titanium-containing strip can be omitted with a decrease in the decontamination factor of only about 10 to 20. Ingrowth of 239Puinto the 239Nptracer must be carefully controlled when using 239Np tracer in a sample also being analyzed simultaneously for 239Pu.After 5 days’ growth, an initially pure solution of 239Np will contain 2.2 counts of 239Pu per l o 3 minutes at 25% counting efficiency for each 104 dpm of 239Nppresent. If 239Pu is to be determined on the same sample being traced with 239Np,a correction will have to be made if the tracer is used for much longer times or if more than lo4 dpm is used. However, because 77% of the 239Npwill have decayed in 5 days, it will be preferable
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to prepare new tracer than to purify the old. The tendency observed previously ( 3 ) for neptunium t o become less extractable with increasing time since the last milking has been confirmed. An 243Am solution about 2 years old containing a small quantity of dirty-colored organic phase from the previous extraction gave a recovery of only 27% of the 239Npin 1 minute of vigorous shaking with an equal volume of 25% triisooctylamine in xylene. After evaporating the solution with 5 ml of 72% perchloric acid to near dryness and dissolving the residue in 50 ml of concentrated hydrochloric acid, the original efficiency of over 98% extracted in 1 minute with only 5% extractant was restored completely. The remedial effect of oxidation by perchloric acid strongly suggests that the obvious decomposition products of triisooctylamine are responsible for the decreased extractability, probably through formation of a competing nonextractable complex. Phase separation was markedly better from 9M hydrochloric acid than from the concentrated hydrochloric acid used previously ( 3 ) .The recovery of 239Npis also better than 99% a t the lower hydrochloric acid concentration, a t least partly because of the improved phase separation. Most importantly, the aqueous phase, which separates nearly clear, shows no organic decomposition products after storage for a t least 6 months, and no detectable decrease in extractability of 239Np. Plutonium-236 Tracer. Plutonium-236 decays with a 2.85-year half-life into 72-year 232Uwhich in turn decays t o 1.91-year 228Thand subsequent daughters in the natural thorium chain. Consequently, when uranium isotopes are being determined on the same sample being traced with 236Pu, ingrowth of 232U must be considered. About 10 dpm of 236Puis recommended for tracing 239Puin 10-gram samples of soil ( I ) . At 45 days after purification, 10 dpm of per lo3 minutes a t 25% 236Pu will contain 3 counts of 232u counting efficiency. If 232U is being determined, larger quantities or longer growth times will require correction for the 232Ubeing added with the 236Putracer. However, 232U is used much more frequently as a tracer in the determination of the other uranium isotopes, about 25 dpm being rec-
ommended for 10-gram samples of soil ( I ) . Even a t 2 years after purification, 10 dpm of 236Pu will contain only 0.25 dpm of 232U.Although giving an easily detectable 61 counts in lo3 minutes, this quantity of 232Ucauses an error of only 1%in the yield determination when 25 dpm of 232Uis used. Such a small error can usually be ignored and even larger ones corrected for adequately by calculation from the date of purification. Americium-243 Tracer. Americium-243 is widely used as a tracer in the determination of 241Am, and decays through 239Npto 239Pu.Because of the increasing practice of determining both 241Amand 239Pusimultaneously in the same sample, it is indeed fortunate that the rate of ingrowth is negligible. About 42 years are required for 10 dpm of 243Amto produce 3 counts of 239Puin I O 3 minutes at 25% counting efficiency. Although the separation of curium from americium is normally very difficult, the separation described removes 99.99% of the curium easily and reliably in a single operation (8). No indication of inhomogeneity has ever been observed as a consequence of having evaporated any of the tracer solutions just to dryness to remove perchloric acid, so long as the heating is kept minimal and the residues are subsequently boiled extensively with concentrated nitric acid. Using pipets treated with a silicone water repellent to eliminate drainage and demonstrated to give aliquots reproducible to 0.1%, triplicate 1-ml aliquots standardized by direct evaporation on stainless steel plates and 2-.rr counted have invariably agreed with each other within the counting statistics, even when counted to as many as 106 total events each. However, it should not be inferred that the final tracers are necessarily in any particular ionic form. I t is assumed that the analyst will take whatever steps might be necessary to ensure complete exchange and/or the necessary ionic species before expecting the tracers to undergo chemical reactions characteristic of the particular element.
RECEIVEDfor review November 30, 1973. Accepted April 30, 1974.
Automated Atomic Absorption Determination of Arsenic, Antimony, and Selenium in Natural Waters P. D. Goulden and Peter Brooksbank Canada Centre for Inland Waters. P.O. Box 5 0 5 0 . Buriington. Ontario. Canada
A method is described for the determination of sub-microgram levels of antimony, arsenic, and selenium in natural waters. Stibine, arsine, and hydrogen selenide are produced from the samples in an automated system and passed to a tube furnace mounted in the light path of an atomic absorption spectrophotometer. The use of a tube furnace as a covalent hydride decomposition device gives an increase in sensitivity over a conventional hydrogen-argon-entrained air flame of at least two orders of magnitude. The method will analyze 40 samples an hour with a limit of detection of 0.1 pg/l. for arsenic and selenium and 0.5 pg/l. for antimony. With a dual-channel spectrophotometer, simultaneous determinations of arsenic and selenium have been made on a large number of natural water samples.
The levels in the environment of such elements as arsenic, selenium, and antimony are of considerable interest because of their potential toxicity and carcinogenic properties. In this laboratory, concerned with the monitoring of natural waters across Canada, a need is seen for sensitive automated methods for the determination of these elements. There are sensitive methods available for selenium and antimony, e . g . , fluorometry ( I , 2 ) , while arsenic can be determined colorimetrically ( 3 ) . However, these methods, involving several procedures are quite time-consuming. Another technique used is the conversion of the (1) J. W. Wiersrna and G. F. Lee, Environ (1971). (2) T. D. Filer, A n a / . Chem . 43, 725 (1971). (3) J . F. K o p p . A n a / Chem.. 4 5 , 1786 (1973).
S o . Techno/. 5 , 1203
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