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Anal. Chem. 1980, 52, 1452-1459
Determination of Gross Alpha, Plutonium, Neptunium, and/or Uranium by Gross Alpha Counting on Barium Sulfate Claude W. Sill' Radiological and Environmental Sciences Laboratoty, Department of Energy, Idaho Falls, Idaho 8340 1
Procedures are presented for rapid and specific separation of uranlum, neptunium, and/or plutonium, either singly or sequentialty by precipitationwith barium sulfate under controlled oxidation conditions in a form suitable for direct a counting. I f desired, ail a emitters can be precipitated and counted simultaneously to permit a true gross cy determination to be made. When isotopic identificationis necessary or desirable, the radionuclides other than uranium can be separated easily from the barium sulfate and precipitated wlth a small quantity of cerium hydroxide for analysis by alpha spectrometry with a resolution of about 65 keV. The samples are dissolved by fusion with anhydrous potassium fluoride and pyrosuifate to ensure complete dissolution of refractory oxides, carbides, and silicates. The effect of hydrochloric acid on the precipitation of barium sulfate, of radon and thoron daughters on cy counting, and of various experimental conditions on the counting efficiency are also discussed.
T h e determination of environmental levels of plutonium and other actinides in soil is generally complicated and expensive because of the large samples and extensive separations required to achieve t h e desired sensitivity and specificity. Most procedures include electrodeposition of the separated activity and final measurement by a spectrometry ( I ) , both of which are time-consuming and require relatively expensive instrumentation. However, there are many situations in which either much higher levels are present or in which only higher levels are of interest, permitting use of much smaller samples and simpler separations. Such situations might include mapping and/or surveillance activities around radioactive waste burial grounds, accident sites, experimental sites, etc. If isotopic identification on every sample is not necessary, both electrodeposition and a spectrometry can be avoided and the final separated fraction counted in a gross a counter, resulting in substantial savings in time and expense. When necessary, the isotopic composition can be determined or verified by further chemical separations and a spectrometry. T o use overly elaborate and sensitive procedures in such situations is not only unnecessary but extremely wasteful in terms of both effort and expense that could be used more profitably in other ways. Similar considerations apply to other types of samples such as urine, feces, water, vegetation, soft tissue, etc. in which small samples would suffice, or to samples such as seston, plankton, air dusts, etc. in which only small samples are likely to be available. Rapid methods are also particularly desirable in emergency situations. I t has been shown previously that small quantities of all elements from lead through californium, except astatine, radon, and francium, can be carried efficiently in a sufficiently small quantity of barium sulfate to permit direct cy counting Present address: EG&G Idaho, Inc., P.O. Box 1625, Idaho Falls, Idaho 83415.
of the precipitate ( 2 , 3 ) . It has also been shown that uranium, neptunium, plutonium, and americium are the only elements that can be oxidized to a higher state that is not precipitated with barium sulfate and subsequently reduced to one that is carried quantitatively ( 2 , 3 ) . Any of these four elements can be separated either from other elements or from each other by proper choice of oxidizing and reducing agents to control the relative oxidation states before precipitation of barium sulfate. However, hydrogen peroxide should not be used t o reduce neptunium and plutonium while keeping uranium in the hexavalent state as recommended previously (2, 3 ) . In the presence of iron, hydrogen peroxide oxidizes quadrivalent plutonium to the hexavalent state in part ( 4 ) , and seriously low results are obtained. In applying these principles to soils, feces, water, etc., i t is generally not possible t o oxidize the transuranium elements before addition of barium because the calcium invariably present precipitates from the sulfate solution required and causes significant precipitation of ter- or quadrivalent neptunium, plutonium, or americium before their oxidation can be achieved. Hydrochloric acid must be used to prevent precipitation of calcium sulfate which then interferes with the use of the powerful oxidants such as permanganate and silver-catalyzed peroxydisulfate required for the oxidations. In addition, barium is frequently present in the sample in sufficient quantity to precipitate as barium sulfate, and the occluded elements are not available for oxidation even if hydrochloric acid were not present. Consequently, a reverse order must be used in which all precipitable elements except uranium are precipitated on barium sulfate from a hydrochloric acid solution and then the desired elements are removed from the barium sulfate by dissolution and selective oxidation. Uranium can be precipitated separately from the filtrate from the original precipitation after reduction.
EXPERIMENTAL Instrumentation. The instrumentation and technique used for gross a counting using individual die-cut phosphors in a scintillation counter have been described ( 2 ) . Gas-flow proportional counters should not be used because the soft barium sulfate tends to blow around, giving significant losses and contamination of the counter. The results are also more erratic, probably due to charge effects of the nonconducting precipitate and filter support. The a spectrometer using a 450-mm2 surface-barrier detector and a 1024-channel pulse height analyzer has also been described ( I ) . A windowless 2-n proportional flow counter using methane as the counting gas was used in the standardization of the tracers. Reagents. Seeding Suspension. Add 50 mL of 0.45% barium chloride dihydrate, 12 g of anhydrous potassium sulfate, and 6 mL of concentrated sulfuric acid t o a 250-mL Erlenmeyer flask and evaporate the solution until the barium sulfate just redissolves. Do not allow much of the sulfuric acid to evaporate or some of the barium sulfate will precipitate from the concentrated acid in the wrong crystalline form. Cool the flask, add 80 mL of water, and heat the solution to boiling. As soon as the cake has dissolved, cool the suspension of barium sulfate t o room temperature and dilute to 100 mL. Shake the suspension thoroughly before each use. The seeding ability of the suspension does not decrease significantly for a t least several months.
This article not subject to U.S. Copyright. Published 1980 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
Reprecipitating Solution. Dissolve 60 g of anhydrous potassium sulfate in 400 XI& of water and 20 mL of concentrated hydrochloric acid and dilute to 500 mL. Tellurous Acid, 5 mg/mL Te. Add 25 mL of concentrated hydrochloric acid to 625 mg of Te02in a 50-mL beaker and heat until the solution is completely clear. Cool to room temperature and dilute to 100 mL. Uranium Reductant. Mix equal volumes of 20% aqueous titanium trichloride and 1-to-1 sulfuric acid. Titanous sulfate precipitates from the solution on standing for a few hours so that only the quantity needed in a short period of time should be prepared. Radioactioe Tracers. Prepare solutions of purified uranium233, neptunium-237, and/or plutonium-239 in 5% nitric acid containing about 5 x lo4 dpm/mL as described previously ( 5 ) . Prepare a solution of natural uranium by dissolving 0.118 g of pure U308in hot concentrated nitric acid. Adjust the volume of concentrated nitric acid remaining to about 5 mL and dilute the solution to 100 mL to give a final concentration of 1 mg/mL of uranium in 5% nitric acid. Although generally not necessary, the uranium can be purified from its thorium and radium daughters by extracting with Aliquat-336 from 10 M hydrochloric acid as described for uranium-232 (5). However, the barium sulfate separation must be omitted because the quantity of uranium present is too large to be carried on a resonable quantity of barium sulfate. Standardization. Standardize the purified tracer solutions by evaporating 250-pL aliquots directly onto stainless steel plates and counting in a 2-a counter that has been standardized against a National Bureau of Standards source (1, 6). Determine the isotopic composition by counting the plates in an CY spectrometer and correct the 2-n values for the fraction of the total activity coming from the nuclide of interest. If the resolution of the CY spectra from the evaporated sources is not good enough to resolve the isotopes present completely, e.g. with natural uranium, an electrodeposited source will also have to be prepared. Instrument Calibration. Determine the counting efficiency of the scintillation counter for each radionuclide to be determined by precipitating aliquots of the standardized tracer solution on barium sulfate and counting for sufficient time to obtain the statistical precision desired. Differences of 5 to 25% in the counting efficiency can easily result from small changes in the experimental conditions so that precipitation, mounting, and counting of the barium sulfate must be carried out under the exact conditions described below for analysis of actual samples. All significant random uncertainties incurred anywhere in the entire measurement process from instrument calibration and standardization of tracers through the analysis must be propagated (7) to give an estimate of the random uncertainties in the final analytical result ( 2 , 8). Most commercially available die-cut phosphors change markedly from one to another in both background and sensitivity, and must be screened before use for the most accurate and reliable results. Prepare an electrodeposited source of plutonium-239 or americium-241 containing at least 2 x IO4 cpm, preferably distributed over an area approximately the same as that of the barium sulfate precipitates to be cou@d. Rub the deposited area strongly with coarse laboratory tissue until little or no more activity can be removed. Count a group of phosphors with this source for a t least 10 min each and reserve them for use with standards and other higher-level samples substantially above background on which higher precision and accuracy is desired. The values obtained can then be used to correct subsequent measurements for the relative sensitivity of each individual phosphor used to some arbitrary value. Preserve both the source and a standardized phosphor for subsequent use in periodically rechecking the intrumental sensitivity a t which the counting efficiency on barium sulfate was determined. Do not use an actual barium sulfate source for this purpose. Barium sulfate precipitates containing CY activity almost invariably increase in counting rate each time they are used, or on standing after mounting, apparently because of slight uncontrollable pressure on the mounted source or other subtle changes in the intimacy of contact between the soft precipitate and the phosphor. Counting Efficiency for Uranium. Add 1 mL of the natural uranium solution, 250 pL of the standardized uranium-223 tracer,
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1mL of a solution of 7.5 g of calcium carbonate in 100 mL of 15% nitric acid, 4.5 g of anhydrous potassium sulfate, and 2 mL of concentrated sulfuric acid to a 250-mL Erlenmeyer flask. Evaporate the solution to near dryness on a hot plate and then heat carefully over a blast burner while swirling the flask continuously until a pyrosulfate fusion is obtained. Add 2 g of anhydrous sodium sulfate and continue heating strongly until both the added salt and the calcium sulfate that will have precipitated have dissolved completely. Cool the flask and add 35 mL of water, 5 mL of concentrated hydrochloric acid, and 2 or 3 silicon carbide boiling chips. Heat the solution to boiling as rapidly as possible with frequent swirling to facilitate complete dissolution of calcium sulfate. When the cake has dissolved, add 1g of potassium nietabisulfite slowly while swirling the solution continuously. Rinse the walls with 5 mL of water to remove any solid metabisulfite and boil the solution actively for 3 min to volatilize sulfur dioxide. Add in order 2 drops of 1%aqueous Safranine 0 (National Aniline Division, Allied Chemical and Dye Corp., New York, N.Y., C.I. 841),0.25 mL of 3 M chromous chloride (9) (8 small drops from the polyethylene delivery tube), 0.20 mL of 20% titanium trichloride (4 large drops from a polyethylene medicine dropper), and 1 mL of the seeding suspension. Add two 1-mL portions of 0.45% barium chloride dihydrate to the boiling solution a t a rate of one drop every 2 or 3 s while swirling the solution vigorously. After each 1-mL addition, boil the solution actively for 1 min after boiling resumes. Precipitate only two samples a t a time so that the schedule of boiling one while precipitating the other can be maintained and excessive evaporation avoided. Centrifuge, wash, mount, and count the barium sulfate on a 47-mm filter as described previously ( 3 ) . However, let the solution cool for only a couple of minutes, just so that it can be handled comfortably, and centrifuge while still hot to minimize the chance of precipitation of calcium sulfate. The VM-6 vinyl membrane filter called for has been redesignated as DM-450 (Gelman Sciences, Inc., Ann Arbor, Mich.). Add the activity of the natural uranium to that of the uranium-233 in calculating the efficiency. Natural uranium emits 1516 dpm/mg of total uranium 01 activity if the uranium is in secular equilibrium. However, the uranium isotopes in many uranium ores are not in secular equilibrium so that the fact of equilibrium should be confirmed, or a correction determined, by a spectrometry on an electrodeposited source. The counting efficiency obtained as above should be used with samples of uranium ore containing about 30 mg of calcium (Le., 1 g of sample containing 3% calcium) and a total of 1 mg of uranium as dicussed below. When soils or other samples containing very little uranium are to be analyzed, omit the natural uranium in the determination of the counting efficiency. Barium sulfate precipitated in the presence of the seeding suspension is much more finely divided, and has a much greater tendency to run around the edge of the fiiter in a filtering chimney, than with the coarser precipitate obtained from hot dilute hydrochloric acid without seeding. Using ethanol from a polyethylene squeeze bottle, position the filter and chimney in place, apply suction from a water aspirator, and suck the filter tightly against the support. Disperse the barium sulfate thoroughly in about 15 mL of ethanol by directing a forceful jet directly at the precipitate in the bottom of the centrifuge tube. Gently disconnect the suction, squirt a few milliliters of ethanol into the filtering chimney, and transfer the barium sulfate suspension to the chimney using more ethanol from the squeeze bottle to make the transfer quantitative. Direct a small stream of ethanol randomly into the chimney to homogenize the suspension. Without delay, hold the chimney down tightly against the filter with one hand and connect the suction with the other hand as quickly as possible to minimize leakage around the filter. Otherwise, losses of 25 to 50% have been obtained. A filtering chimney with a clamp is highly desirable. Draw the ethanol through the filter. Remove the filter and air dry for 5 min, then heat under a 250-W infrared lamp a t a distance of about 6 inches for not longer than 10 min to avoid curling. Mount and count as described. Correct the counting efficiency for losses occurring in the separations. Decant the supernate and wash from the barium sulfate precipitation back into the same Erlenmeyer flask in which the precipitation was made. Place the same flask under the filtering chimney to receive the ethanol transfer liquid during
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
filtration. If any small specks of barium sulfate adhere to the underside of the filtering chimney, rinse them into the flask with a little ethanol. After the barium sulfate has been removed from the centrifuge tube for mounting, rinse the residual ethanol from the tube with a few milliliters of water and add to the flask. Add 1 mL of concentrated sulfuric acid and 1 drop of concentrated nitric acid to the centrifuge tube to oxidize residual ethanol and heat carefully with vigorous agitation to prevent bumping until the refluxing acid has dissolved any barium sulfate remaining on the walls of the tube. Cool the acid and rinse into the flask containing the other solutions. Add a couple of silicon carbide boiling chips and heat the solution to boiling. Shortly before the solution begins to boil, ignite the vapors coming from the flask to avoid any possibility of formation of explosive esters from ethanol and nitric or particularly perchloric acids that might be in the ducts. Evaporate the solutions as far as possible on the hot plate and then to a pyrosulfate fusion strongly over the full heat of the blast burner to ensure subsequent complete dissolution of anhydrous chromic sulfate. Dissolve the pyrosulfate cake in 35 mL of water and 5 mL of concentrated hydrochloric acid. Add 2 drops of Safranine 0 and 4 drops of excess chromous chloride to reduce the titanium present as a holding reductant and repeat the precipitation of barium sulfate under the same conditions used initially. Count the precipitate and add its activity to that from the main precipitation. With care, losses can be kept to 170or below, but the correction should be made to ensure that the correct counting efficiency is used. Counting Efficiency for Gross Alpha. Follow the same procedure described above for uranium, omitting addition of the natural uranium except when the procedure is to be applied to uranium ore. Similarly, omit the calcium when the samples to be analyzed do not contain significant quantities. Tracers other than uranium-233 can also be used as desired. If polonium-210 is to be included in the determination, add 200 pL of tellurous acid solution just before the addition of Safranine 0. Polonium is precipitated with barium sulfate to about 92% in a straight sulfate system (3)but scarcely at all in the presence of hydrochloric acid. Counting Efficiencyfor Plutonium plus Neptunium. Add 2 mL of 72% perchloric acid to 250 pL of the standardized plutonium-239 tracer solution in a 50-mL beaker and heat to strong fumes of perchloric acid to remove nitric acid. Add 25 mL of water, 0.5 mL of concentrated sulfuric acid, and 3 g of ammonium sulfate, and proceed with reduction and precipitation as described in the third paragraph under “Determination of Plutonium plus Neptunium” below. Recover the activity lost in all minor fractions in this and subsequent procedures to correct the counting efficiency similarly to that described for uranium above. Counting Efficiencyfor Plutonium without Neptunium. Add 250 pL of the standardized plutonium-239 tracer, 2 mL of 0.45% barium chloride dihydrate, and 1mL of concentrated sulfuric acid to a 50-mL beaker and heat until the nitric acid has been volatilized and the sulfuric acid begins to boil to dissolve the barium sulfate and to decompose thermally any plutonium that might be present in the hexavalent state. Cool and add 25 mL of the reprecipitating solution all a t once to reprecipitate the barium sulfate containing the plutonium. Add 2 drops of 25 % potassium metabisulfite, boil the solution for 1 min and centrifuge, mount, and count the barium sulfate for plutonium isotopes as described below. Counting Efficiencyfor Neptunium without Plutonium. Add 250 pL of the standardized neptunium-237 tracer and 2 mL of concentrated sulfuric acid to a 50-mL beaker and heat until the nitric acid has been volatilized and the sulfuric acid begins to boil. Cool the solution for 2 min, add 10 mg of solid potassium dichromate, and allow the solution to cool to room temperature without further heating. Add 25 mL of water, 3 g of anhydrous potassium sulfate and 5 drops of 25% potassium metabisulfite and heat to boiling to reduce all chromic acid before addition of ferrous ammonium sulfate. Proceed with further reduction and precipitation as described in the third paragraph under “Determination of Plutonium plus Neptunium” below. Soil a n d Ores. With most soils and low-grade ores, a 1-g sample can be accommodated easily. With higher-grade ores, particularly samples containing high concentrations of iron and/or other metals, a 0.5-g sample will dissolve more completely and
easily in the quantity of flux used below. When uranium is being determined, the sample size should be limited to that containing about 1 mg of uranium, e.g., 1 g of 0.1% uranium ore, because of the limitation on the quantity of uranium that can be precipitated quantitatively in the barium sulfate permitted for direct (Y counting. Dissolve the sample in potassium fluoride and pyrosulfate fusions, and precipitate barium sulfate as described in the first four paragraphs under “Determination of Thorium-230 in Uranium Ores and Mill Tailings” (IO), omitting the thorium-234 tracer. However, reduce the volume of 25% potassium metabisulfite to 6 drops (0.25 mL). On blanks and low-iron samples or on small samples of higher iron concentrations, omit the potassium metabisulfite and add 3 drops of 20% ferrous ammonium sulfate in 1% sulfuric acid. By reducing the quantity of metabisulfite used relative to the iron present, and using ferrous ammonium sulfate on blanks and low iron samples to reduce plutonium, neptunium, etc., reduction of platinum introduced from the dish during the pyrosulfate fusion to the elemental state can be Virtually eliminated (11). Color photographs of the potassium fluoride fusion and the transposition to the pyrosulfate fusion have been published (22). If the solution of the pyrosulfate cake in dilute hydrochloric acid is clear, indicating the absence of significant quantities of alkaline earths or the light lanthanides, add 1mL of the seeding suspension before precipitating barium sulfate as described in the fourth paragraph of the section in Ref. 10 quoted above to speed up the precipitation of barium sulfate in the presence of hydrochloric acid and heavy elements. Treat the filtrate from the barium sulfate separation for the determination of uranium and the barium sulfate for the determination of neptunium and/or plutonium as described below. If a heating block is available to carry out the perchloric acid oxidation in the next step in a centrifuge tube, suspend the barium sulfate in 10 mL of 0.5% sulfuric acid and recentrifuge, leaving the barium sulfate in the tube. If a heating block is not available, suspend the barium sulfate in 10 mL of 0.5% sulfuric acid and filter on a 47-mm GA-6 membrane filter (Gelman Sciences Inc., Ann Arbor, Mich.) in a filtering chimney, using an additional 10 mL of 0.5% sulfuric acid to wash the centrifuge tube and filter. Miscellaneous Samples. Add 10 mL of concentrated nitric acid to up to 100 mL of water or 50 mL of urine containing less than about 50 mg of calcium in a 150-mL beaker and evaporate to about 10 mL. If much siliceous material is present, continue boiling with concentrated nitric acid as necessary to ensure complete elimination of chlorides and transfer the solution to a 50-mL platinum dish. Evaporate to dryness, add 3 g of anhydrous potassium fluoride and proceed as described for Soil and Ores. If the water sample contains a considerable quantity of iron, e.g., from rusty pipes, and little siliceous material, add 0.25 g of silica gel to neutralize the alkali produced in the potassium fluoride fusion to ensure complete dissolution of the iron (11, 12). If siliceous material is virtually absent, add 2 mL of concentrated sulfuric acid and 4.5 g of anhydrous potassium sulfate to the evaporated solution and continue the evaporation to a pyrosulfate fusion in the 150-mL beaker. Add a few drops of nitric and/or perchloric acids as necessary after the sulfuric acid begins to fume to oxidize all organic matter. After the fusion has been obtained, add 2 g of anhydrous sodium sulfate and continue the fusion until the salt has dissolved in the molten flux. Boil the pyrosulfate cake in 40 mL of water and 5 mL of concentrated hydrochloric acid for 5 min to hydrolyze condensed phosphates, precipitate barium sulfate, and finish as described above for Soil and Ores. Small samples of vegetation ash, soft tissue ash, seston, plankton, air dusts, etc. can be ignited or wet ashed with nitric, perchloric, and/or hydrofluoric acids directly in a 50-mL platinum dish, evaporated to dryness, and fused with potassium fluoride as described for Soils and Ores. Ten-gram samples of feces or other samples low in siliceous material can be wet-ashed and fused with anhydrous potassium sulfate directly in a 150-mL beaker. Determination of Uranium. If uranium is to be determined, treat the supernate decanted from the barium sulfate precipitation without delay. If the solution is allowed to cool and/or stand for some time, calcium sulfate and a small quantity of barium sulfate generally post-precipitate, Because uranium is reduced to the quadrivalent state by either chromous chloride or titanium trichloride, much of the uranium precipitates with the alkaline earth
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
sulfates and will be filtered off with the metallic tellurium in the subsequent step for the removal of polonium and platinum (I). Severe loss of uranium will result as was demonstrated with protactinium-231 (13). If i t is not possible or desirable to proceed with the uranium determination immediately, or if a precipitate forms after centrifuging that does not redissolve completely on heating back to boiling, let the solution stand at room temperature for several hours, or overnight if convenient. Filter the solution through a DM-450 fdter while the uranium is still in the hexavalent state to remove all insoluble sulfates that have precipitated before proceeding with the reduction and filtration of polonium, platinum, and tellurium. Hexavalent uranium precipitates with the insoluble sulfates to only a few tenths percent. The Teflon chimney used for filtering ethanolic suspensions of barium sulfate for a counting (3)can be used if the stainless steel support plate is replaced with one of porous polypropylene to avoid reactions with the strong acids present. Even if the determination of neptunium or plutonium is not desired, the barium sulfate separation must still be made to remove other precipitable ions while the uranium is in the oxidized state. In fact, if relatively large quantities of other non-uranium a emitters are present, or if the most accurate and reliable determination of the smallest quantities of uranium is desired, the filtrate should be cleaned up even further. Evaporate the solution back to 35 mL if necessary, scavenge the solution with two more 1-mL portions of 0.45% barium chloride dihydrate before reduction of uranium, centrifuge, and discard the barium sulfate. Any alkaline earths precipitated from the previous step can be removed simultaneously with the newly precipitated barium sulfate. Cool the filtrate from the barium sulfate separation to at least 50 “C, add 200 pL of the tellurous acid solution and 1 g of solid potassium metabisulfite added slowly while swirling the solution gently. Add two or three silicon carbide boiling stones, cover the beaker with a cover glass, and heat the solution slowly to boiling over a period of about 5 min to reduce most of the iron before allowing the sulfur dioxide to be boiled off. The reduction of iron by sulfurous acid is strongly catalyzed by tellurous acid (1, 13), and the solution becomes black and turbid because of precipitation of elemental tellurium when reduction of iron is essentially complete. Boil the solution vigorously for a t least 5 min until the excess sulfur dioxide has been volatilized, the elemental tellurium has flocculated well, and the volume has been reduced to about 35 mL. Elimination of sulfur dioxide must be relatively complete because both tervalent titanium and particularly divalent chromium added in the next step are oxidized by sulfurous acid with precipitation of elemental sulfur. For the same reason, nitrates and/or nitro compounds from oxidation of organic compounds including filter paper must also be completely absent. When all sulfur dioxide has been expelled, add an additional 2 mL of the tellurous acid solution and 2 drops of 1% aqueous Safranine 0 indicator, and heat the solution back to boiling. Insert the delivery tube of the chromous chloride container (9) under the cover glass a t the pouring lip so that the smooth evolution of water vapor is not disturbed to minimize air oxidation of the chromous ion. Add chromous chloride dropwise until a dense black precipitate of elemental tellurium containing polonium and platinum forms, the last trace of indicator color has been discharged, and 4 drops excess have been added. Sulfurous acid does not reduce polonium to the elemental state under these conditions as it does tellurium so that the additional tellurous acid must be added as carrier before reduction with chromous chloride. Otherwise, precipitation and scavenging of polonium will be seriously incomplete. Add 0.20 mL of 20% titanium trichloride (4 large drops from a polyethylene medicine dropper) as a holding reductant and set the beaker at the edge of the hotplate without further boiling to retard flocculation of the elemental tellurium for 3 or 4 min until precipitation of polonium is complete. Boil the solution gently for 3 min to obtain complete flocculation. Allow the solution to cool for a couple of minutes so that the beaker can be handled comfortably, and filter through a 25-mm DM-450 membrane filter into another graduated 100-mL beaker using an all-glass filtering chimney (Millipore Filter Corporation, Bedford, Mass., catalog no. xx 10 025 00). The Teflon chimney used for mounting barium sulfate (3)should not be used because of the difficulty in removing elemental tellurium from its surface. Wash the beaker and precipitate with a few milliliters of 0.5%
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sulfuric acid. The cover glass, beaker, and filtering chimney must be cleaned with concentrated nitric acid before being reused to remove a slight scum of metallic tellurium invariably dried on the sides or subsequent contamination with polonium-210 will invariably result. Add a couple of boiling chips to the filtrate from the tellurium precipitation and evaporate back to about 35 mL. Add 1 drop of Safranine 0 indicator to c o n f i i complete reduction of uranium and the titanium holding reductant, 1 mL of the seeding suspension, and repeat the precipitation of barium sulfate with two 1-mL portions of 0.45% barium chloride as before. Centrifuge, wash, mount, and count the barium sulfate on a 47-mm DM-450 filter in the scintillation counter as described above for the instrument calibration. However, low-counting samples on which an error of 1 or 2 cpm would be unacceptable should be placed in a vacuum desiccator immediately after filtering to reduce pickup of radon and thoron daughters as discussed below. When 1 mg of uranium is present, the barium sulfate will have a light green color in comparison to a white surface. A rough indication of the isotopic composition can be obtained by examination of the barium sulfate precipitate directly on the filter in an a spectrometer. However, the uranium must be separated from the barium sulfate if well resolved spectra are desired. Traces of both tervalent titanium and chromium will be present in the barium sulfate, and both interfere with electrodeposition of uranium and add considerable absorber. It will generally be preferable to wet ash the filter in nitric and perchloric acids, extract the uranium into Aliquat-336 from 10 M hydrochloric acid as described in the second paragraph under “Uranium-232” ( 5 ) ,and electrodeposit. The barium sulfate can also be dissolved in aluminum nitrate for the extraction as described previously (IO),but the blanks will be much higher because of the uranium present in the aluminum nitrate. Determination of Gross Alpha Activity. The sample taken for analysis must not contain more than a total of 1 mg of thorium, uranium, cerium, lanthanum, and all other elements precipitated efficiently with barium sulfate or low results will be obtained. Dissolve the sample in potassium fluoride and pyrosulfate fusions as described in the first three paragraphs under “Determination of Thorium-230 in Uranium Ores and Mill Tailings” (IO), except omit the 234Thtracer and add 200 pL of telluorous acid solution immediately before addition of the potassium metabisulfite to catalyze reduction of most of the iron. After boiling the hydrochloric acid solution of the pyrosulfate cake, add 2 drops of 1% Safranine 0 and precipitate tellurium with chromous and titanous chlorides as described in the third paragraph under Determination of Uranium above. Do not add the additional tellurous acid. Without filtering off the black precipitate, add 1 mL of the seeding suspension, two 1-mL portions of 0.45% barium chloride dihydrate and finish as described for uranium except using a 47-mm Tuffryn HT-100 membrane filter (Gelman Sciences) in place of the DM450. The DM-450 filter containing a black precipitate melts or curls badly if an infrared lamp closer than about 22 inches is used for drying. The HT-100 Tuffryn filter can be used up to 138 “C but will still melt if placed closer than about 10 inches from the 250-W lamp, apparently owing to the low heat capacity of the membrane filter in intimate contact with the black absorbing precipitate. Before mounting the final precipitate for a counting, suspend the precipitate in ethanol and place the centrifuge tube in an ultrasonic bath for a few seconds to clean the finely divided platinum off the sides of the centrifuge tube and to disperse the dense metallic tellurium for more uniform homogenization and deposition. The precipitate must be dried in a vacuum desiccator if the activity level is expected to be very low to reduce pickup of radon and thoron daughters. Reprecipitation of Barium Sulfate. Barium sulfate can be reprecipitated as described previously (14) and carry lanthanum, cerium, and all the actinides except uranium quantitatively provided that two drops of 25% potassium metabisulfite and 1 mL of 20% ferrous ammonium sulfate are added to keep the transuranium elements in their reduced states. Reprecipitation has been found necessary to obtain additional decontamination from calcium and uranium ( I , IO, 1 4 ) but also can be used to obtain more uniform distribution of the activity by precipitation from homogeneous solution.
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When uranium is to be reprecipitated quantitatively in the barium sulfate, some significant changes are necessary. Add the 1 drop of 72% perchloric acid a t the same time as the 1 mL of concentrated sulfuric acid and heat the solution carefully with continuous vigorous agitation until all barium sulfate has dissolved and a strong yellow color of decomposition products of perchloric acid is produced. It is necessary to oxidize the uranium as the barium sulfate is being dissolved to prevent precipitation of anhydrous uranous sulfate on subsequent heating in concentrated sulfuric acid, particularly when 1 mg of uranium is present from uranium ores. As soon as the barium sulfate has dissolved and the yellow gaseous decomposition products have volatilized, cool the concentrated sulfuric acid to about 50 "C and add 200 pL of the uranium reductant by discontinuous drops while agitating the sulfuric acid vigorously to prevent local precipitation of barium sulfate around the drops. Add 35 mL of the reprecipitating solution all at once and finish as described (14). Barium sulfate precipitated from sulfuric acid on addition of water packs very tightly on centrifugation, and generally must be redispersed using a stirring rod and/or ultrasonic bath. Determination of Plutonium plus Neptunium. Place the filter containing the barium sulfate into a 50-mL beaker, add 3 mL of 72% perchloric acid and 1mL of concentrated nitric acid. Cover the beaker with a watch glass and boil the solution until the last of the nitric acid has been expelled, all the organic material has been oxidized, and the barium sulfate has dissolved to give a clear and colorless solution. If a heating block is available and the barium sulfate was left in the centrifuge tube, add 3 mL of 72% perchloric acid and a small silicon carbide boiling chip, and heat in the block until clear. In either case, continue boiling the 72% perchloric acid for 15 min to oxidize neptunium and plutonium quantitatively to the penta- and hexavalent states, taking care that the acid does not evaporate too rapidly and dry on the bottom of the beaker or centrifuge tube. Also, keep the solution boiling smoothly so that the sides do not go dry. Otherwise, some of the hexavalent plutonium will be decomposed thermally to the quadrivalent state and the separation will be seriously incomplete. With soil samples, a very light yellow color will generally develop on boiling due to partial oxidation of the cerium present. Cool the perchloric acid in air for about 1 min and add 25 mL of a solution containing 3 g of ammonium sulfate, 0.5 mL of concentrated sulfuric acid, and 2 drops of 0.5% potassium permanganate while swirling the hot solution continuously to reprecipitate the barium sulfate. If the solution is still in a centrifuge tube, transfer the solution and boiling chip without rinsing to a graduated 100-mL beaker. Add 2 more boiling chips, cover with a watch glass and boil the solution for 1 min. Add 1mL of 0.48% barium nitrate (not the chloride) a t about 1 drop every 3 s while swirling the solution rapidly to precipitate the last traces of precipitable ions and boil for 1 more minute. Transfer the hot solution back to the same 40-mL conical centrifuge tube, rinse the beaker twice with 2 to 3 mL of 0.5% sulfuric acid, taking care not to transfer the boiling chips, and centrifuge for 5 min, again without mixing. Decant the supernate back into the same graduated 100-mL beaker. If desired, the barium sulfate can be washed, mounted, and counted to obtain the total activity of all thorium, radium, protactinium, and transplutonium isotopes present, using an appropriately decreased counting efficiency for the larger quantity of barium sulfate employed. Otherwise, discard the barium sulfate. The precipitate will generally have a pale yellow color due to the quadrivalent cerium. When large quantities of thorium or radium activities are present, such as with natural thorium or uranium ores, or when the most accurate and reliable determination of the smallest quantities of neptunium or plutonium is desired, evaporate the permanganate supernate back to about 25 mL, scavenge the boiling solution with two more 1-mL portions of 0.48% barium nitrate, centrifuge, and discard the barium sulfate. The extra scavenge is not generally necessary for routine work with normal soils. Add 5 drops of 25% potassium metabisulfite, 1 mL of 20% ferrous ammonium sulfate, 3 mL of concentrated hydrochloric acid to prevent precipitation of lead, bismuth, and polonium, and a couple of silicon carbide boiling chips. Evaporate the solution to about 25 mL in the covered beaker. The evaporation must take at least 5 min of vigorous boiling to ensure complete reduction of plutonium to the ter- or quadrivalent state. Add 3 drops of
seeding suspension and precipitate the plutonium and/or neptunium by slow, dropwise addition of two 1-mL portions of 0.45% barium chloride dihydrate with rapid and continuous swirling with 1 min of boiling after each addition. Centrifuge, wash, mount, and count the barium sulfate in a scintillation counter as described previously (3). However, centrifuge the solution while still hot to prevent precipitation of ammonium perchlorate. Small quantities of ammonium perchlorate will be removed in the wash. When sufficient activity is present, an estimation of the composition can be obtained by direct a spectrometry on the barium sulfate, using the high-energy intercept of the degraded spectrum to identify the components (15). However, more accurate and precise determination of the isotopic composition will require separation of the plutonium or neptunium from the barium sulfate as described below before analysis by a spectrometry. This procedure gives the sum of all a-emitting neptunium and plutonium nuclides. However, because neptunium-237 is the only common a-emitting isotope of neptunium, and because its long half life of 2.14 X lo6 years makes it very unlikely to be encountered in environmental samples in significant activities, the procedure described is virtually specific for plutonium isotopes and is somewhat simpler than one requiring removal of neptunium. However, when the presence of neptunium-237 is suspected, or must be positively excluded, the following procedure provides for its separation. Determination of Neptunium. If determination of neptunium alone is desired, place the filter containing the barium sulfate from the original precipitation into a 50-mL beaker. If separate determinations of both neptunium and plutonium are desired, use the barium sulfate from the separation of plutonium plus neptunium. Add 2 mL of concentrated sulfuric acid and 1 mL of concentrated nitric acid. Heat the solution gently until the last of the nitric acid has been expelled and the sulfuric acid begins to fume. Add 1 or 2 drops of a 1-to-1 solution of nitric and perchloric acids to oxidize the last of the organic matter and continue heating until the barium sulfate has dissolved to give a clear and colorless solution. Heat the sulfuric acid to boiling to decompose excess perchloric acid and any traces of plutonium that might have become oxidized by the perchloric acid, particularly in the presence of cerium, and allow to cool for 2 min. Add 10 mg of solid potassium dichromate to oxidize the neptunium, swirl to dissolve, and allow to cool to room temperature spontaneously without further heating. Add 25 mL of the reprecipitating solution all a t once to reprecipitate barium sulfate containing the plutonium, boil for 1 min, and centrifuge as before. If small quantities of neptunium are to be determined in the presence of large quantities of plutonium or other a emitters, scavenge the neptunium supernate with two 1-mL portions of 0.45% barium chloride dihydrate and discard the precipitate to remove traces of plutonium before proceeding with the reduction of the dichromate. Reduce the supernate containing the neptunium and precipitate, mount, and count barium sulfate as described above for plutonium plus neptunium. Determination of Plutonium. If determination of plutonium alone is desired, proceed as described for the determination of neptunium using the barium sulfate from the separation of plutonium plus neptunium. Centrifuge, wash, mount, and count the barium sulfate obtained on addition of aqueous reprecipitating solution to the sulfuric acid solution of barium sulfate containing chromic acid. Separation of Actinides from B a r i u m Sulfate. When isotopic identification is desired, the actinides other than uranium can be separated easily and completely from the barium sulfate by coprecipitation with a small quantity of cerous hydroxide from a strongly alkaline solution of the barium sulfate in ethylenediaminetetraacetic acid (EDTA) (16). The precipitate can then be analyzed by a spectrometry. With 50 pg of cerium carrier deposited over an area 7 / 8 inch in diameter on a 25-mm Tuffryn HT-100 membrane filter, the resolution of the resultant spectrum is about 65 keV FWHM for plutonium-239 from pure solutions. This resolution is almost as good as the 50 keV obtained for pure plutonium-239 from a source electrodeposited on polished stainless steel for the same detector and source distance. However, when applied routinely to real samples, the resolution obtained with cerous hydroxide precipitation has been as good as that obtained from electrodeposited sources, apparently because of electrode-
ANALYTICAL CHEMISTRY, VOL.
Table I. Distribution of Uranium Obtained in Analysis of Soil fraction original barium sulfate tellurium precipitate supernate from barium sulfate precipitation of uranium all other minor fractions main barium sulfate containing uranium material balance uranium recovery
uranium present, % 0.38
i
0.60
*
0.02 0.017 t 0.003 0.048 * 0.004
99.05 100.10 98.95
t t t
0.02 0.15
0.15 0.15
position of traces of heavy metals remaining from the chemical separations. Uranium is not precipitated efficiently under the conditions used apparently because of the strong water-soluble complex formed with carbonates invariably present in strong alkalis.
RESULTS AND DISCUSSION Samples of soil were spiked with about 2 x lo4 cpm of uranium-233 and plutonium-239 in separate runs and carried through the entire procedures exactly as described, including the initial sample decomposition. Certain minor fractions of interest were analyzed separately to determine the recovery and distribution obtained. All other minor fractions resulting from the analytical procedure, including those remaining in all glassware used, were recovered and analyzed together as described under Instrument Calibration to provide a complete accountability of the tracer added. The results obtained are expressed as a percentage of the tracer used by comparing each result with a standard prepared under the same conditions as the major fraction. Counting times were generally about 30 min so t h a t both the major fraction and the material balance could be compared with the standard with a relative standard deviation of about 0.2%. All uncertainties given in this paper are standard deviations resulting from propagation of all random uncertainties in the entire measurement process. T h e results of the material balance study on uranium are given in Table I and show acceptably small losses in all fractions. T o prove that the counting efficiency for the uranium determination had been determined correctly and that no systematic errors were still present, a sample prepared by exact gravimetric dilution of a standard pitchblende containing a n exactly known activity of uranium in secular equilibrium (10, 14) was analyzed. T h e value obtained was 1166 f 14 dpm/g in excellent statistical agreement with the known value of 1152 f 6 dpm/g. T h e results of the material balance study for plutonium in both soil and feces are given in Table 11, and show the excellent recovery of plutonium in the expected fraction. In addition, several standard soils containing exactly known quantities of plutonium prepared in connection with other work (6) and
52, NO. 9,
AUGUST 1980
some unspiked soils known to contain no plutonium were also analyzed. All results agreed with the known values within the statistics of the measurement a t the 95% confidence level.
Interference of Radon and Thoron Daughters in Gross a Counting. T h e contribution of radon daughters from the air to the activity obtained on low-level a counting was pointed out previously (2). However, the only recommendation made was to delay counting for 2 or 3 h after mounting the sample to permit most of the activity to decay (2,3).I t will now be shown t h a t pickup of thoron daughters rather than radon daughters from the air becomes the limiting consideration when long counting times are used to obtain maximum sensitivity. Because of the lower activity levels to which the present procedures are to be applied, a n d the much longer time required for adequate decay of thoron daughters, more positive and satisfactory control of all extraneous a activity from the air is necessary. Radioactive daughters of both radon-222 from the natural uranium-238 chain and radon-220 (thoron) from the natural thorium-232 chain are produced through radioactive decay as positively charged ions, and spend a large part of their existence in the charged condition. Ambient concentrations are particularly high during periods of strong temperature inversions, in confined spaces, or in the vicinity of higher concentrations of uranium, thorium, or their daughters. On the other hand, the membrane filters and zinc sulfide phosphors used in gross a counting are poor electrical conductors and build up a strong negative charge, particularly in dry weather. The charge increases with the amount of handling and movement through air. Consequently, both articles attract the positively charged ions strongly by merely being exposed to the air. If air is drawn through the filter, or if the filter is moved through the air, the rate of collection is obviously much higher. For sampling or contact times less than a few hours, 90 to 95% of the a activity collected on a filter is generally due to radon daughters. Over 98% of this activity will decay within 3 h because of the approximately 30-min half life of the lead-bismuth-polonium-214 system. As the time of sampling increases, both the total activity collected and the fraction coming from the 10.64-h thoron daughters, lead-bismuthpolonium-212, also increase. However, when the sampling time is short compared to 30 min, the accumulation of radon daughters on a filter a t constant flow rate and concentrations becomes nearly linear with time rather than exponential. Because this is also true for thoron daughters for times short compared to 10.64 h, the ratio of thoron daughters to radon daughters becomes essentially constant for short times. In other words, any exposure however short can never prevent the activity of thoron daughters collected on a filter from becoming greater than some minimum fraction of that of the radon daughters present. If the activity of the radon and thoron daughters in air were equal and constant, the minimum
Table 11. Distribution of Plutonium in Analysis of Soil and Feces fraction filtrate from first barium sulfate precipitation wash of first barium sulfate precipitate first barium sulfate after oxidation and reprecipitation wash of first barium sulfate after oxidation and reprecipitation filtrate from precipitation of plutonium on barium sulfate wash a n d alcohol transfer liquid of barium sulfate containing main plutonium fraction barium sulfate containing main plutonium fraction material balance plutonium recovery
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plutonium present, % soil, 1 g feces, 1 0 g 0.41 i 0.02 0.084 i 0.004 0.49 0.02 0.29 i 0.02
0.048 t 0.003 0.023 i 0.003 0.63 i 0.02 0.38 5 0.02
0.22
i
0.90
f
0.090 * 0.004 0.30 t 0.02
*
97.0
t
99.7
*
97.5 i
0.02 0.2
0.2
98.0
0.2 0.2
99.5 t 0.2 98.5 i 0.2
i
0.2
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
activity of thoron daughters collected in the sample would be about 4.7% of t h a t of the radon daughters, or 4.5% of the total. Although the actual activity and composition of a given air sample varies widely with the sampling time and rate, the strength of the inversion, a n d the concentrations of natural uranium and thorium in the particular geographical area, an example will be helpful. After 10-min decay, a 30-min air sample collected at 1.2 cfm, or about 1 m3 of air, gave about 500 cpm of a activity a t 45% counting efficiency of which about 6 % , or 30 cpm, came from the thoron daughters bismuth- and polonium-212, and the remainder from the radon daughter, polonium-214. After 3-h decay, only 7 cpm of the 30-min radon daughters but 25 cpm of the 10.64-h thoron daughters would still remain. A 3-day decay would reduce t h e 30 cpm of thoron daughters to about 0.28 cpm, which would still give 170 counts in lo3 min compared to an instrumental background of about 10 counts in the same length of time with a 2-inch phosphor. If a 30-min air sample contains 6% thoron daughters, a membrane filter being dried in air for 10 min under the same conditions will collect nearly the same fraction of t h e longer-lived daughters. When low-levels of activity are t o be gross a counted for relatively long periods of time, remove the filtered precipitate from the filtering chimney as soon as the liquid phase has been drawn off to avoid filtering activity from the air. Place the filter on a paper hand towel in a vacuum desiccator nearby and evacuate it immediately. Do not carry the exposed filter about the laboratory. When the filter is adequately dried, mount the filter a n d precipitate with a phosphor, preferably without even removing it from the desiccator. Finally, let the mounted sample stand as long as possible before counting, certainly for a t least 3 h to let the radon daughters decay, and for 2 or 3 days if time will permit to decrease the residual thoron daughters as much as possible. When little activity is present, a statistically lower count in lo3 min is invariably obtained on a repeat count the second day than was obtained the first day. P r e c i p i t a t i o n of B a r i u m S u l f a t e i n t h e P r e s e n c e of H y d r o c h l o r i c Acid. Most of the early work from this laboratory in separating small quantities of large ter- and quadrivalent elements with barium sulfate was carried out almost exclusively from a straight sulfate system (2,3). Virtually all of the information about efficiency of precipitation of the heavy elements, the optimum experimental conditions, and effects of interferences refers t o such a system. Since then, most applications t o actual samples have required addition of hydrochloric acid to prevent precipitation of calcium sulfate (I) or t o prevent carrying of lead, bismuth, or polonium (13, 17). T h e presence of hydrochloric acid retards precipitation of barium sulfate significantly, particularly in the presence of heavy elements. When the quantity of barium is kept small to permit direct a counting, precipitation of the actinides can become seriously incomplete and the effect of heavy elements increases drastically. Precipitation of barium sulfate was observed in a previous investigation (13)to be slowed significantly in the presence of 3 mL of concentrated hydrochloric acid in a volume of 30 mL, even though 3 g of potassium sulfate was present. Characteristically, barium sulfate precipitated from hot solutions containing hydrochloric acid has a thin translucent appearance instead of the milky one obtained in a straight sulfate system, is more coarsely crystalline, and has a much smaller volume. Many ter- and quadrivalent elements cause a further decrease in the rate of precipitation, and their effect is additive both to each other and to that produced by the hydrochloric acid. For example, in the presence of 1 mg of uranium and 4 drops of 20% titanium trichloride under the
conditions described above for precipitation of uranium, no precipitation of barium sulfate is obtained on addition of two consecutive 1-mL portions of 0.45% barium chloride with I-min periods of boiling after each addition. A third I-mL portion produces a slight turbidity after which a fourth 1-mL portion produces a nearly normal rate of precipitation. In contrast, a distinct turbidity is produced by dropwise addition of 0.5 mL of 0.45% barium chloride in a straight sulfate system. The effect is clearly a rate problem with condensation nuclei being important in obtaining rapid precipitation, and is one of fundamental importance. T h e efficiency of precipitation of the actinides and the tolerance to other interfering elements are both highly dependent on rapid precipitation of barium sulfate during a slow dropwise addition of barium ion to obtain an exponential precipitation of the large ter- and/or quadrivalent ions involved ( 3 ) . Even the proper concentration of barium chloride t o be used and its rate of addition depend greatly on the rate of precipitation obtained (3). This exponential process is responsible for the higher efficiency of the precipitation and greater tolerance for other elements, but also decreases the counting efficiency and increases its variability somewhat owing to nonuniform distribution of the activity throughout the barium sulfate lattice. T h e rate of precipitation of barium sulfate from sulfate solutions of soils containing hydrochloric acid to keep calcium sulfate in solution ( I ) has been observed to vary from one sample to another for some previously unknown reason. On some samples, distinct precipitation occurs with the first few drops of barium solution added. Other samples remain perfectly clear until several milliliters of barium solution have been added and the solution has been boiled for a considerable period of time. A similar and even more obvious effect is observed when recovering activities from the minor fractions from a previous barium sulfate separation to correct for losses as described above under Instrument Calibration. Frequently, after reevaporation to a pyrosulfate fusion, the reworked solutions give immediate precipitation of barium sulfate, even in the presence of hydrochloric acid, while the original precipitation under supposedly identical conditions was typically slow. Furthermore, the precipitate that forms rapidly has the milky appearance and relatively larger volume characteristic of straight sulfate systems while the precipitate t h a t forms slowly has the fine translucent appearance, coarse crystals, and smaller volume characteristic of hydrochloric acid solutions. Rapid precipitation has been shown in both cases t o have been caused by the presence of an almost invisible quantity of barium sulfate, either from the sample itself, as in the soils, or from the few tenths of a percent not removed during the previous barium sulfate separation (13). It is important to emphasize that the curative effect of a small quantity of barium sulfate on the rate of precipitation is not obtained unless the barium sulfate had been through a pyrosulfate fusion to give the finely-divided milky form. This is why the seeding suspension is prepared by precipitating barium sulfate from a pyrosulfate cake by addition of water. Consequently, if a small quantity of barium is present in the sample or added before the fusion, or if a small quantity of a barium sulfate suspension prepared in the same way is added to the solution, subsequent precipitation of barium sulfate proceeds nearly as fast in the presence of hydrochloric acid as in straight sulfate systems, even in the presence of moderate concentrations of heavy metals. C o u n t i n g Efficiency of a-Emitters i n B a r i u m Sulfate. There is probably no part of a procedure involving gross a counting that is more important than the determination of the correct counting efficiency to be used if accurate a n d reliable results are to be achieved. Because of the limited
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
range and severe attentuation of a particles on passing through matter, the counting rate of a given quantity of radioactivity will obviously be strongly affected by the experimental conditions. Unless all such sources of variability are defined and controlled, substantial systematic errors will result. T h e most obvious sources of variability are those affecting the mass with which the radioactivity is associated. T h e quantity a n d thickness of the barium sulfate used, the additional mass contributed by other elements from the sample t h a t are also carried in barium sulfate such as cerium, lanthanum, thorium, etc., and the distribution of the activity throughout the mass are particularly important. When long-lived elements such as natural uranium and thorium are being counted, their own mass also changes the counting efficiency significantly and must be included in the calibration. Differences in sensitivity of phosphors from one to another have been mentioned above. Identical aliquots of standardized tracer solutions were precipitated with two, three, and four successive 1-mL aliquots of 0.45 70barium chloride dihydrate under the exact conditions specified under Instrument Calibration above. With carrier-free uranium-233, the counting efficiencies obtained were 0.429 f 0.001,0.408 f 0.001, and 0.383 f 0.001 with 6.25, 8.75, a n d 11.25 mg of barium, respectively, including the seeding suspension, as the barium sulfate containing potassium obtained under the conditions described. These results are only 8.3%, 12.8% and 18.2%, respectively, lower than the value of 0.468 f 0.001 obtained for the carrier-free tracer evaporated directly onto a stainless steel plate. In the presence of 1 mg of natural uranium, the corresponding values are 0.412 f 0.001, 0.394 f 0.001, and 0.368 f 0.001 for the counting efficiencies, a n d 12.070, 15.8%, a n d 21.4% for the decrease compared to the carrier-free deposit on stainless steel. Either with or without 1 mg of uranium, the difference in counting efficiency between two and three 1-mL portions of the barium chloride is about 5% for the extra 2.5 mg of barium, or a change of only 2% for a change of 1mg in the quantity of barium added. T h e error from this source should be easily controlled since 1 mg of barium represents about 16% of the total barium added. T h e 1 mg of uranium decreased the three counting efficiencies by 4.0%, 3.4%, and 3.9%, respectively. The effect of 1 mg of uranium might be expected to be about half again worse than with 1mg of barium because of the extra sulfate ion required to maintain electrical neutrality with quadrivalent uranium than that with divalent barium. Because the quantity of uranium present must be kept below 1 mg to avoid severe losses in the barium sulfate precipitation, the counting efficiency needs to be determined only for the carrier-free and 1-mg levels and counting efficiencies for other concentrations obtained from them by interpolation. T h e approximate quantity of uranium actually present can be determined from the final counting rate and either of the two counting efficiencies with sufficient accuracy to determine the correct one t o be used in the calculation, even if the conditions of equilibrium among the isotopes are unknown. All other elements precipitated efficiently with barium sulfate, such as cerium, lanthanum, thorium, etc., will have been removed during the initial precipitation, and will not be present t o precipitate with uranium after its reduction by titanium trichloride. Similarly, both plutonium and neptunium will also have been separated from these elements by oxidation prior to reprecipitation of barium sulfate containing
1459
the ter- a n d quadrivalent elements. However, all such elements will be precipitated in the gross a determination and an estimated quantity should be included in the calibration. One milligram of cerium decreased the counting efficiency by about 5 t o 6% so t h a t the uncertainty of estimating small quantities of such elements should not cause prohibitive errors. Elements t h a t are carried inefficiently in barium sulfate such as calcium and tervalent titanium cause small but detectable decreases in the counting efficiency when present in relatively large quantities and should be included in the calibration. The calcium coprecipitated from a total of 30 mg, about t h a t present in 1 g of an average soil, decreases the counting efficiency by about 1 to 2%. However, if too large samples are taken, or if calcium sulfate is allowed to precipitate because of excessive evaporation, cooling, or standing time, much greater errors will result. If there is any question of precipitation of calcium sulfate, the barium sulfate should be reprecipitated to eliminate all calcium, and the appropriate counting efficiency for reprecipitated material then used. As pointed out previously (13),meticulous care must be exercised to avoid any pressure on the mounted sample, either from the hands during mounting or insertion into the counter, or from contact with the phototube or sample positioning mechanism during counting. If the soft precipitate of barium sulfate is pressed repeatedly into the pores of the phosphor, the counting efficiency has been increased by as much as 16% (13). Generally, the counting rate of a given mounted barium sulfate source has been found to be reproducible within the statistics of counting 4 x IO5 events, i.e., a relative standard deviation of about 0.2%, provided that the source is not removed from the counter between counts. If the source is removed from the counter and reinserted before each count, the counting rate increases progressively with each count by a few tenths percent.
LITERATURE CITED (1) Sill, C. W.; Puphal, K. W.; Hindman. F. D. Anal. Chem. 1974, 4 6 .
..-..
177!i-17?7
(2) sill, C. W. Health Phys. 1969, 17, 89-107. (3) Sill, C. W.; Williams, R . L. Anal. Chem. 1969, 41. 1624-1632. (4) Sill, C. W.; Percival, D. R.: Williams, R. L. Anal. Chem. 1970, 4 2 , 1273-1275. (5) Sill, C. W. Anal. Chem. 1974, 4 6 , 1426-1431. (6) Sill, C. W.; Hindman. F. D. Anal. Chem. 1974, 4 6 , 113-118. (7) Bevington, P. R. "Data Reduction and Error Analysis for the Physlcal Sciences"; McGraw-Hill Book Co.: New York, 1969; p 56. (8) Sill, C. W. Anal. Chem. 1975, 4 7 , 192. (9) Williams, R. L.; Sill, C. W. Anal. Chem. 1974, 4 6 , 791-794. 10) Sill, C. W. Anal. Chem. 1977, 49, 618-621. 11) Sill, C. W. Anal. Chem. 1979, 51. 1307-1314. 12) Sill. C. W. "Problems in Sample Treatment In Trace Analysis", in Prcceedings of the 7th IMR Symposium, National Bureau of Standards Special Publication 422, LaFleur, P. D., Ed.; U.S. Government Printing Office: Washington, D. C., 1976; Vol I.pp 463-490. 13) Sill, C. W. Anal. Chem. 1978, 50. 1559-1571. 14) Sill, C. W. Health Phys. 1977, 33, 393-404. 15) Sill, C. W.; Williams, R. L. "Rapid Identification and Determination of Alpha Emitters in Environmental Samples", Rapid Methods for Measuring Radioactivity in the Environment; International Atomic Energy Agency: Vienna, Austria, 1971; pp 201-211. 16) Sill, C. W.; Williams, R. L. "Separation of Actinides from Barium Sulfate for AlDha SDectrometw or Gross AlDha Counting", Radiological and Environmental Sciences Laboratory, US. Department of Eneriy, Idaho Falls, Idaho, 1980, unpublished document. (17) Sill, C. W.; Willis, C. P. Anal. Chem. 1977, 4 9 , 302-306.
RECEIVED for review February 11,1980. Accepted May 2,1980. Use of commercial product names is for accuracy in technical reporting and does not constitute endorsement of the product by the United States Government.
