Radiochemical determination of plutonium in environmental and

was made of anionexchange separation of plutonium for the routine analysis of environmental and biological samples. Procedures based on the adsorption...
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Radiochemical Determination of Plutonium in Environmental and Biological Samples by Ion Exchange N. A. Talvitie Western Environmental Research Laboratory, Environmental Protection Agency, P.O.Box 15027, Las Vegas, Nev. 89114

An anion exchange method applicable to the routine determination of plutonium in environmental and biological samples is presented. Samples are initially prepared as azeotropic 6M hydrochloric acid solutions. Pu(lV), stabilized with hydrogen peroxide, is adsorbed on anionic resin from 9M hydrochloric acid solution. Coadsorbed iron is removed from the resin with 7.2M nitric acid. Plutonium is reductively eluted with 1.2M hydrochloric acid-O.6% hydrogen peroxide and electrodeposited from 1M ammonium sulfate at pH 2 for alpha spectrometric determination. Sample preparation procedures are given for urine, animal tissue, bone, saline and nonsaline water, siliceous and limestone soil, and glass fiber air filters. Mean recoveries of 236Pu internal tracer standard from the various types of samples were 83 to 102% with an overall mean of 94%. Minimum detectable activity for 1000-minute counts is 0.02 pCi of Z39Pu.

A STUDY WAS MADE of anion exchange separation of plutonium for the routine analysis of environmental and biological samples. Procedures based on the adsorption of anionic complexes of plutonium from nitric and hydrochloric acid media have been reviewed (I). A procedure is proposed in which the usual technique of adsorption from nitric medium and conversion to a hydrochloric system has been reversed to one of adsorption from hydrochloric acid medium followed by a nitric acid wash for removal of iron. The oxidizing and reducing properties of hydrogen peroxide in hydrochloric acid medium are utilized for the adsorption and desorption of plutonium. In the presence of hydrogen peroxide, plutonium has been shown to exist primarily in the trivalent state when the hydrochloric acid concentration is less than 6 M and primarily in the quadrivalent state when the acid concentration is greater than 7 . 5 M ( 2 ) . EXPERIMENTAL

Apparatus and Material. The chromatographic apparatus is an integral unit consisting of a 250-mm by 14.5-mm i.d. tube, a stopcock with a Teflon (Du Pont) plug, and a 250-cm3 reservoir (Kontes Glass Co. and Scientific Glass Apparatus Co.). A plug of glass wool supports the resin bed. A low cross-linked resin was chosen to speed the exchange reactions. This is AG 1-X2, chloride form, 50-100 mesh, an analytical grade of Dowex 1-X2 obtained from Bio-Rad Laboratories. Spherical-grained silica sand was obtained from the St. Peter strata at Minneapolis, Minn., through the courtesy of the University of Minnesota Foundry. The 60- to 200mesh sieve fraction was washed to remove clay particles and heated with an equal weight of molten potassium pyrosulfate to remove acid-soluble impurities. Teflon beakers ( lOO-cm3 Chemware) were used wherever possible in lieu of platinum for hydrofluoric acid digestions

because of the greater ease of decontamination and sample handling. Solutions were centrifuged in 50-cm3, disposable polypropylene tubes (Falcon Plastics No. 2070). Alpha spectrometric measurements using 236Puas an internal tracer standard were made with 450-mm2, 300-pm depletion depth Ortec silicon surface barrier detectors operated in vacuum chambers. A source to detector distance was used which gave a nominal resolution of 35 keV full width at half maximum for the 5.3-MeV monoenergetic 210Poalpha and 21 counting efficiency. Procedure. IONEXCHANGE COLUMN.Remove fines from the resin by repeated suspension in distilled water and decantation. Add 12M HC1 equal to 10% of the volume of slurry to shrink the resin. Transfer the resin to the column in slurry form to give a settled resin bed of 20-cm3 volume. Add dry 60- to 200-mesh silica sand to a depth of 15 mm through a layer of 1.2M HCl. The sand prevents resuspension of the resin and, by its capillarity, stops the flow between additions of reagents enabling unattended operation, Flush the reservoir and resin bed with about 20 cm3 of 12M HCl to solubilize any impurities introduced during preparation of the column. Fill the reservoir with 1.2M HC1 and adjust the flow rate to 6 cm3 min-l. Immediately prior to use, condition the resin at the same flow rate with 100 cm3 of 9 M HCl containing a drop of 30 % H202. ION EXCHANGE SEPARATION. Prepare the sample initially as an azeotropic 6 M HC1 solution having a 40- to 60-cm3 volume, as described under Sample Preparation. Add a volume of 12M HCl equal to the volume of the 6 M solution to adjust the acid concentration to 9M. Add a drop of 3 0 x H 2 0 2for each 10 cm3 of 9 M solution, cover with a watch glass, and heat the solution at 80 to 90 “C for 1 hour. When cool, transfer the solution to the reservoir using 9 M HC1 as a rinse. If barium chloride, sodium chloride, or other solid matter is present, filter the solution into the reservoir through a plug of glass wool in the stem of a funnel. Adjust the flow rate to 3 cm3 min-l. Flush the reservoir three times with 15-cm3volumes of 9 M HC1 and drain each rinse at a flow rate of 3 cm3min-’. Flush the reservoir twice with 15-cm3 volumes of 7.2M nitric acid and drain at a slower flow rate of 1.5 cm3 min-1. Complete the removal of iron with an additional 70 cm3 of 7.2M nitric acid at the slower flow rate. Rinse nitric acid from the reservoir with about 5 cm3, but not over 10 cm3, of 1.2M HC1. Elute the plutonium at 3 cm3 min-l with a fresh solution prepared by mixing 1 cm3of 30% H 2 0 2with 50 cm3of 1.2M HCl. Collect the eluate in a 50-cm3beaker, add 0.5 cm3of 18M H2S04,and evaporate on a steam bath or at an equivalent temperature on a hot plate until only sulfuric acid remains. Neutralize the acid to pH 2 with ELECTRODEPOSITION. ammonia using thymol blue as an indicator and electrodeposit from 10-cm3volume for 60 minutes at 1.2 amperes. Details of cell construction and of the electrodeposition technique have been reported (3). Sample Preparation. URINE. Transfer the sample to a

(1) G. H. Coleman, “Radiochemistry of Plutonium,” National Academy of Sciences, Nuclear Science Series, NAS-NS 3058

(1965). (2) A. S. Ghosh Mazumdar, P. V. Balakrishnan, and R. N. Singh, Vijrtarta Parishad Aizusaridhaiz Pafrika, 4, 149 (1961); UCRL-

Trans-398(L).

(3) N. A. Talvitie, presented in part at the 24th Annual Northwest

Regional Meeting of the American Chemical Society, Salt Lake City, Utah, June 1969.

ANALYTICAL CHEMISTRY, VOL. 4 3 , NO. 13, NOVEMBER 1971

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borosilicate beaker using 40 cm3 of 12M HCl per dm3 of sample volume to rinse the sample container. Add 236Pu tracer, 10 cm3 of 1 M calcium chloride, and a volume of 30% H202 equal to the volume of hydrochloric acid. Place a Teflon stirring rod in the beaker and heat to the boiling point. When foaming subsides, cover the beaker and allow the solution to simmer for 1 hour. Add 60 cm3 of 16M "01 per dm3 of sample and allow the solution to simmer for another hour. While stirring the hot solution, add 14M NH40H until precipitation begins and then add an excess equal in volume to the volume of nitric acid added. Set the beaker aside to cool, remove the supernatant liquid by aspiration, and transfer the slurry to a centrifuge tube. Centrifuge, discard the supernatant liquid, and dissolve the precipitate with about 10 cm3 of 16M H N 0 3 . Transfer the solution to a 250-cm3 graduated borosilicate beaker using as rinse. Cover the beaker with about 5 cm3 of 16M " 0 3 a watch glass and boil on a hot plate until the residue is dry. Wet the residue alternately with 30% H 2 0 2and 16M H N 0 3 with intervening evaporations until a white ash is obtained and then allow all of the nitric acid to evaporate. Add 50 cm3 of 6 M HC1, replace the watch glass, and boil until the volume is reduced to 25 cm3to remove remaining nitrates and to hydrolyze polyphosphates. Add 6 M HCl to increase the volume: to 50 cm3. BONEASH. Weigh 1 to 10 grams of ash into a tared 100cm3Teflon beaker. Add 40 cm3of 6MHCI plus an additional 2 cm3 of 6 M HCl for each gram of ash in excess of 1 gram. Add 236Putracer and a few drops of 30% H202. Cover with a watch glass and digest on a hot plate until the ash has dissolved. Continuing the digestion overnight will ensure the precipitation of any silica which might be present. Transfer the solution to a centrifuge tube using 6 M HC1 and centrifuge. Pour the supernatant liquid into a 150-cm3 graduated borosilicate beaker and return the residue to the Teflon beaker using 10 cm3 of 48% H F and 5 cm3 of 16M "03. Evaporate to dryness. If any organic matter derived from carbon remains, wet the residue with 30% H z 0 2 and evaporate to dryness. Add 10 cm3 of 6 M HC1 and evaporate to dryness. Add 5 cm3 of 6 M HCl and a drop of 30% H202. Heat to dissolve the residue and add the solution to the supernatant liquid. Evaporate the combined solution to 60 cm3. ANIMAL TISSUE ASHAND VEGETATION ASH. Weigh 1 gram of ash into a tared 100-cm3 Teflon beaker. Add 20 cm3 of 16M H N 0 3 and 236Putracer. Cover, digest near the boiling point for as long as necessary to solubilize and decompose traces of carbon, and then evaporate the solution to dryness. If traces of organic matter remain, wet the residue with 30% H 2 0 2 and evaporate to dryness. Decompose and dissolve the residue as described below for ignited glass fiber filters. If greater sensitivity is required, add 236Putracer and 40 cm3of 16M H N 0 3to 5 grams of ash in a 600-cm3borosilicate beaker. Digest as above and then evaporate to dryness. Destroy any remaining organic matter by adding and evaporating 5-cm3 volumes of 30% H2O2and 16M "01 alternately. Add 20 cm3 of 6 M HC1 and evaporate to dryness to dehydrate the silica. Add 25 cm3of 1.2M HCI, digest on a hot plate, and transfer the solution and insoluble residue to a centrifuge tube. Centrifuge, return the supernatant liquid to the 600-cm3 beaker, and transfer the residue to a 100-cm3Teflon beaker with 15 cm3 of 48% H F and 10 cm3 of 16M "Os. Evaporate the contents of the Teflon beaker to dryness. Add 10 cm3 of 6 M HCl, evaporate to dryness, and dissolve the residue in 5 cm3 of 6 M HC1. Add the solution to that contained in the 600-cm3 beaker and dilute the combined solution to 500 cm3 with 1.2M HCI. Add 10 cm3 of 1 M CaCh and heat to the boiling point. While stirring, add 14M N H 4 0 H until precipitation begins and then add 20 cm3 in excess. Set the beaker aside to cool, remove the supernatant liquid by aspiration, and transfer 1828

the slurry to a centrifuge bottle. Centrifuge, discard the supernatant liquid, and dissolve the precipitate with a volume of 12M HC1 equal to that of the precipitate. Transfer the solution to a 150-cm3 graduated borosilicate beaker and dilute to 60 cm3with 6MHCI. SALINEAND SEA WATER. Add 236Putracer, 2 cm3 of a 0.2M FeC13-2M HCl solution, and 20 cm3 or more of 30% H202 to 1 dm3 of sample preserved by the addition of 20 cm3 of 12M HCl per dm3 of sample. Simmer until the hydrogen peroxide has decomposed. While stirring, add 14M ",OH to the hot solution until precipitation begins and then add 15 cm3 in excess. Heat until the precipitate has coagulated and allow to cool. Remove the supernatant liquid by aspiration and transfer the slurry to a centrifuge tube. Centrifuge, discard the supernatant liquid, and add a volume of 12M HC1 equal to that of the precipitate. Dissolve the precipitate adhering to the beaker with about 15 cm3 of 6 M HC1 and add this to the centrifuge tube. When the hydroxides have dissolved, centrifuge the solution and pour the supernatant liquid into a 150-cm3 graduated borosilicate beaker. Transfer the residue to a Teflon beaker and decompose as described above for the insoluble residue from bone ash samples. Combine the resulting solution with the supernatant liquid and dilute to 60 cm3 with 6 M HCl or evaporate to 60 cm3. NONSALINE WATER. Add 236Putracer and 1 cm3 of 30% H 2 0 2to 1 dm3 of sample preserved by the addition of 20 cm3 of 12M HCl per dm3 of sample. Evaporate to dryness on a steam bath. Moisten the residue with 30% H 2 0 2 and evaporate to dryness. Add 20 cm3 of 6 M HC1 and heat until the salts have dissolved. Using a policeman and 6 M HCl, transfer the solution and insoluble residue to a centrifuge tube. Centrifuge the suspension and pour the supernatant liquid into a 150-cm3 graduated borosilicate beaker. Transfer the residue to a Teflon beaker and decompose as described above for the insoluble residue from bone ash samples. Combine the resulting solution with the supernatant liquid and dilute to 60 cm3 with 6 M HC1 or evaporate to 60 cm3. GLASSFIBER FILTERS.Fold a 4-inch filter or one quarter of an 8- by 10-inch filter into a wad, keeping the dusty side to the interior of the wad, and place in a platinum dish or crucible. Ignite in a muffle furnace for 1 hour at 550 "C. Place the ignited filter in a 100-cm3 Teflon beaker. Add 23+5Putracer, 15 cm3 of 48% HF, and 10 cm3 of 16M "03. Swirl the beaker until the filter has disintegrated and then evaporate to complete dryness. Add 10 cm3 of 48% H F and 5 cm3 of 16M H N 0 3 . Evaporate to complete dryness. Add three successive 10-cm3volumes of 6 M HCl with intervening evaporations to dryness. Add 40 cm3 of 6 M HC1 and a few drops of 30% H202. Cover the beaker with a watch glass and heat until the residue has dissolved. SILICEOUS SOIL. Weigh 1 gram of 100-mesh sample into a tared porcelain crucible. Ignite in a muffle furnace at 700 "C until free of carbon. Transfer the sample to a lOO-cm3 Teflon beaker and add 236Pu tracer. Decompose and dissolve as described above for ignited glass fiber filters. CORALLIMESTONE SOIL. Weigh 1 to 10 grams of 100mesh sample into a tared porcelain crucible. Ignite in a muffle furnace at 700 "C until free of carbon. Follow the procedure given above for bone ash samples. RESULTS AND DISCUSSION

The ion exchange study was directed primarily to the routine analysis of samples containing iron in an amount which would interfere with the electrodeposition of plutonium and its subsequent determination by alpha spectrometry. TWO techniques were considered: adsorption of Pu(IV) from 6 to 8M nitric acid followed by the selective elution of coadsorbed Th(1V) with 9 to 12M hydrochloric acid, and adsorption of Pu(1V) and Fe(II1) from 9 to 12M hydrochloric acid followed

ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971

by selective elution of Fe(II1) with 6 t o 8M nitric acid. The second was chosen because the more rapid reaction rate of the chlorocomplex with resin ensures quantitative adsorption of plutonium in the presence of competing complexes-e.g., at a flow rate of 6 cm3 min-I, the retention from 100 cm3 of 9M hydrochloric acid on a 20-cm3 volume of resin was 100 but only 88% from 100 cm3 of 7.2M nitric acid. Once adsorbed from hydrochloric acid solution, the plutonium is retained when the resin is washed with 7.2M nitric acid at a flow rate not exceeding 1 t o 2 cm3min-I. A 50-cm3 volume of 0.36M hydrochloric acid-0.01 M hydrofluoric acid solution (4) was effective in eluting plutonium from resin in the nitrate form except for a slight tailing effect caused by the low acidity o f t h e reagent. Disadvantages were the solvent effect on glass and the interference by traces of fluoride with electrodeposition. Inasmuch as hydrogen peroxide has reductive properties in dilute acid solution and would not introduce nonvolatile extraneous matter, its effectiveness for the elution of plutonium was investigated. Preliminary data showed that the presence of hydrogen peroxide in either nitric or hydrochloric acid eluents increased the rate of elution from the nitrate form of resin and that the combination with hydrochloric acid was more effective than with nitric acid. A comparison of hydrochloric acid concentrations showed a more pronounced tailing of the elution peak with 0.12M than with 1.2M acid. Table I shows the elution pattern of plutonium obtained with the 1.2M hydrochloric acid-0.6 hydrogen peroxide reagent composition which was adopted. The data show that the first 5 t o 10 cm3 of eluate can be discarded and that a 50-cm3 volume contains about 99 % of the plutonium. N o gases were released in the resin bed during elution of plutonium. Although reuse of the resin is not advisable for low-level analysis, immediate regeneration of the resin with 250 cm3 of 1.2M hydrochloric acid prevented the later formation of gas bubbles. Hydrogen peroxide has been used for the stabilization of plutonium in the tetravalent state during solvent extraction (5). For adsorption of plutonium on resin, the excess peroxide must be destroyed to prevent gas release in the resin bed and subsequent channeling of liquids. Heating the sample solution for 60 minutes between 80 and 90 "C reduced the concentration of peroxide t o a safe level without eliminating the last traces. Later tests have shown that if hexavalent plutonium has been reduced by the use of hydrogen peroxide during the preparation of 6M solution, one drop of 30z hydrogen peroxide in 100 cm3of the 9M solution and a few minutes of reaction time at room temperature are adequate t o ensure quantitative adsorption of the plutonium as the quadrivalent ion. Because the adsorption of plutonium on the resin increases with hydrochloric acid concentration ( 6 ) , acidities higher than 9M are advantageous in the presence of other complexing ions but sodium chloride is less readily retained in solution. The separation from sodium by coprecipitation techniques could be omitted when the inorganic content of the sample was restricted t o 1 gram. Any sodium chloride which precipitated was then removed by filtering the solution into the column reservoir through a plug of glass wool. Supersaturation was prevented by initiating precipitation in the cool solution by ultrasonic agitation or by seeding with solid (4) I. K . Kressen and G. R. Waterbury, ANAL.CHEM.,34, 1598 (1962). (5) F. E. Butler, HealthPhysics, 15, 19 (1968). 31, 326(1959). (6) L. Wish, ANAL.CHEM.,

Table I. Elution Pattern of H202Eluent Plutonium with 1.2M HC1-0.6

z3sPu in fraction, %

Eluate fraction, cm3 0-5 5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-75 75-100

0.0 0.0 0.0

0.2 12.3 36.8 32.0 13.8 3.4 0.8

0.6 0.2

Table 11. Removal of Uranium and Polonium from Resin by 7.2M HNO, 7.2M HN03, cm3 232U removed, 210Poremoved, 0-50 13.8 0.3

50-100 100-150 150-200 200-250

85.3 0.9 0.0 0.0

1.2 67.0 28.9 0.6

sodium chloride. The filtration also removed barium chloride derived from barium which is invariably present in glass fiber filter material. Neither salt coprecipitates plutonium but, if not removed, they dissolve in the nitric acid wash and interfere with the flow rate by progressing through the resin bed by a continual reprecipitation process. The equilibrium condition between valence states of plutonium in the presence of hydrogen peroxide assists interchange of the 236Putracer with the plutonium in the samplee.g., hydrogen peroxide added t o acidified sea water containing 236Putracer and iron carrier serves the same purpose as a n oxidation-reduction cycle. In addition, the catalytic oxidation of organic matter provided by the combination of peroxide and iron (7) tends t o release bound plutonium when the sample is heated t o destroy the excess peroxide. The breakthrough capacity of a resin bed having a 20-cm3 volume was 1.25 grams for Fe(II1) in 9M hydrochloric acid. Iron is completely removed from the resin by four column volumes of 7.2M nitric acid. The quantitative elution of iron is assisted by the sharpening effect of the sand layer on the elution band. The mean recovery in four determinations of plutonium in the presence of 45 to 560 mg of iron was 94% when adsorbed from lOO-cm3 volumes of 9M hydrochloric acid solution. The mean recovery from 11 t o 12M hydrochloric acid solutions containing the same amounts of iron was 95 %. The deposits on the planchets showed neither the presence of iron nor an adverse effect on resolution. An increase in sensitivity can be obtained, when necessary, by coprecipitation of the plutonium in 10 grams of dissolved soil or in 10 dm3of water on ferric hydroxide. Adsorbed uranium and polonium are removed from the resin by 7.2M nitric acid as shown in Table 11. Although 75 cm3 of nitric acid wash adequately removes iron, 100 ,ma is required for removal of 99% of uraniuln leaving most of the polonium on the resin t o be eluted along with plutonium. Because the energies are resolved from those of plutonium, neither uranium nor polonium interferes at levels normally found in environmental samples. A high level of zlOPo contributes some counts t o analyzer channels in which Z39Pu (7) V. L. Miller and F. Swanberg, Jr., ANAL.CHEM., 29, 391 (1957).

ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971

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Table 111. Recovery of Plutonium from Urine

precipitants and carriers added 1 cm3 of 8 5 % H3PO4 1 maof 85% Hap04 20 mg of Y 10 cm3 of 1M CaCh 10 cm3 of 1M CaCh 20 mg of Y 200 pCi 23*Pu in 250 cm3of urine.

+

+

Pua recovered, 89.7 93.4 93.4 94.7

Table IV. Recovery of 236PuTracer from Environmental and Biological Samples N ~ of. 236Purecovered, Description detns Range Mean Animal tissue ash 12 62-93 83 Animal tissue ash 23 43-104 93 Urine 24 84-102 91 17 Bone ash 87-103 96 87-100 6 Nonsaline water 92 8 1-99 6 Simulated sea water 90 61 66-107 Glass fiber filter 94 89-103 9 98 Ignited cellulose filter 92-102 12 98 Siliceous soil5 94 78-104 Siliceous soilb 12s 93 90-97 Coral limestone soil 4 Vegetation ash 88 2 86-91 Wet-ashed fish 2 99-104 102 a Electrodeposited 90 min on abrasive-polished planchets. Electrodeposited 60 min on electropolished planchets.

appears. In such case, the peaks can be resolved by increasing the source to detector distance. The decontamination factor for 41Am, determined by analysis of spiked glass filters, was in excess of lo4. The permissible amount of phosphate was determined from the recoveries of 2 3 e Ptracer ~ in the presence of 2 t o 20 grams of bone ash. The mean recovery from six samples containing up t o 10 grams of bone ash in 120 cm3 of 9 M hydrochloric acid was 96% with no evidence of decreasing recovery with increasing phosphate content. Higher than 10-gram amounts of bone ash exceeded the solubility in 60 cm3 of 6M hydrochloric acid and would require a proportional increase in volume of sample solution. A recovery of 10% has been reported for the adsorption of plutonium from nitric acid medium in the presence of 10 grams of bone ash (8). Because of the low solubility of sodium chloride in hydrochloric acid, the direct ion exchange separation of plutonium from wet-ashed urine is practicable only for small samples. Plutonium and other actinides can be concentrated from (8) E. E. Campbell and W. D. Moss, Healrh Phys., 11,737 (1965).

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urine by coprecipitation on calcium phosphate after release from complexes by digestion with nitric acid (9). The precipitate carries down a considerable amount of organic matter not readily destroyed by wet-ashing. If the urine were partially oxidized with hydrogen peroxide in the presence of hydrochloric acid prior to digestion with nitric acid, the precipitate was light in color and easily ashed. Calcium phosphate (IO) and phosphoric acid (8) have been added to urine t o increase the recovery of plutonium. Because of the association of plutonium with phosphate as a complex ion, it was felt of more importance t o ensure complete precipitation of phosphate rather than of calcium. To test this point, comparative recoveries in the presence of excess phosphate and excess calcium were determined. The results given in Table 111 support the premise. The results also show the effect of yttrium as a nonisotopic carrier. Yttrium is added only when trivalent actinides or thorium are to be determined. Table IV gives the recoveries of 236Putracer obtained in the routine analysis of typical environmental and biological samples. Only those results were included for which the amount of 236Puactivity or the counting time was sufficient t o give a counting error not exceeding +475. The relative standard deviation of the recovery in the analysis of 125 soil samples was =t4.4z compared t o a mean counting error of A2.3 75. The weighted mean recovery for all types of samples was 9475. The occasional low recovery values had slight effect on the means and were apparently related as much t o varying sample composition as t o deviations from the established analytical technique inasmuch as consistent, but not always high, recovery values were found when samples were analyzed in replicate. The combined counting error for 1000-minute counts of sample and reagent blank, each containing 4 pCi of 236Pu tracer and having 0.003 cpm of background activity, was k0.006 pCi. At three standard deviations this gave 0.02 pCi of 239Pu as the minimum detectable activity. This was adequate t o detect worldwide contamination from nuclear testing in 1 gram of surface soil or in a n air filter representing 500 m 3of air.

RECEIVED for review March 22, 1971. Accepted July 19, 1971. Presented in part a t the American Industrial Hygiene Conference, Toronto, Ontario, Canada, May 24-28, 1971. Mention of trade names of products or sources of supply does not constitute endorsement of the products by the Environmental Protection Agency. (9) J. C. Dalton in “The Determination of Radionuclides in Materials of Biological Origin,” A. Holmes, Ed., AERE-R 5474, Harwell, England, 1967. (10) J. D. Eakins and P. J. Garnm, ibid.

ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971