Determination of thorium and uranium isotopes in ores and mill

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Determination of Thorium and Uranium Isotopes in Ores and Mill Tailings by Alpha Spectrometry Claude W. Slll Health Services Laboratory, U S . Energy Research and Development Administration, Idaho Falls, Idaho 8340 1

Procedures are presented for the determination of thorlum-230 and Isotopesof uranlum In uranium ores and mill talllngs, and of Isotopes of thorium in thorlum ores, all by alpha spectrometry. All samples are decomposed completely by fuslon wlth potasslum fluoride In platinum dishes. The cakes are then transposed with suHurlc acld and sodlum sulfate to pyrosulfate fuslons with simultaneous volatlllzatlon of hydrogen fluorlde and silicon tetrafluorlde. Afler dissolving the pyrosulfate cakes in dilute hydrochlorlc add, thorium and uranlum are extracted into Allquat-336 from an aliquot of their respectlve samples In strong, acldlc aluminum nltrate. Each element Is then stripped from Its respectlve extract, electrodeposlted, and analyzed by alpha spectrometry. Thorium-230 Is preconcentrated from relatively larger samples by precipitation on barlum sulfate. Chemical yields are determined using thorium-234 and uranium-232 tracers, and are generally at least 90% or hlgher.

The determination of small concentrations of natural thorium and uranium is most commonly made using a wide variety of colorimetric or fluorometric procedures. Many of these procedures have excellent sensitivity, precision, and accuracy but all have the disadvantage of not providing any information about the distribution of the several isotopes of each element normally present. Procedures employing more expensive and sophisticated instrumentation capable of isotopic identification such as alpha spectrometry and mass spectroscopy are generally used only when isotopic fractionation is known or expected to have occurred, such as with enriched uranium or transuranium elements in the atomic energy industry, geochemical fractionation, etc. In many cases, much valuable information is gained by choosing an analytical procedure that will give isotopic distribution data during the analysis routinely. For example, the uranium in most soils and ores is commonly deficient in uranium-234 relative to the uranium238, sometimes by as much as 60% ( I ) ,or occasionally enriched by as much as 70% (2). Conversely, ground water is correspondingly enriched, occasionally by as much as 1300% (3). Consequently, humans and other animals also excrete uranium of the same enrichment as that ingested. Thus, isotopic data are of considerable help in understanding both biological and geological processes. Isotopic information is also necessary to assess the lung dose resulting from exposure to uraniumcontaining dusts, and to establish guide values and/or exposure limits for such chemically and radiologically toxic substances ( 4 ) . Information on the isotopic distribution of thorium is as useful and necessary as with the uranium isotopes. Thorium-230 is one of the most toxic radionuclides known, being surpassed only slightly for inhalation by a few of the transuranium nuclides, and has been determined widely by alpha spectrometry for years. However, thorium-232 is generally determined as natural thorium by chemical procedures, and determination of thorium-228 is largely neglected except in a few special cases in which a particular need for isotopic identification had been realized beforehand. For example, the 618

ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977

alpha counting rate of some samples of low-thorium optical glass from various industrial sources was several times higher than would be expected from the thorium concentration given on the label and was initially attributed to the presence of other radioactiveheries. This laboratory was asked to investigate the source of the excess alpha activity. Until recently, the maximum permissible concentration of thorium in ophthalmic glass was defined in terms of natural thorium with the concentration being determined by chemical means. Using the alpha spectrometric procedure described below, ratios of thorium-228 to thorium-232 as high as 20.3 have been found in ophthalmic glasses. Because four additional alpha-emitting daughters grow into the thorium-228 with the 3.64-day halflife of radium-224, the total alpha emission of this particular glass is about 17 times that expected on the basis of secular equilibrium with the thorium-232 present, a substantial difference in the actual dose received by the eye compared to that expected from the natural thorium present. Obviously, the mass of 1.91-year thorium-228 responsible for even excessive radioactivity is far too small to be detected by the usual chemical procedures. In a previous investigation, a procedure was developed for the determination of all elements from radium through Californium simultaneously in a single 10-g sample of soil (5). However, with thorium and/or uranium ores that are of any economic value, the sample siTe required to give good statistics of counting will seldom need to be larger than a few hundred milligrams. In fact, much larger samples cannot be used because of the decreased electrodeposition yields and resolution of the subsequent alpha spectra that result if more than about 100 hg of uranium or thorium is present on the electrodeposited plate. Consequently, the previous procedure can be simplified considerably for application to ores when small samples can be used.

EXPERIMENTAL Instrumentation. The alpha spectrometer using a 450-mm2 surface-barrier detector and 1024-channel analyzer has been described previously ( 5 ) . Reagents. All reagents and radioactive tracers are prepared and standardized as described in the separate references given below. Determination of Thorium-230 in Uranium Ores a n d Mill Tailings. Weigh 0.5 g of sample into a 50-mL platinum dish and add 0.5 mL of concentrated nitric acid a t one edge of the powder. Smell cautiously to see if hydrogen sulfide is being evolved. If not, add 3 mL of 48% hydrofluoric acid to wet the entire sample thoroughly and evaporate to near dryness, leaving the cake just barely moist with acid. Add 250 WLof 234Thtracer in 10%nitric acid (6) containing at least lo4 dpm and reevaporate gently. If sulfides are present, add excess nitric acid and evaporate to near dryness before evaporating with hydrofluoric acid to avoid adverse effects on the platinum dish. A small quantity of fixed nitrate is desirable to assist in the complete oxidation of organic material in the subsequent fusion, but larger quantities cause formation of bubbles in the fusion that makes it difficult to tell when dissolution is complete. Sprinkle 3 g of anhydrous potassium fluoride over the residue, mix coarsely with a stirring rod, and fuse on a ring stand over a blast burner until a clear melt is obtained. Cool the melt, add 3.5 mL of concentrated sulfuric acid and heat gently on a hot plate until all hydrogen fluoride, silicon tetrafluoride, and water have been expelled, taking

precautions to prevent frothing over as described previously (5, 7). Heat the dish over a blast burner until copious fumes of sulfuric acid are evolved, add 2 g of anhydrous sodium sulfate, and continue heating until a clear pyrosulfate fusion is obtained. Do not heat any longer than necessary to minimize dissolution of platinum from the dish. Use a pair of crucible tongs to grip the dish gently but firmly around the outside to avoid contamination that invariably results when the tips are allowed to go inside the dish. Cool the melt with gentle swirling to deposit the cake in a thin uniform layer about a quarter of the way up the sides of the dish to facilitate removal of the cake. Flex the sides of the dish to break the cake loose and transfer the cake to a graduated 100-mL beaker. Add 35 mL of water and 5 mL of concentrated hydrochloric acid to the platinum dish and heat nearly to boiling to dissolve any cake remaining in the dish, and to preheat the water to ensure subsequent rapid and complete dissolution of calcium sulfate. Pour the acid rinse into the beaker, add two 8-mesh silicon carbide boiling chips, cover the beaker with a watch glass, and heat back to boiling as rapidly as possible until the cake has dissolved completely. If necessary to dissolve calcium sulfate completely, add additional hydrochloric acid not to exceed a total of 10 mL. Add 1 mL of 25% potassium metabisulfite solution as a general mild reducing agent, and boil vigorously for 2 min to dissolve anhydrous metallic sulfates and to hydrolyze any condensed phosphates that might be present. Precipitate barium sulfate by addition of four successive 1-mL portions of 0.45% barium chloride dihydrate added at a rate of 1drop every 3 s to the boiling solution with rapid swirling and 1-min boiling periods after each 1-mL addition as described previously (7). Transfer the hot solution to a nominal 40-mL conical centrifuge tube and rinse the beaker with 2 or 3 small portions of 0.5% sulfuric acid. Add the rinses to the centrifuge tube but do not mix by swirling to minimize the quantity of barium sulfate scum at the surface that will be lost by decantation. Centrifuge at 2000 rpm for 5 min. Decant the supernate into another graduated 100-mL beaker for the determination of uranium as described below or discard as desired. Suspend the precipitate in a few milliliters of 0.5% sulfuric acid and recentrifuge. The precipitate contains the cerium, lanthanum, and all elements except uranium from radium through californium quantitatively when present in quantities less than about 1 mg. Bismuth and polonium are not precipitated with barium sulfate to more than a few tenths percent from solutions containing 2 M hydrochloric acid but lead is precipitated to about 10 to 15%. When analyzing samples containing high concentrations of uranium and low concentrations of thorium230 such as yellow cake or other uranium concentrates, the barium sulfate must be reprecipitated ( 4 ) to remove the approximately 1% of the uranium that will have carried in the barium sulfate from the hexavalent state (71, and which will interfere if not removed. All beakers, flasks, centrifuge tubes, etc. in which barium sulfate has been precipitated while hot must be cleaned with hot sulfuric or perchloric acids before reuse to remove traces of barium sulfate that postprecipitate on cooling and will inevitably cause contamination if not removed. Add 1 mL of 72% perchloric acid to the centrifuge tube and heat gently to boiling until excess water has been expelled and the barium sulfate has dissolved completely.Add 10 mL of 2.2 M acidic aluminum nitrate ( 5 ) and transfer the solution to a 60-mL separatory funnel. Rinse the centrifuge tube with another 5 mL of aluminum nitrate solution and add the rinse to the separatory funnel. Add 10 mL of 30% Aliquat-336 in xylene and extract, wash, strip, and electrodeposit as described previously ( 5 )except use 5 mL of 8 M nitric acid in each of the three washes and 10 mL of 10 M hydrochloric acid in each of the three strips. Count the electrodeposited plate in an alpha spectrometer for whatever time is required to give the desired statistical precision. Correct the observed disintegration rate obtained from the counting rate and known counting efficiency of the spectrometer for the chemical yield as described below. Determination of Thorium-232, -228, and -230 in Thorium Ores. Fuse a sample of ore containing approximately 2.5 mg of thorium-232 (e.g., 250 mg of 1%ore) in 2 g of anhydrous potassium fluoride in a 50-mL platinum dish, transpose the cake with 3 mL of concentrated sulfuric acid and 1g of sodium sulfate, and dissolve the cake in alkaline diethylenetriaminepentaacetic acid (DTPA) to accommodate possible rare earths as described previously under Thorium Ores (8).However, break the cake loose from the dish by flexing the sides, and transfer to a 100-mL beaker. Add the DTPA, triethanolamine (TEA), and water to the dish and heat to boiling. When all the cake has dissolved from the dish, transfer the solution to the beaker and continue boiling to dissolve the cake. After acidification to the acid side of quinine sulfate as described, transfer the solution to a 100-mL volumetric flask and dilute to volume. If rare earths are

known to be absent, the cake can be dissolved directly in 35 mL of water and 5 to 10 mL of concentrated hydrochloricacid before diluting to volume. With some samples, larger samples and higher dilutions might have to be used to avoid inconsistencies due to sample inhomogeneity. Transfer an aliquot containing not more than 75 fig of thorium or 600 mg of potassium sulfate to a 30-mL beaker. Add 250 pL of thorium-234 tracer in 10% nitric acid (6)containing at least lo4 dpm, 0.25 mL of concentrated sulfuric acid, 2 or 3 drops each of concentrated nitric and perchloric acids, and enough anhydrous sodium sulfate to make a total of 600 mg with the potassium sulfate present. (Two grams of potassium fluoride gives 3 g of potassium sulfate after transposition. Consequently,the diluted solution will contain about 40 mg of sodium and potassium sulfate per milliliter). Reevaporate the solution back to a pyrosulfate fusion-to destroy the DTPA, to volatilize chlorides, and to obtain complete exchange of the thorium-234 tracer with the thorium isotopes in the sample. Cool the cake, add 10 mL of 2.2 M acidic aluminum nitrate ( 5 )and heat gently,just enough to dissolve the cake. Cool the solution to room temperature and transfer to a 60-mL separatory funnel. Rinse the beaker with another 5 mL of the aluminum nitrate and add to the separatory funnel. If the aluminum nitrate solution should crystallize, warm enough to reedissolve, and transfer the solution and rinse immediately. Add 10 mL of 30% Aliquat-336in xylene and extract, wash, strip, and electrodeposit as described previously ( 5 )except use 5 mL of 8 M nitric acid in each of the three washes and 10 mL of 10 M hydrochloric acid in each of the three strips. Count the electrodeposited plate in an alpha spectrometer for at least IO3 min. Correct the observed disintegration rate obtained from the counting rate and known counting efficiency of the spectrometer for the chemical yield as described below. Ophthalmic glasses can be analyzed similarly after dissolution of the glass in hydrofluoric acid Determination of Chemical Yield. When alpha-emitting radionuclides are determined by alpha spectrometry using an isotopic alpha-emitting tracer, correction for chemical yield is virtually automatic because both the tracer recovered and the radionuclide being sought are measured simultaneously in the same sample by the same detector. Consequently, counting time, counting efficiency,chemical yield, and uniformity of electrodeposition are identical for both isotopes and cancel. However, when an alpha-emitting isotope must be traced by a beta- or gamma-emitting one, a separate tracer standard must be prepared for counting along with the samples with a different detector but under the identical conditions used in preparing the samples for measurement by alpha spectrometry. Obviously, the standard must be corrected for losses sustained on electrodeposition, which can amount to 5% or more depending on the procedure, even on pure tracer solutions. This correction requires complete recovery of the tracer from the electrodepositioncell, “0”-ring seals, electrode, and electrolyte, and comparison of its counting rate against that of another aliquot of the standard, prepared and counted under identical conditions. The effort involved to obtain accurate results is rather substantial, particularly if the half-life of the tracer is relatively short and the standard must be remade frequently. The following procedure involving direct evaporation gives results in good agreement with those from electrodeposited plates after careful correction, and is much faster and simpler to use. Place a polished stainless steel plate like those used for electrodeposition of thorium on an inverted 100-mL beaker as a pedestal. Add as much concentrated nitric acid as the plate will hold without overflowing, and heat strongly under a 250-watt infrared lamp a t a distance of about 4 inches until the nitric acid fumes strongly for several minutes to clean the plate thoroughly of traces of grease or adhesive from the paper covering. Rinse the plate with water and wipe dry with a piece of clean tissue. Add a 250-fiLaliquot of purified thorium-234 tracer in 10%nitric acid (6) containing at least lo4 dpm as near to the center of the plate as possible. Evaporate the solution carefully to dryness under the infrared lamp at a distance of 8 to 12 inches, taking care during the early stages that the aliquot does not move significantly from the center of the plate nor spread to a circular area larger than about ?$ inch. When dry, place the plate first on a hot plate covered with a piece of asbesabout 5 min and cool. Count both standard and samples on a thallium-activated sodium iodide crystal under identical conditions for whatever time is required to achieve the necessary statistical significance. Triton X-100 is very effective in ’ obtaining more uniform deposition of activity over the plate to simulate better the distribution of an electrodeposited source. However, small losses occur when the relatively high-boiling wetting agent is removed by evaporation to dryness. A wetting agent is unnecessary under the condition described. ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977

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Table I. Ratio of Activity Obtained on Standard Pitchblende to Known Value 23SU 234U 23SU 230Th 0.989 f 0.017 0.990 f 0.017 1.003 f 0.017

0.981 f 0.017 0.987 f 0.017 1.003 f 0.017

1.00 f 0.05 0.97 f 0.06 1.02 f 0.05

a Uranium-238 series, 6.06 f 0.04 X series, 2.83 f 0.09 X lo2dpm/g.

1.016 f 0.011 1.008 f 0.011 1.010 f 0.011

l o 3 dpm/g.

Uranium-235

Substantial differences in counting rate have been observed due to small differences in either vertical or horizontal position of the active deposit when the steel plates are counted either in a jig over the open top of a thallium-activated sodium iodide well crystal or in contact with a solid crystal. Many plates have been observed to be sufficiently bent during stamping, bowed during tightening of the electrodepositioncell, or having a slight burr at the edge to cause easily detectable variations in the counting rate. The most reproducible results and the highest counting rates were obtained when the plates were counted with the activity side toward the bottom of a well counter, using a 1/4-in.plastic spacer ring in the bottom to keep the sample from touching the detector. The counting rate changed by less than 0.75%per millimeter change in vertical position and by less than 2% when the source was moved horizontally halfway between the center and edge of the plate. The steel plates used were l1%6-in.in diameter with an electrodepositedarea in the center 5'4-in. in diameter, and fit snugly into the well of a 3-in. by 3-in. thallium-activated sodium iodide well counter. A dart with a small suction cup at the end like that used with a toy indoor dart gun is used to insert and remove the plates from the well. The suction cup must be removed before counting by squeezing the cup with a pair of heavy-duty forceps to prevent significant changes in the counting rate due to shadowing of the crystal from the gamma rays coming back up through the plate. Determination of Uranium-238, -236, and -234 in Uranium Ores. Fuse a sample of ore containing approximately 2.5 mg of uranium-238 (e.g., 1 g of 0.25% U)in 3 g of anhydrous potassium fluoride in a 50-mL platinum dish, transpose with 3.5 mL of sulfuric acid and 2 g of sodium sulfate, and dissolve the cake in 35 mL of water and 5 to 10 mL of hydrochloric acid as described in the first three paragraphs under Determination of Thorium-230 in Uranium Ores and Mill Tailings. After the 2-min boiling to hydrolyze condensed phosphates, add 30 mL of water and cool to room temperature. Transfer the solution to a 100-mL volumetric flask and dilute to volume. To keep the yield on electrodeposition and the resolution of the resultant alpha spectra highand reproducible, the aliquot taken for the uranium determination should not contain more than about 100 wg of uranium, equivalent to 74 dpm of uranium-238 if the uranium is natural. On the other hand, a quantity as near this value as possible is desirable to keep the sensitivity and precision as high as possible for a given counting time. Place the appropriate aliquot selected into a 50-mL beaker. Add about 75 dpm of uranium-232 tracer (6),0.25 mL of concentrated sulfuric acid, and enough anhydrous sodium sulfate to make a total of 600 mg with the potassium and sodium sulfates already present. (Three grams of potassium fluoride gives 4.5 g of potassium sulfate after transposition. Consequently, the diluted solution will contain about 65 mg of sodium and potassium sulfates per milliliter.) Reevaporate the solution back to a pyrosulfate fusion to ensure complete exchange of the uranium tracer with the uranium isotopes in the sample, and to eliminate chlorides and excess acid. Add 2 mL of concentrated sulfuric acid and 25 mL of water to the cake, cover the beaker with a watch glass, and heat the solution to boiling. Add 1mL of 0.625%solution of tellurium dioxide in 25%hydrochloric acid, mix thoroughly, and add 25% stannous chloride in 25% hydrochloric acid dropwise until all yellow iron color has been discharged and a permanent black turbidity of elemental tellurium has been produced. Add 4 drops excess stannous chloride and boil the solution vigorously for 5 min. Allow the solution to stop boiling and add 1 additional drop of stannous chloride to be sure that no additional darkening can be produced. Cool the solution to room temperature and filter through a DM-450 membrane filter in a glass filtering chimney into another 50-mL beaker. Discard the tellurium precipitate containing the polonium-210. Evaporate the solution to near fuming, add 5 mL of 48% hydrobromic acid and 2 drops of 30% hydrogen peroxide to oxidize the divalent tin, and reevaporate to a pyrosulfate fusion to volatilize the quadrivalent tin and prevent 620

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subsequent problems due to hydrolysis. Remove the cover glass when most of the hydrobromic acid has been removed. Dissolve the cake in 10 mL of 2.2 M acidic aluminum nitrate and finish as described beginning with the third paragraph under Determination of Thorium-232, -228, and -230 in Thorium Ores except use only 2 mL of 8 M nitric acid in each of the three nitric acid scrubs, and strip the uranium from the organic phase after the hydrochloric acid scrubs with 10 mL of the perchloric-oxalic acid solution (5)and then with 5 mL of water.

RESULTS AND DISCUSSION The accuracy of the procedures for determination of thorium-230 and the uranium isotopes in uranium ores was checked by analysis of a standard pitchblende sample. This material was prepared from ore known to contain primary, unaltered pitchblende in secular equilibrium, and was standardized for natural uranium by an accepted volumetric procedure. The activities calculated for both the uranium-238 and -235 series from accepted values of the half-lives and isotopic abundances have been confirmed repeatedly by extensive determinations of each of the main radionuclides by several different investigators over a period of 12 years (5, 9-11). The ratio of the activity of each radionuclide obtained in the present investigation to the calculated value is shown in Table I. T h e uncertainty given is the standard deviation resulting from propagation of all random uncertainties incurred in the entire measurement process. Every analytical result obtained obviously agrees with the known value within the statistical uncertainties of the measurement at the 95% confidence level. Apparently, there are no significant systematic uncertainties remaining in the procedures. The spectrum of uranium-235 overlaps those of both uranium-238 and -234, but the corrections are small with natural uranium except when uranium-235 is being determined. If a well-resolved spectrum of natural uranium is integrated from the low points on both sides of the visible uranium-235 peak, only 84.4% of the alpha branches from uranium-235 will be included, and the activity in the visible resolved peak should be increased b y 18.5%. Without the correction, the ratio of uranium-235 to -238 will not agree with the theoretical value of 0.0467 for natural uranium. Similarly, the activity under the uranium-238 and -234 peaks should be decreased by 7.3 and 11.2%, respectively, of the activity in the visible uranium-235 peak. Protactinium-231 is extracted by Aliquat-336 with uranium virtually quantitatively, particularly in the absence of nitrous acid (5),and gives a peak in the uranium spectrum between those of uranium-234 and -232. Although well resolved in most cases, the visible peak contains only 88.7% of the alpha branches from protactinium-231, with the other 11.3% occurring at energies overlapping those of uranium-234. However, with most uranium ores, even if substantially depleted in uranium-234, the correction is very small. The specific activity of the uranium-235 chain is only 4.7% of that of the uranium-238 chain. Consequently, the protactinium-231 contribution to the uranium-234 peak is only 1% even if the uranium-234 were depleted by 40% relative to the uranium238. Correcting the uranium-234 peak for 12.7% of the visible protactinium peak will be more than adequate for most naturally-occurring ores. For mill tailings or other mill products having undergone recent chemical alteration in which the protactinium-231 might have become substantially enriched relative to the uranium-234, it will be more accurate to remove the protactinium-231 than to correct for it. T h e protactinium-231 can be removed easily by adding two or three SUCcessive 1-mL portions of 0.45% barium chloride dihydrate after hydrolysis of condensed phosphates as described under Determination of Thorium-230 in Uranium Ores and Mill Tailings beginning with the fourth paragraph. Particular attention must be given to the complete removal

of polonium-210 in the tellurium precipitation. Because polonium-210 emits alpha particles having energies only 20 keV lower than those of uranium-232, any polonium-210 not removed will add to the uranium-232 peak and make the recovery of tracer appear to be higher than it really is, giving low results. For example, the solutions can be centrifuged rather than filtered if desired, but about 1%of the polonium-tellurium precipitate will be decanted with the supernate because of a scum a t the surface that is not pulled down by centrifugation. A considerably larger error, and one that is not readily apparent, is produced in the presence of hydrochloric acid. If 5 mL of concentrated hydrochloric acid is used instead of the 2 mL of concentrated sulfuric acid specified, the reduction of the tellurous acid and subsequent agglomeration of the elemental tellurium takes place so much more rapidly that much of the polonium will not have been precipitated and/or included in the tellurium carrier by the time the available surface of the carrier has been reduced to negligible proportions. The results will be 5 to 10%low. The accuracy of the procedure for determination of the three main thorium isotopes in thorium ores was verified by comparing the results obtained for thorium-232 by the present alpha spectrometric procedure with those obtained for natural thorium by a fluorometric procedure of previously demonstrated accuracy (B), and by direct measurement of the lead212 daughter by gamma spectrometry. On 24 measurements of 8 different solid aliquots of a standard monazite ore, the fluorometric procedure gave a mean of 1.404 f 0.017 (f0.004)% thorium, where the first uncertainty is that of an individual about the mean and the one in parentheses is that of the mean itself. Using a value of 2428 f 21 dpmlg of thorium-232 per percent natural thorium, the percentage values convert to 3409 f 51 ( f 3 1 ) dpm/g of thorium-232. On 20 measurements on the same 8 aliquots of the sample by the present alpha spectrometric procedure, a mean of 3421 f 74 ( f 1 6 ) dpm/g was obtained, in excellent statistical agreement

with the chemical values. Four measurements by direct gamma spectrometry gave a mean of 1.444 f 0.018 (f0.009)% thorium, also in acceptable agreement. In addition, the mean ratio of thorium-228 to thorium-232 for the 20 measurements was 0.996 f 0.024 (f0.005),showing that the measurement of thorium-228 is no less exact and that the system is in equilibrium so that the gamma spectrometric measurements can be related unequivocally to the thorium-232 concentration. Although no independent method was available to check the thorium-230 result in thorium ores, there is no reason to believe that its determination will be any less accurate than the other two isotopes.

ACKNOWLEDGMENT The author wishes to express his appreciation for the assistance of his associates, particularly to F. D. Hindman for the thorium and uranium separations, to R. L. Williams for the electrodepositions and alpha spectrometry, and to J. S. Morton for the gamma spectrometry.

LITERATURE CITED (1)J. N. Rosholt, A. P. Butler, E. L. Garner, and W. R. Shlelds, €con. Geol., 60, 199 (1965). (2)J. N.Rosholt, E.N. Harshman, W. R. Shlelds, and E.L. Garner, €con. GeoL, 59, 570 (1964). (3)J. Kronfeld, Nucl. Sci. Abstr., 26, 2982 (1969),Abstract No. 31018. (4)C.W. Sill, "Simultaneous Determlnation of U-238.U-234,Th-230, Ra-226 and Pb-210 in Uranium Ores, Dusts and Mill Tailings, U. S. Energy Research and Development Administration, Health Services Laboratory, Idaho Falls, Idaho. (5) C. W. Sill, K. W. Puphal, and F. D. Hlndman, Anal. Chem., 46, 1725

(1974). (6)C.W. Sill, Anal. Chem., 46, 1426 (1974). (7)C.W. Sill and R. L. Williams, Anal. Chem., 41, 1624 (1969). (8)C.W. Slll and C. P. Willis, Anal. Chem., 34,954 (1962). (9)C. W. Sill and F. D. Hlndman, Anal. Chem., 46, 113 (1974). (10)D. R. Perclval and D. 8. Martin, Anal. Chem., 46, 1742 (1974). (11)C.W. Silland C. P. Willis, Anal. Chem., 37, 1661 (1965).

RECEIVED for review October 29,1976. Accepted January 14, 1977.

Separation and Determination of Nanogram Amounts of Inorganic Arsenic and Methylarsenic Compounds Robert S. Braman,* David L. Johnson,' Craig C. Foreback,2 James M. Ammons, and Joseph L. Bricker Department of Chemistry, University of South Florida, Tampa, Fla. 33620

Arsenate and arsenite ions, methylarsonlcacid, and dimethylarslnic acid in aqueous solutions are reduced to arslne and the corresponding methylarslnes respectlvely at pH 1-2 by sodlum borohydrlde In a reaction chamber. Entrained by He carrier gas the arsines are frozen out In a liquid nltrogen cooled U-trap. Their separation Is accompllshed by volatlilzatlon upon warming the U-tube trap. Arslnes carried out of the trap by the carrler gas are passed through a direct current electrlcai dlscharge. Arsenic atomic emlsslon lines produced in the discharge are detected by a recording, scannlng monochromator system. Llmlts of detectionfor arsenic are approximately 1 ng for each of the arsines.

Present address, State University of New York, College of Environmental Science and Forestry. Syracuse, N.Y. 13210. * Present address, Chemistry Division, Henry Ford Hospital, Detroit, Mich. 48202.

Considerable interest in the environmental chemistry of arsenic stems from the toxicity of its compounds and their use as silvicides or pesticides. An extensive annotated bibliography on arsenic in the environment has been prepared (1). Nearly all of this referenced work used analytical methods for total arsenic with no differentiation of arsenic by chemical form except for thin-layer and paper chromatography separation methods reported by Sachs, Anastasia, and Wells ( 2 ) . Methylarsenic acids are important compounds of arsenic produced by biomethylation and could play an important role in its environmental chemistry. Concentrations of arsenic found in the environment are generally small, several parts per billion in sea water to less than 1ppb in many fresh waters and parts per million in soil. Prior to the work of Braman and Foreback (3) little had been available for the determination of the arsenic forms in environmental samples. Johnson and Pilson ( 4 ) developed a spectrophotometric method for differentiation of arsenic(II1) ANALYTICAL CHEMISTRY, VOL. 49, NO. 4, APRIL 1977

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