present largely by the availability of suitable calibrated standards of the isotopes sought. \\’here the results are expressed relatively in per cent of some known level of original activity, such as in isotope dilution or activation analysis, this method seems especially suitable. Applications of this type are numerous in industry and medicine (5,9,11, I d ) . ACKNOWLEDGMENT
The author thanks Daniel L. Love for furnishing the gamma standards and sample mixtures used in this work. LITERATURE CITED
( 1 ) Connally, R. E., Leboeuf, M. B., A N A L . CHEM. 25 1095 (1953). (2) Cook, C. S., Am. Scientist 45, No. 3, 245 (1957).
(3) Covell, D. F., “Determination of Radionuclide Abundances from GammaRay Pulse Height Distribution Data,” Diesion of Analytical Chemistry, Symposium on Radiochemical Analysis, 133rd Meeting, ACS, San Francisco, Calif., April 1958. (4) Covell, D. F., Sandomire, M. M., Eichen, M. S., Zbid., “Automatic Compensation of Dead Time in CountingEquipment.” (5) Francis, J. E., Bell, P. R., Harris, C. C., Nucleonics 13, No. 11, 82 (1955). (6) Heath, R. L., Schroeder, F., U. S. Atomic Energy Comm. Rept. IDO16149 (1st rev., 1955); Nuclear Sci. Abstr. 9, 5424 (1955).
(7) Koch, H. W.,Johnston, R. W.,eds., Natl. h a d . Sci.-Yatl. Research Council, Publ. 467, 182 (1957). ( 8 ) Lazar. N. H.. Davis. R . C.. Bell. ‘ P. R., li’ucleonics 14, No: 4, 52 (i056). ’ (9) Leddicotte, G. W., Ibid., 14, S o . 5, 46 (1956). (10) LidBn, K., Starfelt, N., Arkiv F y s i k 7, 427 (1954).
(11) McCormick, J. A., compiler, “Bib-
liography of Uses of Radioactive and Stable Isotopes in Industry,” Technical Information Services Extension, U. S. rltomic Energy Comm., Rept. TID-3511; Nuclear Sci. Abstr. 12, 377
(1958). (12) Meinke, W. W.,ANAL.CHPM.28, 736 (1956).
(13) Schumann, R. W., McMahon, J. P.,
Rev.Sci. Znstr. 27, No. 9, 675 (1956). (14) Siegbahn, K., “Beta- and Gamma-
Ray Spectroscopy,” Interscience, New York, 1955. (15) Travis, C. M., Kelly, B. A., Hearne, S. M.,Baker, B. R., Convair, Fort Worth, Tex., NARF-57-34T ; Nuclear Sci. Abstr. 1 1 , 11300 (1957).
RECEIVED for review September 19, 1958. Accepted December 24, 1958. Presented in part before Division of Analytical Chemistry, Symposium on Radiochemical Analysis, 133rd Meeting, ACS, San Francisco, Calif., Spril 1958.
Analysis of Fission Product Mixtures ERIC L. GEIGER’ Savannah River Planf,
E. 1. du Pont de Nemours & Co., Inc., Aiken, S. C.
b A procedure has been developed for the rapid, sequential separation of radioactive ruthenium, strontium, rare earths, zirconium-niobium, and cesium with recoveries greater than 97%, and individual decontamination factors greater than 1000. High decontamination factors are also obtained with radioactive iodine, cobalt, iron, chromium, and zinc. The precision of each analysis is within 2~4% a t the 90% confidence limit. Designed for the examination of air-filter, soil, sediment, vegetation, and aqueous Samples; the analyses are made on 3 N hydrochloric acid-0.1 N hydrofluoric acid solution. With this solution, fission products are quantitatively leached from samples which cannot b e easily dissolved. A complete analysis can b e made in 16 hours, and a trained technician can analyze 40 samples per 40-hour week. By providing nearly quantitative recoveries and high precision, these procedures eliminate the necessity for gravimetric yield determinations without sacrificing specificity. Considerable application to other areas of radiochemistry is anticipated.
D
and evaluation of fission product procedures began with the Manhattan Project (1). Some of these procedures have been modified and new ones have been developed. Radioisotopes of almost any element can EVELOPMENT
Present address, Reynolds Electrical and Engineering Co., Las Vegas, Nev. 1
806 *
ANALYTICAL CHEMISTRY
now be separated from those of almost any other element ( 3 ); however, very few procedures are quantitative or simple. These attributes have been sacrificed to obtain the desired specificity. Being nonquantitative, such procedures require that the carriers be standardized and gravimetric yield be determined for each sample. This investigation was made to develop quantitative procedures that are specific and relatively simple for the analysis of aged fission product mixtures. I n health physics environmental monitoring programs, air-filter, soil, sediment, vegetation, and water samples are analyzed for fission products. Fission products of interest include radioisotopes of ruthenium, strontium, yttrium, rare earths, cesium, zirconium, and niobium. These fission products and their daughter isobars represent the major components of a fission product mixture that has aged 2 months or more. Fission product mixtures that have aged less than 2 months contain radionuclides other than isotopes of the elements listed above. Such radionuclides may or may not interfere with the procedure that was developed. However, any interference will be eliminated through radioactive decay within 2 months. EXPERIMENTAL
Filters used to monitor air for radioactive particulate matter include Hollingsworth Vose No. 70 (HV-70) filters
and Mine Safety Appliances S o . 1106B (MSA-1106B) filters (4). The HV-70 filter is impregnated with silver nitrate to facilitate the collection of gaseous radioiodine. Because this filter is difficult to dissolve, a leaching procedure was developed for the recovery of fission products from the insoluble residue. A 3N hydrochloric acid-0.1.1‘ hydrofluoric acid solution leaches more than 99.9% of the ruthenium, strontium, yttrium, rare earths, cesium, zirconium, and niobium from the insoluble residue. Solutions of nitric, hydrochloric, and nitric-hydrofluoric acids do not leach zirconium and niobium quantitatively. The MSA-1106B filter is used nhen collection of iodine-131 is not necessary. This filter dissolves completely in 3N hydrochloric acid-0.1N hydrofluoric acid solution after first being treated nrith a nitric acid-hydrofluoric acid solution. The 3N hydrochloric acid0.1N hydrofluoric acid solution is also used to leach fission products from soil and ashed vegetation. This technique was quantitative for soil samples contaminated with fission products and for vegetation grown in a contaminated environment. For the evaluation, the amount of radioactive tracer added and the amount recovered were compared using a sodium iodide (thallium) detector 13,/4 (diameter) x 2 inches n.ith a central well 5 / 8 (diameter) X 11/* inches. Tracer solutions were counted in polystyrene vials (inside diameter) x 2 inches. Tracer concentrations that provide lo4 to 106 counts per minute (elm) were used to obtain recovery and decontamination factor (DF) data, the higher concentrations being used to determine the decontamination factor. No at-
acid solution. Add the wash solution to the 150-ml. beaker. Place the beaker on a hot plate and carefully evaporate the solution to dryness. Reagents. CARRIER SOLUTIONS. Allow the residue to char. Retain this portion for subsequent determination These are not standardized, because of strontium, zirconium-niobium, cerecoveries are nearly quantitative and sium, and rare earths. gravimetric yield determinations are SECONDPRECIPITATION OF RUTHEnot required. Chemically pure rePlace the filter containing the NIUM. agents were used to prepare the folruthenium sulfide inside a 50-ml. Erlenlowing meyer flask. Insert filter in such a way as to have the precipitate side down and flat on the bottom of the flask. Add 2 or 3 ml. of 8N nitric acid; this reacts Solution vigorously, dissolving the precipitate. EleConcn., The reaction should be completed bement Rlg./Ml. Solute and Solvent fore the flask is placed on a hot plate. R 11 25 Boil to dryness and leave on the hot Cr 10 plate until fuming ceases. Add 3 or 4 Zr 20 ml. of 1.5N hydrochloric acid and Ce 15 evaporate to dryness on the hot plate. Sr 20 When the fuming ceases, remove from Ce 5 the hot plate and allow to cool. Dissolve the residue in 25 ml. of 3N cs hydrochloric acid-0.lN hydrofluoric Zr 100 ZrO(S'03)2.2H20in 1N HKOa acid solution. Add 1 ml. of chromium carrier, 1 ml. of ruthenium carrier, and 1 ml. of 20 mg. per ml. zirconium carrier, and repeat the thioacetamide precipitation. SPECIAL SOLUTIONS.Thioacetamide, Mount the ruthenium precipitate for 0.1 gram per ml. Dissolve 10 grams of beta counting. Discard the filtrate. thioacetamide in slightly 15-arm distilled PRECIPITATION OF STRONTIUM AND water. Mix thoroughly, allow to cool, RAREEARTHS.To the charred filtrate and dilute to 100 ml. from the first thioacetamide precipitaNitric Acid-Boric Acid Solution. tion, add 2 to 3 ml. of concentrated Transfer 50 ml. of a saturated boric acid nitric acid and again evaporate to drysolution, 50 ml. of distilled water, and ness. Do not char. Dissolve the resi100 ml. of concentrated nitric acid to a due in 3 to 4 ml. of 3N hydrochloric labeled reagent bottle. Mix thoroughly acid-0.lN hydrofluoric acid solution and by swirling gently with the glass stopper again evaporate to dryness. Remove removed. Alloiv to cool and replace the beaker from the hot plate and the glass stopper. allow to cool 3 to 4 minutes. Add 10 Phosphotungstic Acid, 0.05M soluml. of O.1N sulfuric acid-0.3N hydrotion. Transfer the contents of a freshly fluoric acid solution and 1 ml. of 20 mg. opened, 100-gram bottle of Eastman Kodak Co. phosphotungstic acid, per ml. zirconium carrier. Mix thoroughly, then add 1 ml. of 15 mg. per ml. P206.2dR03.44H20(molecular weight cerium(II1) carrier. Cover n-ith a = 6500), to a 500-ml. graduated natch glass and allow the sample to cylinder. Dissolve in distilled mater digest 30 minutes on a hot plate adand dilute to 300 ml. Mix thoroughly and transfer to a labeled reagent bottle. justed to maintain the temperature of Procedure. FIRST PRECIPITATIONthe solution between 90" and 100" C. O F RUTHEMUM. Transfer 25 ml. of Transfer to a 50-ml. round-bottomed the 3 N hydrochloric acid-0.1N hydroglass centrifuge tube. R a s h with small fluoric acid sample solution t o a 50aliquots of 0.1N sulfuric acid-0.3N ml. Erlenmeyer flask. Add 1 ml. of hydrofluoric acid solution until the total ruthenium carrier and 1 ml. of chrovolume of the sample is 20 ml. mium carrier. Adjust to 35 ml. with Centrifuge and decant the supernate into a Dowex-2 anion exchange column distilled water. Mix by swirling gently. Add 1 ml. of 0.1 gram per ml. of thio0.64 cm. in inside diameter X 12.7 em., acetamide. Place the flask on a hot previously prepared by washing with 10 plate adjusted to maintain the temperaml. of 0.l.V sulfuric acid-0.3N hydroture of the solution between 90" and fluoric acid solution. Wash the precipi100" C. Do not boil. tate with 5 ml. of 0.1N sulfuric acidAllow to remain on the hot plate until hydrofluoric acid solution, centrithe precipitate settles to the bottom of fuge, and decant the wash solution to the flask and the supernate clears the reservoir of the anion exchange (approximately 45 minutes). Filter column. Collect elutriant from the with HA Millipore paper, collecting the column in a 100-ml. beaker. filtrate in a clean 250-ml. suction flask. SEPARATIOK OF STRONTIUM AND RARE Use 2hr hydrochloric acid-0.08N hydroEARTHS.Place the centrifuge tube in a fluoric acid to transfer the precipitate hot water bath and dissolve the fluoride quantitatively to the filter and wash the precipitate in 3 ml. of the nitric acidprecipitate and filter. Wash finally boric acid solution. Add 0.5 ml. of 20 with a small amount of ethyl alcohol. mg. per ml. strontium carrier and mix Transfer the filtrate from the suction thoroughly. Add 20 ml. of fuming flask to a 150-ml. beaker. Wash the nitric acid in 5-ml. increments. Let suction flask with small portions of 2N stand for 10 minutes in an ice bath. hydrochloric acid-0.08N hydrofluoric Centrifuge and decant the supernate tempt was made to detprmine the decontamination factor exactly if it exceeded lo4.
into a 100-ml. Lusteroid tube containing 65 ml. of distilled water. Retain this portion for rare earth determination. PURIFIC.4TIOS O F STROKTIUJI. Dissolve the nitrate precipitate in 15 ml. of distilled water and add 1 nil. of 5 mg. per ml. cerium(II1) carrier. Add 1.5 ml. of concentrated ammonium hydroxide. Centrifuge and transfer supernate to a conical, 5O-ml. centrifuge tube. Wash the precipitate with 5 ml. of dilute (1 to 100) ammonium hydroxide solution. Centrifuge and add the supernate to the conical centrifuge tube. Save the precipitate for rare earth determination. To the solution in the conical tube add 1 ml. of ethyl alcohol, then 5 ml. of saturated ammonium oxalate. Place in a n ice bath for 10 minutes. Centrifuge and discard the supernate. Mount the strontium precipitate for beta counting and count immediately. PURIFICATION OF RARE EARTHS. From the 100-ml. Lusteroid tube containing the supernate from the strontium nitrate precipitation,. pour 25 to 30 ml. into the tube containing the hydroxide precipitate obtained in the purification of the strontium portion. Pour this back into the Lusteroid tube and wash the glass tube with 5 nil. of distilled water to complete the transfer. To the Lusteroid tube add 1 ml. of 100 mg. per ml. zirconium carrier. Add 15 ml. of concentrated hydrofluoric acid. Stir with a polyethylene-encased stirring rod. Let stand 15 minutes. Centrifuge and discard the supernate, allowing the tube to drain thoroughly. Dissolve the precipitate in 5 ml. of the nitric acid-boric acid solution with the aid of a hot water bath. Add 50 ml. of distilled water and 1 ml. of saturated strontium nitrate solution. Add 3 ml. of concentrated ammonium hydroxide. Centrifuge and discard supernate. Dissolve the precipitate in 5 ml. of nitric acid-boric acid solution and repeat the precipitation. Mount the rare earth precipitate for beta counting and count immediately. ANON EXCH.4SGE REMOVAL O F ZIRCOKIUM-KIOBIUJI.\Then the supernatant solution from the strontium and rare earths precipitation has passed through the anion exchange column, wash the column with three 10-ml. aliquots of 0.1A' sulfuric acid-0.3N hydrofluoric acid solution, allowing each portion to drain through thc column before addition of the next aliquot. Set aside the combined column elutriant and washes for cesium determination. Add 25 ml. of 6N hydrochloric acid0.5N hydrofluoric acid solution to the column reservoir and collect the elutriant in a 50-ml. beaker. Add 10 ml. of 6N hydrochloric acid-0.5S hydrofluoric acid solution to complete the elution of zirconium-niobium from the column. Place the 50-ml. beaker on a hot plate and carefully evaporate to 1 to 2 ml. Do not allow the sample to evaporate to dryness. Transfer the sample quantitatively to a labeled 5-ml. polystyrene vial, using small pliquots of 2N hydrochloric acid0.08N hydrofluoric acid solution to complete the transfer. Adjust the volume VOL. 31. NO. 5. M A Y 1959
807
0.08iY hydrofluoric acid solution, rutheTable 1.
nium is quantitatively from - seDarated 99.9% of all other contaminants normallv encountered in fission Droduct mixiures that have aged 2 mokths or more. The supernate from the first ruthenium sulfide precipitation is evaDorated to dryness and then carried thiough a series of acid evaporations to eliminate
Recovery and Precision
Recover)., Precision, Element % %" Ruthenium 98 8 *3 4 Strontium 97 0 Rare earths 99 3 Zirconium-niobium 99 0 23 Cesium 99 2 3 8 a 90% confidence level.
Table II.
Tracer Ru'03
SP5
Ce-Pr144
Zr-Nb96
Cs'37
Fe59
coco
Cr 5 1
ZnG5 1131 a
Ru
... > 104 > 104 2 x 103 >io4 > 104 2 x 103 1 x 103 > 104 > 104
Decontamination Factors
Sr > 104 4
Zr-Nb
> 104
~~
x
103
1 . 4 X-103
2 xi03 6 X lo3
2 ' x 103
4
2 X
RE5 >io4 .
io3
x 103 > 104 > 104 > 104 > 104 >104 >104
Rare earths. Sample did not contain Ag+.
to exactly 4 ml. Place the cap on the vial and mix thoroughly. Count the sample in a well-type, sodium iodide (thallium) crystal, gamma counter. This portion contains zirconium-95 and niobium-95. PRECIPITATION OF CESIUM. Add 16 ml. of concentrated nitric acid and 1 ml. of cesium carrier to the 0 . l N sulfuric a c i d 4 3 N hydrofluoric acid column elutriant. Add 1 ml. of 0.05M phosphotungstic acid. Let stand for 10 minutes. Filter with HA Millipore paper. \Trash with 4.1- nitric acid and dry the precipitate with suction continued for 3 or 4 minutes. RIount the sample for beta or gamma counting. For gamma counting, allow 30 minutes for barium-137 to establish equilibrium with cesium- 137.
> 10' >io4 4 x 103 >104 >io4
1.1 x 103
1 . 5 x 103 > 104 > 104b > 104 > 104
CS ,104- _ ~
'i x 103 9 x 103 >lo4
...
x 103 2 x 103 1 . 2 x 103 > 104 1
>lo4
radioiodine. This treatment also prepares the sample for removal of rare earths and strontium on cerium fluoride precipitation, which is made from a solution that contains no strontium or barium carriers. The precipitation of strontium nitrate from concentrated nitric acid solutions n-as first described by Willard and Goodspeed in 1936 ( 7 ) . This strontium nitrate precipitation is followed by a hydroxide scavenge precipitation from the strontium portion to complete the separation of strontium and rare earths. Strontium is then precipitated as strontium oxalate (2, 6). Because barium-140 is reduced by radioactive decays to a negligible amount in aged fission product mixtures, the necessary steps for the separation RESULTS AND DISCUSSION of barium-140 from the strontium porRuthenium sulfide is homogeneously tion were omitted. Should barium-140 precipitated from the hydrochloric acidbe present, the strontium portion should hydrofluoric acid solution by thermal be purified by barium chromate scavdecomposition of thioacetamide. From enge or repeated oxalate precipitations. a hydrochloric acid-hydrofluoric acid Lanthanide rare earths are determixture the precipitation is very specific mined as a group that includes yttrium. for ruthenium. From 2 S hydrochloric They are purified with a fluoride preacid-0.08S hydrofluoric acid solution, cipitation containing zirconium hold0.4% of the zirconium-niobium-95 is back carrier and two hydroxide precarried on the sulfide precipitate; cipithtions with strontium holdback hom-ever, this interference is easily carrier. The zirconium-95 and niobium-95 eliminated by a second sulfide precipitation and addition of holdback carriers. isobars are determined together; however, they are easily separated by The specificity increases with increasing hydrochloric acid concentration. HOW- Dowex-2 anion exchange using hydrochloric acid-hydrofluoric acid solutions, ever, at acid concentrations much above if desired. 2 N , ruthenium sulfide has a tendency The absorption of zirconium and to decompose with heating. Therefore, niobium from 0.1?: sulfuric acid-0.3N a concentration of 2A' hydrochloric hydrofluoric acid solution by Dowex-2 acid-0.08.Y hydrofluoric acid solution anion exchange resin was reported by is considered optimum. The presence Wish and Rowell (8)in 1956. After exof a small amount of hydrofluoric acid tensive evaluation, this technique was inenhances the specificity of the precipitacorporated in this procedure. The solution for ruthenium. K i t h two precipition mas allowed to pass through the coltations from 2.Y hydrochloric acid808
ANALYTICAL CHEMISTRY
umn under gravity flow conditions. A 200- to 400-mesh resin was used, Zirconium and niobium \$-ereeluted quantitatively from the column with 6Al'hydrochloric acid-0.5N hydrofluoric acid solution. No additional purification was necessary. Phosphotungstate precipitation is a modification of a procedure reported by Mizzan (6). For samples containing fission products as the only beta-gamma emitters, a more specific precipitation is obtained in hydrochloric acid-hydrofluoric acid solution than in nitric acidhydrofluoric acid solution. However, because most of the fission products have been removed prior to the cesium precipitation, a higher degree of specificity is not normally required. If the sample contains chromium-51, which is produced by neutron irradiation of stainless steel, the nitric acid-sulfuric acid-hydrofluoric acid solution is better than a hydrochloric acid-sulfuric acidhydrofluoric acid solution. To evaluate the procedure, more than a hundred silver nitrate-impregnated HV-70 filters were spiked with radioactive tracers, leached m-ith 3N hydrochloric acid-0.1X hydrofluoric acid solution, and analyzed for ruthenium, strontium, rare earths, zirconium-niobium, and cesium. Gamma emitters, ruthenium-103, strontium-85, cerium-praseodymiuni-144, zirconium-niobium-95, and cesium-barium-137 were used to determine chemical recoveries and decontamination factors. Recovery and precision data are summarized in Table I. The decontamination factor data are summarized in Table 11. Individual decontamination factors greater than l o 3were considered adequate, except for iodine-131. Being volatile, iodine-131 is sometimes released to the atmosphere in much greater quantities than other fission products. For this reason, a decontamination factor greater than lo4 is considered desirable for iodine. The procedure provides adequate decontamination from iodine-131 in every case. Occasionally, samples are analyzed that contain, in addition to fission products, radionuclides which are produced by neutron irradiation of structural components of a reactor-e.g., stainless steel. Radioactive isotopes of iron, cobalt, chromium, and zinc were considered as possible contaminants; therefore, decontamination factors mere determined for iron-59, cobalt-60, chromium-51, and zinc-65. These data are included in Table 11. For samples containing silver ion in large amounts-e.g., silver nitrateimpregnated filter samples-0.1 to 0.2% of any chromium-51 that may be present contaminates the zirconiumniobium portion. For samples that do not contain silver, decontamination factors greater than l o 4 are obtained
for chromium-51. In every case, adequate decontamination factors are obtained for iron, cobalt, chromium, and zinc. A complete analysis requires approximately 16 hours; however, approximately 50y0 of this time, the analyst is waiting for digestion or ion exchange steps. -4 trained technician can analyze 40 samples in a 40-hour work week. LITERATURE CITED
(1) Coryell, C. D., Suga,rman,N., “Radio-
chemical Studies. The Fission Products,” hIcGrawHill, New York, 1951. (2) Hillebrand, W.F., J . Am. Chem. SOC. 16, 83 (1894). (3) Kleinberg, J., U. S. Atomic Energy Comm., Research and Development Rept. LA-1721 (rev.) (September 1954, unclassitied). (4)Little, A. D., U. S. -4tomic Energy Comm., Tech. Information Service Rep!. AECU-3119 (October 1955, unclassfied). (5) hlizzan, E., “Phosphotungstate Precipitation Method of Analysis of Radioactive Cesium in Solutions of Long Lived Fission Product Activities,” Katl. Researrh Council of Canada. Atomic Energy Project, Chalk River, Repi. PDB-128 (July 1954, unclassified).
(6) Peters, C. A., Am. J . Sci. ( 4 ) , 12, 216 (1901). ( 7 ) Willard, H. H., Goodspeed, E. \J7., ISD. ESG. CHEM.,ANAL. ED. 8, 414 (1936). (8) Wish, L., Rowell, M., G. S. Saval Radiological Defense Laboratory, San Francisco, Rept. USNRDLTR-117, 21 (October 1956, unclassified).
RECEIVEDfor review October 17, 1958. Accepted February 11, 1959. Division of Bnalytical Chemistry, Symposium on Radiochemical Analysis, 133rd Meeting, ACS, San Francisco, Calif., April 1958. Information developed during work under Contract AT(07-2)-1 q-ith the Atomic Energy Commission, Those permission to publish is gratefully acknowledged.
X-Ray Emission Spectrographic Analysis of High-Purity Rare Earth Oxides FARREL W. LYTLE and HOWARD H. HEADY Rare and Precious Metals Experiment Station, U.S. Bureou o f Mines, Reno, Nev. b M i n o r amounts of rare earth elements (0.01 to 1.O%) in high-purity rare earth oxides may b e determined by a fluorescent x-ray procedure. Basic to the method is accurate determination of spectral line intensity above background. Sample and standard preparation, choice of analytical lines, and utilization of a helium path to increase x-ray intensities are discussed. Precision and accuracy were evaluated by analyzing samples of known composition. The average of five separate determinations showed an average error of about 10%.
I
analysis has become a n indispensable tool in research and production of the rare earth elements. This development can be attributed mainly to a combination of two factors-the marked chemical similarity of the rare earth group of elements, which excludes classical analysis methods, and the speed and accuracy of instrumental analysis techniques. K i t h continued improvements in methods of producing high-purity rare earth elements, keeping pace instrumentally is becoming exceedingly more difficult. Accordingly, more and more emphasis is being placed on improving instrumental limits of detection and accuracy. The methods employed most frequently for determining impurities in rare earth elements involve use of the optical emission spectrograph (3, 4) and the spectrophotometer (9). dlthough numerous x-ray emission spechave been trographic methods (6%) developed for analyzing rare earths, NSTRUXCSTAL
these techniques have not included detection of low-level impurities. This investigation vas conducted to find the precision and accuracy obtainable by x-ray fluorescence in determining impurities in rare earth oxides in the 0.01 to 1.0% concentration range. INSTRUMENTATION
A General Electric XRD-5 spectrograph, equipped with a tungsten target x-ray tube, was operated a t 50-kv. peak and 40 ma. Lithium fluoride was used as the diffracting crystal, and collimation was provided by an 0.005-inch Soller slit. An argon-filled Geiger proportional counter was operated in the proportional region in conjunction with a decade-type scaler. The analytical lines were chosen from the rare earth L spectra. Table I shows the tabulations of the analytical lines for each matrix. Calibration v-as accomplished by counting each standard twice for 4000 counts (probable error equals l%), averaging, and computing intensity in counts per second (c.p.s.). Usually, one calibration sufficed for a few months’ constant instrumental operation, if no changes were made in the instrument. Periodically, standard samples were rerun to check the calibration. Some of the rare earth lines used were of long wave length. T o decrease absorption effects and increase intensity, a helium atmosphere was introduced by enclosing the optical path in a plastic bag and maintaining a positive pressure a t low flow rates. Each element analyzed indicated a n appreciable gain
in net intensity over the air path, varying for lanthanum, praseodymium, neodymium, and samarium by a factor of 3.6, 2.6, 2.1, and 1.8, reqpectively. This would appear to increase sensitivity levels by a like factor. However, only a slight increase in peak-tobackground ratios was obtained; accordingly, no significant increase in sensitivity or accuracy was noted. The net intensities above background and peak-background ratios for several elements in ceric oxide, with air and helium atmospheres, are summarized in Table I. K’et intensities and peakbackground ratios in air for elements in other rare earth matrices are also included. PREPARATION OF STANDARDS
Standards were prepared by precipitation to simulate the type of samples submitted for analysis. Each set of standards mas precipitated from acid solution as the oxalate and converted to the oxide by ignition for 1 hour at 1000” C. Some rare earth oxalates were slightly soluble and required evaporation and reprecipitation. Concentrations of the impurity elements were varied in convenient steps from 0.005 to 1.0%. Rare earth oxides to be analyzed were ground under alcohol in a mechanical mullite mortar for 20 minutes to achieve uniformly fine particle size. Because of the abrasive quality of the oxides, dilution with a n abrasive as an aid in grinding 11-as not necessary. After evaporation of the alcohol, each sample was ignited a t 1000’ C. for a short time and kept in a tightly capped bottle in a desiccator until packed into VOL. 31, NO. 5, MAY 1959
809
