predictable, but addition of aluminum to complex the fluoride satisfactorily increased recoveries. I n general, high concentrations of cations reduced the measurable americium, although not to the extent that this extraction would be unsuitable for low level waste streams. Although the true effect of foreign cations could not be explicitly determined, the author believes reduced americium recoveries were not caused by loss 011 extraction, but rather by absorption of the alpha particle on the sample disk. Since a fraction of several cations did follow americium through the separation, they mould necessarily be present on the sample disk to serve as a partial barrier between the americium and the detector. Such inert solids were visually detectable in several cases, and always discernible by loss of resolution in the alpha energy spectra. Additionally, a second extraction with fresh D 2 E H P h yielded no further americium-241 activity in the organic phase. The ferric ion presented by far the most serious cationic interference. Al-
though complexing with oxalate or thiocynate did improve the americium yield, phase separation was too difficult to be practical. Fluoride satisfactorily complexed the iron a t mole ratios of the fluoride to ferrous ions between 1.3 and 1.6. Table IV shows the improvement in americium yields. As the ratio exceeded 1.6, fluoride appeared to interfere in the americium extraction. ACKNOWLEDGMENT
The author thanks C. ;i.Radasch for his technical assistance in the development of this procedure. LITERATURE CITED
(1) Britt, R. D., Jr., ANAL.CHEY. 33,
602 (1961).
( 2 ) Campbell, 11. H., C. S . At. Energy ,Comm. Rept., HW-64619 (1960). (3) Chem. Eng. S e w s 41, S o . 24, 52
11963). ( 4 j Keenan, T. K., J . Chem. Educ. 36, 27 (1959). (5) Moore, F . L., ANAL. CHEY. 35, 715 (1963). (6) Zbid., 33, 748 (1961).
Table IV. Americium(lll) Yield as a Function of F-/FeC3 Mole Ratios
(0.01 9g. of americium-241 taken) F-/Fe Americium-241 mole ratio recovered, yo 0 25 4 0 4 54 9 77.3 0.8 1.0 87.3 1.3 91.9 91.9 1.6 87.3 1.9 (7) Penneman, R. A., Keenan, T. K., “The Radiochemistry of Americium and Curium,” NAS-NS-3006 (1960). (8) Peppard, D. F., LIason, G. W., Maier, J. L., Driscoll, W. J., J . Inorg. Sucl. Chem. 4 , 334 (1957). (9) Schneider, R. A , , Harmon, K. bl., USAECD-HW-53368 (1961). RECEIVEDfor review l l a y 1, 1964. Accepted July 31, 1964. 15th Pittsburgh Conference on Analytical Chemistry and rlpplied Spectroscopy, March 2 , 1964. Work performed under Contract KO. hT(45-1)1350 between the Atomic Energy Commission and the General Electric co.
Determination of Silver, Cadmium, Indium, Tantalum, Rhenium, Gold, and Rare Earths at Low Concentration by Neutron Activation and Radiochemical Analysis CHESTER
E. GLEIT,’
PHILIP A. BENSON, and WALTER D. HOLLAND
Tracerlab, A Division of Laboratory for Electronics, Richmond, Calif. IRVING J. RUSSELL Air Force Weapons Laboratory, Kirtland Air Force Base, N . M. Five types of organic filter media and high purity quartz were analyzed for Ag, Cd, In, Tb, Hot Er, Tm, Ta, Re, and Au. Organic matter was removed by reaction withe lectrically excited oxygen before neutron irradiation. After irradiation, radiocheimical separations stressing rigorous purification were utilized. A combination of beta-ray counting and gamma-ray spectrometry was employed to determine the activity of the separated (elements and to check sample purity. Concentrations as low as one part in 10” were measured. Large differences in traceelement concentration were observed. In the most extreme case, four sheets of I.P.C. grade 107 filter paper displayed a 30-fold variation in Ag and Au concentrations. A study of the sources of experimental error indicates that the technique is #capableof yielding quantitative data and that the observed variations are caused b y nonuniformity among the specimens.
A
employing high intensity neutron sources permits accurate determination of over 30 elements in quantities of less than 10+ gram. By minimizing chemical treatment before irradiation and by using rigorous radiochemical decontamination procedures after the neutron bombardment, concentrations of less than 1 p.p.b. can be measured. As part of a program to determine the minimum blank correction which could be obtained in a study of the atmospheric concentration of trace elements, a high purity quartz and five types of organic filter media were analyzed for nine extremely rare elements. The combined requirements of specimen purity and elements of low abundance suggest the concentrations measured in this program may be the lowest ever determined by activation analysis. To achieve this goal, novel manipulation and incineration techniques were employed before irradiation to avoid CTIVhTION AKALYSIS
contamination of the filters from reagents or dust. To increase the sensitivity and accuracy of measurements of weak activities in the presence of large concentrations of other activation products, high specificity radiochemical separations followed by beta- or gammaray counting were used rather than a purely instrumental method based on gamma-ray spectrometry. Direct activation of large organic specimens is undesirable because intense neutron bombardment degrades organic compounds producing tars and volatile gases. The generation of gas in the sealed samples has been known to produce hazardous pressures. Furthermore, a large variation in neutron energy may occur throughout a bulky hydrogenous sample and lead to erroneous results. To obviate these difficulties, all samples were ashed bel Present address, Dept. of Chemistry, Sorth Carolina State, University of North Carolina, Raliegh, S . C.
VOL. 36, NO. 1 1 , OCTOBER 1 9 6 4
2067
Table I.
Element Silver Cadmium
Neutron Activation Products
Indium Terbium Holmium Erbium Thulium
Species measured 253-day Agl1’Jm 2 %day Cd116 43-day CdLism 50-day I n 1 1 4 m i3-day TblM 2 i 2-hour Ho166 9 4-day Erie9 12i-day Tm170
Tantalum Rhenium
3 i-day Re186
Gold
115-day Ta1S2 li-hour Re1*8
2 i-day Au’98 3 15-day Auig9
Interfering reactions 7 %day Aglll PdIlO (n, y) Pd1l1 U(n, F ) 2 3-day Cd”6 U(n, F ) 43-day Cd115m V(n, F ) Cd115$ 4 5-hour In116 Gdlw (n, 7 )GdlS1 6 9-day Tb161 Sone Sone Er170 (n, y ) Erl;1 1 9-year TmI7l Tm1G9(n,y ) 2 1 9-year Tm171 Tal81 (n, y ) 2 5 0-day Tala3 Sone
a,
5
Pt198 (n, y ) Ptlg9
fore irradiation. To minimize volatility losses and a t the same time to eliminate reagent, contamination, an electronic ashing method was employed (6). This method uses a stream of pure oxygen which has been excited by passage through an inductive field as an oxidant. Seutron activation analysis has been applied previously to the measurement of trace-metal constituents in a variety of materials ( I d ) . Table I lists the nuclides measured and principal interfering reactions. .kctivation products were principally produced by simple neutron capture reactions. The high capture cross sections of the produe?$ TmI7O. Tals2, and Au19* also lead to appreciable yields of Tml’l, Tals3, by secondary neutron capture. and Ah199 Although TblE1 may be produced by niultilile neutron capture from Tb1j9,the irradiated terbium slandards did not display this activity. The Tb161 observed in the specimens arose from the decay of 3.6-minute Gd161. In general, a methane end-viindoiv beta-counter was used to determine radioactivity, with obaen-ations of ganima spectra or beta-decay rate employed to confirm chemical purity. For elements, such as rhenium and gold, which possessed two activation Iiroducts of similar half life, the isotopes were separated by mathematical resolution of the twocomponent decay curve (14 ) . Fire types of filter media were selected for analysis. The Institut’e of I’aI’er Chemistry, Aplileton, \Vis., has dei-eloI)ed filter mcdia specifically for atmos1)heric aanq)ling. IPC grade 107 is made from second cut cotton-linters which are iniliregnnted with dibutoxycthyl phthalate (DIW) to improve wiention caharacteristics. .I specially iml)arrd lot of II’C grade 107 from which the 1113P coating !vas omit’ted wab also analyzed. The third media, ~Iic~roaorban(Delhag), is a recently develolied filtcr de5igned for the collection of solid ])articles in the upper atriiohpherc ( 3 ) . The filter is produced frvni .;ubmicron-dianleter polyqtyrene f i h L . Dacron felt is an ethylene 2068
ANALYTICAL CHEMISTRY
3 15-day Aulg9
Reference 10 10 10
9,16 9,16 9,16 9,16 8 6, 13
r, ir,18
terephthalate polymer containing small quantities of titanium dioxide and vegetable oils. The fifth media was a felt consisting of polypropylene fibers mounted in a reinforcing fabric of polypropylene. EXPERIMENTAL
Standards. l p p r o p r i a t e amount,s of h g S 0 3 , Cd(N03)2, InCI3, and T a 2 0 5 were dissolved and diluted to 1 liter with conductivity water t,o yield solut.ions cont,aining approximately 20 mg. of metal per ml. Redistilled HSOS, 5 ml., was added to prevent hydrolyzation. Gold and rhenium solut’ions were prepared by dissolving t8hepure metals in mineral acids and diluting with conductivity water. Redistilled HC1, 1 nil.! was added to the gold carrier-solution to maintain the gold as the complex chloride. By successive dilution, solutions containing approximately 40 gg. of the element per nil. were prepared. Rare earth standards containing approximately 10 mg. of the element per ml. were prepared by dissolving 99.970 pure terbium, holmium, erbium, and
thulium oxides, obtained from Baker and hdamson, in a minimum of redistilled 6 S HC1 and diluting to 1 liter. These solutionc were diluted to obtain solutions containing approximately 10 fig. of the element per ml. The rare earth oxides contained less than 0.1% of each of the elements of adjacent atomic numbers. Irradiation standards were prepared by evaporating 50-gl. aliquots of the standard solutions to dryness in Suprasil quartz ampoules (hniersil Quartz Division, Englehard Industries, Hillside 5 , X. J.). The standards were prepared in a glove box with nitrogen blown over the tubes to hasten evaporation and prevent bubble formation. Scavenge Carriers. Solutions of metal nitrates containing approximately 10 mg. per ml. were prepared by dissolving the salts in mater or dilute nitric acid. The barium scavenge carrier contained 50 mg. per ml. =inion carriers were prepared from ammonium salts in a similar manner. Preparation of Filter Media. IPC grade 107 filter paper was obtained from Knowlton Brothers, Inc., Watertown, Mass. Microsorban was obtained from the Gelman Instrument Co., Chelsea, 1Iich. The Dacron and polypropylene felts were supplied by Troy Mills, Inc.. Ken- York. S . Y. Each filter was individually inserted into borosilicate sleeves, 4 cm. in diameter and 20 em. long, and placed into the gas-flox section of a n electronic ashing system. This system consisted of an oxygen tank separated from the reactor section by a liquid nitrogen trap and glass wool filter to prevent contaminants in the oxygen tank or valves from reaching the specimens. Two dry ice-acetone traps in series separated the reaction chamber from the exhaust pump and prevented Contamination of the sample by back diffusion of oil from the pump. - i s previously described (e), oxygen gas flowing a t a rate of 20 cc. per
HF, HNO, 6NHCI
S UPE R NATE
P R E C IPI TATE
Ag,R.E.
I
A
A.
JNH40H
NH40H
AT. SOAPT. SOL.
SOL.
A g (NH&
Cd. R e
Rare
Earths
SOL. Re
Figure 1 .
CdS
In*’
Ta,Au IHCI
Flow sheet depicting dissolution and chemical separation procedure
minute STP was excited by inductire coupling to a solenoid which surrounded the borosilicate tube approximately 5 em. above the filter media. Generally. 250 watts a t a frequency of 13.56 nic. per second were delivered to the solenoid. To ensure that all traces of organic matter were decomposed, each filter was exposed to the electronically excited oxygen stream for 3 days. The ash was transferred 'to Suprasil quartz ampoules, 3-ninl. i.d. by 4-mni. o.d. by 2 em. long, that weighed approximately 300 mg.; the ash was then sealed under a nitrogen atmosphere. Irradiation Procedure. T h e standards, samples, and blank quart'z t'ubes were wrapped in quartz ~ o o l placed , in a 1-inch diameter by +inch long aluminum cylinder, and transferred to t h e shuttle tube facility of the General Electric Test Reactor, Pleasanton, Calif. T h e capsule was irradiated for a period of 4 days a t a n equivalent thermal neutron flux of 5 x l O I 3 neutrons per sq. cni. per second. h t the end of the irradiation, the aluminum capsu1.e was transferred to a high-level radioactivity laboratory and immediately opened. Dissolution and Separation. -412proximately 20 mg. each of Ag, Cd, I n , T b , H o , E r , T m , T a , Re, and =lu carriers were placed in a platinum dish, acidified with nitric acid, and evaporated to dryness. -4 quartz ampoule was transferred t'o the platin u m dish and dissolved by heating with H F and HSO3. Figure 1 depicts t h e initial separation scheme. T h e diluted solution was transferred to a centrifuge cone and 0.3 nil. of 6 S HC1 was added. The precipitate containing the rare earths and :silver was washed with 2 ml. each of 1-Y HC1 and 1 S HF and silver was redissolved by the addition of SHdOH. The supernate was combined with the acid washes and made basic with concentrated S H , O H . The resultant precipitate was remored and the supernate saved for separation of the cadmium and rhenium. After 0.1 nil. of HZSO? was added, the precipitate was heated and gold was reduced to the metal. Indium wa5 separated by dissolution in HCI. The supernate and a precipitate wash consisting of 3 nil. of water and 1 ml. of 6 5 HC1 were combined, and indium hydroxide was precipitated. Gold \ w s separated from tantalum by dissolution in aqua regia. Cadmium was recovered from the Cd-Re solution by precipitation from the neutral solution with H2S. The supernate n-as made 4 5 in HCI and Re& was precipitated with HZS. Purification. Iron scavenge carrier, 5 nig., was added i o the Ag(SH3)6solution and Fe(OH)3 \vas precipitated. Silver was precipitated from the supernate with 2 nil. of 3S (?I"?)$.The Ag2S was dissolved in 4 inl. of H S 0 3 , and an iron scavenge was again 1)erformed. The sulfide precipitation and iron hcarenge stel)s were repeated. The silver solution was acidified with G-\. H?;03, and &lgCI\\-as precipitated with HC1. The precipitate mediatelj- onto a 2.3-cni. filter paper
disk. washed with water and methanol, dried, weighed, and mounted for counting. The CdS precipitate boiling HBr. th inl. of concentrated HDr, cadmium was absorbed on a 1.25-inch column of nowex 1 X 10. 50 to 100 mesh anion exchange resin. The colunin was washed with 10 nil. of concentrated HBr, 10 nil. of 1-Y HRr, and 10 nil. of H20. The cadmium was eluted with 20 nil. of 6 S S H I O H . S H 4 0 H ,5 ml., ivas added to the eluate. Iron carrier, 5 nig., was added, and Fe(OH)3 was precipitated. Cadmium was absorbed on a column of Don-ex 50 x 12 cation exchange resin. 50 to 100 mesh, washed with 10 nil. of 6&\-S H , O H , 10 nil. of I S S H , O H , and 15 nil. of H20, and eluted with 20 nil. of 0 . 5 s HBr. The eluate was adjusted to p H of 8 by addition of 6 S ?;&OH, and 2 mi. of 3 S S H d C l were added. Three nil. of 1.5-11 (SH,)*HPO, were added, and the precipitate was digested for 15 minutes at 80" C. The solution was maintained a t a p H of 8 throughout the digestion by additions of 6 S S H 4 0 H or 6 s HCI. The C d S H , P 0 4 precipitate was filtered through a stainless steel filter-tower onto a prepared 2.3-cni. K h a t m a n S o . 42 filter disk. The precipitate was dried, weighed, and mounted for counting. The In(OH)3precipitate was dissolved in 4 nil. of HC1. Ten milligrams of zirconium and lanthanum and 50 nig. of barium seal-enge carriers were added, and the solution was diluted t o 20 ml. LaF3 and 13aZrF6were precipitated with 1 ml. of HF and discarded. The lanthanum scavenge was repeated. Excess barium was precipitated xvith 1.5X HPSOI. One milliliter of a 10% ethylenedinitrilo(tetraacetic) acid solution \vas added to the supernate and the solution \vas neutralized with sodium . ~ added, acetate. Cadmium, 10 i i ~ gwas and CdS was precipitated. The p H of the suliernate was adjusted to 8 to 10 with S a O H . Indium was lireciliitated with 5 nil. of a 1% solution of 8-hydroxyquinoline, dissolved in dilute acetic acid, and relirecipitated with H2S. The In2& precipitate was dissolved in 3 5 HCI. and the solution brought to a p H of 2.4 with 6-Y S a O H containing a buffer of 10% potassium acid phthalate. Indium was extracted with three 8-m1. portions of a 0 . 4 X thenoyltrifluoroacetic acid in benzene and back-extracted with two 10-nil. portions of lA\-HC1. Tin, 10 mg., was added, and then precipitated with H28. &Ifter neutralization with sodium acetate, In& n'ab preciIJitated. The sulfide vias dissolvd. and ain estracted. The CdP repeated. Indiuni was reprecipitated with 8-h?-drosyyiiinoline, converted to the oxide a t 800" C., ground to a fine powler methanol onto a lire Whatman S o . 42 filter dihk. The rare Parth fluorides !\-ere disiolved in a saturated solution of H3130d and HSOa. The rare earth. were 1)recil)iS a O H anti rrdissolved tated with GAIT in 15 nil. o f 2A\-HS03. Ten milligrams of zirconium and strontium \!-ere added
and the rare earths were precipitated with 2 ml. of HF. The precipitate was redissolved and the rare earth hydroxides were precil~itated with TH,OH. The fluoride-hydroxide precipitation cycle was repeated. The rare earth hydroxides were dissolved in 5 ml. of HC1, passed through a colunin of Dowex 1 X 10, 100 to 200 mesh anion exchange resin, and reprecipitated. The individual rare earths were separated chromatographically using a modification of Wolfsberg's method (19). The rare earths were loaded on a 6O-cin. long column of Dowex 50 X 4, minus 400 mesh cation exchange resin in 6 drops of 6 s HCl and eluted ivith 0.5M a-hydroxyisobutyric acid a t pH 2.9 to 3.5. Thulium eluted in 16 hours, erbium in 20.5 hours, holmium in 24.5 hours: and terbium in 31 hours. The rare earths were precipitated 17-ith 1 ml. of saturated oxalic acid, and the precipitate was ignited a t 800" C. The rare earth oxides were ground to a fine powder, filtered in methanol onto a 2.3-em. K h a t m a n S o . 42 filter-disk, washed, dried, weighed, and mounted for counting. The tantalum oxide precipitate from the separation procedure was dissolved in 1 ml. of HF and 1 i d . of 6 S HC1 and diluted to 20 ml. Lanthanum, 10 mg., and barium, 50 mg., were added, and LaF3 was precipitated. Ten milligrams of copper were added and precipitated n-ith H2S. Tantalum was precipitated with ,I;H4S03and washed with 5 nil. of a hot, basic S H 4 S 0 3solution to prevent peptidization of the precipitate. The LaF3 and CuS scavenges were repeated. The tantalum oxide precipitate was dissolved in several drops of HF and diluted to 20 nil. with 9 X HCI. The tantalum was absorbed on a 1-inch column of Dowex 1 X 10, 100 to 200' mesh anion exchange resin. The column was washed with 80 nil. of a 4-11 HCI0.5-11 HF solution to remove niobium, and tanaluiii was eluted with 80 nil. of HF. The eluate was evaporated to near dryness, and tantalum was precipitated with SH,OH. The tantalic oxide precipitate \?-a%wa.shed and ignited in a muffle furnace a t 800" C. The anhydrous oxide \?-as ground to a fine powder and filtered onto a 2.3-em. disk of K h a t m a n S o . 42 filter 1)aper. Rhenium was absorbed from a 0 . 1 s HC1 solution on a colunin of Don-ex 1 X 10 anion eschange resin, 100 to 200 mesh. The column \va\ washed with 10 nil. of 0 . 1 S HC1 and 50 nil. of 1 s HCIO?. and rheniuni was eluted with 50 nil. of l.\- HCIO?. The eluate was made 4 S in HCl, and Re& was precipitated. The precipitate was washed with 10 nil. of hot water. dissolved in 1 nil. of H S 0 3 , and diluted to 20 nil. Rhenium was purified by passage through a 1.2.i-inc*h long column of D o n e s 50 x 8 cation exchange resin, 100 to 200 mesh, which !vas naihed n-ith 10 ml. of O . l L \ - HC1. The eluate 1% made 4\- in HC1 and 2 nig. of 1 1 0 - 6 scarenge carrier were adtled. Molybdenum was precipitated with 5 nil. of a 6y0 solution of a-henzoinosime. Rhenium sulfide was precipitated. i , d a t e d , and retiissolved in 5 nil. of 1.Y KaOH containing 3 drops of 30y0 H,O,. Iron, VOL. 3 6 , NO. 11, OCTOBER 1964
2069
Table
II.
Concentration of Ag, Cd, Tm, Re, and Au in Selected Media Ag Cd Tm Re Au Sample Ash concn., concn., concn., concn., concn., at., grams wt., mg. X lo8 X 108 X 1011 X 10” X log
Media IPC with DBP 14.1408 IPC without DBP 9.2095 Dacron felt 2.8714 Polypropylene felt 2.7846 8,0251 Microsorban Suprasil, lot I 0,2963 Suprasil, lot I1 0,3494
9.1 6.5 13.4 6.0 3.6 ... ...
25. 88.
14. 28, 1.3 2.3 0.54
10 mg., was added to the diluted solution and Fe(OH)3was precipitated. The solution was filtered, heated, and 400 mg. of XaC1 and 5 ml. of a 1% solution of tetraphenylarsonium chloride were added. The mixture was digested for 1 hour, then cooled for 5 minutes in an ice bath. The (C6H6)4+k~Re04 precipitate was collected on a Whatman No. 42 filter paper disk and washed with three 5-ml. portions of ice water and iced methanol. After the precipitate was dried a t 90’ to 100’ C. for 10 minutes in a desiccator, a 1-minute timed weighing was made. The drying, desiccating, and weighing cycle was repeated until the weights agreed to within 0.1 mg. The gold solution was boiled to and diluted to 20 ml. remove ” 0 3 Two milliliters of 5y0 sulfosalicylic acid and 1 drop of Tet6 holdback carrier were added, and gold was precipitated with 1.5 ml. of HI. The solution was boiled to coagulate the precipitate. The precipitate was washed with water and H S O s . The dissolution-precipitation cycle was repeated twice. Silver, 3 mg., was added, and AgCl was removed. Four milligrams of iron were added and precipitated with 6 X NaOH. The supernate was immediately acidified with HC1, and gold was reprecipitated with HI. The gold precipitate was dissolved in 3 ml. of HCl and 3 drops of HXO,, extracted into ethyl acetate, and washed twice with 2N HC1. The ethyl acetate was evaporated, and the residue was dissolved. Gold was precipitated with HI and filtered onto a 2.3-cm. Whatman No. 42 filter paper disk. The precipitate was washed
Table 111.
AI3
1
15. 6.4
Suprasil I1 Ng. per sq. ft. a
n.a.
2070
Cd 8.6 101.
1 ClA
Av.
=
9.7 7.9 6.3 190 2.2 4.0 1.4
1.4 5.2 8.7 6.1 3.4 61. 34.
4.0 6.9 3.8 16. 5.0 0.21 0.25
three times with 5 ml. of 1N HC1, three times with 5 ml. of water, and finally with acetone. The sample was dried, weighed, and mounted for counting. RESULTS AND DISCUSSION
The second and third columns of Table I1 list the init a1 and ash weight of the filter media. Microsorban had the lowest ratio of ash to initial weight, 0.045%. Standard I P C grade 107 paper impregnated with dibutoxyethyl phthalate (DBP) contained fromO.O57’% to 0.089% ash. The average value was 0.065’%. The special I P C paper which did not contain D B P contained 0.071% ash. This difference is not statist cally significant. The ash content of the polypropylene felt was 0.22y0, and that of Dacron felt was 0.47%. Table I1 also contains average concentrations of Ag, Cd, T m , Re, and Au in the filter media. Microsorban displayed the lowest concentrations of Ag, Tm, and Re. The Institute of Paper Chemistry formulations contained slightly greater levels of impurities. The Dacron and polypropylene felts displayed considerably higher impurity levels for all elements, Suprasil quartz contained the lowest weight concentrations for all the measured trace metals. Two lots of Suprasil quartz were employed. These data are presented separately. To determine the extent and nature of variations in trace element concentration, a more comprehensive study of the standard I P C grade 107 paper
Concentration of Trace Elements in IPC G r a d e 107 Filter Paper and Suprasil Quartz (P.P.B.)
Sample 2 3 4 5 6
61, 23. 45, 190. 32. 1.3 0.51
was performed. Results of this study are presented in Table 111. For comparison, the last line of this table contains average values in units of 10-9 gram per sq. foot. These va!ues were calculated on the basis of a unit weight of 12.9 grams per sq. foot. Although samples 3, 4, 5, and 6 were drawn from a single lot, large variations in the concentration of the individual elements were observed among these papers. An analysis of the experimental sources of error indicates that the observed variations are caused by nonuniformity among the specimens. Samples were prepared by ashing a weighed quantity of filter with excited oxygen and then transferring a weighed quantity of ash to the quartz ampoules. The principal source of error in this procedure is loss of material during ashing by diffusion or volatilization. I n preliminary experiments specimens of filter paper containing radioactive tracers were ashed, and the residual activity was measured. I n all tests more than 95% of the Cd, In, rare earth, and T a activities were recovered from the ash. Volatility losses had been observed previously during ashing experiments with radioactive Ag and Au from chloride-rich biological material (6). However, more than 90% of the initial Ag and Au activity was consistently recovered from I P C filter paper ash. Rhenium recovery ranged from 9Q t o 98%. Immediately after ashing, the residue was transferred t o quartz ampoules. These manipulations were performed in a clean room and employed routine laboratory techniques. The relative error introduced by the preparation procedure is estimated to be less than 10%. So appreciable error is attributed to variations in neutron flux or energy during irradiation. The specimens were in close proximity t o each other, and random variations in flux tend to eliminate matrix effects. Replicate standards spaced throughout the sample container yielded similar quantities of induced activities, indicating that significant variations between samples
3.1 25. 0.54 322.
61. 61. 0.51 787.
not analyzed.
ANALYTICAL CHEMISTRY
In n.a.* n.a.
Tb n.a. n.a.
0.92
6.4
0.47 0.90 0.11
5.6 5.7 0.035
11.6
73.
Element Ho Er
