Rapid Radiochemical Separation and Activation Analysis of Rare Earth Elements KRISHNASWAMY RENGANI and W. WAYNE MEINKE2 Department o f Chemisfry, University o f Michigan, Ann Arbor, Mich.
b Rapid separation of different rare earth elements has been effected in 10 to 16 minutes with a small cation exchange resin column and a-hydroxy-isobutyric acid as eluent. Individual rare earth fractions could be obtained with controlled eluent concentrations and pH. Good separation with peak to valley ratio between 50 and 100 in the ehtion curves was accomplished in aboul 10 minutes for rare earth elements with atomic numbers differing by three or more. Less complete but uscible separations of adjacent rare earths such as gadolinium and terbium could be made in about 16 minutes. This rapid separation procedure was coupled with gamma spectrometry for determination of rare earths in samples such as monazite and the U. S. Geological Survey samples G-1 and W-1 by activation analysis. The applicability of this method for the entire group has been investigated by separating rare earth elements ranging from praseodymium to ytterbium.
T
have become increasingly important in industry, especially in the nuc tear energy field (19). The high neutron capture cross section of certain of the rare earths makes the determination of very small amounts present in reactor construction materials important. Furthermore their analysis in many other types of samples from rocks to tissues is of potential interest. The different methods currently used to separate and determine the rare earths have been reviewed recently in two monographs (14, 16). The most sensitive methods for determining rare earths are the spectrographic and the activation analysis methods. Cornish (9) compared the semitivities of these two methods and indicated that activation analysis offers the greater sensitivity. Full advantage has !lot been taken of the activation analysis of these elements, however, because of the difficulty of separating individual rare earths. For HE RARE EARTHS
On leave from Atomic Energy Establishment, Trombay, India. Present address, Analytical Chemistry Division, National Bureau of Standards, Washington 1, D. C.
some rapid determinations the gamma spectrum or gross decay curve of the activated sample, without any chemical separation or after a simple group separation, has been used for the measurement (6, 7, 10, 11). When a determination requires separation into individual rare earths, long and tedious cation exchange separation procedures of several hours duration have been used (S, 17). In the past few years rapid separations of a number of tracer level rare earths have been reported by several authors (g, 1.2, 16). The work reported here was undertaken to determine optimum conditions for separating any particular rare earth element from milligram amounts of total rare earths in 10 to 20 minutes. Such a separation will also be applicable to the study of short-lived nuclides in this region. EXPERIMENTAL
Apparatus. Samples were irradiated in polyethylene snap-type “rabbits” in the pneumatic tube system of the Ford Nuclear Reactor of the University of Michigan. This system permits irradiations at thermal neutron fluxes of about 1OI2 neutrons per sq. cm. second (when the reactor is operating a t full power of 1 megawatt) and delivery to a hood in the neighboring Michigan Memorial Phoenix Laboratory within 3 seconds after the end of
BALL JOINT
1, I
RESIN COLUMN 2mm I D
’
Figure 1.
Cation resin column
irradiation. Samples were then worked up chemically and were measured by a 3- X 3-inch NaI(T1) crystal coupled with a special 100-channel pulse height analyzer with duplicate memories. This equipment has been described in detail elsewhere (8, 9). An end window, methane, proportional flow counter was used for beta measurements. The ion exchange column is a thickwalled capillary tube of 2-mm. internal diameter having a reservoir with a side tube, a semiball joint a t the top, and a sharply drawn taper a t the bottom (Figure 1). The top is clamped on the ball joint during operation. An automatic fraction collector, locally made, with sample collector disk of 18-inch diameter and provision for collecting 60 samples in tubes was used. pH determinations were made with a Beckman Model G pH meter. Reagents. Dowex 50WX12, -400 mesh cation exchange resin, Dow Chemical Company and Bio Rad, graded; fraction settling a t the rate of 0.5 t o 1.5 cm. per minute, washed with hydrochloric acid and ammonium hydroxide and then stored in distilled water prior to use. a-Hydroxy-isobutyric acid, Eastman Kodak; purified from metallic impurities by passing through a Dowex 5 0 x 8 column in H+form. All other reagents were of analytical reagent grade. Procedures. ELUTION CURVE. Mix a n y two rare earth tracers of approximately equal activity, evaporate to near dryness, and add -5Op1 of 0.03M hydrochloric acid. Load on the resin column which has been previously equilibrated with the appropriate eluent. Apply slight air pressure through the side tube to force the solution through. Rinse the sides of the capillary above the resin bed with a drop of water and allow that to pass through. Start elution with the appropriate eluent maintaining a flow rate of three drops per minute. Collect individual drops on aluminum planchets, dry under infrared lamp, and count in a beta counter. ACTIVATION ANALYSIS:RAREEARTH GROUPSEPARATION.Irradiate a known weight of sample (0.1 to 1 gram) for the required time (irradiation time depends upon the amount of the particular element present and the half life of the radioisotope formed) together with a gold foil as flux monitor. While the sample is being irradiated, combine in a nickel crucible 3 to 4 mg. of lanthanum carrier with a known amount of VOL 36, NO. 1, JANUARY 1964
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ELUTION CURVE OF THULIUM AND TERBIUM
L .
Column: 2 8 c m Dowex 50W x 12 Eluent O 2 O M Q hydroxy isobutyric acid, p H 4 0 Separation Time. IO Minutes
ELUTION CURVE CF TERBIUM AND GADOLINIUM
I
I 1
€ i u e n t ' 0 20M Q hydroxy isobutyl IC acid, pH 4 20 Separation Time 16 PPinutes
d o
a
W
-4
I
t 0 0
IO
2
-~
E
i,-
1
IO'
-'
0
5
IO
15
20
25
30
35
DROP NUMBER
Figure 2. (Z = 65)
IO'
Elution curve of thulium (Z =
DROP NUMBER
69) and
terbium
Figure 4. Elution curve of terbium (Z = 64)
(Z = 65) and gadolinium
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E L U T I O N CURVE OF T E R B I U M AND NEODYMIUM C o l u m n ' 2 8 cm Dawex 5 0 W x 12 E l u e n t : 0 2 0 M Q hydroxy isobutyric a c i d , p H 5 08 Separation Time I O Minutes
I 0' W
t
3
z5
a W n (0
c Z
3 0 0
IO'
I
ti
U
IZ
I
I
I8
24
I
30
DROP N U M B E R
Figure 3. Elution curve of terbium (Z = 6 5 ) and neodymium (Z = 60) -
.-
158
-.
ANALYTICAL CHEMISTRY
I 02 03
YTTERBIUM STANDARD
I
04 0 5
1
1
0 6 0 7 0 8 0 9 IO ENERGY ( M E V )
1
/I
12
13
14
Figure 5. Purity comparison for ytterbium fraction from rare earth mixtures
+
long-lived Eulj2 E u ' ~ tracer ~ mixture (for determining chemical yield). Carefully evaporate to dryness, then add sodium peroxide (10 tiines the weight of the sample). Add the irradiated sample, fuse, and cool by dipping and rotating the crucible in cold water. iidd water to the melt, transfer the contents to a tube, and centrifuge. Dissolve the rare 'earth hydroside precipitate in 2 ml. of 6JI hydrochloric acid, dilute with ]vatel*, and precipitate lanthanum fluoride by adding a few drops of concentrated hydrofluoric acid. Centrifuge and wash the precipitate with 1J1 nitric acid-lJf liydrofluoric acid mixture. Dissolve in 6JI nitric acid saturated ivith boric acid and precipitate hydrosides by addit.ion of 611 ammonium hydroxide. Repeat the fluoride and hydroside precipitations. Dissolve the hydroside precipitate in 611 hydrochloric acid, evaporati: to near dryness, and take up in -50 pl. of 0.03.11 hydrochloric acid. I O l i EXCHANGE SEPARATIOS. Load chloride solution on pre-equilibrated column, and proceed a i above for elution curve, using conditilms specified in Table I for specific rare earths. Obtain appropriate 1111by adding ammonium hydroside to acid eluent. Collect the required fraction anli the europium fraction. The exact position of these peaks is determined with known samples of the element (as in Figures 2-4) and remains constant for a given batch of re& under ider tical conditions. Take the gamma-ray spectrum of both fractions. (Correct ljhe total counts under the photopeaks for decay and flus.) Calculate the chemical yield Eu1jl from the amount of Eu162 tracer recovered, being careful to correct for any contribution to the 1.28- and 1.42-m.e.v. photopeaks from Eu1jZtrL formed by activation of any europium present in the sample.
+
DISCUSSION Ah D RESULTS
Ion Exchange Separation. The elution experiments show t h a t with '1 Dowes 50IV-Xl2 column 2.8 cm. in length and 0.2 cm. Ln diameter and a-hydroxy-isobutyric acid as eluent, good separation bet w e n rare earth elements with atomic numbers t h a t differ by threc or more can be accomplished in about 10 minutrs. T h e elution curves obtained for a mixture of thulium ( Z = 69) and terbium ( Z = 65) :ind for terbium ( Z = 65) and neodymium (Z = 60) are shown in Figureq 2 and 3, respcctively. I n t h e case of adjacent rare earths. lesq complete b u t usable separation could be obtained in about 16 niinuteq. Figure 4 ihol\-s the separation obtained between terbium (Z = 65) and gadolinium (Z = 64). typical adjacent rare earths. Froni such a curve the samplc fraction giving optimum separation from the neighboring rare earths could be selected arld used for the characterization of radiations emitted from the isotopes of thc element.
Table I.
Summary of Values Obtained for Rare Earths in Different Samples
Element determined Sample Yb Yttrium group Oxalate concentrate YbC1, mixture (Yb Tm E r ) Dy 1 7 K ) 3 (997, pure)
+ +
Monazite G-le
Isotope used for Amount Time determifound, Eluenta taken* nation rg.c 0.15'11 15 2 . 0 h Pblii (or 8.1 pH 3.95 4 . 2 cl Yb175) 8.6 0.15x 1: 2 . 0 h Tb17' (or 233 pH 3.95 4 . 2 d Tb'ij) 215 0 20M 20 2 32 h DyLfi5 1 .36 pH 4 20 1.60 0.20.11
20
0 2031
50
pH 1 . 2 0 pH 4 20
2 32 h L>J
165
Remarks d
d
ii
d
2 40 2 43 2 40 1 32 1 36
1 36
Group separation was done
131
R-18
Eu Yz03(99% pure)
Monazite
G-le
0 20N
50
0 2051 p H 4 20
30
0 25'11
20
pH 4.19
pH 4 42 0.2031
p H 4 20
0.2051
pH 4.19
3 33 Group separation *3 27 was done d 0 043
0.041 Dv was also deter-
"mined in the same run
60
60
0 313
9 . 2 h EU'j?'"
d
0 293 0 320 0.302 0 970 Group separation 0 895 was done 0 975 Dy vias also deter-
mined in the same run 1 17 Group separation 1 Ob was done. Dy was also deterrnined in the sime run
Pr Monazite
0.30.11 25 19.1 h P r I 4 ? 2s 0 d pH 4.92 2s T a cu-Hydroxy-isobutyric acid used as eluent in all cases. The conrentration and pH of the eluent is given in this column. * Time between the end of irradiation and the collection of the fraction containing the particular element is given. c Microgram of the element per milligram of the sample is given except for G-I and W - I analysis where microgram of the element per gram of sample is given. In these cases no rare earth group separation was made. -4known amount of the sample in solution was irradiated and a known aliquot loaded on the inn exchange column. e G-1 and W-1 are the granite and diabase rock standards of the L* S. Ge~ilogicalSurvey Department.
Some runp were made n-ith high temperature columns of other dimensions. Sufficiently rapid separations were obtained a t room temperature, holyever, with this small column so that high temperature operation 11as avoided in the final procedureq. Activation Analysis. The rapid ion exchange procedure was coupled with gamma spectrometry for the determination of rare earths in different samples. Figure 5 s h o ~ v st h e gamma spectrum of t h e ytterbium fraction separated from a mixture containing ytterbium, thulium, and erbium. The standard ytterbium spectrum is also given for comparison. The gamma spectrum of dysprosiuni and europium fractions obtained from irradiated R-1 rock samples are shown in Figures 6 and 7 , respectively, while Figure 8 shows the gamma spectrum of the
praseodymium fraction obtained from irradiated monazite. These spectra shon that the separations are clean enough that quantitative determination of different rare earths can be made from one gamma-ray spectrum. To obtain 100% "yield" of an element in the elution some contamination of the neighboring elements must aIqo be collected. Fortunately, for most qamples the effects of thew contaminants can be easily reduced or eliminated by gamma spectrometry. I n our program rare earthq ranging from ytterbium to praqeodj mium have been separated, thus indicating the applicability of the method for the entire rare earth group. The actual procedure to be uqed and the time required for the separation depend upon the nature of the sample to be analyzed. sample containing small amounts of rare earths in a non+\
VOL. 36, NO. 1 , JANUARY 1964
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c
1
1
4
W
E
lo3-
DY S PROS I U M STANDARD
r-
2
lo
04
06 ENERGY ( M E V )
-1
4\
L
02
-A
---i
A
0 8'
160
ANALYTICAL CHEMISTRY
1
IO
Figure 6. Purity comparison for dysprosium fraction from W-1 rock samples
rare earth matrix requires a rare earth group separation (as outlined under procedure) and chemical yield determination. Here the time from the end of irradiation to the beginning of counting is approximately 50 to 60 minutes. The elution conditions can be adjusted so that two rare earths differing in atomic number by two or more can be determined in the same experiment. The amounts of dysprosium and europium present in G-1 and m-1 were determined in this manner. The results shown in Table I indicate the reproducibility of the method. Determination of a minor rare earth element in a "pure" rare earth matrix does not require group separation. Since the lanthanides are known to be carried down the column quantitatively even in the carrier-free state ( I ) , chemical yield determination can be avoided by irradiating a standard solution of the sample and loading a known aliquot on the column. This method was applied to the determination of ytterbium, dysprosium, europium, and praseodymium in various samples (Table I). The time from the end of irradiation to the beginning of counting varied from 15 to 30 minutes. Details of the experiments and the results are given in Table I. This method should be applicable to
4~
IO
lo 0
1
02
, 04
I
1.b
0'8 12 ENERGY ( M E V )
26
14
16
1'8
50
Figure 7. Purity comparison for europium fraction from W-1 rock samples I
,
o3
E
PRASEODYMIUM FRACTION
I
1;
I
4 \ 0 4
1
-
--I
a' 0
w c
zh
l
-
t-
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-4
-
_. -.
PRASEODYMIUM STANDARD
-
I
!
I I
i10
I 12
t 14
16
,110
18
ENERGY (FJEV)
Figure 8. Purity comparison for praseodymium fraction from monazite sample
samples containing fissionable materials such as uranium or thorium if one group separation is made prior to irradiation. For example, t o determine dysprosium or europium in a samjde of uranium or thorium, a known amount of lanthanum may be added as carrier and a group separation from urar ium or thorium accomplished by stzndard methods (e, 6, 14). (The reagents and lanthanum used should not contain europium or dysprosium.) The final solutions can then be irradiated, one aliquot taken for ion exchange separation and another foi. chemical yield determination by colorimetric determination of lanthanum. Sensitivity will varj’ among the rare earths, depending upcln the activation cross section of the element, the decay schemes, and half lives of the isotopes produced. The minirium amounts of ytterbium, dysprosium, europium, and praseodymium detectable by the method outlined in this paper are given in Table 11. This is based on a 25minute irradiation in a thermal neutron 2 x lo** neutrons em.+ flux of second-’. Examples given in this paper are representative of different portions of the rare earth region r n d illustrate the applicability of the method to any rare earth element. Any specific laboratory application will, however, require prior work with known elements to standardize the procedures for the particular batch of resin available.
-
ACKNOWLEDGMENT
The authors thank William Dunbar and the staff of the Ford Nuclear Reactor for their help in making the irradiations. LITERATURE CITED
(1) Alstad, J., Pappaa, A. C., J. Inorg. Nucl. Chem. 15,222 (1960). (2) Choppin, G. R., Silva, R. J., U. S. Atomic Enerav Comm. Rept. UCRG 3265 (1956); -J. Inorg. Nuci. Chem. 3, 153 (1956). (3) Cornish, F. W., Brit. Atomic Energy Research Establishment Rept. AERE C/R 1224 (1956). (4) Gindler, J. E., Nuclear Science Series Rept. NAS-NS-3050, “The Radiochemistry of Uranium,” Office of Technical Services, Department of Commerce, Washington 25, D. C., 1962. (5) Hyde, E. K., Nuclear Science Series Repd. NAS-NS-3004, “The Radio-
chemistry of Thorium,” Office of Technical Services, Department of Commerce, Washington 25, D. C., 1960. (6) Kawashima, T., Osawa, M., Mochieuki, Y., Hamaguchi, H., Bull. Chem. SOC.Japan 34,701 (1961). (7) Kohn, H. W., Tompkins, E. R., U. S. Atomic Energy Comm. Rept. ORNG390 f 1949).
(8j-Mei;lke1 W. W., Nucleonics 17, No. 9,
86-9 (1959). (9) Meinke, W. W., U. S. Atomic Energy Comm. Rept. AECU 3887 (1958). (10) Okada. M.. ANAL.CHEM. 33, 1949 (1961). . l ) Phillips, G., Cornish, F. W., Brit. I
.
Atomic Energy Research Establishment Rept. AERE C/R 1276 (1953). -2) Preobraehensky, B. K., Kalyamin, A. V., Lilova, 0. M., Soviet J . Inorg. Chem. 2, 1164 (1957); Translation of
Table 11. Experimental Sensitivity for the Determination of Ytterbium, Dysprosium, Europium, and Praseodymium
Amount, Isotope Pg.” 2 . 0 h Yb177 1.5 Yb 4 . 2 d Yb116 0.4 Dy 2.32 h Dy1s6 0.004 Eu 9 . 2 h EuIs2 0.001 Pr 1 9 . 1 h Prlra 3 5 Based on 25-minute irradiation in a flux of 2 X IO1*neutrons cm.-* sec.-l Element Yb
Brit. Atomic Energy Establishment Rept. AERE-IGRL-T/R-81 (1958). (13’1 SDeddina. F. H . , Daane, A. H., “The ‘ Rare Eart&,” Wiley, New York; 1961. (14) Stevenson, P. C., Nervik, W. E., Nuclear Science Series Rept. NAS-NS3020, “The Radiochemistry of the Rare Earths, Scandium, Yttrium and Actinium,” Office of Technical Services, Department of Commerce, Washington 25. D. C.. 1961. (15)’Stewa;t, D. C., ANAL. CHEM. 27, 1279 (1955). (16) Vickery, R. C., “Analytical Chemistry of the Rare Earths,” Pergamon Press, Kew York, 1961. (17) Wong, K. M., Voigt, A. F., U. S. Atomic Energy Comm. Rept. IS 376 (1961). RECEIVEDfor review June 25, 1963. Accepted August 30, 1963. Division of Analytical Chemistry 144th Meeting, ACS, Los Angeles, balif., April 1963. Work supported in part by the U. 8. Atomic Energy Commission. Part of the stay of one of us (K. R.) wm supported by a training fellowship from the International Atomic Energy Agency.
Atomic Fluorescence Spectrometry as a Means of Chemica I Analysis J. D. WINEFORDNER and T. J. VICKERS Department o f Chemistry, Universify of Florida, Gainesville, Fla.
b A new method of flame spectrometric analysis i s intrcduced in which the intensity of fluorescent emission i s measured when atoms in a flame are excited b y the absorption of radiation of the proper frequency. Theoretical principles are discussed, and equations are derived relating )he intensity o f fluorescent emission to the concentration o f atoms. Experimental requirements are considered, and the method i s compared to atomic ubsorption and atomic (thermal) emission methods. Fluorescence technique: are shown to have several advantciges over a b sorption and emission techniques which should make the method a valuable tool for chemical analyzis.
I
N ATOMIC FLUORESCE,NCE SPECTROMETRY atoms are excited by the
absorption of radiation of t h e proper
frequency and then are deactivated by the emission of radiation of the same or lesser frequency. The frequency of t h e emitted radiation is characteristic of the absorbing atoms, and the intensity of the emission may be used as a measure of their concentration. The intensity of the emitted radiation should be dependent on the fraction of excitation radiation absorbed, on t h e efficiency of conversion of absorbed radiation to emitted radiation, on the fraction of emitted radiation self-absorbed by similar ground state atoms, and, just as in all fluorescence techniques, on the intensity of the radiation from the source of excitation. Thus, atomic fluorescence spectrometry has characteristics which resemble atomic emission (thermal emission) and atomic absorption, as well as molecular fluorescence techniques, but, as will be pointed
out in this paper, it offers several advantages uniquely its o m . The experimental arrangement used in atomic fluorexence spectrometry is basically similar t o that of molecular fluorescence spectrometry. Holyever, the individual components are qiniilar to those used in atomic emission and atomic absorption flame spectrometry. A typical experimental arrangement Yould consist of an intense source of radiation with its emitted beam focused on a suitable sample cell containing atomic vapor of the element of interest. The fluorescent radiation at right angles t o the exciting beam would be focused on the entrance slit of a monochromator, and the a m p l i e d signal from a photomultiplier detector would be displayed on a meter or recorder. I n the event the sample emits thermal radiation of the same frequency as the fluorescent VOL. 36,
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