Nov., 1963
ION-EXCHANGE SEPARATION OF TRACE IMPURITIES
ments which you do determine currently by reactions other than neturon capture reactions. LEDDICOTTE.-At present, the only trace element which we determine by a reaction other than a neutron capture reaction is sulfur. In the n-p reaction, sulfur will produce phosphorus-32. This reaction allows us to achieve a greater sensitivity for sulfur since the sensitivity for the n-gamma reaction on sulfur is limited by t,he length of time necessary for irradiation (usually 2 to 3 weeks) and the fact that the radiations from sulfur-35 are quite low in energy and aren’t readily detectable.
F.J. NORTON(General Electric Co., Schenectady).-Have you dealt with gases? LEDDIC0TTE.-That is one of the things we hope to try. We have been talking about it for some time but we are limited to some extent by the lack of proper irradiation facilities. However, there are certain nuclear reactions i n a cyclotron that are of interest and work with these is being planned for the future.
s19
COOKE.-I would like to make a comment here. Even though this method can obviously not be used in a truly routine fashion, it can be used for research samples. However, I think the point is being overlooked in that it is possible to send the sample t o this analysis group and get back a standard. Then using this standard, work out some other routine methods. Sometimes these standards might be estremely difficult to obtiiin. For example, the one concerned with the vanadium in motor oil. I am sure that you can’t take a vanadium salt and mix it in the motor oil and expect to have the same type of sample that you had in the oil crude. I t is not known i n what particular form vanadium actually exists in the crude. I think that a very important point might be made here in that you can have a method for getting an original standard and then from there on your own analytical lahoratory can take over the problem of trying to work up a more suitable analysis from this original standard. LEDDICOTTE.-ThiS is one of the possibilities in which activation analysis might be useful.
Coom.-would you like to comment on the length of time required for servicing samples?
F. A. LONG(Cornel1 University).-How service cost?
LEDDICOTTE.-The results on arsenic and germanium can be returned in about 5 days from the time the sample is received by the laboratory. I t is our hope, with the exception of those elements that are determined by long lived isotopes, that most results can be returned within a period of one to two weeks after the sample has been received.
LEDDICOTTE.-I think it is rather reasonable in cost. The average charge for the analysis of a single element in a sample is about $40.00. (All assays are made on a t least duplicate portions of the sample.) Of course the costs will vary with the length of irradiation as well as the separation time.
much does this
ION-EXCHANGE SEPARATION OF TRACE IIYIPUltITIES BY W. A. BROOKSBANK AND G. W. LEDDICOTTE Analytical C’lwtnistry Division, Oak Ridge National Laboratory, Operated by Carbide and Carbon Chemicals Division, a Divisioii of Union Carbide and Carbon Corporation, Oak Ridge, T e n n . Received October 6,1065
The use of ion-exchange separations in determining trace elements by neutron activation analysis is described. Activation analysis, coupled with ion-exchange techniques, becomes an extremely sensitive and specific method for the determination of small quantities of the rare earth and alkali elements in the presence of each other. This method has been applied to the analysis of rare earths in animal tissues and to the analysis of the alkali metals in pure chemical compounds and synthetics.
Analysis for trace elements can be performed in many instances by neutron activation analysis, LE., by the production of a radioisotope of the element under investigation. Basically, as described elsewhere, l activation analysis is a specific and sensitive method for determining many trace elements as impurities in a variety of materials. It consists, in principle, of exposing a sample to a source of nuclear particles (neutrons, in this instance) from a nuclear reactor. I n this exposure, or irradiation, the element whose concentration is being sought must be capable of reacting with the nuclear particles in the reactor to produce a radioactive isotope of the element. The radioactivity associated with this isotope is determined by means of counters or ionization instruments and thus becomes a measurement of the amount of the element present,. Although the conoentration of the impurity can be calculated from nuclear data, sample weight and flux measurement, it is much simpler to irradiate a series of standard samples (1) W. A. Brooksbank, G. W. Leddicotte and H. A. Mahlman, paper reviewed this meeting, entitled “Analysis for Trace Iinpurities by Neutron Activation,” T H I BJOURNAL,67, 815 (1953).
simultaneously with the unknown, and determine the concentration of the elenient in the unknown by comparison with these standards. When several elements in a sample become activated, chemical separations must be used to isolate the radioactivity associated with the element under investigation. The selected examples of analysis by neutron activation, described in the previous paper,’ utilized the fact that the trace impurities under investigation were very different from the matrix and could be separated and isolated by simple precipitation techniques. However, if the trace constituent is similar in chemical behavior and has similar radioactive properties, these techniques are not always applicable. The chemical separation must give a complete decontamination of the radioactivity associated with the trace element from the radioactivity associated with the matrix. For example, in the analysis of Rb2C03 for trace potassium content, a suitable chemical separation of these two elements was lacking until the advent of cation exchange. In effect, other separations had not been capable of giving a complete decontamination of the small
820
W. A. BROOKSBANK AND G. W. LEDDICOTTE
amounts of radioactive potassium from the large amounts of radioactivity associated with nibidium by the chemical methods used in a separation and isolation by means of "isotopic carrying" (a known weight of the desired element is added as an inactive ion to the solution of the irradiated material t o carry the radioactivity through the chemical processing). Since it is almost always necessary to utilize a separations procedure capable of being completed in a reasonable time and with the highest possible decontamination from interfering ions, it seemed obvious that an ion-exchange separation using a cation exchanger could be best used to effect such a separation. Similarly, it has been shown2that a separation of the rare earth elements can be effected by an ion-exchange technique. Thus, it is the intent of this paper to show the potentialities and limitations of utilizing ionexchange separations as a means of separating and identifying the radioisotopes used to determine trace m o u n t s of the rare earth and alkali elements after neutron bombardment. In practice, ionexchange separations have been used to determine trace amounts of the rare earth elements in biological materials and have also been used to determine trace alkali elements (especially K and Rb) in pure chemical compounds, A. Principles of the Ion-exchange Method.The theory of ion exchange is beyond the scope of this paper, but a brief discussion of the way a resin operates must be included. The cation exchanger, in a primary sense, is an insoluble compound with a readily exchangeable positive ion, such as H+, held in place by an electronegative charge. As the solution enters the resin bed, certain cations from the sol will replace these protons so that these ions are now insoluble and, as they cling t o the resin, will liberate H + to the solution. As the solution runs through the resin bed, all of the cations will he replaced by H+, and the solution flowing from the bottom of the resin will not have any other cation but H + in it. If there are two cations, A+ and B+, in the solution, there will be a competition set up for the place of the H+ on the resin. The ion with the greatest affinity, A+, for the resin will remain in the'insoluble resin form longer, and will therefore lay behind B+ in the moving effluent stream. Conversely, since B+ spends more time in the solution, it will leave the bottom of the resin bed sooner than A+. I n this way, A+ is separated from B+. In effect, the separation depends upon the "degree of affinity" of the exchangeable cation for the resin. This "degree of affinity'' is merely an expression of the effects due to the concentration of H+, change of ions in competition, ionic size, degree of hydration, ionic diffusion rate, complexing degree of elutriant, etc. Since Boyd3 has recently given an excellent review of the properties of synthetic organic polymers, no attempt is made to discuss the effect of the above mentioned factors in this present paper. B. The Determination of Trace Amounts of the Rare Earth Elements.-The analysis of the rare (2) B. H. Ketelle and G. E. Boyd, J . Am. Chrm. Soc., 69, 2800 (1947). (31 0.E. Boyd, Ann. Rcv. Phus. Chem., 2 , 309 (1951).
Vol. 57
earth elements has always been a difficult chemical or physical procedure. The subsequent separation of one rare earth from another by fractional recrystallization is a highly unsatisfactory analytical procedure because the purity of the resulting compcund is usually doubtful. Likewise, the physical methods used t o detect impurities in separated rare earth conipounds are not always capable of being sensitive. For instance, spectrophotometric and copper spark spectrographic techniques in simple routine procedures do not approach limits of detection as low as 1 p.p.m. for the detection of one rare earth in another, and these limits of detection are usually dependent on the individual species present. The usefulness of radioactivation analysis in determining trace quantities of many of the elements has been reported by a number of investigator~.~-' Hevesy and Levis have shown that individual rare earths can be determined in rare TABLEI NUCLEAR DATA FOR T H E DETERMINATION O F THE RARE EARTHS BY NEUTRON ACTIVATION ANALYSIS Stable isotope
Active isotope
La139 La1@(40 h ) Ce138 Ce140 Ce142 Pr141 Nd14'3 Nd14S Sm162 Sm Eu161 E11153 Gd"8 Gdl60 Th16B Dy1B4 Dy164 Dy
LaI40
TVr
Sensitivity of detection4 for a 62-hr. bombardment
40 h 0.003 pg. b 3.6h Ce139 140 d 1350 Ce141 28 d 1.2 Cel43 33 h 0.230 Prl4a 19 h .002 Nd147 11 d ,420 Nd"9 1.7 h * 090 Smlba 47 h .om9 Sm1s 25 m .016 E~162 9.2 h .00003 0 Eu 164 5 . 4 !f Gdl69 18 h 0.023 Gd161 3.6 m .064 Tb160 73 d .004 Dylu 1.3 m .00005 Dyl@ 2.4 h .00008 81 h d Dy16s Ho166 27 h 0.0005 H0165 .021 Er171 7 h Er 110 Tml" 129 d ,009 Tm169 P b 168 Yb18@ 33 d .027 Yhl75 4 d .004 YhlT4 Yblll 1.8 h .026 Yb Lu'l6 3.7 h 0009 Lu"5 Lu '71 6.7 d .0009 LUl'fl " T h a t weight of element considered as a practical limit for separation and measurement. In this instance, 40 disGrowth of Lal40 integrationsj'second of j3 radiations. needed for production. Half-life too long for practical detection in this study. d Growth of Dyl" needed for production.
.
~~
( 4 ) G.E. Boyd, Anal. Chem., 21,335 (1948). (5) G. W.Leddiootte and 8. A. Reynolds, Nucleonics, 8 , No. 3 , 62 (1951). (6) G.W.Leddicotte and S. A. Reynolds, A.S.T.M. BUZZ.,No. 188, 29 (1953). (7) T. I. Taylor and W. W. Havens, Jr., Nucleonicr, 6, No. 4 , 54 (1950). (8) G. Hevesy and H. Levi, K a l . Danske Videnekab, Selekab, Mat.-/us., Medd., 16, No. 11 (1938).
8
ION-EXCHANGE SEPARATION OF TRACE IMPURITIES
Nov., 1953
utility with which acti100,ooo vation analysis could be - I I 1 I I I I1 I I I I I I I 1 I I I I applied to the quantitative determination of trace rare earths samples of biological material, metals and alloys, ores, etc., has been greatly Lu enhanced by adapting 10,000 t Ketelle and Boyd's2 Ho method of ion exchange 0 to these analyses. How- .g! ever, certain factors required consideration before these analyses could be effected. Much of T b m the data used in our con1,000 sideration of these fac-tors is given in Table I. The material used in this tabulation can be found -Anlons in the nuclear data compilations by Way, et aL9 Theoretically, the rare I I I 1 I I I I It I I I I I I I I I I earth elements can be 100
I I
-
821
I I I I I I I 1 ? 1 1 ( I I 1 I I I I 1 1 I I I 1 t-I I
--
--
Tb
3
-
-
I
can be used for the determination of dysprosium, gadolinium and neodymium, respectively. The interference from the end product of the first reaction must be considered in the holmium determination, which employs the reaction H O '+ ~~
(27.3 h)
(4)
The decay schemes of all the rare earth radio(9) K. Way, K.Fano, M. R. Scott and K.Thew, National Bureau of Standards Circular No.499 (1950).
II
I1I
I I
11
jt I
ill'l I I , I I I
separation to the quantitative determination of trace rare earths in biological materials, metals and alloys, ores, etc. C. Procedure for Determining Rare Earth Elements by Activation Analysis and Ion Exchange.-The ion-exchange procedure described by Ketelle and Boyd2 showed that a separation of the rare earths could be made on a 270-325 mesh Dowex-50 resin column, having an area of 0.26 cm.' and a length of 97 cm. The elution of each rare earth fraction was made at a temperature of 100' by use of 5% ammonium citrate buffer with a pH range 3.2 t o 3.4. Utilizing this information, the present method of separation employs
822
W. A. BROOKSBANK AND G. W. LEDD~COTTE
two identical columns operating at different pH values (3.26 and 3.43). The samples are irradiated for at least 62 hours in the Oak Ridge National Laboratory Graphke Reactor (neutron flux = -10'2 neutrons Der Der second). After'discharge from the reactor the sampleis put int80solution in mixture of concentrat etl hydrochloric and nitric acids. Known amounts of inactive cerium (10 mg. as Ce) and lanthanum (20 nig. as La) "carriers" are added and the rare earths precipitated with concentrated KaOH. The hydroxide precipitate is dissolved in 6 N HCI and transferred t o a lusteroid container. The rare earths are precipitated with a small amount of concentrated hydrofluoric acid. After washing the fluoride precipitate with water, i t is dissolved in saturated boric acid and concentrated nitric acid and reprecipitated as rare earth hydroxides with concentrated ammonium hydroxide. The hydroxide precipitate is then dissolved in HCI and transferred to a small Dowex-50 column (270-350 mesh; 7.5 cm. long; 11 mm. diameter; flow rate 0.708 ml./min.). The column is eluted with cold 5% ammonium citrate buffer solution a t a pH of 3.26. Tinder these conditions and a t a total time of seven hours. for collecting the elutriant, a separation of the La (as La140) from Ce and the other rare earth elements is obtained. The lanthanum fraction from this column is then processed by precipitating the lanthanum fluoride from the bulk of the citrate first and then finally determined as lanthanum oxalate. The final precipitate is then weighed, mounted and counted in some type of counter. Chemical recovery through the column is 95-9975.
VOl. 57
ization. This means an identification can be made during the separation. Table I1 shows some typical data obtained in the analysia of an animal tissue by tthis method.
TABLEI1 THEDETERMINATION OF RAREEARTHS I N ANIMAL TISSUE (BONE)BY ACTIVATION ANALYSISUSINGA N ION-EXCHANGE SEPARATION Rare earth elementa
Concn., pdg.
Rare earth elementa
La 0.27 Ho Sm ,009 Er Eu .20 Tm Gd Not found Yb Tb 0.0004 Lu DY .00 Y Ce, Pr and Nd were not determined.
Concn., w/g.
0.50 2.20 1.30 1.30 0.08 .04
D. The Determination of Trace Amounts of the Alkali Elements.-With specific reference to the determination of the alkali metals by activation analysis, Table I11 shows the products of slowneutron bombardment as reported in the compilation of nuclear data by Way.9 TABLE I11 NUCLEAR DATAFOR THE DETERMINATION OF THE ALKALI ACTIVATION ANALYSIS METALSBY NEUTRON Rad. Sena., pg.a Target Product TI/, Li(6) H(3), He(4) ... ... Li(7) Li(8) ' 0.9s p-,2a Na( 24) 0-, y 0.007 Na( 23) 15.0h p-, y .08 K(42) 12.4h K(41) Rb( 8 6 ) p-, y .03 Rb( 85) 19.0d Rb(87) Rb(88) 17 m p.3 Cs(133) Cs(134 m) 3 h I.T., e1.0 p-, y 0.03 Cs(134) ~2 y a Sensitivity of detection: weight of element necessary to give 40 disintegrations per second of @-radiation.
...
...
It is evident that NaZ4and K42 are desirable nuclides in activation analysis assays. H3and He4 (for the determination of Li6) and Lis require' snecial techniaues for analvsis. Rbss is too short5 10 15 20 30 35 40 li'ted for most kork, while RbS6and CslS4are rather long-lived although the high Cs cross-section parElution time (hours). tially compensates for this. The sensitivity indiFig. 2.-Elution of rare earth element; Dowex-50 column, is that amoul't Of the in the 270-325, mesh, 0.26 cm.2 X 100 cm., elution with ammonium which will nroduce in ten half-lives. or in one month. citrate pH 3.43. whichever ;s shorter, a readily measurable amount The cerium fraction is treated with HF and this precipitate of the radioauclide, Le., 40 disintegrations per dissolved in boric acid and nitric acid. Following this the second. The flux is that normally obtained in the rare earths are precipitated with ammonium hydroxide and this precipitate is dissolved in 6 N HCl and diluted to a ORNL Reactor. It is usually possible to employ precipitation known volume and made 1 M in HCI. Equal aliquots are then taken from this solution and placed on the two columns methods for the determination of NaZ4activity in mentioned previously and are eluted with 5% ammonium the presence of the other elements. However, citrate buffer solutions at a p H of 3.43 and 3.26, respectively. The elutriants are passed through the columns in sufficient some difficulty is experienced (as pointed out in the volume and a t a time interval comparable t o that used in the introduction of this paper) in the determination of calibration of this method with samples of high purity rare potassium in the presence of rubidium by precipiearths. tation. Hence, considerable interest has been At the present time, the elutriant flowing through the columns is passed through a counting device capable of shown in the use of a cation separation of Na, K, measuring the radioactivity. This monitor gives a record Rb and Cs. of activity us. time on a Brown recorder. The strip chart Beukenkamp and Rieman'O have demonstrated data can then be analyzed by comparing its data with that that an ion-exchange separation is specific for alkali obtained from an analysis of a comparison sample in the 8ame manner. Any rare earth in the unknown will elute during the same interval as the rare earth used in standard-
(10) J. Beukenkamp and W. Rieman, 3rd., Anal. Chem., 22, 582 (1950).
.
ION-EXCHANGE SEPARATION OF TRACE IMPURITIES
Nov., 1953
823
105
104
d
*g 103
2
102
I
101 0
Fig. 3.-Alkali
500
1000
1500
2000
2500
5500
6000
6500
7000
MI. of 0.1 N HCI. separation by means of ion exchange: IR-1 acid form, 100-120 mesh column, 1 cm. i.d. X 140 em., flow rate 1 ml./min., elutriant 0.102 N HCI.
metals. They have also shown that the separation is adequate from magnesium, the most difficult polyvalent cation to separate from the alkalies. I n separate investigations, Brooksbank’l and Kayas12 proposed conditions which were almost identical. The separation was excellent, with peak to valley ratios of -lo4. Numerous absorption and decay studies demonstrated the purity of the fractions obtained during analyses. It has been shown also that 97-100% of alkali activity added to an ion-exchange column is recovered in the proper fraction. E. Procedure for Determining Alkali Elements by Activation Analysis and Ion Exchange.-Figure 3 shows a typical ion-exchange separation of the alkali metals. The data were obtained from an ion-exchange separation method using a glass column, having the dimensions of 1.0 em. i.d. by 170 em. IR-1 resin, 100-120 mesh, in the acid form, was used in a bed 1 em. in diameter and 140 em. in length. One-tenth normal HCI was the elutriant, and the flow rate was approximately 1 ml./min. All runs were made at room temperature. Recent work has shortened the analytical time from >lo0 to N 24 hr., and the volumes from liters to several hundred milliliters. The eluate went from the bottom of the column through a pipet counter and was collected on an automatic sampling table. The pulses from the pipet counter were counted by a radiation scaler and permanently recorded on a pen-type recorder. Although elution curves of counts/min. vs. time (11) W. Brooksbank, Oak Ridge National Lab. Rept. No. ORNL867.52 (Oct. 10,1950). (12) G. Xayas, J . chim. p h y s . , 41,408 (1950).
were plotted automatically by the recorder, gross beta counts were run on each fraction for confirmation. This column was used in the analysis of a sample of K&03, containing supposedly > 0.001% Na. Fifty milligrams of K&03 was irradiated for 16 hours in the ORNL Graphite Reactor, dissolved in dilute HCI, and transferred quantitatively to the column, and Na24 eluted with 350 ml. of 0.1 N HC1. The volume was reduced by evaporation, and the solution assayed for sodium. The Na24 activity corresponded to 2.19 X gram ( 2 ~ 2 0 % )of sodium present in the KzCOs, or 0.005%. Sodium has also been determined in LiCl to the extent of 0.001%. Potassium has been determined in RbzCOa by both a spectrographic method and by activation analysis with an ion-exchange separation. Results were 0.06 and O.OSyo,respectively. Potassium has also been determined in water “synthetics” in the range of 10-500 p.p.m. with an average error of ~ 5 % . Rubidium has been sought, but not found, in samples of NasCOa by the ion-exchange method. Cesium has been determined in RbzC03 by activation and by spectroscopy in the order of 0.02%, respectively.
Conclusions.-Activation analyses coupled with ion-exchange techniques offers a specific method for determining small quantities of one or more of the rare earth elements, or one or more of the alkali elements, in a variety of materials. These techniques afford an analytical method which is not only sensitive but specific and which can give analytical results having a precision of 10% or better. Acknowledgments.-The able analytical assistance of J. H. Oliver has been most valuable in obtaining the data reviewed in this paper.
