NOTES
Cation Exchange Separation-X-Ray Spectrometric Determination of Scandium in Plutonium E. Arnold Hakkila, Ronald G . Hurley, and Glenn R. Waterbury The Los Alamos Scientific Laboratory, University of California, Los Alamos, N.M. 87544 A RAPID METHOD was required for determining scandium present in concentrations between 0.05 and 0.4 % in alloys with plutonium. X-ray fluorescence methods were considered because generally they are specific, rapid, and easy to use. Scandium intensities measured with a helium-path spectrometer from plutonium-containing solutions were approximately 10 cps for 1 mg of scandium per ml, indicating the required sensitivity of 0.05% could not be obtained without separations. Precipitating agents such as fluoride, hydroxide, or oxalate, normally used to separate rare earths, did not quantitatively precipitate scandium. Ion exchange separations were then considered. Korkisch and Ahluwalia ( I ) studied the cation exchange behavior of various ions from hydrochloric acid-organic solvent media. Although scandium and plutonium were not investigated, studies with uranium(V1) and cerium(II1) suggested application of cation exchange filter papers to remove scandium from plutonium(V1) solutions in hydrochloric acidorganic solvent media. Applications of ion exchange filter papers to the separation of trace elements from aqueous media prior to X-ray fluorescence measurements were reported by Campbell, Spano, and Green ( 2 ) . EXPERIMENTAL
Apparatus and Reagents. A Philips Electronics Corp. vacuum X-ray spectrograph equipped with a chromium target X-ray tube, LiF analyzing crystal, and a flow proportional detector with P-10 gas was used for X-ray measurements. Adequate intensities for the Ka X-ray of scandium were obtained operating with an air path (20 cps per pg of scandium). Therefore, helium or vacuum path operation were considered unnecessary. Reeve-Angel SA-2 cation exchange filter papers were cut to 15/16-inch diameter, and discs, selected at random, were weighed to assure that weights ranged between 60 and 70 mg. The exchange capacity of a disc was calculated to be 1.5 to 2 mg of scandium. The filtering apparatus consisted of a 125-ml Erlenmeyer flask, a modified size OOOA Hirsch funnel, and a glass chimney held in place over the Hirsch funnel with a No. 35 pinch clamp. The flow rate was adjusted to approximately 50 cc/min using a controlled air leak to the vacuum. The plutonium metal was prepared by a Los Alamos electrorefining process (3) and contained 100 ppm of americium, less than 200 ppm of other detected metallic impurities, and no scandium. Scandium solutions were prepared from 99.9 % pure scandium oxide. Solvent System. The studies of Korkisch and Ahluwalia ( I ) with uranyl and cerous ions indicated that the following (1) J. Korkisch and S. S. Ahluwalia, Tuluntu, 14, 155 (1967). (2) W. J. Campbell, E. F. Spano, and T. E. Green, ANAL.CHEM., 38, 987 (1966). (3) J. A. Leary and L. J. Mullins, U S . At. Energy Comm. Rept. LA-3356-MS, 1965.
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Figure 1. Effect of acid concentration on Sc/La intensity ratio (100 SC) hydrochloric acid-organic solvent systems might be used for separating scandium from plutonyl ion: (1) 90 isopropanol0.3 to 1.2M hydrochloric acid, (2) 90% 1,2-propanediol-0.5 to 1.2M hydrochloric acid, (3) 90% acetic acid-0.6 to 1.2M hydrochloric acid, (4) and 90% acetone-0.1 to 1.2M hydrochloric acid. Good separations of scandium from plutonium were obtained only with the acetone-hydrochloric acid system at low acid concentrations. The optimum hydrochloric acid concentration for the separation of 100 pg of scandium from plutonium was determined by analyzing a set of solutions in which the final acidity varied from 0.05 to 0.9M. Lanthanum was added to serve as internal standard in each test. The scandium and lanthanum were adsorbed onto SA-2 filter papers and the scandium and lanthanum X-ray intensities measured. The data (Figure 1) showed that the scandium/ lanthanum intensity ratio did not vary appreciably for hydrochloric acid concentrations between 0.1 and 0.2M; therefore, an acidity of 0.1M was selected for all further work. Internal Standard. Approximately 90 to 95% of the scandium was adsorbed onto SA-2 filter papers in six passes from solutions containing 100 pg of scandium in 50 ml of 90% acetone-0.lM hydrochloric acid. The adsorption was reduced if strongly absorbed cations were present in amounts greater than a few hundred micrograms. An internal standard technique was utilized, therefore, to reduce the effect of variable adsorption of scandium. The internal standard selected was lanthanum which had adsorption and chemical properties similar to those of scandium. In addition, lanthanum has the longest wavelength Lal X-ray of the rare earth elements. The distribution coefficient for lanthanum in acetone-0.1 M hydrochloric acid is >lo$ ( I ) ; the distribution coefficient for scandium was not measured. Cation exchange studies of scandium and rare earths in sulfuric acid ( 4 ) showed that scandium was not adsorbed as strongly as the rare earths. This may also be the case in the organic-hydrochloric acid system. (4) F. W. E. Strelow and C. J. C. Bothma, ANAL.CHEM.,36, 1217 (1964). VOL. 41, NO. 4, APRIL 1969
665
Table I. X-Ray Lines that Overlap the L a l Line for Lanthanum and the Ka Line for Scandium Diffraction angle, Relative Element X-Ray degrees 20, LiF intensity La Ce Pt
La 1 Ltl La, (n
=
2)
Sb
LY2,3 La2 (n
=
2)
Pt cs Nd cs
82.85 81.15 81.38 81.86 82.28
LP4 Ll LP 1
82.88
Sb
LY2
Sn Sb
KCU Lr 1 LP 1
83.99 97.66 96.32 96.49 96.54 96.67 97.27 98.17 98.26
sc
La Te Sb
Ta Te
83.23 83.53
Li LP3 LBZ
Lal (n LP 4
=
2)
100 10 100
10
3.5
/
E c
20 30 80 10
100 40
80 30 30 60 100 20
Effect of Sample Size. The range of sample sizes that could be analyzed by the recommended method was investigated by determining known amounts of scandium between 0 and 400 pg in 100, 200, 500, or 1000 mg of plutonium. Ratios of scandium intensity to lanthanum intensity (Figure 2) showed that the method tolerated only small variations in sample size; the intensity ratio decreased as the plutonium content or sample size increased. This was caused by incomplete oxidation of the plutonium(1V) which also was adsorbed. The adsorbed plutonium displace the more weakly cationic scandium and tended to partition the lanthanum and the scandium on the filter paper. By proper preparation of standards, the method was readily applied to determination of as little as a few ppm of scandium in 1-gram samples of plutonium. Recommended Procedure. Weighed 100 i 10 mg samples of plutonium were dissolved in hydrochloric acid and 2 ml of 70 % perchloric acid. Five hundred micrograms of lanthanum were added, and the solutions were fumed to incipient dryness at a temperature of 225 "C. The residue was dissolved in 5.0 ml of 1M hydrochloric acid, and 46 ml of acetone were added. The solution was passed through an SA-2 cation exchange filter paper six times at a flow rate of 50 ml per min. The paper was washed with 5 to 10 ml of 90% acetone-0.1M hydrochloric acid and dried under an infrared lamp. The dried paper, held between two layers of 0.25-mil Mylar in the Philips standard sample holder, was irradiated using a chromium target X-ray tube operated at 48 kV and 28 mA. The intensities of the K a X-ray for scandium and the Lal X-ray for lanthanum were determined by measuring the number of counts accumulated in 10.00 seconds while the sample was rotated under the X-ray beam. The sample was inverted and the intensities from the back of the disc were measured in the same manner. The scandium-to-lanthanum intensity ratio was calculated from the total counts accumulated from fronts and backs of the papers (without background corrections), and the scandium content was determined by comparing the intensity ratio to ratios obtained for standards prepared by separating known amounts of scandium from 100 mg of plutonium. RESULTS AND DISCUSSION
Reliability. The reliability of the method was determined by repeated analyses of solutions containing known quantities of scandium and 100 mg of high purity plutonium. Fourteen samples were analyzed at each scandium concentration. The data showed that the relative standard deviation of the method was approximately 2% for determining 50 to 400 pg of ANALYTICAL CHEMISTRY
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scandium. The standard deviation for measuring a blank was 0.15 pg, indicating that the lower limit of reliable measurement was approximately 0.5 pg (5 ppm for 100-mg samples). The method was unbiased. Interferences. Expected interferences in this method were: X-ray line overlap by elements adsorbed on the ion exchange resin, and swamping of the resin by elements which had greater distribution coefficients than that for scandium. Because of the limited exchange capacity of the discs, microgram amounts of strongly absorbed cations were expected to interfere. Overlap interference may be caused by elements that have X-ray lines within approximately two line half-widths (1.9" 20) of the K a X-ray for scandium or the La1 X-ray for lanthanum, providing these elements are retained on the resin. Possible interfering X-rays are listed in Table I. Only first order lines having a relative intensity of 10 or greater are listed, except for tantalum and platinum which have intense second order Lal lines near the lines used in the analysis. Of these elements, the effects of cesium, neodymium, and platinum on the determination of scandium were studied. Results showed that no greater than 50 pg of cesium, 100 pg of neodymium, or 500 pg of platinum can be tolerated. The second order X-ray for platinum was not completely eliminated using the pulse height analyzer. The decrease in scandium/lanthanum intensity ratio with increasing sample weight (Figure 2), as discussed previously, indicated that lanthanum is more tightly adsorbed on SA-2 papers than is scandium. It would be expected, therefore, that cations such as other rare earths would tend to partition scandium and lanthanum. To study the effect of a typical rare earth on the determination of scandium, a series of samples was prepared to contain 0 or 100 pg of scandium, 500 pg of lanthanum, 100 mg of plutonium, and between 100 and 5000 pg of cerium. The results show that greater than 100 pg of cerium interferes. In addition to cerium, the effects of sodium (Kd > 103), aluminum ( K d > 109, calcium ( K d > 109, nickel (Kd > loa), terbium (Kd > IO$), thorium (Kd > 109, cobalt (Kd = SOO),
copper ( K d = 3), cadmium (Kd = 2), iron (Ka < 1). zinc (Kd < l), gallium (Kd < I), bismuth ( K d < l), ( K d < 1) on the separation of scandium from pl studied. These elements were selected to incluue munu- LU tetravalent elements having Kd values in the range between 1000 (strongly adsorbed) and < 1 (weakly or not adsorbed), These studies showed that 100 pg of elements having Kd > 1000 can be tolerated in the determination of scandium. For copper, which has a Kd value of 3 , l mg can he tolerated. For those elements having Kd of 2 or less, 100 mg did not interfere. Although the method applies specifically to the determination of scandium in plutonium, it can he readily adapted to determining other strong cations, such as rare earths, or
scandium in other materials that are not adsorbed on cation rate and measure ACKNOWLEDGMENT
The authors are indebted to Dr. C. F. Metz, who supervised this work, for valuable suggestions, and to Mi!is H. L. Barker L-"-, -P"n.re.-.oJ for performing some of the separations and : ments.
.. .
RECEIVED for review November 15,1968_ 14ccepted January 8, ^. 1969. The Los Alamos mentmnc Iaboratory IS ooeratea unaer the auspices of the Atomic Energy CommissioiR.
Infrared Spectra of Compounds Separated by Thin Layer Chromatography Using a Potassium Bromide Micropellet Technique W. J. de Klein Laboratory of Organic Chemistry, State University of Leiden, Leiden
COMBINATION of thin layer chromatography (TLC) and infra1mertmrmnu ;q In intprprtinolnrl nrnmirino eutpn.in Ir r, (IR) ,__., ._.______l.... r____..l... _.__..I._ n of both techniques. The separation of the components of a mixture hv TLC and examination of the resultine mots hv IR ~, is one of the most useful ways of identifying the unknown compounds. Several procedures have been described in the literature. These include scraping of the sample from the support,
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from the support material ( I ) or followed hy packing the suooort .. into a Pasteur tvoe .. Dioet . . and elution of the ComDound into the tip of the pipet, evaporation of the solvent and dissolution of the compound in a solvent which is suitable for IR measurement (2); and scraping the support surrounding the spot from the chromatoplate, building a dam of KBr powder onto the tip of the spot, and eluting the compound directly onto the KBr powder (3). Experience has shown that the method of Sturn, Parkhurst, and Skinner ( I ) is not directly suited to the infrared experiment for it was developed as a combination with ultraviolet spectroscopy which permits a much larger dilution. The method of McCoy and Fiehig (2) is rather time consuming ( 4 M 5 minutes) and suffers from the restriction that spectra can only he measured in a solvent. Rice's method (3) is a rather promising technique but is limited by the skill of the individual worker for considerable loss occurs when the elution solvent spills from the support on to the glass plate. A method is now described which is rapid and generally applicable for it uses the KBr technique and gives relatively high recovery. Garner and Packer ( 4 ) have described a similar technique employing Harshaw's Wick-Stick.
Figure 1. The shape of the KBr wall around the tip of the spot depends somewhat on the size of the spot. The upper spot is small and requires a relatively larger envelope EWERIMENTAL
Glass plates (5 x 20 cm) were coated with a 250-p layer of either silica gel (Merck HR), or silica gel containing an inorganic fluorescent indicator and gypsum (Merck G F 254), or aluminium oxide (Merck G F 254). The plates were air dried and heated as described. They were then stored in an air-tight box. Glass plates coated with cellulose (Merck, Fertigplatten F, thickness 80 p ) were used in some experiments. One to fifty micrograms of a compound in solution were applied to the plates with a micropipet (Camag AG, Muttenz, Schweiz). The plates were developed in the usual manner in covered chromatographic tanks. The walls of the tanks were not paper covered for vapor saturation. Location of separated components was done hy irradiation with ultraviolet radiation at 254 or 350 mp. The spots were outlined hy scratching the support. After evaporation of the elution solvent, the TLC support was removed around the spot. The glass around the spot was tarefullv, cleaned with tissue oaoer or wash leather. A wall of (1) P. A. Sturn, R. M. Parkhurst, and W. A. Skinner, ANAL. GEM., ~~~.~~ KBr powder was built around ihe tip of the spot and an open 38, 1244 (1966). space of 1-2 mm left between the spot and the KBr wall (2) R. N. McCoy and E. C. Fiebig, ibid., 37, 593 (1965). (Figure 1). This wall can easily he built hy lightly tapping a (3) D. D. Rice, ibid., 39, 1906 (1967). (4) H. R. Gamer and H. Packer, Appl. Spectrosc., 22, 122 (1967). Pasteur pipet, containing finely divided KBr powder, with the ~
~~
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VOL. 41, NO. 4, APRIL 1969
.
667
