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 spectrographic methods (6%) have been 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
Table
I.
Analytical Angles for Each Matrix, Net Intensity, and Peak-Background Ratios‘ a t 1 % Concentration Level
La
Ce
La,, 82.85
78.96
LUl,
Element Pr Nd Analytical Angle LB1, LPI, 68.23
Sm LPi,
Y Kal,
65.06
59.48
23.75
CeOz Matrix
Air Net intensity, C.P.S. Ratio Helium Netintensity, C.P.S. Ratio
6.74 3.29
...
...
15.43 1.46
20.80 1.22
20.29 3.20
.*.
24.00 5.10
...
...
39.90 1.80
42.96 1.23
37.34 3.81
...
Yet intensity, C.P.S. Ratio
4.55 3.03
20.33 4.52
20.58 3.14
55.85 5.82
30.10 0.96
LB1--2, 144.85 4.65 1.21
Gd
Y
144.85
LBl-2,
Lffl, 61.05
Ka, 23.75
6.45 1.29
63.96 3.85
32.24 6.89
Er
Yb
LPl, 46.41
112.25
81.77 3.33
12.35 1.08
...
...
Pr6OI1Matrix 6.94 1.81
... *..
Eu Net intensity, C.P.S. Ratio
5.03 0.59
8.00 2.32
Ce
Pr
La1, 78.96
68.23
L81,
14.00 2.33
... .,.
Element Nd EU Analyt,ical Angle Lcul, 72.08
Sm203Matrix Net intensity, c.p.s. Ratio
6.86 2.25
Eu La1, 63.51
14.46 1.50
Elemen tt Tb DY Analytical Angle La1, La1, LB1 , 61.05 58.76 50.24 Y20rMatrix 73.17 62.74 48.27 8.88 7.20 2.19 Gd
Netintensity,c.p.s. 55.49 Rat.io 6.80 gross c.p.s.-bg. C.P.S. a Peak/bg. = bg. c.p.s.
a sample holder and analyzed. This was done to minimize hydration and carbonation of the oxide material. BACKGROUND DETERMINATION
In any trace-analysis technique, determination of spectral line intensity above background is very important (1, 2 ) . The usual method of determining background consists of counting in an interference-free area near, and on both sides of, the analytical line. An average value is used as the background intensity. However, this approximation method was insufficiently accurate for determining minor amounts of rare earths in a rare earth oxide. The absolute background intensity of each analytical line in a particular matrix was evaluated by using an extremely high-purity sample of the type to be analyzed as a blank. Highpurity samples were obtained from commercial sources or prepared by ion exchange. Purity was checked by spectrographic and spectrophotometric methods. However, it was sometimes impossible to obtain a standard pure enough for use as a blank with respect I
810
ANALYTICAL CHEMISTRY
18.51 6.11
Lal-2,
to all of the impurity elements. The procedure followed was to determine background intensity for the desired analytical line in several of the highest purity samples available. The lowest intensity obtained was used as the background value for that analytical line in that particular matrix. Interelement effects due to the presence of impurities in the 0.01 to 1.0% concentration range were negligible. The worst interference possibility, for example, was in the determination of cerium in samarium oxide. Enhancement of the CeLa line by the PrLr, NdLr, SmLP, SmLr, EuLa, EuLp, EuL-y, GdLa, GdLLp, GdLr, and Y K a - 2 and KP-2 lines was possible. However, a samarium oxide containing 1% of each of these elements caused an increase of only 0.5 c.p.s. in cerium intensity, representing about 0.02% cerium. Obviously, in high-purity samarium these impurities would be present a t much lower levels and would consequently make an insignificant contribution to the cerium intensity. Because the availability of highpurity oxide was limited, standard series were prepared from rare earth
oxides of 99.9+% purity. After x-ray intensities were obtained, the background intensity of each analytical line for that particular lot was determined, and was then subtracted from each standard. The net intensity thus obtained for each standard shown in Table I1 was due only to the added amount of rare earth impurity. Per cent oxide was plotted against net intensity for each impurity to give a standard curve through zero. Although this curve was completely valid, the background intensity due only to the major element was not known. To analyze unknown samples, the background, as determined from the high-purity sample, was subtracted from the scaled intensity of each unknown. I n Table I1 the background intensity determined from a high-purity sample, Ce-JM, is equal to 11.14 C.P.S. By subtracting this value from the background intensity of the standard material, 11.79-11.14 = 0.65, the residual per cent La203 = 0.01 may be read directly from the standard working curve. Although the data were accurate t o only I%, one more figure than significant mas carried through the calculations. Determined percentages were rounded off to the number of figures significant. RARE-EARTH ANALYSIS
I n developing a routine analytical method, the degree of accuracy desired and the counting time necessary to attain it must often be compromised to make the method practical. It was possible with the instrumental arrangement used to operate a t 1% probable error in determining all intensities, The counting time necessary for accumulating the required number of counts (4000) varied from 1 to 10 minutes, averaging about 3. The error inherent in each analysis depended on the background and net intensity of that particular case. Statistical methods were invaluable in predicting errors a t a given concentration level for a particular count situation. The inherent error was conveniently estimated by extrapolating data more easily determined a t a higher concentration. For example, the net intensity and background a t the 1.0% level for LaLal line in cerium oxide were, respectively, 6.74 and 2.05 c.p.s. (see Table I). An estimate of the error a t 0.01% was calculated thus: The net count a t 0.01% would be 6.74/100 = 0.07 count per second, or a total count of 2.12. All data are good to 1.0% or 2.12 z!= 0.02 and 2.05 + 0.02. A statistical consideration (6) of adding and subtracting intensities leads to: Probable error = 2/3(0.02* 0.022)1’2= 0.019 C.P.S.
+
Finally, considering only the error involved in counting, 2.12 i 0.02 - 2.05 f 0.02 = 0.07 f 0.019 = f27%
Table II. Background Determination b y High-Purity Sample, Lanthanum Oxide in Cerium Oxide
Standard No. Ce-1 Ce-2 Ce-3 Ce-4 Ce-5 Ce-6 cc-7
Less Matrix Background, 11.79 C.P.S. 58,88 29,23 11.26 5.26 2.32 0.23 0.21
La203 Intensity0 Added, LaLal, % C.P.S. 1.0 70.67 0.5 41.02 0.2 23.05 0.1 17.05 0.05 14.11 0.01 12.02 0.005 12.00
This analysis is applicable to all data in Table I and may be used to estimate the inherent error in a given analytical situation. Table I11 shows the predicted probable error a t O . O l ~ o concentration for elements in ceric oxide in air and helium. Synthetic samples of known composition were prepared and analyzed to determine the actual precision and accuracy of the method. The precision was determined by analyzing five individual samples from each synthetic mixture. The accuracy of individual and replicate analyses was calculated, using statistical methods (6),from the five individual analyses of each known sample. The significance of statistical evalu-
Ce-8 (blank) 0 . 0 11.79 ... Ce-JMa ,.. 11.14 ... Values taken in helium atmosphere with 0.010-inch Soller slit. * Johnson-Matthev Catalog No. 304, 99.99% pure CeOz, rklative to rare earths present; analysis by supplier indicated no lanthanum.
Table 111.
Predicted Probable Error at 0.01 7 0 Concentration Level in Ceric Oxide Matrix
La
Pr
Element Sm La
Xd In Air
Gross intensity,
Nd In Helium
Pr
S m
2.12
10.73
17.27
6.55
4.95
22.61
35.39
10.17
c p.s.
2 05
10 58
17 06
6 35
4 71
22 21
34 96
9 80
c.p.s
0 07
0 15
0 21
0 20
0 24
0 40
0 43
0 37
C.P.8.
Background intensity, Net intensity, Probable error,
%
rt27 0 f 6 9 0 A76 0 f 3 0 0 f 2 0 0 f 5 3 0 f 7 6 0 f25 0
Table IV.
Precision and Accuracy in Samarium Oxide Matrix"
Ce Known Foundb Error Accuracv" Precision0 Known Foundb Error Accuracyc Precisionc Knonn Foundb
0 418 0 404
3 3 f13 0
f11 0 0 09s 0 097
1.0 f 9 ,S f9.0
0.034
0,037
Pr 0 617 0 560 9 2 f21 0 f7 4 0 137 0 138 0.7
f26.0 f25.0 0.041 0,046
Element Nd Eu 0 822 0 500 0 491 0 738 10 2 1 8 f25 0 1 88 f 7 4 178 0 100 0 093 7.0
f17.0 3~7.8
0.020
0.022
0 182 0 179
1.6 f21.0 +21.0 0.054 0 058 7 4 3L56.0 h50.0
Gd
E'
0 307 0 302
1 6 112 0
111 0 0 067 0 067 0
114.0
0 204 0 204 0 f2 0 130 0 044 0 047
6.8
*26.0
dzl4.0 0.019
0.025 31.6 rt127.0
h 2 1 .o 0.012
0.012 0
10.0 8.1 12.0 rt140.0 f56.0 1113.0 Accuracy0 f l 2 S . O 146.0 h69.0 1140.0 3L95.0 Precisionc f 1 1 4 . 0 All figures as yooxide. An average of 5 dekrminations. 0 95V0 confidence level; precision and accuracy of individiial analyses lie within limit's listed 95% of time. Error
Table V.
Accuracy of Analysis Summary"
Concn. Element, Average yo*Error Range La Ce Pr Nd Sm Eu 0.3-0.8 15 2 4 4 1 6 0.06-0.2 21 9 7 4 5 6 0.01-0.05 29 5 14 19 15 15 a All figures aa yooxide. * Based on comparison of average of 5 determinations to known value.
Gd 2 0
32
Y 0 7 0
ations based on only five determinations might be questioned: however, accumulation of many similar sets of data tends to validate the precision and accuracy thus determined. The composite gives a rather accurate representation of the precision and accuracy attainable by this x-ray technique for various concentration levels of impurities. These data for a samarium oxide matrix are presented in Table IV. I n all instances the impurity elements are those expected when the element was purified by ion exchange. The known percentage was obtained by adding various amounts to a high-purity material and determining the residual per cent by the method described. Each per cent found is the average of five individual determinations. The accuracy and the precision were evaluated a t + 2 sigma, or the 95% confidence level; errors of individual analyses lie within the limits listed 95% of the time. Where precision and accuracy are about the same, the maximum accuracy has apparently been attained. However, in some instances precision is good, but there is a fairly large error. An analysis summary in Table V shows the average error of analysis for several rare earth elements a t different concentration levels. For samples analyzed in qu,intuplicate, the average error varies from 0 to 30% of the amount present, depending on the element and concentration. The overall average error is about 10%. The fluorescent x-ray analysis technique is applicable to the determination of rare earth impurities in any single rare earth oxide. Individual sensitivities of about 0.01% oxide were attained, but may vary depending on the element and matrix. Although fluorescent x-ray analysis of high-purity rare earth oxides is rapid and reasonably accurate, the best analytical situation is realized when a combination of methods (emission spectrography, x-ray spectroscopy, and spectrophotometry) is used conjointly to complement each other. LITERATURE CITED
(1) Brissey, R.
IT.,ANAL.Cmm. 25, 100 (1953). (2) Campbell, .'%1 J., Carl, H. F., White, C. E., Zbid., 29, 1009 (1957). (3) Fassel, V. A., Cook, H. D., Krotz, 1,. C., Kehres, P. W., Spectrochina. A c t a 5,201 (1952). (4) Fassel, V. A., Wilhelm, H. A,, J. Opt. SOC.Am. 38,518 (1948). (5) Friedlander, C., Kennedy, J. W., "Introduction to Radiochemistry," p. 203, Wiley, Xew York, 1949. (E) Heidel, R. H., Fassel, V. A., ANAL. CHEM.30, 176 (1958). (7) Lytle, F. W., Botsford, J. I., Heller, H. A,, Bur. Mines Rept. Invest. 5378 (1957). (8) Salmon, M. L., Blackledge, J. P., Norelco Reptr. 3, 68 (1956). (9) Stewart, D. C , Kato, D., ANAL. CHEM.30, 164 (1958). RECEIVEDfor review September 8, 1958. Accepted January 15,1959. VOL. 31, NO. 5, MAY 1959
811
