Determination of Samarium and Gadolinium in Rare Earth Ores by Neutron Capture Gamma-Ray Activation Analysis S. M. Lombard and T. L. Isenhour Chemistry Department, Uniuersity of Washington, Seattle, Wash. 98105
NEUTRON CAPTURE gamma-ray activation analysis differs from conventional activation analysis in that the irradiation of the sample and the analysis of the resultant gamma radiation are performed simultaneously. The ability to isolate the various gamma-ray photopeaks produced by a complex sample is entirely dependent upon the resolution of the gamma-ray detector because post-irradiation chemical separations or resolution based on differing half-lives are not possible. The rare earths have been determined in their ores by many techniques. Methods involving chemical processing of the ore samples such as polarography or colorimetry are time consuming and subject to a variety of interferences. Emission spectroscopy is adequately sensitive for the purpose but its accuracy is seldom better than *lo% for a complex sample. Cobb ( I ) has used conventional neutron activation analysis with direct gamma counting using a Ge(Li) detector to analyze for several rare earths in rocks. Garbrah and Whitley (2) have analyzed various types of rocks for the rare earths by the capture gamma-ray method using a NaI(T1) scintillation detector with a multichannel analyzer. Background spectra obtained with a cadmium shutter over the beam port were subtracted from each capture gamma-ray spectrum. Total counting time for each sample was in excess of 30 minutes. Several sets of standards were prepared and the spectra of the elements in each set were combined by a computer program to form a synthetic spectrum which was compared to that of the sample to find the “best fit” as determined by the chisquared criterion. These workers reported precisions of 1 4 % for Sm and G d in five minerals and 10-18y0 from Dy and Er in three. They concluded, however, that the time required for the preparation of standards was prohibitive for routine analysis. This paper describes the nondestructive neutron capture gamma-ray activation analysis of four of the most common ores of the rare earths for samarium and gadolinium. A large coaxial Ge(Li) detector provides adequate resolution of the capture gamma-ray spectra and a high speed chopper permits automatic background subtraction. Data from 10minute live-time irradiations of samples from 10 to 100 mg of each ore are fitted by weighted least squares and the error for each determination is calculated. EXPERIMENTAL
The apparatus used in this work has been described in detail (3). It includes a high resolution cadmium chopper and a 1024 channel analyzer. The thermal neutron flux at the sample is 1.1 X lo7 n/cmz sec over a 0.5-inch diameter beam. The advantages of the use of a Ge(Li) semiconductor detector over a NaI(T1) scintillation detector have been demonstrated (4). The Ge(Li) diode used in this work is an Ortec (1) J. C. Cobb, ANAL.CHEM., 39, 127 (1967). (2) B. W. Garbrah and J. E. Whitley, _ .I n t . J . ADDI. _ . Radiaiion Isotopes, 19,605 (1968). (3) S. M. Lombard, T. L.Isenhour, P. H. Heintz, G. L. Woodruff, and W. E. Wilson, ibid., 19, 15 (1968). (4) S. M. Lombard and T. L. Isenhour, ANAL.CHEM., 40, 1990
30-cm3 true coaxial type mounted in a gravity feed dewar/cryostat with an Ortec Model 118A preamplifier. System resolution with this detector is -4 keV (FWHM) at 663 keV with a peak to Compton ratio of 13/1 and an efficiency of 4% relative to a 3- X 3-inch NaI(T1) detector at 1.33 MeV. The typical compositions of the four rare earth ores selected
Table I. Compositions of the Four Ores Analyzed Ore
Natural
Synthetic
25.7% 42.5 f 5.5 15.6
23.6 f 1.9% 38.4 f 7 . 3 9.8 f 0 . 7 1.1-2.6 0.09-1.8 20.0 f 1 . 6 7.5 f 2.0
1.8
0.9 20.0 7.0 24.0% 15.0 10.0 23.0 2.0 5.0 5.0 trace 5.0 6.0 1.o 4.0 0.6
27.7% 8.0 19.0 13.0 29.0 2.0
25.0 dz 3.9 14.0 f 2.2 10.3 f 1.6 24.0 f 3.8 2.8 f 1 . 1 5.2 f 0 . 9 1.3-7.1 0 0.9-9.7 5.33-9.6 1 . 3 dz 0 . 5 4.1 f 0.5 0.7 f 0.3
1.o 0.05 0.1 0.05
29.1 f 3.5% 8.7 f 0 . 9 20.7 f 2.2 15.2 f 2.6 31.7 f 3.3 1.2-7.4 0.5-3.8 0.06 f 0.01 0.1 f 0.01 0.05 f 0.01
0.03 0.005
0 0
32.6 40.0 4.4 2.8 1.2 0.1
2.8 7.6 1.8 5.9 0.8
33.2 f 4.9 40.7 f.4.1 4.5 0 . 4 2.8 f 0 . 3 0.7-2.4 0.1-0.4 2.0-4.3 3.6-1 3.5 1.8 f 0.2 5.2-6.7 0 . 8 f 0.1
(1968). VOL. 41, NO. 8,JULY 1969
1113
Table 11. Date for Analysis of Synthetic Rare Earth Ores Bastnasite Sample # 1
2 3 4 5 6 7
Actual 893 f 9 876 f 9 868 f 9 838 f 8 846 f 8 1540 =k 15 75 f 1
Gd Weight (fig) Calcd 765 833 838 799 926
e
(%I
-14.3 -4.9 -3.5 -4.6 +9.5
a
87 -
+16.0
Actual 2210 f 10 1770 f 10 1460A 10 2230 f 10 960f 5 1500 f 10 1670 f 10
11.0
u =
Sm Weight (Po) Calcd b
1740 1570 2220 950 1360
-1.7 +7.5 -0.4 -1.0 -9.3
C __
u =
6.0
Gadolinite 1
2 3 4 5 6 7 8 9
477 f 5 474 f 5 483 f 5 410 & 5 421 f 5 80 f 1 235 f 3 624 f 7 847 f 9
470 506 461 416 431 84 219 621
-1.5 +6.7 -4.5 +1.5 f2.4 +5.0
-6.8 -0.5
b
-
479 f 5 108 f 1 248 f 3 608 f 7 725 f 8 455 f 5 441 f 5 390 f 4 386 f 4
u = 4.2
Monazite 1 2 3 4 5 6 7 8
198 f 4 186 f 4 181 f 4 177 f 4 176 f 4 80 f 2 313 f 6 530 f 11 u
Xenotime 1 2 3 4 5 6 7 Q
b c
194 189 183 181 185 73 319 516 = 4.2
-2.0 $1.6 +1.1 +2.3 +5.1 -8.7 +1.9 -2.6
471 f 3 496 f 3 529 f 4 525 f 4 740 f 5 635 f 4 351 f 2
for analysis were obtained from Kolthoff and Elving (5); Palache, Berman and Frondel (6); and Deer, Howie and Zussman (7). Synthetic ores were prepared to match the composition of the natural ores as closely as possible but with varying concentrations of the elements to be determined. The compounds used in the preparation of the synthetic ores (with the exception of DyP04, ErP04,LaP04 and YPo4) were obtained commercially with 99.9% or better purity. Y 2 0 3Nd203, , NdF3,CeF3,H0203, Tm203,Er203,Gd203,GdF3, Dy203, La203 and EuP03were obtained from Allied Chemical; Sm208,SmF3, Tho,, and La2(C0& from American Potash; CeO, from Matheson, Coleman and Bell; B e 0 and SiO, from Spex Industries; and, Fe203from J. T. Baker. The com( 5 ) I. M. Kolthoff and P. J. Elving, “Treatise on Analytical Chemistry,” Part 2, Vol. 8, Interscience, New York, 1961, p 7.
(6) C. Palache, H. Berman, and C. Frondel, “The System of Mineralogy,” Vol. 11, 7th Ed., Wiley, New York, 1951. (7) W. A. Deer, R. A. Howie, and J. Zussman, “Rock Forming Minerals,” Vol. 5, Wiley, New York, 1962. ANALYTICAL CHEMISTRY
-
+1.2 +3.7 -2.0
+2.5 +0.5 $0.2 +2.3 -3.8 -4.4
u = 2.8
375 f 4 201 f 2 528 f 6 758 f 9 1287 f 14 345 f 4 403 f 5 424 f 5
404 208 475 810 1298 359 389 395 = 6.5
+7.7 +3.5 -10.0 +6.9 +0.9 $4.1 -3.5 -6.8
230 391 328 115 270 362 303 u = 8.4
$15.0 -7.1 +1.2 -9.4 -1.5
u
b 200 f 3 518 +4.4 421 f 5 531 +0.4 324 f 4 503 -4.2 127 f 2 717 -3.1 274 f 3 666 $4.9 362 f 5 345 -1.7 282 f 4 u = 3.8 The quantity of Gd in this sample exceeded the upper limit of the linear portion of the curve. The results for this determination were unexplainably poor. The extremely low Gd content of this sample caused the results for Sm to be very high.
1114
485 112 243 623 729 456 451 375 369
0.0
4-7.4
pounds were powdered in an agate mortar and pestle when necessary. The four phosphates which were not available commercially were prepared in this laboratory by the method of Buyers and his coworkers (8). Mixtures of 750 mg or larger were mixed in an agate mortar and pestle and smaller quantities were mixed in a Wig-1-Bug amalgamator. The final samples used for analysis were mixed with pure S O , powder to a total sample weight of -200 mg. This procedure was to aid the even distribution of the ore over the entire area of the neutron beam. Two hundred milligrams of SiOz produced no detectable contribution to the capture gamma-ray spectra. Table I lists the typical compositions of the naturally occurring ores along with those of the synthetic ores used for analysis. Bastnasite is a fluocarbonate of lanthanum and cerium with (8) A. G. Buyers, E. Giesbrecht, and L. F. Audrieth, J. Znorg. Nucl. Chem., 5, 133 (1957).
0
I
100
I
I
I
I
200
300
400
500
p9 G d
Figure 1. Calibration curve for the determination of gadolinium in monazite small amounts of the lighter rare earths. The synthetic ore was prepared by mixing La2(C0&, Ce2(C03)3,CeF3, NdF3, SmF3 and GdF3. For analysis, -100 mg of the ore was mixed with -100 mg of Sios. Gadolinite is composed of the oxides of yttrium, iron, beryllium, and silicon with small amounts of most of the rare earths. The synthetic ore was prepared from Y203, SO2, BeO, Ce02, Nd203, Ho203, Tm203, Er203, SmnOs, Gd203,and Dy203. Samples for analysis consisted of -10 mg of the ore with -190 mg of S O 2 .
Monazite is probably the most common source of most of the rare earths. Its principal constituents are the phosphates of lanthanum, cerium, and neodymium with thorium oxide and varying amounts of the phosphates of most of the other lanthanides. The synthetic ore was prepared with Tho*, LaP04, NdP04, CeP04, Sm203,Gd203, Eu203, Dy203, and Er203. Samples consisted of -20 mg of the ore mixed with -180 mg of SOP. Xenotime is a phosphate ore consisting mainly of yttrium with varying amounts of the heavier rare earths. It is com-
pL9 S m
Figure 2. Calibration curve for the determination of samarium in monazite VOL. 41, NO. 8, JULY 1969
1115
monly found associated with monazite from which it is separated by magnetic means. The synthetic ore was prepared from YPO4, CeP04, DyPOa, ErP04, Nd203, Sm203, EU203, Gdz03,Ho203,and Tm203. Xenotime samples consisted of -20 mg of the ore with -180 mg of S O 2 . Each sample was analysed for 10 minutes live-time at a chopper frequency of 170 Hz using a quartz container as described in (4). The reactor flux was sufficiently constant so that no separate flux monitor was required. The spectra were printed and the photopeak areas determined manually. The photopeaks used-334 and 440 keV for Sm, and 79 and 180 keV for gadolinium-were those proven best in an earlier study (4). In each case both photopeak areas were combined before further data treatment. No blank sample was used and background was determined by constructing a base line for each photopeak. This technique is readily adaptable to computer handling and would certainly be the preferred method if large numbers of samples were to be analyzed. RESULTS AND DISCUSSION
The data for samarium and gadolinium in each synthetic ore were subjected to a weighted least squares analysis and the error for each sample as well as the relative standard deviation of all samples for each ore were determined from the least squares best fit to the data. The analysis of elements other than samarium and gadolinium in these ores was not possible because the combined quantity of these two elements reduced substantially the analytical sensitivity for the other rare earths. Individually,
cerium could be determined easily in monazite and bastnasite at its normal concentration by chemically separating it from the other rare earths prior to irradiation. The concentrations of neodymium in bastnasite and monazite and of dysprosium and erbium in xenotime and gadolinite are such that these elements could also be determined in larger samples, once separated from samarium and gadolinium. The contribution of the other rare earths to the photopeaks of samarium and gadolinium in the spectra of the samples studied was found to be substantially less than 1% of the total photopeak and therefore no corrections for this effect were made. The data for the analysis of samarium and gadolinium in the four synthetic rare earth ores are presented in Table 11, and Figures 1 and 2 show typical calibration curves. The failure of some calibration curves to pass through the origin is caused by reproducible but imperfect background subtraction. The per cent error, E , is relative to the actual weight of the element in each sample. The limits given for the actual weight of the elements are calculated on the basis of a weighing error of 10.1 mg on the Mettler balance used. The standard deviation for the determination of 75 to 893 pg of Gd in 28 samples is 5.9y0 and that for 108 pg to 2.2 mg of Sm in 29 samples is 5.8%.
RECEIVED for review January 9, 1969. Accepted April 23, 1969. Research supported by the National Science Foundation.
1,2,3-Tris-(2-diethylaminoethoxy)benzene Hydrochloride as Reagent for the Determination of Nitrite and Bromate Ions Ivan OdIer’ Bratislava, Czechoslovakia 1,2,3-TRIS-(2-DIETHYLAMINOETHOXY)-BENZENE, C24Ha503N3n, is a triamine prepared first by Protiva et al. (1, 2). Common with it is the quaternary base C ~ O H S ~ O ~ N ~the ( O iodide H ) ~ , of which [1,2,3-tris-(2-triethylammoniumethoxy)-benzenetriiodide] is a medicine known under the names of Gallamine, Triethiodide, Flexedil, Retensin, Relaxan, Remyolan, etc. Lestrange prepared it for the first time (3). The possibility of using 1,2,3-tris-(2-diethylaminoethoxy)benzene hydrochloride as a reagent for the identification of certain anions has been examined. For example, nitrite ions give a brown color and bromate ions give a yellow color.
EXPERIMENTAL
Reagents. A 0.1M solution of 1,2,3-tris-(2-diethylaminoethoxy) benzene hydrochloride is prepared by neutralization of the necessary quantity of the corresponding base with hydrochloric acid and dilute with water to the final volume. Analogous analytical reactions are also obtained using 1,2,3-tris-(2-triethylammoniumethoxy)-benzene triiodide or 1
Present address, Clarkson College of Technology, Potsdam,
N.Y. 13676. (1) J. Kolinsky and M. Protiva, cas. &sk. Lbkam., 6 0 , 2 5 (1947). (2) M. Protiva et al., Collections, 13, 326 (1948). (3) D. Lestrange, A m . Pharm. Fr. 6,450(1948). 1116
e
ANALYTICAL CHEMISTRY
VAVELENGTH t n f l )
Figure 1. Characteristic absorption spectrum for product of nitrite-reagent reaction (lO-‘M NO23 the corresponding trichloride. In preparing the reagent, the preparation “Remyolan” from the firm SPOFA-Leciva, Prague which is a 2% aqueous solution of 1,2,3-tris-(2-triethylammoniumethoxy) benzene triiodide is used. The corresponding trichloride was prepared by reacting with an equal amount of mercury(I1) chloride and by separation of the precipitated mercury(I1) iodide.
