of oxygen, fluorine, sodium, scandium, germanium, selenium, bromine, yttrium, rubidium, rhodium, silver, indium, erbium, ytterbium, hafnium, tungstrn, iridium, and gold (Figures 1 to 6). The examples given in Table I mere chosen as typical to demonstrate the method and indicate the sensitivities obtainable for seven elements. To facilitate comparisons, the counting data from the thermal neutron activations were normalized to a flux of approximately 1 x IO8njsq. cm./sec. The data are expressed as counts per minute, obtained by dividing the deadtime corrected cumulated 30-second spectra from eight irradiation-counting cycles by 4. This is a somewhat arbitrary procedure, since the count rates vary markedly during the time of each of the counts. For the illustrations the data of the standards were further normalized to 1-gram sample size. For the determinations of fluorine and oxygen the fast neutron reactions
F19 (nJo)XI6and 0 I 6 (n,p)N16 were used. I n the fluorine case the same flux was used as for the other elements, but a Cd foil was wrapped around the rabbit tube in the irradiation position to exclude the thermal neutrons inside the paraffin cube. The 14-m.e.v. neutrons for the oxygen determinations were obtained by irradiating a tritiated titanium target with 60 pa. of 400-k.e.v. deuterons and deuterium ions. I n both cases the 6.1-m.e.v. photopeaks and pair peaks of the N16 were used for the analysis. Because of the high Q value of the oxygen reaction, no interference has been observed in the fluorine determinations. The oxygen samples used were free of fluorine. It is important that the standards are of approximately the same weight and geometry as the unknown, whenever materials of high cross section are involved. This is seen from the cases of silver and tungsten. The self-shielding effect otherwise experi-
enced can cause errors of more than 100%. Most of the error encountered is due to statistics, This is seen in the case of gold, where only relatively few counts were collected for the samples employed. ACKNOWLEDGMENT
The author thanks R. Wayne Miller for his assistance with the operation of the Van de Graaff accelerator. LITERATURE CITED
(1) Anders, 0. U., ANAL.CHEAT. 52, 1368
(1960).
(2) Anders, 0. U., “Gamma-Ray f3pectra of Neutron-Activated Elements, Dow Chemical Co., May 1960. (3) Anders, 0. U., Beamer, W. H., ANAL.CHEM.33,226 (1961).
RECEIVEDfor review June 9, 1961. Accepted July 3, 1961. Division of Analytical Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1961.
X-Ray Spectrographic Determination of Thorium in Uranium O r e Concentrates W. C. STOECKER and C.
H. McBRlDE
Uranium Division, Mallinckrodt Chemical Works, Saint Charles, Mo.
b
Thorium is determined rapidly and precisely in uranium ore concentrates by an x-ray spectrographic measurement of the intensity at the Thla peak. The intensity is compared to that of the U 11 peak as an internal standard. Background interference due to the nearby U l a peak is compensated by a simple mathematical correction applied to the intensity ratio. The variable (but accurately determined) internal standard compensates for differences in composition of ore concentrates from various sources, and the effects of the common contaminants are eliminated. No elements interfere at concentration levels ordinarily encountered.
T
was determined in rock samples by Adler and Axelrod ( I ) , using thallium as an internal standard on a two-channel spectrograph. They used either the Lpl or the La peak of thorium, depending on interferences, and compared x-ray intensity with that of the thallium I&/32 peaks. Pish and Huffman (8) determined thorium and uranium in aqueous solutions and in a nonaqueous solution by measuring the La peaks of both elements. Strontium was used as the internal standard for HORITJM
aqueous solutions, and bromine (as bromobenzene) for the nonaqueous. The K a peaks of both internal standard elements occur in the desired region. Campbell and Carl (4) combined x-ray and radiographic measurements to determine small quantities of thorium and uranium in ores. The weight ratio of the two elements, together with a total radioactivity measurement, enabled a determination of the concentration of both elements. Uranium, in particular, is often determined on the xray spectrograph, usually through a measurement of La peak intensity. The authors have determined minor amounts of uranium in several materials using strontium as an internal standard and measuring the U La/Sr K a x-ray intensity ratio. Although in uranium ore concentrates, the thorium La peak intensity is a function of thorium concentration, it is also dependent on other factors such as instrumental variables and difference in sample composition. An internal standard is therefore required for the necessary precision arid accuracy. The use of uranium ( 7 ) as the internal standard eliminates the need for sample preparation except for the reduction of coarse lumps. Figure 1 shows a spectrum of a typical ore concentrate, and
indicates the need for a correction of background due to the U La peak shoulder. EXPERIMENTAL
Apparatus. The work was done on a Philips Electronics x-ray spectrograph, Type 52260, operated with the usual power supply and scaling units. A scintillation counter detector and a lithium fluoride analyzer crystal were used throughout the investigation. The sample boat used was somewhat smaller than that supplied with the instrument. It has a depth of l:g inch and a capacity of 3 ml. and was machined from a 3/*-inch sheet of Plesiglas. Mising operations were done on a Spex Industries KO. 8000 ?vlixer/hIill using the No. 6135 polystyrene vial and four Plexiglas balls. Some grinding tests were also made with the steel vial. Hand grinding in a glass mortar was used in most cases for preparing samples for packing into the sample boat. Analytical Procedure. Grind 5 to 7 grams of sample in a glass mortar until a smooth surface can be formed by pressing with a spatula. The Mixer/Mill may be used if desired. Pack the sample into the Plexiglas sample boat and form a smooth level surface with a spatula or glass slide. VOL 33, NO. 12, NOVEMBER 1961
1709
U
La
Th La
A THORIUM IURANIUM
Figure 2. Calibration lines for x-ray spectrographic determination of thorium in uranium, showing effect of differeni methods of preparing standards. 4 I
I
I
A.
Mixed 10 minutes in plastic vials Reground to ultimate intensity in steel grinding vial C. Coprecipitoted b y ammonium hydroxide addition to elements in solution
B.
Figure 1. Partial x-ray spectrum of uranium ore concentrate containing 0.72% thorium Arrows indicate angles a t which intensities a r e measured Interpolation of thorium background
___
Insert into the spectrograph and count in duplicate, using the following settings:
Peak
Th La
Background
2e( ) 27.46
No. of Counts per Determination 64,000" 16,000
29.00 128,000 30.72 Reduce to 32,000 if time required is more than 60 seconds.
u LI
Net peak intensities in counts per second are used in Equation 1. The quantities a and k in Equation 1 are evaluated, respectively, by means of a set of synthetic standards and several chart record scans of unaltered samples. RESULTS AND DISCUSSION
Calibration. A series of standards was prepared by mixing accurately weighed quantities of a finely powdered (-325 mesh) thorium oxide with portions of a sample of Canadian ore concentrate (Lot C-4). This sample, also finely powdered, contained 0.087. thorium according to chemical analysis. The powders were mixed in a plastic vial on the Mixer/Mill for 10 minutes. The results are shown in Figure 2, A , in which per cent thorium on a U308 basis is plotted against the observed Th/U count ratio. The range covered is from 0 to 1.5y0 thorium
1710
ANALYTICAL CHEMISTRY
(sample basis). The background x-ray intensity a t 29.00' 2 8 was deductedfrom each peak count. The points produce a straight line with a slope of 3.72% thorium per unit count ratio. X background correction for the T h La peak due to the U La peak shoulder was estimated from several chart record scans of unaltered samples, as described below. Figure 2, B , shows a replot (with slope of 3.51% thorium per unit count ratio) of the x-ray counts after each standard had been ground repeatedly in the steel vial of the Miser/Mill until the maximum Th/U ratio was produced. This required about 25 minutes of total grinding time. The increase in Th/U ratio was due to the increase in thorium peak count, since neither the uranium nor background count changed significantly. This effect of milling was interpreted as an increase in thorium emission due to reduction of particle size (2, 6). It seemed likely that Figure 2, B, represented a degree of thorium admixture equal to that of the original concentrate, since further grinding had no effect. This was questioned, however, when it was found that x-ray determinations had a relative bias of about 13% higher than the chemical method. A second series of standards was prepared in which the thorium was added to each standard by coprecipitation with uranium. A weighed portion of an ore concentrate was dissolved in
nitric acid and mixed with the calculated volume of standard thorium nitrate solution. Ammonia was added to p H 9, and the precipitate was filtered (not washed), dried, and ignited. Since precipitation with an alkali is used in the manufacture of the ore concentrate, this method should give a uranium-thorium misture similar to the original. Another sample (Lot H-2) containing less than 0.1% thorium by the chemical method was used for this test. The result was curve C of Figure 2, with a slope of 3.22, which is somewhat lower than that of curve B. This indicates an 8% increase in thorium emission over the standards from which curve B was prepared. The precipitated standards were ignited in order to obtain a material which could be ground easily. However, several additional standards were prepared by drying the precipitate a t 115' C. Since results were identical, moisture and other volatiles were shown to be without effect. The validity of this calibration obtained from Lot H-2 sample was tested for the material from Lot C-4 by making three evperiments on the recovery of added thorium. Thus, for 0.79, 1.18, and 1.38% thorium added by precipitation, respectively, 0.80, 1.16, and 1.35% were recovered. Additional justification for the second method of standard preparation is found in the better agreement with the chemical method (Table 11). Determination of Background Correction. It can be shown that thorium concentration may be expressed on a uranium oxide basis by Equation 1
n here
I designates net s-ray intensity
after deducting background a t 29.00’ 28. Equation 1 assumes that the portion of the intense U La peak which is measured a t the T h L a wave length bears a constant relationship to the U Lr intensity despite differences in thorium or uranium content. Chart recordings of spectra of 23 lots showed that this condition was met. The thorium background was determined from the chart recordings by extending the base of the uranium La peak (Figure l), and measuring the height above the background level a t 29.00’ 28. The average of the Z T h baokground/1t’ ratio for the 23 scans Tyas 0.0633. From the differences beh e e n the average and each individual determination the standard deviation as calculated as &0.0054. The precision a t the 95% confidence level of a single run is therefore 2.074 X (=t0.0054) = 10.0112, and the precision of the mean is O.O112/fi3 = 50.0023. From Equation 1 0 = (3.22 X 0.0633) - a a = 0.204
On the basis of the precision of the mean a t a 95% confidence level, and with k = 3.22, the value of a lies between 0.196 and 0.211%. Following is the final working equation :
% Th (UsOs basis)
=
(3.22 X 2)-
0.20 (2) Precision. Precision of the method was determined on material from four different sources. Duplicate determinations were made on samples of 12 or 15 lots from each source, and precisions shown in Table I were computed on the basis of observed agreement between pairs. Comparison with Spectrophotometric Method. Prior t o the development of the x-ray method, thorium in ore concentrates was determined by a chemical procedure which involved digestion in nitric and hydrofluoric acids to produce an insoluble residue of thorium tetrafluoride, separation and dissolution of the residue, and extraction of the thorium into an organic phase (thenoyltrifluoroacetone) and then back again into an acid solution. A color was produced with thorin reagent (3) [1 - ( 0 - arsonopheny1aso)2 - naphthol - 3,6 - disulfonic acid] and measured on a spectrophotometer. Several difficulties are inherent in the method as applied to ore concentrates, one being the measurement of the thorium absorption in the presence of a high reagent blank. The precision is
k0,04% absolute a t the 95% confidence level (sample basis). Table I1 gives a comparison of the xray and chemical determinations for 32 samples from eight different sources. On one sample (Lot D-5, not listed) the chemical method failed because of interference by a large amount of silica in the initial fluoride precipitation step. Several chemical determinations were repeated because the x-ray determination indicated possible low results. The duplicate chemical results were averaged and included in calculating the bias of 0.015% thorium (x-ray-chemical) which is significant a t the 99% confidence level. Interferences. I n the study of interference effects, i t is convenient t o consider the following two groups of contaminants : NONSPECTRAL CONTAMINANTS. Elements or compounds which occur as contaminants but do not critically influence x-ray emission a t the wave lengths used. In ore concentrates all elements of this group are of low atomic number. INTERFERIKG ELEMENTS. Elements for which more or less interference would be predicted owing to the proximity of K lines, L lines, or absorption edges (6) to the wave lengths used in the determination of thorium. Many elements of high atomic number belong to this group, but some of these have not been detected in concentrates, or are too rare to be considered. It would be expected that some elements in this group could be tolerated in fairly high concentration. Each contaminant studied was added to a sample of ore concentrate in the form of a very fine powder of an appropriate compound. When necessary, the compound was ground in the Mixer/Mill to obtain a powder finer than 325 mesh (usually much finer). It was incorporated with the sample by mixing in the plastic vial of the Mixer/ Mill accurately weighed portions of sample and contaminant. A separate test was made for each concentration This technique of level studied. making additions is simple, and if the compound is suitably chosen, produces a homogeneous mixture. Experiments have indicated that errors due to particle size effects are less than 10% (relative) under the conditions used. The reprecipitation method used in preparing standards would not be applicable for most additives studied. The result was calculated from the count ratio just as for an ordinary sample, neglecting any dilution correction. The per cent thorium so obtained is expressed on the “original sample” basis, and in the absence of interference should agree with the per cent thorium on the “unspiked” sample. I n consideration of the precision of the
Table I.
Precision Data on Thorium in Ore Concentrates
No. of Samples
%Th, Ranged
15
0.38-0.67
Precision 95% Confidence Limit, RelaThorium” tive f 0 . 0 1 5 lrt2.7 fO.021 f 9 . 5 icO.028 1 2 . 5 10.008 1 4 . 6
12* 0.14-0.38 12b 1.01-1.37 15 0.14-0.20 Sample basis. b Two operators participated.
method, a difference of 0.02% thorium is significant. To the first group of contaminants belong calcium, sodium, iron, ammonia, silica, and sulfate, any of which may be present up to several per cent. Smaller amounts of chlorine, fluorine, vanadium, copper, phosphate, carbonate, and other light elements are commonly found. Still others are present in amounts spectrographically detectable. Light elements would not be expected to interfere seriously, since all lines and absorption edges occur a t much longer
Table II.
Comparison of X-Ray and Chemical Methods
Per Cent ~.~ .~Thorium Source (Sample Basis) Differand Lot ewe No. X-ray Chemical 0.37 0.00 A-1 0.37 0.17 0.00 -2 0.17 0.01 0.19 -3 0.20 0.29 0.00 -4 0.29 0.42 0.07 -5 0.49 0.40 -0.03 -6 0.37 0.43 0.01 -7 0.44 0.01, 0.04 B-1 0.06 0.04 0.02 -2 0.05
