Simultaneous Determination of Uranium and Thorium by Instrumental Neutron Activation and High Resolution X-Ray Spectrometry Mariana Mantel and Saadia Amiel Nuclear Chemistry Department, Soreq Nuclear Research Centre, Yavne, Israel
Reactor neutron activation followed by high resolution X-ray spectrometry has been applied to the nondestructive and simultaneous determination of uranium and thorium in geological materials. The L X-rays of Np and Pa obtained from neutron activated U and Th, respectively, were measured with a 100 mm2 Si(Li) spectrometer. A pg for U and l o - ’ pg for Th limit of detection of was obtained by the irradiation of samples ( - 1 0 to - 1 0 0 mg) for 20 minutes at a neutron flux of 1013n c m - 2 sec-’ and employing a 4% counting geometry. The accuracy of the assay is within a few per cent. The different sources of error, as well as the possibilities of minimizing them, were studied.
Uranium and thorium are widely distributed in nature and are present in most rocks and ores which constitute the Earth’s crust. Since they generally occur together, and in fact are sought for together, their accurate simultaneous determination is of great importance and has been the subject of intense studies. On the other hand, their concentrations in geological materials are usually so low that a quantitative determination requires trace level techniques. Several methods using chemical separations have been proposed (1, 2) but they all have the disadvantage of being time consuming and inaccurate since they require elaborate chemical processing prior to measurement. The method of thermal neutron activation followed by delayed neutron counting (3) or by gamma spectrometry ( 4 , 5 ) has been found to be most suitable for this purpose. Since uranium-235 undergoes fission with fast as well as thermal neutrons while uranium-238 and thorium fission only with fast neutrons, the removal of thermal neutrons from the irradiation flux (by screens of cadmium or similar material) is required in order to determine uranium and thorium in each other’s presence. This procedure introduces errors in the assay of thorium due to its small cross-section for fission with reactor neutrons as compared to that of uranium. In y-ray spectrometry, the large excess and variety of matrix elements present in geological materials may interfere with the determination of uranium and thorium since the resolution of their characteristic peaks from the background spectrum becomes difficult. Recently Meyer (6) succeeded in depressing somewhat the interfering matrix activities by irradiating the samples with resonance neutrons; nevertheless the y measurements required a delay of at least two days after the end of irradiation. In a previous study (7) the advantages of activation Morgan and J. F. Lovering, Anal. Chim. Acta, 28, 405 (1963); Talanta 15, 1079 (1968). M . Picer and P. Strohal, Anal. Chim. Acta. 40, 131 (1968) S. Arniel, Anal. Chem., 34, 1683 (1962). A . 0. Brunfelt and E. Steinnes, Anal. Chim. Acta, 48, 13 (1969). I. Kuleff and D. Todorowsky, Fresenius’ Z. Anal. Chem., 257, 23
(1) J . W. (2) (3) (4) (5)
(1971). (6) H. G. Meyer, J. Radioanal. Chem. 7,67 (1971). (7) M . Mantel and S. Amiel, Anal. Chem., 44, 548 (1972)
analysis followed by X-ray spectrometry with a high resolution solid state Si(Li) spectrometer as detector has been described. X-Rays are emitted during the decay of the thermal neutron activation products of both uranium and thorium. The nuclear reactions which take place are the following: 238U(n,y )
- z+ - - 239Np
239TJ
23.5 m i n
232Th(n, y)
233Th
a-
233pa
22.1 m i n
B2.35 d
239Pu
3-
233U
27.4 d
The p- decay of 239U and 233Th is accompanied by gamma rays whose internal conversion gives rise to X-rays of the daughter elements Np and Pa, respectively, and half-lives corresponding to the decay of the parent isotope. By measuring these characteristic X-rays and applying the principles of X-ray spectrometry, the difficulty arising from the presence of different matrix elements is overcome. As a result, uranium and thorium may be easily detected and simultaneously and nondestructively determined in various matrices.
EXPERIMENTAL Irradiation. The irradiations were carried out in the pneumatic tube of the IRR-1reactor. Counting. A 100 mmz area and 4 mm depth Si(Li) detector (manufactured by Seforad, Israel) was used. The output signals from the detector were passed through an Ortec 118A preamplifier and an Ortec 410 amplifier, Ortec 408 biased amplifier, and Ortec 411 pulse stretcher. The resulting pulses were analyzed by a T.M.C. 400 channel analyzer. The resolution of the system for the 6.4-keV Fe K X-rays (obtained from W o ) and 31.7-keV Ba K X-rays (obtained from l37Cs) was 450 and 550 eV (FWHM), respectively. Standards. Solutions containing about 1 mg/ml of uranium or thorium were prepared by dissolving appropriate quantities of UaOs in dilute nitric acid or Th(N03)d in water, and standardized gravimetrically. Dilute solutions (a few bg/ml) were prepared by diluting these stock solutions with triple distilled water. T o obtain uranium, thorium, and mixed uranium-thorium standards, aliquots of the dilute solutions were introduced into small polyethylene cups (10-mm i.d.1, weighed, evaporated to dryness under an infrared lamp, and sealed. For the analysis of ores, standard thorium-uranium ores obtained from New Brunswick Laboratories, USAEC, were used. Standards with specific absorption coefficients were prepared by vacuum drying mixtures of calcium and magnesium carbonate (Table VI) in a solution of thorium and uranium of the desired concentration (8). Procedure. The smallest possible quantity of the sample to be analyzed (according to the expected concentration of U and T h ) is weighed into a small polyethylene cup (10-mm i.d.). To prevent dispersion of the powder on the walls of the vessel during irradiation, a few drops of hot paraffin are added as described before (7). The polyethylene containers are sealed and irradiated up to 20 minutes together with a suitable quantity of an appropriate standard prepared in the same way. After a delay of 15-20 minutes, the samples are counted, care being taken to count sample and standard at the same time after irradiation and in carefully defined geometry. The intensities of the Np and Pa X-rays are cal(8) M . Mantel, P. Sung-Tung, and S. Arniel, Ana/. Chem.. 42, 267 (1970).
A N A L Y T I C A L CHEMISTRY, VOL. 45,
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2393
t
a
Pa La, Po La, 16.7 keV
-1
Energy keV Figure 1. Energies and relative abundance of Np and Pa X-rays obtained from U and Th, respectively a, 35 kg Th: b. 19 kg U: c, 19.9 kg U f 30.5 I, Th. Irradiated for 1 rnin. counted for 4 min through a 1.5-rnm plastic absorber after a cooling time of 20 min
culated by subtracting the background from the integrated number of counts under each of the photopeaks concerned. The lower and upper edges of the peak were found using the procedure of Guzzi et al (9). The concentrations of uranium and thorium were calculated by comparing the intensity of the X-rays obtained for the sample t o those obtained for the standard.
RESULTS AND DISCUSSION Nuclear Reactions and Choice of Conditions. As mentioned above, both uranium and thorium produce X-rays during their decay following radiative capture. Figure 1 shows the X-ray energies and relative abundances. From 239U only Np L X-rays are obtained, whereas from 233 Th, L and K X-rays of P a are formed. Because of the low sensitivity obtained for P a K X-rays as compared to L X-rays (Pa L ( a +~ ,,,/Pa K ( a +~ 0 2 ) = 90, determined experimentally), L X-rays were used throughout this work for the determination of both uranium and thorium. This low sensitivity is a result of the preferential emission of L X-rays as well as the decrease by a factor of about 50 in the efficiency of the Si(Li) detector a t the energies of P a K X-rays (92.2 and 95.8 keV). The latter may be used for the determination of thorium [with a Ge(Li) detector] in special cases where the high beta activity obtained from the sample after irradiation or the presence of interfering elements covers the Pa L X-rays and the 29 keV 233Th y-ray. Figures l a and l b show the L X-ray spectra obtained (9) G Guzzi, Y Pauli, F EUR-346ae, 1967
2394
Girardi. and B Dorpema, Euratom Report
from pure uranium and thorium solutions. As was to be expected from the resolution of the Si(Li) detector in this energy range (FWHM -450 eV) LBI, Lgz and L,1 X-rays of Np and P a were well resolved, whereas L,1 and Lo2 Xrays could not be resolved and form a single peak. The uranium and thorium standards were analyzed repeatedly a t different time intervals after irradiation. The results obtained for thorium remain constant for all the X-ray peaks within the limits of error whereas the results obtained for uranium increase with increasing time intervals after irradiation. This increase is not equal for all the Np X-ray peaks considered. A possible explanation could be the appearance of fission products which emit X- or y rays with similar energies to those of Np. To overcome this difficulty samples and standards should both be counted a t the same time after irradiation. To check the possibility of determining uranium and thorium in the presence of each other, several samples containing different ratios of U/Th were prepared by mixing appropriate quantities of pure uranium and thorium standards. The samples were irradiated and counted as described under “Procedure.” Figure IC shows the spectrum obtained for a U/Th ratio of 1:l. As may be seen, uranium can be easily estimated since the Np Lo1 X-ray (17.8 keV) is relatively strong and well resolved from the P a X-rays. The estimation of thorium is more difficult: the P a L D (16.7 ~ keV) and N p Lo2 (16.8 keV) X-rays cannot be resolved and form a single
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
Table I. Results Obtained by Analyzing Samples Containing Different Th/U Ratios Found Taken Th/U ratio
811 211 1.311 111 1/1 113.5 '15
u, kg
U, kg
Th , kg
6.5 9.8 6.7 26.0 11.0 19.0 26.0
49.6 20.5 8.5 23.5 11.4 5.2 5.1
Error, %
U
Th, k g
X-Rays
?-Rays
X-Rays
?-Rays
X-Rays
6.6 9.6 6.5 26.9 10.6 19.5 26.7
6.8 10.2 6.8 25.5 10.5 18.6 26.9
48.7 21.2 8.9 24.4 12.0 4.8 4.5
50.7 19.9 8.8 24.0 11.7 5.5 4.8
+1.5 -2.0 -2.9 f3.4 -3.6 +2.6 $2.7
Th ?-Rays
X-Rays
- 1 .8a +3.5a +4.7a +3.8b +5.2b -7.2b -11.7b
+4.5 +4.1 +1.5 -1.9 -4.5 -2.1 +3.4
Results calculated according to Pa La1 (16.7 keV) after substraction
a Results calculated according to Pa Laz (16.0 keV).
~?-Rays
Of
+2.2 - 2.9 +3.5 +2.1 4-2.5 +5.5 - 5.9
the contribution of Np L a p .
-
Table II. Results Obtained for Uranium and Thorium in Standard Oresa Found, kg Specified,
Yo U
Th
0.04 0.002 0.004 0.05
1 .o 0.05 1 .o Not
Weight of sample, mg
Taken, Pg U
Th
10.04 95.25 40.00 19.19
4.0 1.9 1.6 9.6
100 47.5 400 ..
Error, %
U
.
Th
U
_____ Th
- - ~ -
X-Rays
?-Rays
X-Rays
y-Rays
X-Rays
y-Rays
X-Rays
3.70 1.78 1.32 10.0
4.15 1.80 1.78 9.85
97.2 49.6 385
104 45.6 390
-7.5 -6.3 -17.5 4-4.2
+3.75 -6.2 +11.0 +2.6
-2.8 4-4.4 -3.75 ...
. .
.
...
?-Rays
+4.0 -4.0 -2.5 ...
specified Standard ores: uranium and thorium in about 99% dunite, obtained from New Brunswick Laboratories USAEC
peak; the P a L ( u l + a21 (13.3 keV) and Lo2 (16.0 keV) peaks are relatively weak and only partially resolved from Np L ( m l+ a 2 r (13.9 keV) and Np Lgl + Pa L p (16.8 keV). The possibility of using them for the quantitative evaluation of thorium will obviously depend on the ratio Th/U in the sample. For ratios in favor of thorium, the above mentioned peaks will be well resolved and easily integrated; as the ratio T h j U decreases, the peaks will be only partially resolved and integration will become difficult; finally as the ratio changes in favor of uranium, the resolution will become very poor and integration impossible. In this last case, the contribution of Np may be evaluated by calculating the ratio of N p L,bl/Lp X-rays from the spectrum obtained for pure U (Figure l b ) . The concentration of thorium in the samples may then be calculated by deducting this value from the integrated peak a t 16.7 keV. The concentrations of uranium and thorium were calculated using pure standard solutions of the two elements analyzed in the same way as the mixtures to be determined. Table I summarizes the results obtained. As was to be expected, the errors obtained for thorium increase with decreasing T h / U ratios and whenever the indirect method of calculation is employed, are greater than those obtained for uranium. Yevertheless, they do not exceed * l l % , a value generally considered satisfactory for nondestructive activation analysis ( I O ) . Spectrometers with resolutions better than 450 eV will alleviate the cross interference and allow a more accurate assay of thorium. T o check the results obtained based on the integration of X-ray peaks, the two low energy gamma ray peaks of 239U (44 keV) and 233Th (29 keV) may be used (Figure 1). The concentrations of uranium and thorium in the analyzed samples calculated according to the integration of these y-ray peaks are shown in Table I in comparison with those obtained according to integrated X-ray peaks. For uranium, little difference is found between the results, but for thorium, the accuracy is much better if the results are calculated according to y-rays, whenever the Th/U ratio decreases. It follows that for high U/Th ratios, it is (10)
' Activation Analysis, Principles and Applications ' J M A Lenihan and S J Thomson, Ed , Academic Press, London, 1965. p 89
more convenient to use the value of the integrated 29-keV gamma ray peak for the calculation of T h concentrations. As seen from Figure 1, the L,1 X-ray peaks of P a (19.5 keV) and Np (20.8 keV) are well resolved in all cases. Nevertheless, because of their low intensities, they may be used in calculations only if uranium and thorium are present in large quantities in the sample to be analyzed. Based on the satisfactory results obtained for the uranium-thorium mixtures, different standard ores obtained from New Brunswick Laboratories, USAEC, have been analyzed. Table I1 summarizes the results obtained and Figure 2 shows the spectrum obtained from a standard ore containing 1% T h and 0.04% U. In most of the standard ores analyzed, the thorium concentration is much in excess over that of uranium. It follows that the accuracy of the T h determination should be high, which in fact is the case. Furthermore, it is interesting to note that uranium could be determined even a t T h / U ratios of 25011. In this last case, the Lo1 X-ray of U is poorly resolved and its evaluation becomes inaccurate (Table 11). In contrast the 44-keV y-ray of 239U is well resolved and the results calculated according to it, are more accurate. Since thorium is generally found in nature in excess over uranium, it will be possible by the present method to determine both in the presence of each other in naturally occurring materials. As an example of the practical application of the method, different ores were analyzed for uranium and thorium and the results compared to those obtained previously by activation analysis followed by delayed neutron counting (3) for uranium and thorium and by gamma spectrometry of 233Pa (8) for T h and of 239Np (11) for U. The results, summarized in Table 111, show good agreement among the three methods. Figure 3 shows the spectrum obtained from a monazite sample. This example has been chosen since it illustrates the easy determination of thorium in the presence of rare earths; these elements generally accompany thorium in nature and interfere with its quantitative determination by most methods including y-spectrometry. (11) M Mantel and S. Amiel, IA-1218. Annual Rept., 1970, p 104
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2395
-
PO LB;
2.0
I 80
1.5
7.0 llli
Pa-L,.,
T h- rr o y 29.0
ls.5
I
as*
U-7-roy 44.0
IO
20
30
40
Energy k e V Figure 2. X-ray spectrum of a standard ore ( N B L - 7 9 ) containing 1% Th and 0.04% U 10-mg sample, irradiated for 20 min, counted for 4 min through a 1.5-mm plastic absorber at 5 c m from the crystal after a cooling time of 20 min
6
-29.0
81s f h y -ray
n
P x
m
c
LUI(. I 41.5
4
c 0
u
2
0
lo
20
30
40
50
Enrrgy k a V Figure 3. X-ray spectrum of a monazite sample containing 5.2% Th and 0 . 3 % U 5-mg sample, irradiated for 1 min, counted for 10 min through a 1.5-mm plastic absorber at 2 . 5 cm from the crystal after a cooling time of 15 min
2396
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
Table 1 1 1 . Results Obtained for Uranium and Thorium in Different Rocks and Ores (Comparison of Methods)
Sample
Weight U found, of ppm' sample, mg X-Rays ?-Rays
Th found, ppm"
Delayed neutron counting
X-Rays
?-Rays
U. ppm
205 f 12
220 f 8.5
y-Spectrometry (8, 7 7 )
--
T h ppm
U, ppm
Th. ppm
igneous rock
15.2
1310 f 35
1 2 3 0 f 37
1260
1290 f 27
f 45
231
f 9.2
201
f 8.5
Igneous rock
22
170
f 10
1 6 4 f 7.5
195 f 1 2
Yo
184 f 8.5
160 f 7
155 f 5.1
%
Yo
%
--
Monazite 1
8.5
0.28 f 0.011
0.30 f 0 . 0 1 2
5.2 f 0.15
5.4 f 0.18
5.5 f 0.11
0.29 f 0 01
Monazite 2
Xenotime a
10.2
0.15
14.5
0.38
f 0.007 f 0.02
f 0.008 4.8 f 0.17 4.5 f. 0.15 0.155 f 0.004 f 0.017 Not detected 0.4 f 0 . 0 2 4 0.89 f 0.08
4.6 f 0.12
0.142 0.35
0.96 f 0.016
The results are the mean of 5 determinations.
-
Interferences. Discrete X-rays As mentioned before (7) one of the basic properties of X-rays is the direct relationship between their respective energies and the atomic number of the parent element. I t follows that interferences will result from L X-rays of neighboring elements in the periodic table, or K X-rays in the same energy range, produced by elements with low atomic number. Elements which could produce interfering neighboring L X-rays, like plutonium and americium, do not occur in nature, whereas the possible interference of others, like actinium and radium is negligible, due to their low abundance. Elements which could produce interfering K X-rays are Sr (KBI = 15.83, Kp2 = 16.08), Y (Kpl = 16.7, Kgz = 17.01), Zr (KBI = 17.66, Kpz = 17.96, K,I = 15.77), Nb (K,I = 16.61, K,z = 16.50) and Mo (K,I = 17.47, Ka2 = 17.37 keV). None of these elements are major constituents of minerals. Furthermore, Zr does not produce after neutron activation isotopes which decay by X-ray emission, whereas from the other four elements only Nb, as shown in our previous study ( 7 ) , has a sensitivity which could cause it to interfere seriously. Differences in half-life (6.3 m for 94mNb us. 23.5 m and 22.4 m for 239Np and 233Pa, respectively) could be used to overcome this difficulty. As an example, in the X-ray spectrum obtained from a xenotime sample which contains about 30% Y and 2% Sn, it was very difficult to detect thorium (0.95%) (8); the L X-rays of P a were completely covered by the K X-rays of Y and the 29-keV y-ray of 233Th was covered by the Kp X-rays of Sn (29.1 and 28.4 keV). The great difference in half-lives between 233Pa and 9omY (22.1 min us. 3.1 h) may be used to solve this problem. After the decay of 233Pa, the amount of yttrium in the sample may be calculated according to its K X-rays, obtained by radiative capture (12). The contribution of yttrium is then deduced from the 16.7-keV P a La1 peak. Uranium (0.4%) could be detected and evaluated (see Table 111) since both the 17.8 keV K B L~ X-ray of Np and the 44 keV y-ray of 239U were well resolved. Bremsstrahlung. Another possible source of interference is the bremsstrahlung of the beta particles emitted from the radioactive sample. This may interfere either by upsetting the resolution of the detector by increasing its dead time, or by raising the background substantially and making the integration of the X-ray peaks inaccurate. This problem becomes of importance only for high energy beta activities over 10 mCi; it may be overcome by using (12) M. Mantel and S. Amiel. lnternationai Conference on Modern Trends in Activation Analysis, Saclay, Oct. 1972.
Table I V . Mass Absorption Coefficients Calculated for Different Types of Rocks ( 7 4 ) Mass absorption coefficient, cmz/g Type of roc ka
Sandstone Limestone Sediment Igneous rockb
Shale
16.0 keV
17.8 keV
29 keV
44 keV
5.84 8.85 6.89 7.72 7.27
4.22 6.60
1.09 1.61 1.34 1.47 1.42
0.45
5.20 5.79 5.48
0.65 0.52
0.55 0.53
O1 Average values according to F. J. Pettijohn, "Sedimentary Rocks" ( 7 4 ) . Averaqe basalt.
low detection geometries or plastic absorbers of thicknesses in accordance with the strength and quantity of the beta particles emitted. Obviously, these procedures introduce losses in sensitivity. In the two examples shown, the determination of uranium and thorium in a standard ore (Figure 2) and in monazite (Figure 3), the above mentioned is illustrated. The rise in background may be observed and the sensitivity is reduced; nevertheless the peak-to-background ratio remains high enough to allow an accurate integration of the peaks. Errors. The typical errors may be divided into two groups: the absorption of the emitted X-rays by the elements present in the sample matrix, and the emission of secondary, fluorescent X-rays induced by the radiations in the sample. Absorption hy the Matrix. The mass-absorption coefficient of an element for a given matrix (expressed as cm2/g) will depend on the energy of the X-rays to be measured. The mass absorption coefficient of the constituent elements of a compound are additive. Since tables of mass absorption coefficients for the elements are available (13), those for compounds can be easily obtained. By using data present in the literature (14) on the composition of rocks, the mass absorption coefficient of any rock in the range of uranium- and thorium-produced X-ray energies, can be calculated. Table IV shows the mass absorption coefficients calculated for different types of rocks. The small variations in
(13) E. Strom and H. I . Israel, Nuclear Data Tables, U. S. At. Energy Cornrn., A7, 565 (1970). (14) "Handbook of Chemistry and Physics," The Chemical Rubber Company, Cleveland, Ohio, 1970, 51st ed., p F-144.
A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
2397
Table V. Preparation of Standards with Absorption Coefficients Similar to Those of Different Rocks
Table VI. Interference of Elements with Absorption Edges in the Vicinity of the L X-Rays Energies of Np and Pa
Absorption coefficient Type of rock
CaC03, Oh
MgC03, %
16.0 keV
17.8 keV
100 60 20 43 34 39
10.35 2.37 5.62 8.75 6.92 7.68 7.31
7.82 1.78 4.19 6.61 5.24 5.75 5.45
100
Sandstone Limestone Sediment Igneous rock Shale
40 80 57 66 61
Mass absorption coefficient, cm2/g (73) Element
Y
Sr Rb Br
Se As
Ge Ga the relative abundance of the major constituents, will influence only slightly the overall absorption coefficient for every type of rock. Taking into account the values from Table IV (p = cm2/g) and the surface of 1 cm2 of the polyethylene containers on which the samples are weighed (see “Procedure”), it follows that for 10-30 mg (0.01-0.03 g/cm2) samples there will be no appreciable absorption. If, due to low concentrations of uranium and thorium, it is necessary to weigh a larger quantity, it is possible to overcome the error due to absorption by preparing a standard with a similar absorption coefficient to that of the sample to be analyzed. Such a standard may be obtained by vacuum drying a mixture of CaC03 and MgC03 in the necessary proportion, in a solution of thorium and uranium (8). Table V shows the relative quantities of CaC03 and MgC03 necessary to obtain absorption coefficients similar to those shown in Table IV for different kinds of rocks. Besides the absorption by the matrix as a whole, the absorption of elements having K or L absorption edges in the vicinity of the energies of the L X-rays of uranium and thorium has to be considered. At the absorption edge, the increase in absorption with energy is sharp and the decrease is mild. Thus the highest absorption will be obtained from those elements whose absorption edges will be a t energies just below those used in the determination of T h and U (16.0 and 17.7 keV, respectively). In Table VI, the elements in question are listed, as well as the maximum quantity of each permissible in a 10-mg sample. As may be seen, the quantities obtained do not exceed, in general, the abundances of these elements in nature. The actual quantity of these elements which may be present will be a function of the overall absorption coefficient of the matrix. As an example, in a matrix of limestone with a mass absorption coefficient of 8.85 and 6.60 for thorium and uranium, respectively (Table V), which will result in an absorption of 6-870 in a sample containing 10 mg/cm2, an increase of 1%will be significant. Contrarily in a matrix with a low absorption coefficient (for instance organic materials), such an increase in absorption will be negligible. If a rock containing a high percentage of one or more of the elements listed in Table VI has to be analyzed for uranium and thorium, a special standard must be prepared, as described previously. The possible influence of uranium and thorium themselves on the Np and P a X-rays has to be considered since the L I I Iabsorption edges of both are a t energies similar to those of Np Lo1 and P a L p X-rays (Table V ) . The high sensitivity of the method will allow us to overcome this error using small samples whenever the concentration of U or T h exceeds 1%. 2398
Zn
K absorption edge, keV 17.03 16.10 15.20 13.47 12.65 11.86 11.10 10.36 9.65 LI111 absorption
U Th
17.16 16.30
16.0 keV (Th) 19.30 112.71 97.93 94.96 88.47 82.40 78.02 72.49 69.92
Maximum permissible quantity,a in the determination
of
17.8 keV
(U) 90.77 82.47 76.10 70.84 64.82 62.40 61.40 56.90 52.50
Th
U’
5.0
1 1.2 1.3 1.4 1.5 1.6 1.6 1.7 1.9
1 .o 1 .o 1 .o 1.1 1.2 1.3 1.4 1.4
edge 105.7 102.0
114.9
1 .o 1 .o
1 .o
The quantity of the element which will increase the absorption of the matrix by about 1% (absolute).
In order to assess whether or not there is significant absorption of X-rays in a sample, the ratio X-raysly-rays must be calculated for uranium and thorium (17.7/44 keV and 16.0/29.0 keV, respectively) and compared to that obtained from pure standards (12.5 0.1 and 0.45 f 0.014, respectively). As shown in Table V, the absorption of different matrices a t the y-ray energies is much smaller than that a t the energies of X-rays and becomes negligible. Thus, if absorption occurs, the ratio will be in favor of y rays and the concentrations of uranium and thorium will have to be calculated from the respective y-ray peaks. In this case, the possible interference of I, Sn, and S b X-rays (obtained from Te, Sn, and S b by neutron activation) with the 29-keV peak, and that of T b K, and Pm K, X-rays (obtained from Gd and Nd, respectively) with the 44-keV peak, has to be taken into account. Emission of Fluorescent Secondary X-Rays. The intensities of the X-rays obtained by neutron activation according to the present method are much too low to produce fluorescent X-rays in sufficient yield so as to interfere with the determination of uranium and thorium (15). Precision and Accuracy. As shown in Table 111, the precision of the method, obtained from five determinations, is about &4%. These errors are the result of counting statistics and peak integration based on the prevailing resolution of the detector. Another important factor is the small size of the detector. The latter has great influence on the precision of the results due to irreproducibility in geometry. The precision is dependent on the Th:U ratio in the sample; it is better for Th:U ratios of about 1 and decreases if one element is in great excess over the other. The precision obtained for results calculated according to the two low energy y-rays does not vary with the ratio of the two elements. The accuracy of the method as shown in Tables I and I1 is in the range of 2-470. As discussed before, the accuracy will increase with increasing T h / U ratios and will also depend on the quantity of interfering elements present in the sample. If these elements do not exceed the values shown in Table VI, the accuracy will be a result of all the
*
(15) J. R . Rhodes, Analyst (London), 91, 683 (1966)
ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
errors of the method-counting statistics, geometry, the influence of bremsstrahlung, and the absorption of the matrix.
racy for T h / U ratios up to 25/1 in liquid or solid samples in a wide variety of matrices.
CONCLUSION
The authors wish to thank Raya Nuthman for technical help throughout the experiments.
Uranium and thorium can be determined simultaneously and with great accuracy by activation analysis followed by X-ray spectrometry. Because of the short half-lives of the radioisotopes used in the determination, the method is rapid; one determination may be carried out in about an hour. The proposed technique is suitable with high accu-
ACKNOWLEDGMENT
Received for review March 5, 1973. Accepted July 6, 1973. This work is part of an investigation performed by Mrs. M. Mantel in partial fulfillment of the requirements for a Ph.D. degree, the Hebrew University of Jerusalem.
NOTES
1
Quantitative Determination of Bismuth Trioxide in Teflon Catheters Using Decomposition by Molten Salt Fusion Followed by Standard Addition and Conventional Direct Current Polarography Stephen Sherken and Elaine J. Friedman Department of Health, Education and Welfare, Food and Drug Administration, 850 Third Avenue, Brooklyn, N. Y. 11 232
Bismuth trioxide is incorporated into Teflon-100 in the manufacture of radiopaque catheters for medical use. A method has been developed for the assaying of the bismuth content of these medical devices. The ability to analyze for a metal in an organic matrix requires a prior decomposition. Teflon is a very refractory material and does not yield to wet decomposition ( I ) . (Teflon is DuPont's brand of polyfluoroethylenepropylene.) T. Y. Komentani (2) has reported a method for the determination of trace amounts of alkali ions incorporated into Teflon, wherein the sample is decomposed using a dry-ashing procedure. In an attempt to determine the amount of bismuth trioxide in the Teflon, a dry ashing procedure was attempted, but proved inadequate. Even though bismuth trioxide has a melting point of 810 "C, it was found that during the decomposition of the Teflon, the bismuth was volatilized in spite of the fact that the ashing temperature never exceeded 550 "C. This loss could be rationalized by postulating reaction of the bismuth with one or more decomposition products produced during the ashing of Teflon. A study by R. E. Kupel et al. ( 3 ) describes the decomposition of Teflon in the temperature range of 490 to 565 "C. The decomposition products were analyzed and identified by mass spectrometry. Teflon-100 decomposes into five major and four minor fragments. The use of fusion mixtures for decompositions is a wellestablished practice. A paper by H. J. M. Bowen ( 4 ) out(1) T. T. Gorsuch, Analyst (London), 84, 135-173 (1959) (2) T. Y. Kornentani, Anal. Chem., 38, 1596-8 (1966). (3) R. E. Kupel, M . Noland, R. G . Keenan, M . Hite, and L. D. Scheel, Anal. Chem., 36,386-9 (1964). (4) H . J . M . Bowen, Anal. Chem., 40, 969-70 (1968).
lines the use of an equimolar mixture of sodium and potassium nitrate for the destruction of organic material (including Teflon) containing various added elements. The samples were smoothly decomposed and in the majority of cases the elements contained in the organic matrices were retained. Although bismuth compounds had not been subjected to this procedure ( 4 ) , it was thought that this fusion mixture might be efficacious in preventing the losses of bismuth trioxide during the decomposition of the Teflon. The fusion mixture was used on the bismuth impregnated Teflon. The decomposition process, which occurs quickly, can be carried out in an open 50-ml porcelain crucible. After the decomposition was completed, the resulting melt was dissolved in diluted hydrochloric acid, and the bismuth assayed by conventional direct current polarography. I t was found that the polarographic wave in 0.1N hydrochloric acid was unstable, so a change was made using a supporting electrolyte consisting of 0.1M ethylenediaminetetraacetic acid (EDTA) in 0.4M acetate solution buffered to a p H of 4.6 ( 5 ) . With these changes, the polarographic wave was both reproducible and accurate.
EXPERIMENTAL Reagents and Solvents. Reagent grade bismuth trioxide was analyzed against bismuth metal of 99.999% purity. Sodium nitrate, potassium nitrate, disodium EDTA, anhydrous sodium acetate, glacial acetic acid, and hydrochloric acid were reagent grade, and used without further purification. Buffers and Electrolyte Solutions. Solution A A sample of (5) M . Brezina and P. Zuman. "Polarography in Medicine, Biochemistry, and Pharmacy," Interscience, New York, N . Y . , 1958, p 737.
A N A L Y T I C A L CHEMISTRY, VOL. 45, N O . 14, DECEMBER 1973
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