Both calibrations indicate a linear function between the absorbance and the water content in the region of A = 0.2 to 0.8. The two regression lines are almost identical (Figure 2). Equation l was used for humidity measurements because calibration A was carried out under more favorable conditions (diminution of error possibilities by excluding the distillation step). SPECIFICATION OF METHOD
The acceptable sample size is limited by a moisture content of between 15 and 65 mg. of water. In the range of these possibilities, a sample weight of approximately 2 grams is recommended. ilccuracy depends mainly on the dry state of the materials used. The seneitivity of the method allows a detection of 0.5 mg. of water. -4 great number of determinations used a sample size of 1 gram or less and 5 ml. of collected dioxane distillate, but the deviation of these results was relatively high (&0,05% H20). By employing 2 grams of powder and collecting 10 ml. of distillate, the rate of deviation could be decreased to h0.02% H20
COMPARISON WITH OTHER METHODS
Comparison ob Karl Fischer Table I!. and Spectrophotometric Methods
Double Detn.
Mean Value
Deviation
Karl Fisclier 1.51 1.59 1.75 1.71 1.19 1.13 1.74 1.85 1.32
1.55
0.08
1.73
0.04
1.16
0.06
1.80
0.11
1.37
0.09
IR Analysis of Azeotrope 1.59 1.61 1.77 1.82 1.18 1.18 2.05 2.05 1.81 1.93 1.80 1.76
1.60
0.02
1.80
0.05
1.18
0.00
2.05
0.00
1.86
0.12
1.78
0.04
The results obtained by four methods
of moisture determination are summarized in Table I. This comparison shows considerable deviation of the results from the different methods. The error possibilities of the drying procedures have already been mentioned. The Karl Fischer method has been used by untrained personnel, which could explain a t least partially the differences between the results of the Karl Fischer and the new method. The results from the Karl Fischer titration and the spectrophotometric method are mean values of two determinations. A comparison of the deviation of these results is given in Table II. LITERATURE CITED
(1) Goddu, F., Delker, D. A,, ANAL. GHEM.32, 140 (1960). (2) Horsley, L. H., Advances in Chemistry Ser., No. 6, 8 (1952). (3) Maye, W., Spectrochim. Acta 6, 257 (1954).
RECEIVEDfor review May 23, 1961. Accepted September 6, 1961.
Rapid Microdetermina in Rare-Earth-Rich Ma and Gamma-Ray Spe MINORU OKADA Government Chemical lndustrial Research Instifufe, Tokyo; Hon-machi, Shibuya-ku, Tokyo, Japan
b Activation analysis has been applied to the simultaneous determination of scandium and dysprosium in rare-earth-rich ores, soil, yttrium oxide, and dilute solutions containing both elements. A sample is irradiated for 30 seconds at a flux of about 4 X 1 Ol0 neutrons/sq. cm./second and the induced activity is observed with a welltype sodium iodide crystal attached to a 256-channel pulse height analyzer which is operated in two steps to see the decay. The height of photopeak due to 0.1 42-m.e.v. y radiation from scandium-46m and that due to 0.108-m.e.v. y from dysprosium-1 65m are measured. Concentrations down to a few parts per million of each element can be determined.
both scandium and dysprosium have large neutron activation cross sections (Table I), it was believed that they could easily be detected INCE
through neutron activation analysis. Qne of the radioactive nuclides formed from scandium is 19.5-second scandium46m, and one from dysprosium is 1.26minute dysprosium-165m. Therefore, a short-time irradiation can activate these two elements in a sample without appreciable activation of the other elements which may coexist. Moreover, both of the above auclides are y emitters, and are determined simultaneously if a y-scintillation spectrometer isemployed. EXPERIMENTAL
Comparative Standards. Scandium oxide was dissolved with nitric acid and diluted t o give solutions whose concentrations were 10-l to 102 p g , of scandium per ml. From each solution, a 0.25-ml. aliquot was taken and sealed with 0.1 gram of filter paper in a polyethylene capsule whose diameter was 16.5 mm. The capsule was sealed by warming the plastic with a gas Aame. The form of the capsule after
sealing is shown in Figure 1. The purpose of the filter paper is to prevent the solution from flowing out when the capsule is broken down in the nuclear reactor by an unforeseen occurrence. Dysprosium standards were prepared in the same way as scandium standards. Certain Synthetic Samples. amounts of both elements (in the form of solutions) were sealed in each of several polyethylene capsules in the same way as comparative standards. Unknown Samples. Varying amounts-e.g., 3 mg., 10 mg., 0.03 gram, and 0.1 gram-of each sample to be analyzed were sealed in separate polyethylene capsules. Samples were in the form of powder of about 80 mesh. Irradiation and Counting. A sample was irradiated with neutrons a t a flux of about 4 x 10'0 n/sq. cm./second, whose fluctuation was less khan =t5yo. In the choice of irradiation, cooling, and counting times, signal-to-noise ratio (4)-i.e., the ratio of activity to be detected to interfering activity-was VOb. 33, NO. 13, DECEMBER 1961
e
1
416.5
m m.
L-
swle after sealing
first considered; second, half-life determination. The chosen periods for irradiation, cooling, and first counting, further cooling, and the second counting were 30, 10, 30, 30, and 30 seconds, respectively. The adequacy of these times was examined in preliminary experiments. The scintillation spectrometer used was a 256-channel pulse height analyzer attached with a well-type sodium iodide crystal ( 18/4-inch-diameter 2inch-thick crystal with a 8/4-inchdiameter I1/2-inch-deep well), For the first counting 128 channels in the analyzer were used; the rest were used for the second counting. All of these controls were operated manually. In the resultant y-spectrogram consisting of two successive spectra, the heights of photopeaks due to the 0.142-m.e.v. w a y from scandium-46m and the 0.108-m .e.v y-ray from dysprosium165m were measured. From two values of peak height thus obtained, the half life of each nuclide was roughly determined. After the half life was found to be about 20 seconds for 0.142m.e.v. y and about 1.3 minutes for 0.108-m.e.v. y, peak heights obtained from the first counting were converted to the amount of elements sought by use of calibration curves. M%en scandium interfered with dysprosium, the height of the 0.108-m.e.v. peak oba
Table I.
Target Nuclide “SC
Abundance in Natural Elements (6),% 180
MICROGRAMS OF SCANDIUM OR DYSPROSIUM Figure 2. a.
b.
tained from the second counting was converted to the amount of dysprosium as well. Sample and standard were irradiated a t different times. The energy scale in the pulse height analyzer was corrected a t least once every 20 minutes by the use of standards or synthetic samples. REASONFOR CHOKEOF TIMING. If irradiation is continued for more than 30 seconds, interfering activity increases markedly, in most cases, without marked increase of the activities of either scandium-46m or dysprosium16Sm. If it is less than 30 seconds, the build-up activity of dysprosium-165m remains far from its saturation. The time intervai of 10 seconds was
Nuclear Data
Isotopic a Activation Cross Section for Thermal Neutrons, Barns
Product Of
Radiation and Energy
tion
M.E.V.
40mSc
0.142 0- 0.357 1.48 y 0.885
Neutron Irradia-
10 i.4 ( 9 ) 22.3 f 2.2 (3)
%c
Unknown
167Dy
96 f 20 (3) 2000 i 200 (3) 800 i 100 (3)
lSmDy
(61,
y
Half Life
of Product
Nuclide
(6) 19.5 sec. 84 days
1.119 0.0524 0.0902 28.18
1Wy
la6Dy
0.0608 5 others y 0.0530 y 0.108 8- 1.25 0.83 0.42 y 0.0948 0.175 0.27 y
0.36 -0.67 -0.75
1950
e
ANALYTICAL CHEMISTRY
Calibration curves
Obtained from Arrt counting Obtained from second counting
8 . 2 hr. 134 days 1.25 min. 139.2 min.
enough to carry out such procedures aB ejection of a sample from the pile, taking out a capsule from the “rabbit,” and mounting the capsule in the well of a scintillation crystal. This interval is thought to be enough for very shortlived interfering activity such as 1.0second chlorine or 2.5-second erbium to decay out. The lengths of the first and second counting times were chosen for convenience a t first, and then tested by experiments which showed these were appropriate. When the time interval between the first and second 30-second countings was 10 seconds, it was too short to determine the half life of dysprosium-165% If the interval is longer than 30 seconds, it is too long to determine the half life of scandium46m. Calibration Curves. Comparative standards described above were irradiated and counted, and the heights of the resultant photopeaks were plotted against the amounts of scandium or dysprosium (Figure 2). RESULTS
The examples of the relative standard deviation of single analyses were obtained using five scandium solutions (0.25 ml. each), each of which included 3.07 pg. of scandium, and four dysprosium solutions (0.25 ml. each), each of which included 1.62 pg. of dysprosium. The estimated coefficients are shown in Table 11,which also gives the analytical results of aqueous solutions containing 12.3 p.p.m. of scandium and 6.5 p.p.rn. of dysprosium. These coefficients are thought to correspond to an ideal case in the sense that interfering activity is very weak and the activity levels are suitable for the counter used. Therefore, these coefficients are approximate
2000
461"sc
iooor
1
.. I
.
v)
I-
5
IOOC
0
**
0
I.
.
46m
sc I
0
200
CHANNEL iJUMBER Figure 3. Spectra for ?-ray of activated scandium standard containing 3.07pg. of scandium
lower limits of the relative stpr ' deviation in this method. In activmeu ore samples, however, interfering-activity levels (contribution of other nuclides to the photopeak under observation) are often high; hence, the error arising from y-spectra unscrambling is sometimes large. In such cases, another procedure may be required to obtain more accurate results. Examples of y-ray spectrograms, analytical results, and recovery are shown in Figures 3 t o 6 and Tables I11 and IV. Samples taken are very small (Table 111),to avoid the saturation of the pulse height analyzer. In other words, interfering activity often appeared and the sensitivity of this method was reduced. I n some cases, such as yttrium oxide, interference is relatively small and high sensitivity is attainable. Therefore, the sensitivity depends markedly upon the extent of interference, and a few parts per million of each element can sometimes be determined. DISCUSSION
The procedures required are simple, and can be carried out on a low level of radioactivity. Since this is a nondestructive method, contamination can easily be minimized. On the other hand, this method cannot be applied if a matrix is strongly activated upon short-time irradiation. Such a case is when a sample is rich in vanadium, cobalt, silver, etc. From Table I11 it is deduced that this method may fairly be applicable if the ores to be analyzed are rich in rare earths. When a relatively accurate result is required, it may be necessary to determine both elements separately with longer irradiation for dysprosium and shorter for scandium, and t o correct for the self-shielding effect, which is neglected in the present work. It is desirable to correct for this effect experimentally.
CHANNEL NUMBER Figure 4. Spectra for ?-ray of activated dysprosium . . standard containing 1.62 p i . ofdysprosium
Table II.
Examples of Relative Standard Deviation and Corresponding Size of Photopeak
Relative Standard Deviation of Single Analyses, %
Sample Scandium solution, 0.25 ml., containing 3.07 pg. Sc Dysprosium solution, 0.25 ml., containing 1.62 pg. Dy a Obtained from first counting Obtained from second counting.
3.8"
1 2 C. Orthite 1 2 D. Columbite 1 2 3 4 5
7.4 x 108
4.6 X 10' 2.60 X loe
2.1" 9.5b
Table 111.
Sample A. Soil B. Monazite
Av. Photopeak Av. Area of Height, Counts Photopeak, Counts 1.88 X I O p 4 . 3 x 104 4.7
x
108
Analytical Results
Sample, Mg. 49.6
Scandium Found,a P.P.M. 19
Dysprosium Found, P.P.M.
1.54 3.26
-45
