Uranium Isotope Investigations by Atomic Absorption
SIR: Uranium is one important element for which no analytical work by atomic absorption has been reported in the past. The evident reasons are the general weakness of the lines in the complex spectrum and the fact that its compounds are not readily dissociated in the flames usually used as absorption sources. The sputtering technique described in this communication using a hollowcathode discharge tube as the absorption tube, however, seems to be quite adequate for determination of uranium and uranium isotopic ratios. Figure 1 shows the spectroscopic arrangement used to obtain the uranium absorption. Incident uranium radiation from a Schuler-Gollnow water-cooled hollowcathode tube (3’) was modulated by a BO-cycle mechanical chopper and focussed by lens L1 on the hole of a watercooled cylindrical uranium metal cathode, used as the sample in the absorption tube. The transmitted light was focussed by two lenses in series, Lz and La, on a 15-micron slit of a JarrellAsh monochromator, Model 82-000, equipped with a lP21 photomultiplier tube. The a.c. signal was amplified, rectified, and transmitted to a recorder. The Schuler-Gollnow emission tube contained milligram amounts of the isotope being investigated, and argon as the carrier gas. The tube was operated a t approximately 30 ma. The sample in the hollow-cathode absorption tube was in the form of a uranium cylinder, 1 1 / 2 inches long having an inside diameter of inch, Xenon was used as the sputtering gas, and the tube was operated at approximately 200 ma.
Lens-L, Emission tube
Figure 1 . uranium
Chopper
Absorption tube
Lens-L3 L e n s - L2
Monochromctor detectors
A. C. Arnplifier and rectifier
Recorder
Schematic diagram showing equipment used for the absorption of
One of the differences between this cathodic sputtering technique used for uranium and Walsh’s technique (4) is that our absorption tube a.; well as emission tube was temperature controlled (water-cooled) to reduce spectral line widths. As a result a number of sharp uranium resonance lines, occurring in the very complex and involved spectrum of uranium, were obtained respectively free of interferences and absorbed. Also, by having the two tubes temperature controlled using currents and gas pressures mentioned, reproducible sputtering was observed in each tube and thus stability for atomic absorption measurements was obtained. Table I shows some results obtained with the experimental arrangement shown in Figure 1. The observations indicated (a) cathodic-sputtering inside the surface of a uranium cylinder produced metal atoms in appreciable concentrations; hence. incident uranium resonance radiation was absorbed. (b) T o determine total uranium content by atomic absorption techniques with a hollowcathode tube as an emission source, using the uranium lines shown in Table I, and either flame or nonflame
methods to vaporize the sample, the sample being analyzed should be of a known uranium isotopic coniposition, because the isotope shift is much greater than the line lvidth ( 1 ) . (c) A uranium line 5915,40 A. that exhibits hyperfine splitting ( I ) and therefore is not very useful for isotope determination by emission spectrographic techniques, appears quite useful in atomic absorption work. (d) Since good discharge stability was obtained using uranium c h i p in the emission tube (e),atomic absorption work might be carried out directly on metal sample chips in the emission tube, using uranium standards in the absorption tube; as a result, much sample preparation work presently used to vaporize samples by flame or nonflame methods might be eliminated. A further preliminary investigation, using an enriched uranium cylindrical insert, 93.0% uranium-235 and 7.0% uranium-238, in the absorption tube and the same samples in the emission tube, shown in Table I, shovved very strong absorption for uranium-235 and weak absorption for uranium-238, thus confirming the findings in Table I. LITERATURE CITED
Table 1.
Incident Uranium Light Absorbed by a Uranium Hollow-Cathode Discharge Tube Used as an Absorption Source
Composition of uranium Emission tube Absorption tube
1978
ANALYTICAL CHEMISTRY
Incident light absorbed, %
Uranium line,
60
5915 40
1
5915 40
4‘2
5027 38
3
5027. 38
A.
(1) Xnderson, 0 . E., White, H. E., P h y s . Rev. 71,911 (1917). ( 2 ) Goleb, J. A , , Brodl-, J. I
