Anal. Chem. 1990, 62, 2722-2727
2722
Determination of Uranium, Thorium, and 18 Other Elements in High-Purity Molybdenum by Radiochemical Neutron Activation Analysis Karl-Heinz Theimer and Viliam Krivan*
Sektion Analytik und Hochstreinigung, Universitat Ulm, Oberer Eselsberg, D- 7900 UlmlDonau, FRG
A radlochemkai neutron acthratlon analysis technlque for the determination of uranium and thorium In high-purity molybdenum via the lndlcator radionuclides 290Npfor U and 23sPa for Th has been developed. S k n u l t a w , the ebments Ag, Co, Cr, Cs, Cu, Fe, Ga, In, Ir, K, Mn, Na, NI, Rb, Ru, Sc,Se, and Zn can be determined, too. The elements Hf, Sb, Ta, Sn, and W were determined by instrumental neutron activation analysis. The radloclremical separatlon Is pertormed by anion exchange on a Dowex 1 X 8 column from a 20 M HF/3% H,02 medlum. A h i t of detection of 4 ng/g for uranium and 40 pg/g for tholkwn can be achleved. For the other elements, the limns of detection are between 1 pg/g and 100 ng/g. A m o d W more seiectlve separatlon of the Indicator radlonuelide of Th, =Pa, alkws improvement d the lknit d detecth for Th by a factor up to 5. Thls technique was appHed lo the analysis of high-purity molybdenum, and the results of a number of elements were compared with those of other technlques.
INTRODUCTION Molybdenum, because of its several special properties, has become an important material for advanced technology. Trace impurities have been found to influence many of these properties. Especially in microelectronics research and technology, there is an increasing demand for high-purity molybdenum and its silicide used for very large scale integration (VLSI) applications. In this connection, the purity requirements are extraordinarily high (ppb level) for the a-emitting elements U and Th, which can cause changes in the potential of the data storage cells (I, 2). Further trace impurities of interest include the "mobile ions" Li+, Na+, K+, Ca2+,and Mg2+and many other metallic impurities such as Fe, Mn, Ni, Co, Cr, Cu, and Zn. These purity requirements have led to efforts to develop adequate analytical methods, and during recent years, considerable improvement of trace characterization of molybdenum has been achieved. Atomic spectrometry techniques like flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GFAAS), and inductively coupled plasma atomic emission spectrometry (ICP-AES) suffer from strong matrix interferences and insufficient sensitivity. Recently, an essential improvement of the performance parameters of the solution atomic spectrometry methods as applied to analysis of molybdenum has been achieved by matrix-traces separation (MTS) prior to the determination (3-10). Such separations are a necessity also in the analysis of molybdenum by inductively coupled plasma mass spectrometry (ICP-MS),otherwise considerable matrix interferences occur (11, 12). Recently, a spectrophotometric method for the determination of U (LD = 3 ng/g) and T h (LD = 20 ng/g) in mo-
* To whom correspondence
should be addressed.
lybdenum was proposed by Peters et al. (13). This method is based on the matrix-traces separation using the ion-exchanger cellulose Hyphan followed by formation of arsenazo-I11 complexes of U and Th. Although the authors claim that this method is a very applicable one and i t gives them the potential for standardization of glow discharge mass spectrometry (GDMS), the disadvantages and risks connected with its practical use cannot be overseen: metal powder is oxidized by air to oxide, of which a sample portion of 75 g is used for the analysis involving the utilization of large amounts of reagents. Among the mass spectrometry methods using solid samples (spark source mass spectrometry (SSMS),secondary ion mass spectrometry (SIMS), and GDMS), the last has become the most important one for bulk trace characterization of highpurity materials (14, 15). I t has also been applied to multielement analysis of molybdenum (3). However, all these methods require matrix-containing standards for quantification. Neutron activation analysis has some unique features, mainly the freedom of blank and high sensitivity and the multielement character which make this method very attractive for the analysis of very pure materials (16-18). The formation of short-lived and medium-lived matrix radionuclides of high radioactivity considerably limits the possibilities of application of instrumental neutron activation analysis (INAA) to the determination of trace elements in molybdenum. An improvement can be achieved only by using radiochemical separations. Radiochemical neutron activation analysis (RNAA) based on different separation principles has been applied to the determination of individual elements (19-23) or of a rather small group of elements between 2 and 11 (24-30). In many instances, the limits of detection are not sufficient for molybdenum of high purity, and/or the separation procedure is relatively complicated. Two comprehensive and relatively selective radiochemical separation procedures were reported earlier for NAA of molybdenum in connection with Na(1) y-ray spectrometry and Geiger-Muller counting (31,32). Both are time-consuming multistage procedures, not suitable for modern high-resolution y-ray spectrometry. In this paper, we describe two modifications of an improved RNAA method for the determination of 20 elements including U and Th. The simpler one is based on the separation of the indicator radionuclides (IRN) from the molybdenum and technetium matrix radionuclides in one group. A more selective separation of the indicator radionuclide for thorium, 233Pa,enables a second slightly modified technique. EXPERIMENTAL SECTION Reagents and Radiotracers. All reagents used for the separation procedure were of "pro analysi" quality and supplied by Merck, Darmstadt, FRG. The original concentrations of the hydrofluoric acid, nitric acid, and hydrogen peroxide solutions were about 40%, 65%, and 30%, respectively. The exact concentrations were determined by titrimetry. The anion-exchange
0003-2700/90/0362-2722$02.50/00 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 24, DECEMBER 15, 1990
resin used was Dowex 1 X 8 (100-200 mesh) supplied by Fluka, Buchs, Switzerland. The radiotracers of the elements investigated were prepared by irradiation of pure metals or suitable compounds in the nuclear reactors described below. The radiochemical purity of the radiotracers was checked by high-resolution y-ray spectrometry. Samples and Standards. The sintered molybdenum samples of purity grade 4N5 were obtained from Metallwerk Plansee, Reutte, Austria. Sample portions of 100 mg were cut with a diamond saw. Before irradiation, the samples were etched with a mixture of concentrated HF and concentrated HN03 (1O:l) for 30 s and washed with doubly distilled HzO. Then they were sealed into Suprasil quartz ampoules (Heraeus, Hanau, FRG). Multielement standards were prepared in a clean bench by pipetting a known amount of the respective element stock solution into cleaned Suprasil quartz ampoules. The standards were dried in a desiccator over Sicapent (Merck, Darmstadt, FRG) at reduced pressure and room temperature. The groups of elements in the multielement standards were combined in such a way that no y-line interferences could occur. After irradiation, the surface of the ampoules containing standards were etched before counting. Instrumentation. In the radiotracer experiments, a well-type 3 X 3 in. NaI(T1) detector coupled with a single-channel analyzer (Berthold, Wildbad, FRG) was used for counting the y rays. A high-resolution y-ray spectrometer system (EG&G Ortec, Munchen, FRG), consisting of an intrinsic germanium detector with a fwhm of 1.72 keV at the 1332-keV y ray of @%o, an efficiency of 44% relative to a 3 x 3 in. NaI(T1) detector, and a peak-to-compton ratio of 78:1, and an ADCAM multichannel buffer was used for checking the radiochemical purity of the radiotracers and counting the indicator radionuclides in activation analysis. The evaluation of the y-ray spectra was carried out with the Digital Equipment Professional 350 computer by using the Ortec Geligam program version 1.7. For the elution, a IP4 peristaltic pump (Ismatec, Switzerland) was used for operating several columns simultaneously. Irradiation. For determinations via indicator radionuclides with half-lives between 2.5 h and 2.35 d, sample portions of about 100 mg and standards were irradiated for 6 h at a thermal neutron flux of 1.3 X 1013cm-, s-l in the FRM reactor, Garching, FRG and for the element determinations via long-lived indicator radionuclides, the sample portions and suitable standards were irradiated for 14 d at a thermal neutron flux of l O I 4 cm-, s-l in the FRG-2 reactor, GKSS Geesthacht, FRG. Determination of Distribution Coefficients. The ,distribution coefficients for static conditions were determined by means of the radiotracer technique. The distribution coefficient D is defined as the ratio of the total amount of the element per gram of dry resin to the total amount of the element per milliliter of the solution. For the determination of the distribution coefficients, 2 mL of the solution containing 1-10 wg of the labeled elements and 239Npwere carrier-free) were mixed with 30 mg of the Dowex 1 X 8 resin (100-200 mesh, preconditioned with 2 M HF, dried at 80 "C). To adjust the equilibrium, the test tubes were shaken for 6 h. The solutions were prepared shortly before use. Radiochemical Separation Procedure (A). The separation columns were made of disposable polystyrene pipets having an active bed of 18 cm x 0.8 cm 0.d. packed with the anion-exchange Dowex 1 X 8 resin (about 4.5 g of dry resin of bead size 100-200 mesh). Before use, they were pretreated with 20 mL of 20 M HF/3% HzOzsolution. (Caution: HF solutions are extremely corrosive and appropriate safety procedures must be employed). Figure 1 procedure A shows the flow-chart of the radiochemical procedure for the separation of the radionuclides of 31 elements in one fraction. For surface decontamination, the irradiated samples were etched with a mixture of concentrated HF and concentrated HN03 (1O:l) for 20 s, washed with demineralized HzO, dried, and weighed. The samples were dissolved in 1 mL of concentrated HF under dropwise addition of 0.5 mL of concentrated HN03. The mixture was heated under an infrared lamp for about 5 min, and 2 mg of (NH4),Sz08was added. The solution was then evaporated to dryness, the residue was redissolved in 2 mL of the 20 M HF/3% H202mixture, and the resulting solution was passed through the Dowex 1 X 8 anion-exchange column. The elution was carried out with 27 mL of the 20 M HF/3% H,Oz mixture at a flow speed of 1 mL/min. Finally, the eluate was
2723
decomposition
0.5mL conc. HNOd
procedure A
pmedun B
I
(2 mL 20 M HF/ I
(0.5mL 20 M HF/ 0.5 mL 30 % 60,)
__
elution with 27 mL 20 M HW 3% %02
I
I7 elution with 20mL
I
2 MHF/ 3% H202 elution with 25 mL 1 M HF/ 1M NH4F
-
008
! t
*- adsorbed:Hi,Mo,
Nb, Re, Sb,Ta, Tc,
I
x v
eluted: Ag. Cd. Co. Cr. Cs, Cu, Fe, Cia, In. Lr, K, Mn, Na, Np. Pa, Rb, Ru, Sc, Se, Zn, (Hfj, (Snh (a)
Figure 1. Flow
Eluate 1: Ag, Co, Cr, Cs, Fe, Rb, Ru, Se, Zn,(Sn)
Eluate 2: Pa, Sc
chart of the radiochemical separation procedures.
Table I. Relevant Nuclear Reactions Induced on Molybdenum by Reactor Neutrons
nuclear reacn
isotope cross abundance, section, % mb
half-life time
92Mo(n,y)g3mMo 14.8
99
Zr
62 f 10 75 f 8 95 f 4
As, Mo, Nb, Re, Sb, Ta, Tc, W
99 (fracn 1) >99 (fracn 2)
