oritic fall known as Allende were similarly treated. Belonging to the class of Type I11 carbonaceous chondrites, 6.8 kg of Allende has been recovered and has been prepared by the Smithsonian Institute as a possible standard meteorite powder. Its detailed analysis was therefore most appropriate. Since the powder has not yet been well characterized and suitable internal standards could not be selected, the W-1 and Allende were both analyzed on the same photoplate (4). Analysis of four different samples of the meteorite were performed and results of the mass spectrographic and the neutron activation analysis (included for comparison) are reported in Table 111. The average RSD for the 28 elements determined is 7 %, comparing favorably with the precision of analysis of simpler metallic samples. The higher deviations for Be, Cs, and Pb can be attributed to analytical difficulties associated with their determination at low concentrations. Although comparative data are limited, agreement with the results available from the neutron activation analysis of
the same samples is quite good. A more extensive evaluation of the reported results of the analysis of Apollo 12 soil sample 12070 using the homogenization method has been completed (27). The accuracy of the mass spectrometric determination of 33 elements in 12070, for which comparable precision was attained is 9%. ACKNOWLEDGMENT
The authors thank J. T. Gerard for assistance in developing the crushing procedures.
RECEIVED for review September 3,1971. Accepted November 3, 1971. Financial support was provided by the National Science Foundation under Grant No. GP-6471X and the Advanced Research Projects Agency (DAHC 15-67-C-0214) through the Cornell Materials Science Center. (27) G. H. Morrison, ANAL.CHEM.,43 (7), 22A (1971).
Determination of Technetium by Atomic Absorption Spectrophotometry Willard A. Hareland,' Earl R. Ebersole, and
T. P. Ramachandran
Argonne National Laboratory, Idaho Division, Idaho Falls, Idaho 83401 The atomic absorption characteristics of technetium have been investigated with a laboratory-constructed technetium hollow-cathode lamp as a spectral line source. The sensitivity for technetium in aqueous solution is 3.0 pg/ml in a fuel-rich acetylene-air flame for the unresolved 2614.23-2615.87 A doublet under the optimum operating conditions. The absorption of spectral radiation was studied as a function of wavelength, fuel-to-oxidant ratio, hollow-cathode lamp current, burner height, and spectral bandwidth. Studies with 32 cations at concentrations of 50 and 500 pg/ml indicated that only calcium, strontium, and barium caused severe technetium absorption suppression with 60 pg/ml of technetium in 2N hydrochloric acid solutions. The cationic interferences are eliminated by adding aluminum to the test solutions. The applicability of atomic absorption spectrophotometry to the determination of technetium in uranium and a uranium alloy was demonstrated.
ATOMIC ABSORPTION SPECTROPHOTOMETRY is a rapid, accurate, and extremely versatile analytical method for performing routine analysis and has been applied successfully to the determination of nearly 70 elements. However, no published information exists pertaining to the determination of technetium by atomic absorption spectrophotometry or to the atomic absorption characteristics of this element. Technetium does not exist as a primordial element in terrestial minerals but is produced in high yield in neutron-irradiated uranium and plutonium. However, traces of technetium have been detected in uranium ores as a result of spontaneous fission (I). Several analytical methods are available for the determination of trace quantities of technetium. Neutron actiPresent address, Department of Biochemistry, College of Biological Sciences, University of Minnesota, St. Paul, Minn. 55101.
(1) B. Kenna and P. Kuroda, J. Znorg. Nucl. Chem., 26,493 (1964). 520
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
vation is the most sensitive. For the reaction 99T~(n,y)100T~, the neutron capture cross-section is 30 barns, which permits the determination of 2 x lo-" gram of g9Tc (2). Boyd (3) described a polarographic method capable of determining 5 X lo-* gram. Technetium is determined spectrophotometrically as a thiocyanate (4, 3,or 1,5-diphenylcarbohydrazide(6, 7) complex with a sensitivity of 1 X lo-' gram. Using the 4031 A spectral line, as little as 1 X 10-7 gram can be determined by emission spectrometry (8). However, direct analysis of highly radioactive nuclear fuel by conventional analytical techniques is not practical because of interference from other fission products. Thus, separation of technetium is generally required. A direct atomic absorption method would be a valuable asset to the nuclear power industry since it would provide a rapid and accurate method for determining burnup. In addition, nuclear fuel processing waste could be analyzed conveniently to aid in establishing the economic factors in the recovery of this valuable element. This paper describes the design and construction of a technetium hollow-cathode lamp which was used as a spectral line source for the atomic absorption determination of technetium. The absorption of technetium was studied as a function of wavelength, fuel-to-oxidant ratio, hollowcathode lamp current, burner height, and spectral bandwidth. The optimum parameters were established by varying one factor at a time while keeping the others constant. Studies (2) G. Boyd and Q. Larson, J . Phys. Chem., 60, 707 (1956). (3) G. Boyd, J . Chem. Educ., 36, 3 (1959). (4) C. Crouthamel, ANAL.CHEM.,29,1756 (1957). (5) 0. Howard and C. Weber, ibid., 34, 530 (1962). (6) F. Miller and H. Zittel, ibid., 35, 299 (1963). (7) R. Villarreal, M.S. Thesis, University of Idaho, MOSCOW, Idaho, 1967. (8) W. Meggers, Spectrochim. Acfa, 4, 317 (1951).
TECHNETIUM CATHODE
50
-
I
,
'I1
STOPCCCK SHELL EPON RESIN 815
c
a 40 5
6
F
U
1
Figure 1. Diagram of technetium hollowcathode lamp were conducted to identify those ions which interfere. The applicability of atomic absorption spectrophotometry to the determination of technetium in uranium and a uraniumbased alloy was demonstrated. EXPERIMENTAL Apparatus. A Perkin-Elmer Model 303 atomic absorption spectrophotometer with a recorder readout accessory was used for this investigation. In this instrument, spectral radiation is isolated by means of a 0.4 meter Czerny-Turner grating monochromator with variable slit. This instrument is equipped with a triple-slot burner (10-cm length) and a burner control box for the regulation of acetylene and air flow rates. The sample aspiration rate was maintained at 3.0 ml per minute. A technetium hollow-cathode lamp was constructed from a commercial Perkin-Elmer lamp by removing the quartz envelope and inserting a tschnetium-plated foil into the cathode. A diagram of the lamp is shown in Figure 1. A vacuum stopcock was sealed to the Existing evacuation tube and provided a means of resealing the lamp after the optimum pressure had been obtained. The vacuum system was designed so that spectral radiation from the lamp could be monitored by the atomic absorption instrument while adjusting the pressure. The vacuum system is constructed of 17-mm glass tubing and consists of a liquid argon cold trap, a Pirani gauge to measure the vacuum within the system, and a three-way vacuum stopcock. The vacuum pump and gas supply are connected to the system cia the stopcock. Reagents. Reagent grade chemicals were used for all studies. Technetium was obtained from the Nuclear Science Division of the International Chemical and Nuclear Corporation, Pittsburgh, Pa. Technetium solutions were prepared by dilution of the ammonium pertechnetate stock solution with de-ionized water. Elements tested for interference were prepared from the metal or metal salt. Procedure. Technetium metal was electroplated on a copper foil (1 cm2)from 1 M ammonium sulfate solution (pH 1.0) by a method described by Voltz and Holt (9). The rate of electrolysis was followed by removing aliquots of the electrolyte solution at regular intervals and counting the dried radioactive sample on a scintillation counter. After cleaning the glass surface on the lamp, the copper foil containing -14 mg of technetium was shaped in the form of a cylinder and inserted into the cathode of the modified lamp. The quartz window was resealed with Shell Epon Resin 815. After testing for leaks, the lamp was connected to a vacuum pump and placed in an oven at 100 "C. Continuous evacuation while heating removed adsorbed water and oxygen from the internal surface of the lamp. The lamp was mounted in the atomic absorption instrument and connected to the vacuum system. The system was purged by alternately evacuating and refilling with neon. The vacuum was adjusted to -5 mm of Hg and a glow discharge was produced. After (9) R. Voltz and M. Holt, J . Electrochein. SOC.,114, 128 (1967).
30
0
a
20 P
a
IO 0
ACETYLENE-TO-AIR
RATIO
Figure 2. Atomic absorption of technetium as a function of acetylene-to-air ratio Table 1. Absorbance for Selected Spectral Lines of Technetium Absorbance, Wavelength," A Energy levels,a cm-l 300 pg Tc/ml 2608.86 0-38319 0.1079 2614. 23h 0-38240 0.3665 C38216 261 5 , 87b 3173.30 0-31503 0.0044 3182.37 0-31414 0.0434 3466.28 2572-31414 0.0044 3636.07 2572-30061 0.0391 4031.63 2572-27369. 0.0044 4238.19 0-23588 0.0419 4262.27 0-23455 0.0545 4297.06 0-23265 0.0680 a W. Bozman, C . Corliss, and J. Tech., J . Res. Nat. Bur. Stand., 72A, 559 (1968).
* Unresolved doublet.
maintaining a hollow-cathode discharge for several minutes, the system was evacuated. This was repeated several times to degas the cathode. The anode was degassed by reversing the polarity in the lamp and repeating the process. The intensity of the spectral radiation and the stability of the hollow-cathode discharge varied with the neon pressure. The spectral line intensity was determined by mea?uring the signal-to-background ratio of the 2614.23-2615.87 A doublet. Maximum intensity was obtained between 2 and 5 mm. The arc favored the outside of the cathode below 2 mm. Above 5 mm, the line intensity decreased and became less stable. After adjusting the neon pressure to 3 mm, the lamp was sealed by closing the stopcock. At this point, the lamp was disconnected after the vacuum system was brought to atmospheric pressure. RESULTS AND DISCUSSION Determination of Optimum Conditions. The technetium spectral lines investigated for absorption including the absorbances obtained with a 300 pg/ml aqueous technetium solution are listed in Table I.o All measurements were made at a spectral bandwidth of 2 A using a fuel-rich acetylene-air flame. Maximum sensitivity was obtained at the 2614.232615.87 A doublet which was not resolved by the monochromator in the Perkin-Elmer instrument. The 2608.86 A spectral line is less sensitive by a factor of approximately 3.5 and is useful for analyzing samples containing higher concentrations of technetium. The technetium doublet was used for the remaining studies. Variation of the acetylene and air flow rates indicated that extremely fuel-rich flames are required to produce a maximum concentration of technetium atoms in the absorption system. Figure 2 illustrates the effect of the acetylene-to-air ratio on ANALYTICAL CHEMISTRY, VOL. 44,NO. 3, MARCH 1972
521
I
1
Table 11. Effect of Spectral Bandwidth on Sensitivity and Detection Limit Spectral Detection bandwidth, A Sensitivity, pg/ml limit, pg/ml 2 7 20 14 16 18 20 22 24 26 28 30 HOLLOW-CATHODE LAMP CURRENT (mA)
Figure 3. Atomic absorption of technetium as a function of hollow-cathode lamp current
40
c
HEIGHT OF LIGHT BEAM ABOVE BURNER (rnrn)
Figure 4. Atomic absorption of technetium as a function of burner height
3.0 4.3 1.5
0.9 0.9 1.5
Table 111. Effect of Various Metal Ions on Atomic Absorption of 60 pg/ml of Technetium Percentage change Metal ions Li(1) Na(I) Cs(1) TU) Mg(I1) Ca(I1) Sr(I1) Ba(I1) Mn(I1) Co(I1) Ni(I1) CU(I1) Zn(I1) Pd(I1) Cd(I1) Pb(I1) Al(II1) Cr(111) Fe(II1) Ga(II1) Y(II1) Ru(II1 or IV) Rh(II1) In(II1) Sb(II1) La(II1) Bi(II1) Zr(1V) Mo(1V) Sn(1V) Th(1V) Re(VI1)
-1 -1 -1 -1 -3 - 25 - 17 - 14 -4 -1 -3 -4 0 -4 -1 -3 -1 -1 -1 -4 0 -3 -3 0 -1 -3 -1 -1 0 -1 -4 -4
-4 -1 +1 -1 -9 -49 -40 - 29 +1 -1 -4 -3 -3 -1 -4 +1 -1 +1 +3 +1 +1 +4 -9 0 +1
+1 -1 -4 -4 -5
-3 -8
TECHNETIUM ( u g h l )
Figure 5. Technetium calibration curves as a function of spectral bandwidth the absorption of 60 pg/ml of technetium. For each ratio, the burner height was varied to obtain the position of maximum absorption in the flame. The air flow varied from 12 to 23 liters per minute and the acetylene flow varied from 1.5 to 4.2 liters per minute. Maximum sensitivity was achieved at an acetylene-to-air ratio of 0.185 which corresponds to a highly luminous reducing flame. The effect of hollow-cathode lamp current is illustrated in Figure 3. The lamp was allowed to equilibrate after each increment in current. The optimum flame region was determined by measuring the absorption of technetium at various regions in the flame while the acetylene and air flow rates were held constant. The absorption shows a maximum near the base of the flame and decreases gradually at higher regions as illustrated in Figure 4. The burner height refers to the distance from the base of the flame to the center of the light beam. The beam from the hollow-cathode lamp ranges in vertical height from 5 to 15 mm as it passes through the flame. Thus, measure522
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
ments could not be made at positions lower than 7 mm above the base since the burner head obstructed the light beam. Table I1 lists the sensitivity (pg/ml/l absorption) and the detection limit (pg/ml required to produce a signal equal to twice the background noise) of technetium as a function of spectral bandwidth. The sensitivity varied indicating that the technetium doublet was not completely isolated from extraneous radiation emitted by the hollow-cathode lamp. A scan of the spectral region near the technetium doublet revealed a cFpper spectral line of approximately equal intensity at 2618.4 A. Radiation from copper in the hollowcathode lamp is not absorbed by technetium. This effect can be eliminated by excluding copper from the cathode. The major source of extraneous radiation from the lamp is due probably to adsorbed hydrogen in the technetium cathode. Electroplated metals are likely to contain hydrogen which possesses a very intense continuum throughout the ultraviolet region. Excitation of hydrogen in the lamp produces a continuous background which is not absorbfd appreciably by technetium atoms. At a bandwidth of 7 A, the background continuum represents about one third of the total quantity of radiation emitted at the technetium doublet.
Y
0.10
-
TECHNETIUM (Ug/ml)
Figure 6. Technetium calibration curve Figure 5 shows standard calibration curves as a function of spectral bandwidth. These data were obtained using the experimentally determined instrumental parameters that provided maximum sensitivity and analytical precision. The absorbance is nearly a linear function of technetium concentration. Slight curvature of the standard curves may be attributed to unabsorbed background radiation from the hollow-cathode lamp. Examination of Interferences. Various inorganic acids and metal ions were examined for possible interference. Studies revealed that hydrochloric and phosphoric acid at concentrations between 0.4 and 2.ON did not alter the atomic absorption of 60 pg/ml of technetium. Sulfuric acid interfered by suppressing the absorption. As the nitric acid concentration was increased from 0.4 to 2.ON, the absorption decreased by about 10%. Although variations in sample flow rate resulting from changes in acid concentration were neglected, it is likely that small differences in sample flow could cause significant errors. Table I11 shows the percentage change in the absorption of 60 pg/ml of technetium in 2N HC1 in the presence of various cations at concentrations of 50 and 500 pg/ml. A percentage absorption change less than 5x was not considered significant because of uncertainty in the measurement resulting from instrument noise. Calcium, strontium, and barium severely suppressed the absorption. Magnesium, rhodium, and rhenium interfered slightly when present at 500 pg/ml. The nature of the interference is not known with certainty. However, interferences due to alkaline earth
metals can be eliminated by addition of aluminum to the sample solutions. For example, 100 pg/ml of aluminum completely eliminated the interference from 50 ,ug/ml of the alkaline earths. Solutions of inorganic acids and metal ions which did not contain technetium were analyzed concurrently with the above samples. No measurable absorption changes were observed for the blank solutions indicating the absence of spectral interference at the concentration levels studied. Atomic absorption analysis of uranium and a uranium alloy for technetium was demonstrated. Samples containing 60 pg/ml of technetium and varying amounts of uranium at concentrations from 0 to 20 mg/ml in 2N HC1 were analyzed. The absorption was identical within experimental error for all samples. Technetium standards containing 25 mg/ml of a synthetic uranium fission product mixture were analyzed and the results compared with similar standards in 2N HC1. The uranium fission product mixture is similar to the nuclear fuel used at the Experimental Breeder Reactor-I1 (EBR-11) which is an alloy consisting of 95 % uranium and a 5 % mixture of molybdenum (2.5 %), ruthenium (1.9%), rhodium (0.3 %), palladium (0.19%), zirconium (0.1 %), and niobium (0.01 %). Calibration curves for both sets of standards are shown in Figure 6. The results indicate that atomic absorption spectrophotometry can be applied successfully to the analysis of multicomponent samples in the concentration range studied. ACKNOWLEDGMENT
The authors express appreciation to Robert Villarreal for many helpful suggestions in the course of this work and to W. R. Sovereign and C. C. Miles for their assistance in the construction of the hollow-cathode lamp and vacuum system. RECEIVED for review July 19, 1971. Accepted October 29, 1971. Work preformed under auspices of U S . Atomic Energy Commission by W.A.H. in partial fulfillment of the requirements for the Degree of Master of Science, Major in Chemistry, University of Idaho, Moscow, Idaho. This paper was presented in part at the XXI Mid-America Symposium on Spectroscopy, Chicago, Ill., June 4, 1970.
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