Simultaneous Determination of Tellurium and Uranium by Neutron Activation Analysis Ernest S. Gladney’ and Harry
L. Rook
Analytical Chemistry Division, National Bureau of Standards, Washington, D.C. 20234
A procedure for the simultaneous determination of tellurium and uranium is described. The procedure utilizes thermal neutron activation, followed by sample combustion and a gas phase separation of the volatile radionuclides of lnterest. Thls method is sensitive enough to permit the measurement of tellurium and uranium at their naturally occurrlng levels in materials of biological and environmental origin. The procedure has been successfully employed to determine the tellurium and uranium concentrations in three Standard Reference Materials (SRM’s) currently being offered by the National Bureau of Standards, and in atmospheric particulate material collected in rural Maryland.
Tellurium and uranium have not been shown to play essential roles in the trace element biochemistry of higher animals ( 1 , 2 ) . Rather, both elements are potent biotoxins. Information on the toxicology of tellurium is limited, but it is believed to resemble arsenic more than selenium in its toxic action (3, 4 ) . Tellurium binds uniformly to soluble proteins in the blood, has been implicated in nerve and brain damage in rats, and is readily transported across the placental barrier (5-11). Reduction in general activity and in longevity has been demonstrated in mice fed sub-toxic doses of tellurium in water, but no influence on carcinogenicity or on the production of other tumor types was observed ( I ) . No other data appear to have been accumulated regarding the effects of long-term low-level exposures of tellurium to mammals ( I , 4 ) . The toxicology of uranium has been extensively investigated ( 2 ) .In addition to the radiation hazard, the ingestion of uranium and its compounds has been implicated in arterial, kidney, and liver damage (9, 10). Few analytical techniques have the demonstrated sensitivity to permit the study of tellurium in biological and environmental materials a t naturally occurring levels. Tellurium has been successfully analyzed in geological samples by neutron activation coupled with an elaborate radiochemical separation scheme (12-16). The general procedures utilized either acid dissolution or basic fusion of the sample followed by distillation or solvent extraction of the I3lI produced during neutron irradiation. The I3lI is the daughter activity of the 131Te produced via the 130Te(n,y)131Tereaction. The one disadvantage of using the 1311 daughter of 131Te is that l3II also is produced by 235Ufission. The importance of correcting for the fissionproduced l3II in natural materials has not always been appreciated in past work. A procedure for the determination of tellurium in atmospheric particulate material utilizing neutron activation has recently been reported ( 17). However, the method suffered from severe s2Br interference as well as from losses of tellurium during the chemical manipulations. The reported procedure did not have sufficient sensitivity to determine tellurium in urban atmospheric particulate material. Present address, Group H-8, Los Alamos Scientific Laboratory, Los Alamos, N.M. 97544. 1554
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
Atomic absorption spectrometry has been utilized to determine tellurium, generally above the pg/g concentration level (18, 19), but again this technique does not have the sensitivity necessary to determine tellurium in most biological and environmental matricies without a pre-concentration step. A wide variety of techniques for the determination of uranium has been reported: gravimetric (20), polarographic ( 2 ) , fluorimetric (21), a-particle counting (22), delayed neutron emission (23), mass spectrometry (24), fission track (25), instrumental neutron activation analysis (26), and neutron activation followed by 239U or fission product separation and counting (20,27). Relatively few analyses for uranium in atmospheric particulate material have been reported other than close to nuclear facilities where unnaturally high levels might be expected. Of those few, airborne uranium concentrations of 0.1-1.5 ng/m3 in New York State (28),0.02 ng/m3 in rural England, 0.004 ng/m3 in the North Atlantic, and 0.003 ng/m3 in the Antarctic have been reported (29). In this work, both tellurium and uranium were analyzed using a combustion method followed by gas phase separation of volatile radioiodine. The procedure was adapted from a separation originally developed for the determination of mercury (30). Four principal advantages over currently used techniques have been attained. This procedure requires a minimum of chemical manipulations, thereby reducing the chances of technique related errors; the separation of radioiodine is quantitative, eliminating the need for chemical yield determinations; the procedure can be utilized with a variety of matrices without modification; and the low limits of detection, especially for tellurium, permit the characterizations of these elements a t their naturally occurring levels. Even though the use of iodine fission products for the determination of uranium is itself not new, the combustion technique provides a rapid, extremely sensitive method for measuring uranium simultaneously with tellurium in biological and particulate matrices.
EXPERIMENTAL Apparatus and Reagents. The sample combustion train employed in this study was a modified version of that described by Rook et al. (30) (Figure 1). The liquid nitrogen condenser was replaced by a gas phase adsorption column followed by an iodine trap. The column was made from a 15-cm length of 5-mm (i.d.) glass tubing inserted into a Teflon stopper machined to a 19/38 standard taper. The stopper and trap were fitted into the outer 19/38 standard taper Vycor joint a t the end of the main combustion tube. After combustion, the gases were first passed through a hydrated manganese dioxide (HMD) adsorption column and then through a silvered glass wool iodine trap. Hydrated manganese dioxide was first described by Giradi et al. as an inorganic ion exchange media for solution separations (31). In this work, we have found HMD to be a superior material to effect simple gas phase separations of the other halides from iodine. Standard solutions of tellurium were prepared by dissolving a weighed quantity of 99.9% pure tellurium metal in a small volume of dilute nitric acid and diluting to volume with water. Potassium bromide was added in sufficient quantity to simulate normal bro-
SILVER
WOOL
TRAP
TRAP
O2 I N L E T
/
Table I. Uranium Concentrations of Standard Reference Materials, p g / g
\
SAMPLE
/
QUARTZ WOOL
\ 8 JOINT
Mean Sa
Figure 1. Sample combustion system
mine levels in urban air particulate material. T h e tellurium concentration of the final working standard was 8.64 pg Te/g H20. Uranium standard solutions of known isotopic ratio were prepared by dilution of National Bureau of Standards Standard Reference Materials (SRM's) U0002 and UOlO uranium oxide standards. Both the total uranium content and isotopic fractions are certified in these standards. The 235Uisotopic contents of the standards used were 0.01755 and 1.0037%, respectively. Procedure. For analysis, approximately 0.5 gram of the sample material was encapsulated in acid cleaned quartz vials. Several a t mospheric particulate samples, collected on 0.5-pm pore diameter E H Millipore filters, and filter blanks were similarly encapsulated. One-hundred microliter portions of the standard solutions were pipetted into separate quartz vials, adjusted with water to equal the sample volumes, and sealed. Samples and standards were arranged in a concentric ring inside a polyethylene rabbit and irradiated in the pneumatic transfer facility RT-3 of the NBS reactor for 1-2 hours a t a thermal neutron flux of approximately 6 X l O I 3 n cm-2 sec-I. Flux monitors were not used because the radial flux variation was less than 2% in RT-3 ( 3 2 ) .The samples were permitted to decay for 1 2 hours to minimize personnel radiation exposure without losing significant analytical sensitivity. T h e quartz vials were washed with concentrated "03 and distilled water t o remove surface contamination, cooled t o liquid nitrogen temperature, and broken a few mm from the top. The samples were transferred to a ceramic combustion boat, 5 mg of KI carrier added and both the boat and quartz irradiation vial inserted into the combustion chamber. An oxygen flow of -100 cm3/min was passed over the boat and the samples were ignited with a gasoxygen torch. The samples were allowed to burn freely with the downstream quartz wool plug maintained a t red heat t o ensure complete combustion of the volatile materials during the burn. After combustion, the ash was heated to approximately 900 'C for 20 minutes using a tube furnace. Fly ash samples were baked out a t 1200 O C for 30 minutes. After this treatment, the entire combustion system was heated to the Teflon plug to drive all volatile components of the samples into the gas trap. Since some of the iodine vapor condensed on the HMD trap, the trap was heated gently to revaporize the iodine and ensure its complete collection on the silver wool. T h e HMD trap was heated only enough to observe the off-gassing of the violet 12. The glass tubing containing the gas trap was removed from the Teflon plug and the trap was separated for counting by breaking the tubing between the HMD and the silver wool sections. T h e HMD trap was counted for 10 min on a 60-cm3 Ge(Li) detector to determine the decontamination factor for bromine. T h e I3lI and fission produced 1331activities on the silver wool trap were counted for from 60 to 600 min on the same detector. T h e silver wool trap was recounted after the residual 82Br had decayed to recheck the I3II determination. The 364.5-keV and 531-keV y-rays from I3II and 1331, respectively, were employed in these determinations. The half-lives of these isotopes were followed for several weeks on some samples to ensure that no interfering species were present. Primary standard solutions were irradiated simultaneously with samples and were processed in the same manner as the samples. T h e standard-sample activity ratios, extrapolated back to the end of irradiation, were used individually to quantify each sample set. Bromine Separation. The only important interference with the direct determination of iodine after combustion is s2Br. Since the half-life of 1331is less than that of 82Br,reduction of the iiiterference through decay cannot be employed. In this study, the bromine in the combustion stream was removed from the gas phase
SR,M 1571
S R \ l 1632
S R \ l 1633
0.0217 0.0244 0.0247 0.0319 0.0185 0.0300 0.0254 0.025 0.004
0.99
9.7 11.9 12.5 14.3 12.3 9.5 14.8 12.1 2 .o
1.17 0.95 1.13 1.14 1.08 1.07
1.1 0.08
NBS
certified value
0.027 z 0.003 1.4 i 0.1 11.6 s is the standard deviation of a single determination.
i;
0.2
with a 2-cm long HMD adsorption trap. Solid HMD, of approximately 40-mesh size, was throughly washed with 1N NH40H and dried a t 110 "C for 4 hours in an oven. T h e dried HMD was packed loosely between two glass wool plugs, ahead of the silvered quartz wool trap. I t was necessary t o control the temperature of the HMD adsorption trap to obtain high bromine decontamination. During sample combustion and system bakeout, the trap was kept as cool as possible and was never allowed to exceed 100 "C. Under these conditions, all of the bromine and chlorine, and some of the iodine was adsorbed on the HMD trap. After the system bakeout, the iodine was quantitatively passed onto the silver wool trap by gently heating the HMD trap with a cool Bunsen burner flame. At this stage, care was always taken not to overheat the HMD trap or a portion of the bromine was also transferred t o the silver wool trap. Under normal operation, overheating of the HMD was no problem. However, when a system bakeout temperature above 900 "C was used, as in the case of fly ash or rock samples, the distance between the Teflon plug and the HMD trap was increased. Also, a lower oxygen flow rate, once the sample combustion was complete, was advantageous. With these precautions, bromine decontamination factors >lo4 were routinely achieved. After familiarization with the iodine revaporization process, decontamination factors >lo6 were achieved in most cases.
RESULTS AND DISCUSSION The uranium determination using fission product 1331 does not suffer from direct nuclear interferences. The use of 1311for the tellurium measurement requires that the I3lI contribution from uranium fission be carefully measured. I t is necessary to demonstrate that the fission product 1311 comes solely from 235U in order that assumptions concerning the uranium isotopic distribution in the samples can be employed in the determination. Two standard uranium solutions of widely different isotopic composition were irradiated and the radioiodine was separated using the described procedure. The production of fission product radioiodine from the two solutions was found to be in a ratio of 58 f 2.0 to 1 which compares excellently with the 235U isotopic ratio of the two certified standard solutions of 57.2 to 1. In these standard solutions, the 235Uprovided the only source of 1311. Thus, with the same series of experiments, the 1311/1331fission product activity ratios were established using a fixed counting configuration. These values were subsequently used to correct all tellurium data for the 235U fission component from naturally occurring uranium. Using the analytical procedure developed, the tellurium and uranium concentrations in three NBS Standard Reference Materials were determined. These materials were: SRM 1571 Orchard Leaves, SRM 1632 Trace Elements in Coal, and SRM 1633 Trace Elements in Fly Ash. The individual results are given in Tables I and 11, along with the ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
1555
Table 11. Tellurium Concentrations of Standard Reference Materials, F g / g SR.M 1571
Mean sa
0.012 0.017 0.007 0.010 0.011 0.008 0.011 0.003
S R M 1632
0.56 0.66 0.61 0.60 0.62 0.54 0.60 0.04
Table 111. Uranium and Tellurium Concentrations on Atmospheric Particulates S R V I053
9.5 9.9 12.1 9.9 9 .o 9.1 9.9 1.1
CAI1
14 15 16 23-24 EF =
LU1
[Tel
Sample date
nglm'
pg/m3
U EF"
pg,'m3
Aug. 1973 Aug. 1973 Aug. 1973
400 280 1440 510
39.7 28.9 110 49.3
2.2 2.3 1.7 2.1
260 390 520 290
Aug. 1973
Tc P 10'
2.5 5.4 1.4 2.2
E n r i c h m e n t Factor relative t o Wedepohl's Crustal
Average.
s i s t h e standard deviation of a single determination.
NBS certified values for the uranium concentrations. I t was assumed that the uranium in the SRM's had the natural isotopic abundance. The results compare well with the certified values established by other analytical techniques. The confirmed accuracies of the uranium values obtained by this procedure (Table I) are of vital importance in confirming the reliability of the tellurium analyses. Even though tracer studies using 1251were run to demonstrate that the separation was quantitative, the accurate uranium results indicated that the iodine separation was also quantitative when using real samples. By inference, one may assume that the tellurium results are also accurate, since any errors in the iodine chemistry would have been reflected in the results of the simultaneously determined uranium. This analytical procedure has also been used to measure the tellurium and uranium concentrations on atmospheric particulates collected in rural, southeastern Maryland during August 1973. The sensitivity of this technique permitted these elements to be measured on particulates from less than 40 m3 of air. The concentrations of tellurium, uranium, and aluminum for four different days sampling are reported in Table 111. The aluminum concentrations were measured by non-destructive instrumental neutron activation analysis (33). All data have been corrected for filter blank. Atmospheric enrichment factors (EF) for tellurium and uranium, relative to Wedepohl's crustal averages ( 3 4 ) , have been calculated using the method of Gordon and Zoller ( 3 5 ) .The low uranium EF's indicate that the airborne uranium was probably of crustal origin. This interpretation is consistent with previously reported atmospheric uranium measurements in other uncontaminated areas (28, 29). The high tellurium EF's suggest a strong, non-crustal source for this trace metal in the atmosphere. Similar high EF's have also been observed for selenium in a variety of environmental measurements (36-38). The high EF's for these chemically similiar elements suggest either poorly established crustal levels, an important natural non-crustal source, or anthropomorphic pollution. The tellurium results compare well with the only previously reported atmospheric measurement of
