The analysis of actinides in environmental samples poses a range of chal- lenges. The behaviour of uranium and of plutonium is similar in the mass spec- trometer regardless of instrument type, given that these elements have comparable isotopic mass ranges, chemical behaviours, ionization potentials, etc. Conversely, typical environmental levels of uranium and plutonium differ by orders of magnitude. Soils and natural waters generally contain ppm and ppb levels, respectively, of natural uranium. Plutonium, being solely of anthro- pogenic origin, is present at sub-ppt levels in soils and waters. Accordingly, different methods are needed to prepare environmental samples for uranium and plutonium analysis. For instance, chemical processing can introduce back- ground uranium from sources such as chemical reagents and glassware. Typical process blank levels for bulk uranium analysis are in the picogramme to nanogramme range. Similar difficulties are not encountered with plutonium, and process blank levels are correspondingly lower. Thus, once extracted and separated, the relatively large amounts of uranium in environmental samples mean that overall sensitivity of the instrument measurement is less important than the precision and accuracy of the isotopic analysis. Even typical process blanks contain sufficient uranium for complete isotopic analysis. For plutonium, the main issue for mass spectrometry is sensitivity.
3. THERMAL IONIZATION MASS SPECTROMETRY
Typical environmental samples, such as soils, are prepared for analysis by wet ashing, followed by spiking with isotopic tracers. Analyses of uranium and plutonium require different approaches and, generally, initial sample solutions are gravimetrically split. A typical solution is split into a 90% aliquot for plutonium analysis and two 5% aliquots for uranium analysis. The plutonium aliquot is spiked with 244Pu, while one of the uranium aliquots is spiked with 233U for quantitation and the other is analysed unspiked for accurate isotopic analysis. All aliquots are separated and purified using a combination of precip- itation and anion exchange chromatographic steps. The final purification steps utilize microlitre volumes of analyte. The last step of the chemical procedure consists of quantitatively adsorbing the analyte onto an anion exchange resin bead, which is then loaded onto high purity rhenium mass spectrometer filament. After carburization, the filament is analysed on a triple sector mass spectrometer. The resin bead technique has been combined with a thorough knowledge of the physicochemistry of thermal ion emission [16] to achieve femtogramme detection limits for the TIMS analysis of uranium and plutonium
in environmental samples. Descriptions of the chemical methods, filament preparation and mass spectrometric techniques have been published [10, 17, 18]. Thermal ionization mass spectrometry has been used to measure the isotopic composition of actinide elements in a variety of environmental samples, including soils and groundwater [1–5, 17, 18]. Examples are shown in Figs 1 and 2. Typical concentrations of global fallout 239Pu in soils are ~0.05 pg/g.
4. INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY
Over the past decade, developments in ICP-MS have led to considerable improvements in sensitivity and precision [8, 11–15]. Experiments were conducted using a ThermoElemental PQ ExCel, quadrupole ICP-MS. Experi- ments to measure sensitivity and precision of ICP-MS have been systematically
FIG. 1. The isotopic composition of global fallout plutonium (plotted as 242Pu/239Pu
versus 240Pu/239Pu ratios) in soil samples collected around the world. The samples were
collected by the US Department of Energy Environmental Measurements Laboratory, and analysed at PNNL [4].
conducted using natural and synthetic uranium standards as demonstration vehicles. Recent results at PNNL have yielded several GHz/ppm sensitivity (at very low sample flow rates) for uranium and a background count rate of 0.1 counts per second. This performance is comparable to that reported by McLean et al. [15]. When coupled to low flow rates (20 µL/min), detection effi- ciencies of 0.1% (ion detected/atoms injected) can be achieved for the detection of actinide isotopes. This corresponds to detecting a measurable signal from a 0.01 fg/min sample flow rate. Analyses of the uncertainties associated with the measurement of isotope ratios in low level samples have shown that ICP-MS is capable of achieving precisions near the Poisson limit. For example, Table I shows precisions for uranium isotope ratio measurements for various quantities of total U.
The experiments described above were conducted with pure solutions, with no matrix interferences. For environmental samples, the performance differs due to the presence of matrix constituents and the need to chemically process to extract, purify and concentrate actinide elements. Recent results from IAEA NWAL (network of analytical labs) swipe samples, presented at a
FIG. 2. Isotopic composition of plutonium in groundwater from the Savannah River site F-area. Elevated 240Pu/239Pu ratios are due to the decay of 244Cm. The samples were
collected and processed by the Woods Hole Oceanographic Institute and analysed at PNNL [18].
recent workshop [20], have shown that the performance of ICP-MS compares favourably to TIMS. Detection limits for plutonium on swipe samples were ~10 fg and for process blanks were ~25 fg. Analyses of groundwater samples from the Hanford site [21, 22] have also demonstrated that ICP-MS is capable of providing good sensitivity and precision. Data were obtained on uranium isotopes, as well as on long-lived and stable fission products. These data showed that multiple and distinct sources contributed to a contaminant plume near tanks containing high-level reprocessing waste.
REFERENCES
[1] BEASLEY, T.M., et al., Isotopic Pu, U, and Np signatures in soils from Semipalatinsk-21, Kazakh Republic and the southern Urals, Russia, J. Environ. Radioactivity 39 (1998) 215.
[2] BEASLEY, T.M., et al.,237Np/129I atom ratios in the Arctic Ocean: Has 237Np
from Western European and Russian fuel reprocessing facilities entered the Arctic Ocean?, J. Environ. Radioactivity 39 (1998) 255.
[3] US DOE ENVIRONMENTAL MEASUREMENTS LABORATORY, Heavy
Element Radionuclides (Pu, Np, U) and 137Cs in Soils Collected from the Idaho
National Engineering and Environmental Laboratory and Other Sites in Idaho, Montana, and Wyoming, Rep. EML-599, Department of Energy, New York (1998).
[4] KELLEY, J.M., et al., Global distribution of Pu isotopes and 237Np, Sci. Total
Environ. 237/238 (1999) 483.
[5] COCHRAN, J.K., et al., Sources and transport of anthropogenic radionuclides in the Ob River system, Siberia, Earth Planet. Sci. Lett. 179 (2000) 125.
[6] KERSTING, A.B., et al., Migration of plutonium in ground water at the Nevada Test Site, Nature 397 (1999) 56.
TABLE I. ICP/MS PERFORMANCE
Isotope ratio Precision Efficiency Sample size Reference
235U/238U 0.1% 0.15% 10 pg [19]
(natural)
235U/238U ~1% 0.15% 10 fg [19]
(natural)
234U/236U 3–4% Not measured <5 fg PNNL Instrument
[7] OUGHTON, D.H., et al., Plutonium from Mayak: Measurement of isotope ratios and activities using accelerator mass spectrometry, Environ. Sci. Technol. 34 (2000) 1938.
[8] KENNA, T.C., SAYLES, F.L., The distribution and history of nuclear weapons related contamination in sediments from the Ob River, Siberia, as determined by isotopic ratios of plutonium and neptunium, J. Environ. Radioactivity 60 (2001) 105. [9] DAI, M.H., et al., Size fractionated plutonium isotopes in a coastal environment,