Optimisation and application of ICP-MS and alpha-spectrometry for determination of isotopic ratios of depleted uranium and plutonium in samples collected in Kosovo

The determination of environmental contamination with natural and artificia l actinide isotopes and evaluation of their source requires precise isotopi c determination of actinides, above all uranium and plutonium. This can be achieved by alpha spectrometry or by inductively coupled plasma mass spectr ometry (ICP-MS) after chemical separation of actinides. The performance of a sector-field ICP-MS (ICP-SFMS) coupled to a low-flow micronebulizer with a membrane desolvation unit, "Aridus'', was studied with respect to precise isotopic measurements of uranium and plutonium at the ultratrace level. Th e UH+/U+ formation rate was about 3x10(-5) and a sensitivity for U-238 of u p to 4x10(9) counts s(-1) ppm(-1) was achieved. The limit of quantification (LOQ, 10s) for U-236 and Pu-239 using the experimental arrangement describ ed above was 0.6 pg l(-1) in aqueous solution and 0.13 pg g(-1) in soil, re spectively. ICP-SFMS was used in comparison to alpha spectrometry to measur e the U and Pu concentrations and isotopic compositions in two soil samples and in one penetrator collected in Kosovo. ICP-MS permitted the determinat ion of U and Pu isotope ratios including the U-236 abundance and Pu-240/Pu- 239 ratio at ultratrace levels in soil samples of up to 0.1 g. Depleted ura nium (U-235/U-238= 0.00202 +/-0.00001) was determined in one penetrator and one soil sample. Pu concentrations of (5.5 +/-0.1) x 10(-13) g g(-1) and ( 4.4 +/-0.5) x 10(-13) g g(-1) (Pu-240/Pu-239=0.35 +/-0.10 and 0.27 +/-0.07, respectively) were found in both soil samples from Kosovo. Besides plutoni um, U-236 (3.1x10(-5) g g(-1)) and Am-241 (1.7x10(-12) g g(-1)) were also d etected in the penetrator sample, which indicates the previous existence of neutron-related processes and points to a possible presence of spent react or uranium in munitions. However, the most probable plutonium contamination sources in analyzed soil samples from Kosovo are mixed fallout including s pent reactor fuel due to the Chernobyl nuclear power plant accident in 1986 and plutonium due to nuclear weapon tests. Additional plutonium contaminat ion could not be determined in the Kosovo soil sample containing depleted u ranium with a detection limit of about 10(-13) g g(-1).