The Development and Application of ICP-MS for the Determination of Plutonium and Neptunium in the Marine Environment

[摘要] Inductively coupled plasma-mass spectrometry (ICP-MS) is a rapid and sensitive technique which enables the determination of pg ml-1 concentrations of 237Np, 239Pu and 240Pu from a single sample. It is a versatile technique with the capability of analysing gaseous, liquid or solid samples by the utilisation of different sample introduction techniques. In this study, liquid nebulisation and electrothermal vaporisation were investigated. Liquid nebulisation was found to be the most reliable technique with a typical detection limit for the actinides of 5 pg ml-1 using a minimum sample volume of 3 ml. Inconsistent results were obtained using electrothermal vaporisation, but under optimum conditions, a detection limit of 0.3 pg ml-1 was obtained, using only a 50mul sample. More typically, the detection limit for the actinides was of the order of 1 pg ml-1. It was shown that ICP-MS offers certain advantages over alpha-spectrometric techniques, such as the determination of 240Pu/239Pu atom ratios and 237Np concentrations from the same sample within a matter of minutes. In addition, ICP-MS is more tolerant of sample impurities, reducing the need for lengthy, time consuming sample preparation procedures such as those required for alpha-spectrometry. Application of ICP-MS to the analysis of environmental samples required consideration of a number of additional factors. The introduction of liquid samples into the plasma restricted the dissolved solids content to less than 0.2% (w/v), to prevent the blocking of the nebuliser and overloading of the plasma. This restriction, coupled with the low concentrations of actinides in environmental media, made the separation of the actinides from the bulk matrix necessary. In addition, the high concentrations of 238U in environmental matrices resulted in formation of significant levels of 238UH+ in the plasma which, in turn, caused an isobaric interference on the 239Pu peak. This was overcome by removing uranium from the samples until the final 238Uconcentration was less than 100 ng ml-1 and the 238UH+ contribution to the 239 peak was below the detection limit, enabling accurate determination of 240Pu/239Pu atom ratios. An analytical procedure was developed to isolate nuclides of Pu and Np from the bulk matrix and uranium using a combination of anion exchange and solvent extraction. This procedure was validated by the analysis of a Ravenglass silt sample which had been analysed by independent techniques. It was then applied to the analysis of intertidal surface sediment and core samples from the eastern shores of the Irish Sea. Good agreement was found (with linear correlation coefficients of > 0.95) between 239 + 240Pu specific activities obtained by ICP-MS and independent data obtained by alpha-spectrometry. A systematic difference was found between the results when different yield tracers were used. For some samples, 240Pu/239Pu atom ratio data obtained by Thermal ionisation mass spectrometry (TIMS) were also available. These showed excellent agreement with ICP-MS data, although the precision of TIMS was much greater. To some extent this could be accounted for by the limited sample size (1-5 g) available for ICP-MS analysis. Although TIMS is a more sensitive technique than ICP-MS for the detection of actinides, like alpha-spectrometry, it requires lengthy sample preparation procedures and the sample throughput is less than half that of ICP-MS. Sediment core samples obtained from areas of accumulating sediments at Maryport and the Solway Firth, provided a temporal record of the environmental signature of Sellafield discharges. 240Pu/239Pu atom ratio profiles in these cores indicated that sediment material deposited at the time of the early discharges from Sellafield had low 240Pu/239Pu atom ratios (< 0.10), characteristic of low burn-up material. With time, the 240Pu/239Pu atom ratios in the sediments increased until they attained a value typical of the average burn-up in a nuclear power station (approximately 0.2). Low 240Pu/239Pu atom ratios were also observed in the surface samples of the Maryport core and from some other sites close to Sellafield. Although these could be caused by the incursion of significant quantities of 'older' material, it was concluded that they were more likely to be representative of 240Pu/239Pu atom ratios in the discharges from Sellafield in the late 1980s.(Abstract shortened by ProQuest.).

[发布机构] University:University of Glasgow

[关键词] Analytical chemistry, Nuclear chemistry