A rapid and accurate method for the determination of plutonium in food using magnetic sectorICP-MS with an ultra-sonic nebuliser and ion chromatography

Abstract

In the event of a nuclear incident it is essential that analytical information on the distribution and level of contamination is available. An ICP-MS method is described which can provide data on plutonium contamination in food within 3 h of sample receipt without compromising detection limits or accuracy relative to traditional counting methods. The method can also provide simultaneous determinations of americium and neptunium. Samples were prepared by HNO₃ closed-vessel microwave digestion, evaporated to dryness and diluted into a mobile phase comprising 1.5 M HNO₃ and 0.1 mM 2,6-pyridinedicarboxylic acid. A commercially available polystyrene–divinylbenzene ion chromatography column provides on-line separation of ²³⁹Pu and ²³⁸U reducing the impact of the ²³⁸U¹H interference. Oxidation of the sample using H₂O₂ ensures all Pu is in the Pu⁺⁴ state. The oxidation also displaces Np away from the solvent front by changing the oxidation state from Np⁺³ to Np⁺⁴ and produces the insoluble Am⁺⁴ ion. Simultaneous Pu, Am and Np analyses therefore require omission of the oxidation stage and some loss of Pu data quality. Analyses were performed using a magnetic sector ICP-MS (Finnigan MAT Element). The sample is introduced to the plasma via an ultrasonic nebuliser–desolvation unit (Cetac USN 6000AT+). This combination achieves an instrumental sensitivity of ²³⁸U > 2 × 10⁷ cps/ppb and removes hydrogen from the sample gas, which also inhibits the formation of ²³⁸U¹H. The net effect of the improved sample introduction conditions is to achieve detection levels for Pu of 0.020 pg g⁻¹ (4.6 × 10⁻² Bq kg⁻¹) which is significantly below 1/10th of the most stringent EU (European Union) legislation, currently 0.436 pg g⁻¹ (1 Bq kg⁻¹) set for baby food. The new method was evaluated with a range of biological samples ranging from cabbage to milk and meat. Recovery of Pu agrees with published values (100% ± 20%).