Determination of femtogram quantities of 239Pu and 240Pu in bioassay samples by thermal ionization mass spectrometry

A thermal ionization mass spectrometry (TIMS) method is described for the determination of ultra-trace levels of plutonium isotopes in human urine samples. The method has been validated through the analysis of artificial urine samples spiked with known amounts of Pu-239 ranging from 2.5 fg to 50 fg (6-115 mu Bq). A slight positive bias of 1.7%-2.7% was determined, with a relative precision of 2.2% at 50 fg. increasing to 2.7% for 5-25 fg Pu-239. The detection limit of the method was 0.53 fg (1.2 mu Bq) Pu-239. and the instrumental detection limit was at least 0.1 fg. The determination of the isotopic signature of the sample with Pu-239, Pu-240, and Pu-241 amounts of several femtograms is possible, and was demonstrated with the determination of the 240 to 239 ratio in an inter-laboratory sample comparison. The method is relatively free from interferences, 95% of sample preparations were acceptable both in terms of chemical recovery and lack of isobaric interference. The isotopic abundance of the Pu-242 SRM 4334E of the National Institute of Standards and Technology (NIST) was also determined by TIMS and was found to be 99.99967 atom% Pu-242.