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 239Pu ranging from 2.5 fg to 50 fg (6-115 μBq). 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 239Pu. The detection limit of the method was 0.53 fg (1.2 μBq) 239Pu, and the instrumental detection limit was at least 0.1 fg. The determination of the isotopic signature of the sample with 239Pu, 240Pu, and 241Pu 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 242Pu SRM 4334E of the National Institute of Standards and Technology (NIST) was also determined by TIMS and was found to be 99.99967 atom% 242Pu. (author)