Accurate and precise determination of 90Sr at femtogram level in IAEA proficiency test using Thermal Ionization Mass Spectrometry

A novel method for the determination of ultra-trace level Sr-90 has been recently developed applying thermal ionization mass spectrometry (TIMS). The method includes the chemical separation of Zr (isobaric interference of Zr-90) from the samples followed by determination of Sr-90/Sr-88 abundance sensitivity (2.1 x 10(-10)). The analytical performance of this method was assessed in the IAEA-TEL 2017-3 worldwide open proficiency test. For Sr-90 determination, tap water and milk powder samples were distributed amongst the participant laboratories with reference values of 11.2 +/- 0.3 Bq kg(-1) (2.2 +/- 0.1 fg g(-1)) and 99.9 +/- 5.0 Bq kg(-1) (19.5 +/- 1.0 fg g(-1)), respectively. The stable Sr concentrations were 39.4 +/- 0.9 ng g(-1) and 2.5 +/- 0.1 mu g g(-1) while the Sr-90/Sr-88 isotope ratios were 6.47 +/- 0.17 x 10(-8) and 9.04 +/- 0.45 x 10(-9) in the tap water and milk powder samples, respectively. For TIMS measurement, 50 mL water and 1 g milk powder samples were taken for analysis. This TIMS method demonstrated an impressive accuracy (relative bias of 4.2% and -2.1%, respectively) and precision (relative combined uncertainty of 4.1% and 7.6%, respectively) when compared with radiometric techniques. For the first time in the history of inorganic mass-spectrometry, Sr-90 analysis using a TIMS instrument is confirmed by an independent proficiency test.