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

A novel method for the determination of ultra-trace level ⁹⁰Sr has been recently developed applying thermal ionization mass spectrometry (TIMS). The method includes the chemical separation of Zr (isobaric interference of ⁹⁰Zr) from the samples followed by determination of ⁹⁰Sr/⁸⁸Sr abundance  sensitivity (2.1 × 10^-10). The analytical performance of this method was assessed in the IAEA-TEL 2017-3 worldwide open proficiency test. For ⁹⁰Sr 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 µg g^-1 while the ⁹⁰Sr/⁸⁸Sr isotope ratios were 6.47 ± 0.17 × 10^-8 and 9.04 ± 0.45 × 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, ⁹⁰Sr analysis using a TIMS instrument is confirmed by an independent proficiency test.