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.