Fully resolved high-precision measurement of 36S for sulfur reference materials

Rationale : Advances in sulfur isotope measurement techniques have led to increased analytical precision. However, accurate measurement of S-36 remains a challenge. This difficulty arises primarily from unresolved isobaric interferences of (SF5+)-S-36 at m/z = 131 u, (WF42+)-W-186 and (C3F5+)-C-12, which lead to scale compression. Theoretically, unresolved interference with 2% relative intensity could cause 1 parts per thousand underestimation in a sample with real delta S-36 = +60 parts per thousand. Methods : Our study develops an interference-free four-sulfur isotope measurement method by using the high-resolution mass spectrometer Panorama. The mass resolving power of Panorama allows the distinction of (WF42+)-W-186 and (C3F5+)-C-12 from (SF5+)-S-36. Results : The (WF42+)-W-186 relative intensity was initially 9.4% that of (SF5+)-S-36 but reduced to 1.5% through tuning, while (C3F5+)-C-12 relative intensity dropped from 74% to 40% after flushing with air. Three IAEA standards were analyzed with both Panorama and MAT 253. We obtained Delta S-36(IAEA-S-2) = 1.238 +/- 0.040 parts per thousand and Delta S-36(IAEA-S-3) = -0.882 +/- 0.030 parts per thousand, relative to IAEA-S-1, from Panorama, and Delta S-36(IAEA-S-2) = 0.18 +/- 0.02 parts per thousand and Delta S-36(IAEA-S-3) = 0.11 +/- 0.14 parts per thousand from MAT 253, while delta S-34 values from the two instruments are consistent. Conclusion : The measurement discrepancies on S-36 between Panorama and MAT 253 highlight the impact of scale compression due to unresolved isobaric interferences. Resolving this problem is crucial for accurate S-36 analysis. We recommend replacing the filament material with rhenium, tuning the filament voltage, and avoiding carbon in instruments to eliminate or mitigate interferences. We propose future systematic efforts to calibrate the delta S-33, delta S-34, and delta S-36 of IAEA-S-1, IAEA-S-2, and IAEA-S-3 and advise bracketing all three reference materials in the measurement sequences, to enable calibration.