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

Abstract

Advances in sulfur isotope measurement techniques have led to increased analytical precision. However, accurate measurement of 36S remains a challenge. This difficulty arises primarily from unresolved isobaric interferences of 36SF5 + at m/z = 131u, 186WF4 2+ and 12C3F5 +, which lead to scale compression. Theoretically, unresolved interference with 2% relative intensity could cause 1‰ underestimation in a sample with real δ36S = +60‰. 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 186WF4 2+ and 12C3F5 + from 36SF5 +. The 186WF4 2+ relative intensity was initially 9.4% that of 36SF5 + but reduced to 1.5% through tuning, while 12C3F5 + relative intensity dropped from 74% to 40% after flushing with air. Three IAEA standards were analyzed with both Panorama and MAT 253. We obtained Δ36SIAEA-S-2 = 1.238 ± 0.040‰ and Δ36SIAEA-S-3 = -0.882 ± 0.030‰, relative to IAEA-S-1, from Panorama, and Δ36SIAEA-S-2 = 0.18 ± 0.02‰ and Δ36SIAEA-S-3 = 0.11 ± 0.14‰ from MAT 253, while δ34S values from the two instruments are consistent. The measurement discrepancies on 36S between Panorama and MAT 253 highlight the impact of scale compression due to unresolved isobaric interferences. Resolving this problem is crucial for accurate 36S 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 δ33S, δ34S, and δ36S 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.