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

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 = 131 u, 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 Δ³⁶S_IAEA-S-2 = 1.238 ± 0.040‰ and Δ³⁶S_IAEA-S-3 = -0.882 ± 0.030‰, relative to IAEA-S-1, from Panorama, and Δ³⁶S_IAEA-S-2 = 0.18 ± 0.02‰ and Δ³⁶S_IAEA-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.