Isotope ratio analysis of sulfate using Orbitrap Exploris Isotope Solutions

Significance of the topic

Tracing the isotopic composition of sulfate in water samples reveals sources and processes in the sulfur cycle, aiding environmental monitoring and pollution assessment.

Objectives and study overview

This study demonstrates how Orbitrap Exploris Isotope Solutions provide simultaneous δ34S, δ18O and clumped isotopocule analysis of intact sulfate ions from a single aliquot, using two injection methods.

Methodology and instrumentation

Samples and standards were dissolved at 100 mM and diluted to 50 µM in methanol. Electrospray ionization delivered intact HSO4⁻ ions to the Orbitrap analyzer. Two experiments ('M0' including the monoisotopic peak and 'noM0' excluding it) targeted scan ranges of 93–105 or 97.4–105 m/z with resolutions of 45 000–60 000. AGC target and quadrupole filtering optimized ion packages. Data acquisition used Thermo Xcalibur software; isotopic calculations employed IsoX, custom R scripts and Excel.

Instrumentation

• Thermo Scientific Orbitrap Exploris 120/240/480 MS
• Thermo Scientific Vanquish Neo UHPLC System (in-flow injection)
• Dual Syringe Inlet system with syringe pump

Key results and discussion

The method detected all major sulfate isotopocules (M0 to M+4) in a single run. Standard materials (SMIF-1, SMIF-2, S-hot, S-18O-depleted) showed precise δ34S, δ18O and clumped values. Addition of 0.1 % formic acid suppressed monosodium adducts and improved spray stability without biasing isotope ratios. Linearity was maintained between 1 and 100 µM sulfate. River water samples exhibited δ34S and δ18O trends reflecting mixing between evaporite dissolution and pyrite oxidation.

Benefits and practical applications

• Simultaneous measurement of sulfur and oxygen isotopes from one aliquot
• Analysis time of ~15 minutes per sample with low consumption
• Direct determination of clumped isotopocule distributions
• Reduced sample preparation compared to conventional IRMS

Future trends and potential applications

Integration with chromatographic separation could extend analyses to complex matrices. Further automation, higher resolution and enhanced sensitivity will support in situ studies of sulfate cycling. Application to additional anionic species may broaden isotopic tracing capabilities.

Conclusion

Orbitrap Exploris Isotope Solutions deliver rapid, sensitive and comprehensive sulfate isotope ratio analysis, enabling multispectral isotopic measurements from a single injection and advancing biogeochemical investigations.

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