Zinc/Silver-Based Hydrogen Sulfide Trapping as an Alternative to Cadmium for Sulfur Stable Isotope Analysis

Rationale

Hydrogen sulfide (H₂S) sulfur stable isotope ratio (δ³⁴S) analyses use cadmium (Cd) or zinc (Zn) solutions; however, Cd is toxic, and Zn does not display an obvious change in color at low molar H₂S inputs. This study (i) developed a non-toxic, colorimetric mixed zinc/silver (Zn/Ag)-based method and (ii) evaluated the minimum H₂S input required for analysis.

Methods

Gaseous H₂S was trapped using the Cd- and Zn/Ag-based methods and converted to solid silver sulfide (Ag₂S). δ³⁴S stable isotope analysis was completed using elemental analysis–isotope ratio mass spectrometry (EA-IRMS).

Results

H₂S, from 8.4 × 10⁻⁶ mol up to 1.0 × 10⁻⁴ mol, was trapped using the Cd- and Zn/Ag-based methods. The Zn/Ag-based method retained stable δ³⁴S values at lower molar inputs than the Cd-based method. Environmental sampling had a usable detection range of approximately 4.1 × 10⁻⁴–7.8 × 10⁻¹ mol/m³ (~10–19 000 ppm). The mean δ³⁴S difference between methods was −0.1‰, ranging from −3.0‰ to +3.1‰, with 23 out of 27 samples falling within ±1.2‰.

Conclusions

The Zn/Ag-based method produced δ³⁴S values comparable with the Cd-based method and maintained accuracy with lower molar inputs of H₂S. The minimum molar input for analysis is ~8.4 × 10⁻⁶ mol. The Zn/Ag method is safer for human health and the environment, and a suitable alternative to Cd for H₂S trapping and subsequent δ³⁴S determination by IRMS.