BayCEER Workshop 2015

Experimental confirmation of isotope fractionation in thiomolybdates

Introduction

Confirming the so far only modelled isotopic fractionation among intermediate thiomolybdate species experimentally is an important step towards better understanding the isotopic fractionation of molybdenum (Mo) in sulfidic sediment records.

Material and Methods

A new method for the determination of isotope ratios in individual thiomolybdate species was developed using analytical chromatography (HPLC-UV) to separate the individual thiomolybdates, collect each peak and analyze the isotope ratios with multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS).

Results

Using commercially available MoO₄^2- and MoS₄^2- standards, the method was evaluated and excellent reproducibility and accuracy were obtained for MoO₄^2-. For species with longer retention times, complete chromatographic peaks must be collected to avoid fractionation within peaks. Isotope fractionation during the formation of thiomolybdates could be experimentally proven for the first time in experiments with Mo to sulfide (S) ratios of 1:20 and 1:50. The experiment with Mo to S ratios 1:20 could confirm the previously theoretically calculated fractionation for MoS₄^2- and the result for MoO₂S₂^2- was in the predicted range, though not matching completely. Tetrathiomolybdate was oxidized with H₂O₂ and pressurized air in a second set of experiments. Isotopic fractionation was observed but only kinetic instead of equilibrium fractionation.

Conclusions
While in general HPLC-MC-ICP-MS was confirmed to be a suitable method for Mo isotope determination in intermediate thiomolybdates, transfer to natural samples will require further optimization for low natural conditions.