A new analytical approach to determining Mo and Re speciation in sulfidic waters

- Mo and Re thiometalates cannot be separated using anion exchange chromatography.
- Mo and Re thiometalates are separated using reverse phase ion pair chromatography.
- Chromatographic methodologies differ markedly for Mo and Re thiometalates.
- Addition of p-cyanophenol to the eluent assists in facilitating separation.
- Provides foundation for methods aiming to quantify speciation in natural waters

The distinct reactivities of Mo and Re in oxic versus anoxic waters make these elements ideally suited for use as redox proxies. However, their full exploitation as geochemical tracers requires that their chemical transformations in sulfidic water be well understood. While thermodynamic data have been used to predict Mo and Re speciation within sulfidic waters, these predictions remain unsubstantiated because effective methodologies for separating the thiomolybdate (MoVIOxS4 − x2 −) and thioperrhenate (ReVIIOxS4 − x−) anions within natural settings have yet to be developed. Anion exchange chromatography (AEC) is often employed to provide separation of environmentally important anions. For example, thiometalates of As and Sb have been quantified in sulfidic geothermal waters using AEC coupled with inductively coupled plasma mass spectrometry (ICP-MS). Unfortunately, AEC methods are incapable of separating the thiomolybdate or thioperrhenate anions due to the extreme retention these thiometalates experience on AEC columns. Because reverse phase ion pair chromatography (RP-IPC) offers greatly diminished retention times, we have developed RP-IPC methods that are capable of separating all stable thiomolybdates and thioperrhenates within 40 min. An isocratic method provides effective separation of the thiomolybdates. Thioperrhenates and mixtures of the thiomolybdates and thioperrhenates require gradient methods. Addition of p-cyanophenol (p-CP) to the eluent markedly reduces thiometalate retention and facilitates their separation. Our efforts could lead in the near future to coupling RP-IPC with ICP-MS or multi-collector ICP-MS for characterizing Mo and Re speciation in natural sulfidic waters as well as potential fractionation among Mo and Re isotopes during speciation changes.