59th Annual Report on Research 2014

The aim of this project was to establish the method of vanadium isotope measurements in crude oils and associated organic rich sediments in order to assess the utility of vanadium isotopes to provide information about processes associated with petroleum formation.

The project primarily involved significant method development due to the finding that the mass spectrometer we utilize at Woods Hole (Thermo Finigan Neptune; the most common mass spec used by isotope (geo)chemists globally) generates significant interferences on vanadium from sulfur oxide molecules that have almost the same atomic mass as the two vanadium isotopes. Based on collaboration with colleagues at Imperial College London, UK, we discovered that the other commercially available mass spectrometer (Nu Plasma) does not produce these interferences, which is the reason why the vanadium isotope method works well when using the latter instrument even if some sulfur is present in samples.

Attempts to correct for these interferences on the Neptune proved difficult and we therefore spent a substantial amount of time developing protocols that allowed for the separation of sulfur from vanadium. We found that passing samples through an anion exchange resin (AG1x8) column in a dilute hydrochloric acid medium (0.01M) quantitatively removes all sulfur, thus eliminating any sulfur oxide interferences on vanadium.

The experience gained from these method advances is not only critical to the present project, but will serve all future studies of vanadium isotopes when using a Thermo Finnigan Neptune mass spectrometer. Because this is the most commonly used mass spec by isotope (geo)chemists worldwide, it is crucial to document the specific procedures necessary to generate reliable data for this instrument. We are in the process of preparing a manuscript that outlines these techniques.

In addition to the method development on vanadium isotopes, Postdoctoral fellow Dr. Sen, who was supported by the ACS PRF grant, was also involved in several other studies relevant to the goals of the ACS PRF fund. Together with Dr. Bernhar Peucker-Ehrenbrink (faculty at Woods Hole) he developed a new method for obtaining osmium isotope data from oils, organic rich sediments and other natural materials, much more rapidly than has ever before been documented. These protocols were published in Analytical Chemistry in 2014 [1]. In collaboration with PI Nielsen and Dr. Ehrenbrink, Dr. Sen is also currently preparing a manuscript describing trace metal partitioning between oil and sediments in hydrous pyrolysis experiments.

Unfortunately, it was not possible to obtain additional funds for the continuation of the 8-month postdoctoral appointment of Dr. Sen funded by ACS PRF. Dr. Sen has instead returned home to India, where he currently has a faculty position at the Indian Institute of Technology at Kanpur (IIT-Kanpur). While it would have been ideal for the project that Dr. Sen could have continued his successful method development at WHOI, his new position at IIT-Kanpur will allow him to further develop the research he started at WHOI on vanadium isotopes. He intends to keep strong ties with WHOI and he returned for a short visit to Woods Hole in the summer of 2014. The funds from ACS PRF therefore have provided not only an important springboard for a young scientist, but also given PI Nielsen and WHOI as an institution an opportunity to form new collaborations with a high-ranking institution in a developing country.

1. Sen, I.S. and B. Peucker-Ehrenbrink, Determination of Osmium Concentrations and Os-187/Os-188 of Crude Oils and Source Rocks by Coupling High-Pressure, High-Temperature Digestion with Sparging OsO4 into a Multicollector Inductively Coupled Plasma Mass Spectrometer. Analytical Chemistry, 2014. 86(6): p. 2982-2988.