Sune Nielsen, PhD, Woods Hole Oceanographic Institution
The first year of the project has seen significant analytical challenges that were unexpected based on previous efforts of the PI while at a different institution (University of Oxford, UK). Specifically, we have found 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. Attempts to correct for these interferences have proven difficult, and no consistent results could be obtained when implementing these corrections.
Based on collaboration with colleagues at Imperial College London, UK, we have 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.
The problems relating to sulfur oxide interferences can be overcome by quantitatively removing sulfur from samples prior to introducing them into the mass spectrometer. We have spent the major portion of the first year testing various protocols aimed at separating sulfur from our samples. These protocols include equilibrating the sample with a special Nobias ion exchange resin at different pH, precipitating the sulfur as barium sulfate, and passing samples through an anion exchange resin (AG1x8) column at various hydrochloric acid molarities. We have found that by far the most efficient sulfur removal method is to dissolve samples in dilute hydrochloric acid (0.01M) and pass them through an anion exchange column. This procedure quantitatively removes all sulfur, thus eliminating any sulfur oxide interferences on vanadium.
Subsequent to this advance, we have also discovered that many of the plastic receptacles in our laboratory emit minor amounts of sulfur when immersed in acid and therefore have contaminated many of our acids and standards with sulfur. This sulfur contamination precludes production of reliable vanadium isotope measurements. From our preliminary investigation it appears that Teflon does not emit any appreciable sulfur, we have therefore been forced to replace all non-teflon plastic ware used for vanadium chemistry. At present we have completed the replacement of plastic ware, which we are confident will now allow us to proceed with vanadium isotope measurements of crude oils and organic rich sediments.
PI Nielsen and postdoctoral fellow Dr. Indra Sen, who has been supported by the grant, have primarily conducted the technical developments described above. 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 expect to prepare a manuscript in the coming year that outlines these techniques.
The funds allocated by ACS PRF have been exceptionally important as they have allowed PI Nielsen and Postdoctoral fellow Dr. Sen to not only perform vital method development, but also for the first time conduct independent research on petroleum formation and organic rich sediments.
Unfortunately, it has not been 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 has secured a faculty position at the Indian Institute of Technology at Bhubaneswar (IIT-BBS). 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-BBS will allow him to further develop the research he started at WHOI on vanadium isotopes. He intends to keep strong ties with WHOI and we have planned for him to return as a visiting scientist 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.