Methodology:
This work uses a new mass spectrometric method using medium resolution mode (~4000) of a Thermo Scientific Neptune MC-ICP-MS. The setup presented is able to resolve all interfering molecular species in the V mass spectrum, thus removing the uncertainty of minor isotope effects from combinations of polyatomic C, N, O, S, Ar and Cl. The method utilizes high-transmission interface cones that significantly lower the amount of V required per analysis to roughly 200–300 ng, or almost an order of magnitude less than previous methods.

To test this we use pure vanadium solutions and analyze various instrumental settings to reproduce the known isotopic values of these solutions. Additionally, we test sulfur interferences by spiking the pure vanadium solutions with pure an elemental sulfur solution. Using this new instrumental method we test any potential S interferences. This new method will allow for more accurate and precise seawater analysis.

Sampling and analytical procedures:
Vanadium isotope compositions are measured using a Thermo Scientific Neptune MC-ICP-MS. The front-end is configured with a 'Jet' sampler cone and an 'X'-type skimmer cone in order to obtain the highest possible V transmission efficiency. Vanadium is introduced into the mass spectrometer as a dry aerosol by passing the sample through a CETAC Aridus II desolvator, which is fed by an ESI perfluoroalkoxy alkane (PFA) nebulizer that aspirates the sample solution at ~120 ml per minute. This configuration routinely produces a vanadium ion beam of ~2.5 nA for a solution containing 1 mg per g vanadium when the mass spectrometer is operated in medium resolution, equivalent to a total ion transmission of ~0.7‰.

The masses 48 through 53 are collected simultaneously using the L2, L1, Center, H1, H2 and H3 collectors. The Center cup was equipped with a 10¹² Ω resistor, H1 is equipped with a 10¹¹ Ω resistor, while the rest of the collectors are connected to standard 10¹¹ Ω resistors. Each sample analysis consists of 30 s background that is measured by defocusing and deflecting the ion beam away from the detectors in the electrostatic analyzer, followed by 40 cycles of 4.194 s integrations. Correction for interferences on ⁵⁰V from ⁵⁰Ti and ⁵⁰Cr that cannot be resolved in medium resolution (nor high resolution) mode are performed as outlined in Nielsen et al. 2011. Each sample is bracketed by four (two on each side) measurements of the Alfa Aesar (AA-V) standard that has been defined as δ⁵¹V = 0 (where δ⁵¹V = 1,000 × [(⁵¹V/⁵⁰Vsample - ⁵¹V/⁵⁰VAA-V)/⁵¹V/⁵⁰VAA-V]). In addition, each sample measurement is bracketed by two measurements of the bracketing standard BDH vanadium solution that has an isotope composition of δ⁵¹V = ~1.19 ±0.12 (2sd) (Nielsen et al., 2011).

Methods for V isotope analysis are published in Nielsen et al., 2016 (doi: 10.1039/C5JA00397K).