Electrospray mass spectrometry method to prevent the reduction of hexavalent to pentavalent chromium

Electrospray mass spectrometry (ESI-MS) is one of the most effective methods for assessing the state of metals in solution. For ions with a redox potential close to ~0.55 V, such as Cr⁶⁺ , reduction of the metal in solution occurs in the ESI-MS system. In our studies, it was observed that [HCrO₄ ]^- undergoes reduction, resulting in the formation of [CrO₃ ]^- . The precise mechanism remains ambiguous. The reduction of hexavalent chromium to pentavalent chromium is supported by Frost diagrams, reinforcing our confidence in the validity of the ESI-MS measurement method. The reduction mechanism in ESI-MS was clarified, and a system was devised to eliminate electron donation during the reduction of Cr⁶⁺ in solution.

To determine the state of Cr⁶⁺ by ESI-MS, CrO₃ in solid form was dissolved in ultrapure water to prepare a solution of 500 × 10^-6  mol/L (μM) concentration. The pH was adjusted to 4.0, 5.3, 6.3, 8.2 and 9.1 and subsequently measured. CrO₃ solutions with various concentrations of 10, 100 and 500 μM were prepared and adjusted to a pH of ~7 using tetramethylammonium hydroxide to measure Cr⁶⁺ under different conditions.

Cr⁶⁺ in solution was soluble and existed as an oxoacid with a negative charge independent of pH. Cr⁶⁺ was stable over a wide pH range at various concentrations. The ESI-MS method determined the negative ion [HCrO₄ ]^- as the stable ion, but [CrO₃ ]^- was also present as a byproduct. Therefore, we were interested in the presence of other species, such as [CrO₃ ]^- , which could have formed owing to the reduction of Cr⁶⁺ .

In ESI-MS system, it undergoes reduction to form [CrO₃ ]^- . The high flow rate of ultrapure water in pump insulated the acceptance of electrons by Cr⁶⁺ preventing its reduction. Further in-depth ESI-MS studies could explain the complex formation and behavior of Cr⁶⁺ in aqueous solution.