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

Abstract

RationaleElectrospray 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.MethodsTo 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.ResultsCr6+ 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+.ConclusionsIn 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.