Atmospheric pressure surface ionization mass spectrometry for oxidized SUS304 and nichrome: chromic acids (H2CrnO3n+1) as intermediate precursors for the formation of hexavalent chromium oxide negative ions HCrnO3n+1− and protonated oxide surface as solid acid

Negative and positive ions generated from heated oxidized SUS304 and nichrome filaments were measured by atmospheric pressure surface ionization mass spectrometry. Both filaments produced hexavalent chromium-oxide negative ions of HCr_nO_3n+1⁻ (n = 1–4) and Cr_nO_3n^−• (n = 1–4) and positive ions of A₂HCr_nO_3n+1⁺ (n = 1–3) (A: Na and/or K). While negative ions were detected at temperatures above ∼1000 °C, positive ions were detected at much lower temperature above ∼800 °C. The precursors of negative ions were assumed to be chromic acid formed by the reaction, Cr_nO_3n + H₂O → H₂Cr_nO_3n+1 followed by electrolytic dissociation, H₂Cr_nO_3n+1 → H⁺ + HCr_nO_3n+1⁻ taking place on the solid surface. The proton H⁺ may be captured by the surface oxide instantly to form protonated metal oxide which acts a solid Brønsted acid for the protonation of gas-phase analytes. The formation of hexavalent chromium oxide ions from heated chromium alloys may be hazardous to health because they are carcinogenic at low concentrations due to their strong oxidizing properties.