Solution- and gas-phase behavior of decavanadate: implications for mass spectrometric analysis of redox-active polyoxidometalates

Abstract

Decavanadate (V₁₀O₂₈ ^6- or V10) is a paradigmatic member of the polyoxidometalate (POM) family, which has been attracting much attention within both materials/inorganic and biomedical communities due to its unique structural and electrochemical properties. In this work we explored the utility of high-resolution electrospray ionization (ESI) mass spectrometry (MS) and ion exclusion chromatography LC/MS for structural analysis of V10 species in aqueous solutions. While ESI generates abundant molecular ions representing the intact V10 species, their isotopic distributions show significant deviations from the theoretical ones. A combination of high-resolution MS measurements and hydrogen/deuterium exchange allows these deviations to be investigated and interpreted as a result of partial reduction of V10. While the redox processes are known to occur in the ESI interface and influence the oxidation state of redox-active analytes, the LC/MS measurements using ion exclusion chromatography provide unequivocal evidence that the mixed-valence V10 species exist in solution, as extracted ion chromatograms representing V10 molecular ions at different oxidation states exhibit distinct elution profiles. The spontaneous reduction of V10 in solution is seen even in the presence of hydrogen peroxide and has not been previously observed. The susceptibility to reduction of V10 is likely to be shared by other redox active POMs. In addition to the molecular V10 ions, a high-abundance ionic signal for a V₁₀O₂₆ ^2- anion was displayed in the negative-ion ESI mass spectra. None of the V₁₀O₂₆ cations were detected in ESI MS, and only a low-abundance signal was observed for V₁₀O₂₆ anions with a single negative charge, indicating that the presence of abundant V₁₀O₂₆ ^2- anions in ESI MS reflects gas-phase instability of V₁₀O₂₈ anions carrying two charges. The gas-phase origin of the V₁₀O₂₆ ^2- anion was confirmed in tandem MS measurements, where mild collisional activation was applied to V10 molecular ions with an even number of hydrogen atoms (H₄V₁₀O₂₈ ^2-), resulting in a facile loss of H₂O molecules and giving rise to V₁₀O₂₆ ^2- as the lowest-mass fragment ion. Water loss was also observed for V₁₀O₂₈ anions carrying an odd number of hydrogen atoms (e.g., H₅V₁₀O₂₈ ^-), followed by a less efficient and incomplete removal of an OH^• radical, giving rise to both HV₁₀O₂₆ ^- and V₁₀O₂₅ ^- fragment ions. Importantly, at least one hydrogen atom was required for ion fragmentation in the gas phase, as no further dissociation was observed for any hydrogen-free V10 ionic species. The presented workflow allows a distinction to be readily made between the spectral features revealing the presence of non-canonical POM species in the bulk solution from those that arise due to physical and chemical processes occurring in the ESI interface and/or the gas phase.

Figure 1.

ESI mass spectrum of a 1 mM aqueous solution of sodium V10 in 5 mM ammonium acetate (pH adjusted to 4.7). The inset shows the isotopic distribution of the most abundant mono-anionic species (black trace) and the calculated isotopic distributions for the fully oxidized species (H₅V₁₀O₂₈ˉ, red bars) and a partially reduced one (H₆V₁₀O₂₈ˉ, blue).

Figure 2.

Extracted ion chromatograms representing the fully oxidized V10 (gray) and the partially reduced species incorporating one (red), two (blue) and three (olive) metal atoms at oxidation state IV obtained using ion exclusion LC with on-line MS detection. The inset shows the evolution of the ionic signal within the m/z window 961–967 as a function of elution time.

Figure 3.

Low-energy CAD mass spectra of H₄V₁₀O₂₈²ˉ (red trace), H₅V₁₀O₂₈ˉ (brown) and NaH₄V₁₀O₂₈ˉ (blue) acquired by placing the most abundant isotopologue of each species in the center of a 3-m/z unit wide mass selection window and applying low (5 V) collisional activation. The dotted lines show positions of the three precursor ions, and the stars represent the FT harmonics. The insets show isotopic distributions of the surviving precursor ions (right) and the “terminal” fragment ion, V₁₀O₂₅ˉ (left). The precursor ion isolation window highlighted in orange and the black traces represent isotopic distributions of these ions in MS1.

Figure 4.

Zoomed views of mass spectra of V10 (sodium salt) acquired in H₂O and ²H₂O (black and blue traces, respectively) showing isotopic clusters corresponding to V₁₀O₂₆²ˉ (left) and V₆O₁₆²ˉ (right). The inset on the left panel shows the results of high-resolution measurements acquired in the narrow-band mode.