Analytical applicability of transition metals and boron anionic complexes formed with iodide, chloride, cyanide, and fluoride in electrospray mass spectrometry

Abstract

A negative-mode electrospray ionization mass spectrometry study of some transition metal ions and boron was carried out. Several experiments combining these elements with iodide, fluoride, chloride, and cyanide are presented. Methanol/water was used mainly as the solvent in order to reduce the surface tension and, thus, the voltage at the capillary tip. Some common behaviors could be observed. Metals give more abundant peaks with iodide and chloride, while boron gives an abundant cluster for BF 4 −. In general, the complexes are singly charged, formed by association of the metal ions with the anions present in the solution, or by loss of one or more ligands from species previously present in solution. In some cases, this tendency surpasses the maintenance of the oxidation state of the metal in liquid phase. The interconversion of low and high oxidation states of copper and iron ions depends on the solvent and other species from the solution, but in the gas phase the high oxidation state species can be reduced by collision induced dissociation at low sampling cone voltages. Surprisingly, ferricyanide and ferrocyanide anions render almost the same spectrum. The results suggest that [Fe(CN) 6] 4− loses one electron to a leaving solvent molecule to form [Fe(CN) 6] 3− in the final steps of desolvation. These and other results suggest that, for the ligands studied in this work, quantitation and speciation are not easy tasks, but there is the possibility of performing isotope ratio measurements with the complexes formed with monoisotopic anions. The main advantages in this case would be the shift of the m/z to high mass region, which diminishes the chance of isobaric interference, and the inexistence of hydrides, commonly observed in the positive mode electrospray mass spectrometry spectra of metal ions and that cause isobaric interference.