Behavior of transition metal salts during the electrospray ionization process

Abstract

A study of the behavior of transition metal salts during the electrospray ionization (ESI) process using a quadrupole ion trap mass spectrometer and novel ion source is reported. The unique results in both positive and negative modes for multiple salts of 28 different cations are discussed. In positive mode, the metal cations formed clusters with the solvents, following the pattern [M + (solvent)n]+, with maximum observed values for n of 5. In general, transition metals with more than one oxidation state, such as iron, cobalt, and chromium, produced more complicated mass spectra, because the ions formed also included multiples of the anion. In negative mode, the cations formed clusters with the anions of the salts, following the pattern [My + Xn]-. The relationship between y and n correlates to the oxidation state of the cation, with maximum observed values for y and n of 3 and 7, respectively. For some environmental applications, studies in negative mode demonstrate an alternative to specialized elemental analysis instrumentation such as ICP/MS or AMS, as well as complicated sample preparation, such as the use of chelating complexes. This is especially applicable for the analyses of mercury and lead, as these species form only a few, predictable ions. For instance, mercury present in seawater forms a single ion, [Hg + Cl3]-, which can be readily isolated from the sodium chloride clusters in the background with a non-specialized ion trap mass spectrometer using ESI, and can be verified by its characteristic isotopic pattern.