Lanthanide ions provide a classic example of the distortion of speciation that sometimes occurs during the electrospray ionization (ESI) process. Under normal conditions, M3+ ions are not observed as such from aqueous solution; rather, they appear as charge-reduced species such as [M(OH)]2+ or even [MO]+ ions. However, source conditions can be tuned to a “cold-flooding” mode such that [M(H2O)n]3+ ions can be readily obtained, thus providing spectra that are much more representative of the solution speciation. Examination of heavily hydrated trications by MS/MS enables the step-by-step progression of a small droplet to a solvated ion and eventually to a solvent-free ion to be examined in detail, and the point at which charge reduction occurs can be established. We find a strong correlation between the solution acidity of each lanthanide ion and the number of water molecules at which charge reduction occurs under MS/MS conditions. Examination of previously published data on a range of dications supports this conclusion. So not only can ESI-MS provide an accurate picture of the solution speciation, it can also provide insights into chemical behavior in solution.
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