Solvent effects on the ion formation and fragmentation behaviour of copper(II) acetate in desorption electrospray ionisation mass spectrometry

Solvent selection is one of the most influential parameters in desorption electrospray ionisation (DESI), as it governs the ionisation process. In coordination chemistry, this dependence is particularly pronounced due to possible solvent substitution of ligands. For the first time, DESI was applied to the analysis of copper(II) acetate deposited on paper, with the aim of systematically evaluating the influence of solvent selection on the obtained mass spectrum. Pure water failed to produce stable ion signals, whereas organic solvents enabled efficient ionisation with varying spectral complexity. Methanol and ethanol yielded closely related spectra dominated by ions derived from copper acetate, reflecting partial ligand retention. In contrast, acetonitrile promoted complete ligand substitution, resulting only in the formation of copper-acetonitrile species, while acetone resulted in highly complex spectra containing both solvent and acetate species. Propan-2-ol required increased analyte loadings to obtain detectable signals, consistent with reduced extraction efficiency. High-resolution and tandem mass spectrometry experiments with methanol and ethanol revealed a consistent fragmentation pathway of copper-acetate ions converging to [Cu(CH₃COOH)] + , which has not been reported before. This study establishes DESI mass spectrometry as a suitable technique for conducting detailed studies of coordination compounds. It provides insight into solvent-controlled metal–ligand coordination processes under ambient conditions, highlighting the importance of solvent selection in DESI analysis.