An investigation of the ionicity of selected ionic liquid matrices used for matrix-assisted laser desorption/ionization

Abstract Ionic liquid matrices (ILMs) have been applied for MALDI-MS analysis for more than 20 years. Despite this, there is still insufficient knowledge regarding their properties. In this work, we tested the ionicity of selected ILMs and discussed their impact on the MALDI process. The targets were the following mixtures: CHCA/TEA, FER/TEA, HABA/α-MBA, 2,5-DHB/α-MBA (CHCA - α-cyano-4-hydroxycinnamic acid, FER – ferulic acid, HABA - 2-(4-hydroxyphenylazo)benzoic acid, 2,5-DHB - 2,5-dihydroxybenzoic acid, TEA – triethylamine, α-MBA - α-methylbenzylamine). These are ILMs which in our previous study showed liquidity at room temperature. We decided to apply several empirical and theoretical techniques due to the complexity of the problem. Thermogravimetric analysis (TGA) with IR was used to test the process at a higher temperature (simulation of a laser shot). 1H-NMR was used for the determination in pure ILMs of intermolecular interactions, which have a high impact on ionicity. Conductivity and rheological measurements were performed to determine the Walden plot, from which the excess of ionicity can be concluded. DFT calculations were used to support the statements of dissociation processes in the gas phase and solution. The auto-ionization of matrices in MALDI was also tested. The interpretation of the Walden plot shows that HABA/α-MBA is a very good ionic liquid, which can be derived from a proposed “pedal motion”-like mechanism. The ionicity of other matrices was reduced because of intermolecular interactions (proved by TGA-IR and 1H NMR). The MALDI-MS analysis shows that the obtained mass spectra, namely the base peaks, depend on amine gas proton affinity. Furthermore, the DFT calculation results and the TGA-IR analysis support the mechanism of retro-proton transfer after the laser shot in the gas phase. Finally, it was concluded that the four tested ILMs are differentiated in ionicity, but its extent has no impact on the auto-ionization of the matrix.