Cold-spray ionization mass spectrometry of the choline chloride-urea deep eutectic solvent (reline)

Cold-Spray Ionization mass spectrometry (CSI-MS) that can be compared to an electrospray ionization (ESI) source acting with a nebulizing gas cooled by liquid nitrogen is used for analyzing reline as Deep Eutectic Solvent (DES). The association of cholinium chloride salts with urea molecules is evidenced in negative CSI-MS through the chloride adduct formation. The structure of the supramolecular assemblies forming the reline ions that are observed on CSI mass spectra is rationalized by chemical quantum calculations. The theoretical studies indicate that the ionic network organization is only supported by a maximization of hydrogen bonds of the chlorides with the hydroxyl and methyl moieties of the cholinium cations and the amino groups of urea. The studies of gas-phase fragmentation of the supra-molecular ionic assemblies detected in CSI-MS are performed using the in-source collision-induced dissociation experiments. The experimental measurements in CSI-MS, interpreted at the light of the molecular modelization results, suggest that the insertion of urea in adducts of chlorides with cholinium cations does not lead to the most stable ions.