Salicylaldehyde azine cluster formation observed by cold-spray ionization mass spectrometry

We installed a cold-spray ionization (CSI) source on a mass spectrometer to investigate the self-assembly behavior of an aggregation-induced emission enhancement system. Using a CSI source and the three-dimensional platform, a self-assembly system of a salicylaldehyde azine (SAA) was studied in mixture solution. This method permitted the determination of the structural information of the solution state, which cannot be detected by conventional mass spectrometry. In addition to the [M+H](+) ion (M is the SAA molecule), many major ion clusters such as [2M+Na](+) at m/z 503, [3M+Na](+) at m/z 743, [4M+Na](+) at m/z 983 and higher order aggregates were observed in the CSI mass spectra. However, many fragment ions, with the exception of cluster ions, appeared with high abundance when the ESI ion source was used due to the desolvation chamber temperature, suggesting that some aggregation can be detected at low temperatures. To investigate the effect of solvent on the aggregation, the CSI-mass spectrometry (MS) experiments of SAA in absolute ethanol solution and ethanol/water (good/poor solvent) mixture solution were conducted. The most abundant ion peak was protonated SAA (m/z 241) in absolute ethanol, but many cluster ions and some multiple charged ion peaks were observed after adding a small amount of water into the ethanol solution. The results showed good agreement with that inferred by the combinational analysis of scanning electron microscope and fluorescence microscopy, indicating that CSI-MS is capable of providing self-assembly information of labile molecules in the solution state.