Detection of native proteins using solid-substrate electrospray ionization mass spectrometry with nonpolar solvents

Detection of native proteins, particularly directly from raw biological samples, has been a challenging task for mass spectrometry. In this study, we demonstrated that solid-substrate electrospray ionization mass spectrometry with nonpolar solvents such as n-hexane could allow detection of native proteins and protein complexes directly from raw biological samples. Mechanistic study revealed that the process involved rapid vaporization of the nonpolar solvent, temperature reduction of substrate surface, condensation of water from the ambient air, and spray ionization of the condensed water with analytes under the electric field. The fine spray with water at low temperature allowed the technique to detect native proteins, even directly from viscous samples (e.g., egg white) and solid samples (e.g., bone marrow). This study sheds new insight into the sampling and ionization process of mass spectrometry and provides a technique of great potential for characterization of proteins.