A Sonic Spray Interface for the Mass Analysis of Highly Charged Ions from Protein Solutions at High Flow Rates

We have improved the sonic spray interface to enable the analysis of multiply charged ions of protein from a solution at a flow rate of 1 mL/min using a conventional liquid chromatograph/mass spectrometer. In this interface, we added a multihole plate in front of the sampling orifice of a mass spectrometer. This plate does not have a hole coaxial to the sampling orifice but has small holes around the central region of the plate. The plate reduces the density of the solvent molecules in the sprayed gas introduced into the vacuum region through the sampling orifice from the atmosphere and prevents the ions from being solvated and becoming charged droplets due to the cooling that follows adiabatic expansion of the sprayed gas. With this improvement, multiply charged ions whose charge distribution ranged from 11+ to 16+ were analyzed from a 1 μM cytochrome c solution at a high flow rate of 1 mL/min without using a splitter.