A method of detecting a membrane protein by mass spectrometry comprises: (a) providing a solution comprising a native membrane vesicle having a lipid bilayer to which said membrane protein is attached; (b) sonicating the vesicle in the presence of a mass spectrometry-compatible buffer; (c) providing a mass spectrometer comprising a nanoelectrospray ionisation source, a mass analyser and a detector; (d) vaporising the sonicated solution using the nanoelectrospray ionisation source under conditions such that the membrane protein is released from the vesicle; (e) ionising the membrane protein; (f) resolving the ionised membrane protein using the mass analyser; and (g) detecting the resolved membrane protein using the detector. Also provided are solutions comprising sonicated vesicles for use in mass spectrometry methods.