According to the present invention, there is provided a method of detecting a complex comprising a membrane protein bound to a therapeutic agent by mass spectrometry. The method comprises: (a) providing a solution comprising a detergent micelle in which said complex is contained; (b) providing a mass spectrometer comprising a nanoelectrospray ionization source, a mass analyzer and a detector; (c) vaporizing the solution using the nanoelectrospray ionization source under conditions such that the complex is released from the micelle; (d) ionizing the complex; (e) resolving the ionized complex using the mass analyzer; and (f) detecting the resolved complex using the detector. Also provided is a solution comprising a detergent micelle in which a complex is contained, wherein the complex comprises a membrane protein bound to a therapeutic agent.