A method of detecting a membrane protein by mass spectrometry, wherein the method comprises: (a) providing a solution comprising a detergent micelle in which said membrane protein is contained; (b) providing a mass spectrometer comprising a nanoelectrospray ionisation source, a mass analyser and a detector; (c) vaporising the solution using the nanoelectrospray ionisation source under conditions such that the membrane protein is released from the micelle; (d) ionising the membrane protein; (e) resolving the ionised membrane protein using the mass analyser; and (f) detecting the resolved membrane protein using the detector; wherein the solution contains a phosphate ester detergent which forms said detergent micelle and is present in the solution at a concentration which is greater than or equal to the critical micelle concentration of the detergent in said solution; wherein the phosphate ester detergent is a compound of the formula: wherein n is 1, 2, 3, 4, 5 or 6; each R 1 is independently selected from hydrogen and alkyl; X is an alkylene linker having from 2 to 6 carbon atoms, wherein the carbon atoms of the alkylene linker are optionally interrupted by one or more moieties selected from -O-, -C -, -NH-, -S-, -C -, -S - and -S 2 -; and wherein the carbon atoms of the alkylene linker are optionally substituted by one or more moieties selected from hydroxy, halo and amino; and R 2 is optionally substituted hydrocarbyl. (a) providing a solution comprising a detergent micelle in which said membrane protein is contained; (b) providing a mass spectrometer comprising a nanoelectrospray ionisation source, a mass analyser and a detector; (c) vaporising the solution using the nanoelectrospray ionisation source under conditions such that the membrane protein is released from the micelle; (d) ionising the membrane protein; (e) resolving the ionised membrane protein using the mass analyser; and (f) detecting the resolved membrane protein using the detector; wherein the solution contains a phosphate ester detergent which forms said detergent micelle and is present in the solution at a concentration which is greater than or equal to the critical micelle concentration of the detergent in said solution; wherein the phosphate ester detergent is a compound of the formula: wherein n is 1, 2, 3, 4, 5 or 6; each R 1 is independently selected from hydrogen and alkyl; X is an alkylene linker having from 2 to 6 carbon atoms, wherein the carbon atoms of the alkylene linker are optionally interrupted by one or more moieties selected from -O-, -C -, -NH-, -S-, -C -, -S - and -S 2 -; and wherein the carbon atoms of the alkylene linker are optionally substituted by one or more moieties selected from hydroxy, halo and amino; and R 2 is optionally substituted hydrocarbyl. wherein the solution contains a phosphate ester detergent which forms said detergent micelle and is present in the solution at a concentration which is greater than or equal to the critical micelle concentration of the detergent in said solution; wherein the phosphate ester detergent is a compound of the formula: wherein n is 1, 2, 3, 4, 5 or 6; each R 1 is independently selected from hydrogen and alkyl; X is an alkylene linker having from 2 to 6 carbon atoms, wherein the carbon atoms of the alkylene linker are optionally interrupted by one or more moieties selected from -O-, -C -, -NH-, -S-, -C -, -S - and -S 2 -; and wherein the carbon atoms of the alkylene linker are optionally substituted by one or more moieties selected from hydroxy, halo and amino; and R 2 is optionally substituted hydrocarbyl. n is 1, 2, 3, 4, 5 or 6; each R 1 is independently selected from hydrogen and alkyl; X is an alkylene linker having from 2 to 6 carbon atoms, wherein the carbon atoms of the alkylene linker are optionally interrupted by one or more moieties selected from -O-, -C -, -NH-, -S-, -C -, -S - and -S 2 -; and wherein the carbon atoms of the alkylene linker are optionally substituted by one or more moieties selected from hydroxy, halo and amino; and R 2 is optionally substituted hydrocarbyl.