A method of quantifying the concentration of a protein of interest, or the concentration of a conformational state of the protein of interest, in a mixture, wherein the protein of interest or conformational state has an intrinsic fluorescence decay signature. The method comprises: addressing the mixture with one or more pulses of light, wherein the light has a wavelength in the 240-295 nm range, preferably in the 250-280 nm range, further preferably wherein the laser light has a wavelength of 266 nm. The method further comprises: taking a series of measurements of the fluorescence intensity of the mixture at a series of time points where the time interval between a fluorescence measurement and a preceding light pulse is recorded. The series of measurements comprises measurements for which the time intervals differ from each other by less than a nanosecond, and where the difference between largest and smallest time intervals is at least 10 nanoseconds (ns) and/or a sufficient time to detect a decay of the fluorescence intensity towards a baseline level, such that the series of measurements defines a fluorescence decay curve. The method further comprises quantifying the concentration of a protein of interest or conformational state of the protein of interest in the sample by reference to the fluorescence decay curve.