The present invention provides a method for labelling one or more analytes in a sample, the method comprising: a) contacting the sample with one or more bifunctional linker reagents having the general formula Re1-L1-Re2, wherein Re1 is a first reactive group, L1 is a linker moiety and Re2 is a second reactive group, wherein Re1 reacts with an analyte to form a modified analyte; and b) contacting the sample with one or more mass labels, wherein Re2 of the bifunctional linker attached to the analyte reacts with a mass label to form a labelled analyte, wherein each mass label is relatable to an analyte by mass spectrometry.