Abstract: A reactive mass label for labelling a biological molecule for detection by mass spectrometry, which label comprises a reactive functionality for labelling thiol groups or carbonyl groups. Also provided is a reactive mass label for labelling a biological molecule for detection by mass spectrometry, wherein the mass label comprises the following structure: X-L-M-S—Re wherein X is a mass marker moiety, L is a cleavable linker, M is a mass normalization moiety, S is a mass series modifying group comprising the following group: wherein J is C?O, K is NH, and n is 2 or J and K are both CH2 and n is 1, and wherein m is at least 1; and Re is a reactive functionality for attaching the mass label to a biological molecule.

Abstract: A reactive mass label for labelling a biological molecule for detection by mass spectrometry includes the following structure: X-L-M-S-Re, wherein X is a mass marker moiety, L is a cleavable linker, M is a mass normalization moiety, and S is a mass series modifying group including the following group: wherein J is C?O, K is NH, and n is 2 or J and K are both CH2 and n is 1, m is at least 1; and Re is a reactive functionality for attaching the reactive mass label to a biological molecule.

Abstract: A reactive mass label for labelling a biological molecule for detection by mass spectrometry, which label comprises a reactive functionality for labelling thiol groups or carbonyl groups. Also provided is a reactive mass label for labelling a biological molecule for detection by mass spectrometry, wherein the mass label comprises the following structure: X-L-M-S-Re wherein X is a mass marker moiety, L is a cleavable linker, M is a mass normalization moiety, S is a mass series modifying group comprising the following group: wherein J is C?O, K is NH, and n is 2 or J and K are both CH2 and n is 1, and wherein m is at least 1; and Re is a reactive functionality for attaching the mass label to a biological molecule.