Provided herein are sets of mass labels. Each mass label in a set includes 1) a mass marker moiety; 2) a mass normalisation moiety; and 3) a cleavable linker connecting the mass marker moiety to the mass normalisation moiety. Each mass marker moiety is characterised as having a mass different from that of all other mass marker moieties in the set as determined by mass spectrometry. Further, each mass normalisation moiety ensures that each mass label in the set has substantially the same mass as determined by mass spectrometry.