A set of two or more reactive mass labels, wherein each label in the set is for labelling a biological molecule for detection by mass spectrometry, and each label comprises the following structure: wherein R 1, R 2, R 3 and Z are selected from one of the following definitions (a) to (d): (a) R 1 and R 2 together form: R 3 is absent; Z is O; and R 4 and R 5 may be the same or different and are each independently selected from H, a substituted or unsubstituted straight or branched C 1 -C 6 alkyl group, a substituted or unsubstituted aliphatic cyclic group, a substituted or unsubstituted aromatic group and a substituted or unsubstituted heterocyclic group; (b) R 1 and R 3 together form: R 2 is: Z is N; and each of R 6 to R 9 is independently selected from H a substituted or unsubstituted straight or branched C 1 -C 6 alkyl group, a substituted or unsubstituted aliphatic cyclic group, a substituted or unsubstituted aromatic group and a substituted or unsubstituted heterocyclic group; (c) R 1 is: R 2 is A; R 3 is absent; Z is O; and halo is a halogen; (d) R 1 is: R 2 is A; R 3 is absent; Z is O; and B is -NH 2 or -(CH 2) n -ONH 2, wherein n is from 1 to 6; and wherein in (a) (b) (c) and (d) A comprises the following structure: X-L-M wherein L is a cleaveable linker comprising an amide bond, X is a mass marker moiety comprising the following group: wherein the cyclic unit is aromatic or aliphatic and comprises from 0-3 double bonds independently between any two adjacent atoms; each Z is independently N, N(R 1), C(R 1), CO, CO(R'), C(R 1) 2, O or S; X is N, C or C(R'); each R 1 is independently H, a substituted or unsubstituted straight or branched C 1 -C 6 alkyl group, a substituted or unsubstituted aliphatic cyclic group, a substituted or unsubstituted aromatic group or a substituted or unsubstituted heterocyclic group; and y is an integer from 0-10 and M is a mass normalization moiety comprising a straight or branched C 1 -C 20 substituted or unsubstituted aliphatic group and/or one or more substituted or unsubstituted amino acids, and wherein each mass normalisation moiety ensures that a mass label has a desired aggregate mass, and wherein the set comprises: - a group of labels having a mass marker moiety of common mass, each label in the group having a unique aggregate mass; or - a group of labels having a mass marker moiety, each mass marker moiety having a mass different from that of all other mass marker moieties in that group, and each label in the group having a common aggregate mass; and wherein all the mass labels in the set are distinguishable from each other by mass spectroscopy. (a) R 1 and R 2 together form: R 3 is absent; Z is O; and R 4 and R 5 may be the same or different and are each independently selected from H, a substituted or unsubstituted straight or branched C 1 -C 6 alkyl group, a substituted or unsubstituted aliphatic cyclic group, a substituted or unsubstituted aromatic group and a substituted or unsubstituted heterocyclic group; R 3 is absent; Z is O; and R 4 and R 5 may be the same or different and are each independently selected from H, a substituted or unsubstituted straight or branched C 1 -C 6 alkyl group, a substituted or unsubstituted aliphatic cyclic group, a substituted or unsubstituted aromatic group and a substituted or unsubstituted heterocyclic group; (b) R 1 and R 3 together form: R 2 is: Z is N; and each of R 6 to R 9 is independently selected from H a substituted or unsubstituted straight or branched C 1 -C 6 alkyl group, a substituted or unsubstituted aliphatic cyclic group, a substituted or unsubstituted aromatic group and a substituted or unsubstituted heterocyclic group; R 2 is: Z is N; and each of R 6 to R 9 is independently selected from H a substituted or unsubstituted straight or branched C 1 -C 6 alkyl group, a substituted or unsubstituted aliphatic cyclic group, a substituted or unsubstituted aromatic group and a substituted or unsubstituted heterocyclic group; (c) R 1 is: R 2 is A; R 3 is absent; Z is O; and halo is a halogen; R 2 is A; R 3 is absent; Z is O; and halo is a halogen; (d) R 1 is: R 2 is A; R 3 is absent; Z is O; and B is -NH 2 or -(CH 2) n -ONH 2, wherein n is from 1 to 6; R 2 is A; R 3 is absent; Z is O; and B is -NH 2 or -(CH 2) n -ONH 2, wherein n is from 1 to 6; wherein L is a cleaveable linker comprising an amide bond, X is a mass marker moiety comprising the following group: - a group of labels having a mass marker moiety of common mass, each label in the group having a unique aggregate mass; or - a group of labels having a mass marker moiety, each mass marker moiety having a mass different from that of all other mass marker moieties in that group, and each label in the group having a common aggregate mass;