Embodiments of the present invention relate to reagents and their use for elemental imaging mass spectrometry of biological samples. The embodiments comprising methods for quantifying one or more analytes within a sample, comprising the steps of: (a) providing the sample, wherein the one or more analytes are immobilized to a sample carrier, wherein the sample has been labelled with one or more mass tags comprising one or more labelling atoms, (b) performing mass cytometry on the sample to determine the level of the one or more labelling atoms, wherein the level of the one or more labelling atoms corresponds to the copy number of the one or more analytes.