The disclosure provides a method for assaying for a target analyte, comprising providing a plurality of samples which may comprise the target analyte, wherein each sample is differentially labelled with a mass label or a combination of mass labels, wherein the mass labels are from a set of mass labels, wherein each mass label is an isobaric mass label comprising a mass spectrometrically distinct mass marker group, such that the samples can be distinguished by mass spectrometry and determining from the mass spectrum the quantity of the target analyte in each sample.