Mass spectrometric quantitation

A method of assaying for an analyte, comprising:

• a) covalently, labelling a test sample comprising the analyte with one or more isobaric mass labels to provide labeled test sample;
• b) covalently labelling a calibration sample comprising at least two different aliquots of the analyte, each analyte aliquot having a different known quantity of the analyte, with two or more isobaric mass labels to provide labeled calibration sample;
• wherein the test sample and each analyte aliquot of the calibration sample are differentially labelled with one or more isobaric mass labels, each with a mass spectrometrically distinct mass marker group, such that the test sample and each analyte aliquot of the calibration sample can be distinguished by mass spectrometry;
• c) combining the labeled test sample with the labeled calibration sample;
• d) determining by mass spectrometry the quantity of the analyte in the test sample and the quantity of analyte in each analyte aliquot in the calibration sample, and calibrating the quantity of the analyte in the test sample against the known and determined quantities of the analytes in the aliquots in the calibration sample.