Provided is a method of assaying for an analyte, which method comprises: a) combining a test sample, which may comprise the analyte, with a calibration sample comprising at least two different aliquots of the analyte, each aliquot having a known quantity of the analyte, wherein the sample and each aliquot are differentially labeled with one or more isobaric mass labels each with a mass spectrometrically distinct mass marker group, such that the test sample and each aliquot of the calibration sample can be distinguished by mass spectrometry; b) determining by mass spectrometry the quantity of the analyte in the test sample and the quantity of the analyte in each 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.