Abstract: The present invention relates to a method of determining an analyte in a sample by mass spectrometry (MS), the method comprising (a) admixing a pre-determined amount of internal calibrator to said sample, wherein said internal calibrator comprises at least two non-identical isotopologues of the analyte at predetermined amounts; (b) determining MS signals of ions generated from said analyte (analyte signal) and from said at least two isotopologues (isotopologue signals); (c) providing a calibration based on the analyte signal and the isotopologue signals determined in step (b); and (d) determining said analyte based on the calibration provided in step (c). Further, the present invention relates to devices, systems, and uses related thereto.