A method of determining a constituent related sample property of a multi-constituent sample comprising: subjecting the sample to a perturbation selected to induce a time dependent change in measurement data associated with a constituent related to the sample property to be determined; recording a time-series of measurement data following subjecting the sample to the perturbation; and determining the sample property from the application to the recorded time-series of measurement data of a calibration correlating the sample property with time-series of measurement data, said calibration being empirically derived from chemometric time-series modelling of time-series measurement data recorded for each of a plurality of reference samples following subjecting each reference sample to the perturbation, each reference sample having a different known values of the sample property.