An isotope ratio of a continuous sample is measured in an isotope ratio spectrometer. At least one sample isotope ratio is measured over a measurement time period tn s (n>=1 ) and a sample concentration cn s is measured over at least a part of the measurement time period tn s. A reference gas concentration cn ref for the spectrometer is selected for reference to the sample measured during the measurement time period tn s, on the basis of the measured sample concentration cn ref. An isotope ratio of the reference gas is measured at the selected reference gas concentration cn ref in the spectrometer. The at least one isotope ratio of the sample measured during the measurement time period tn s is calibrated using the measured isotope ratio of the reference gas at the corresponding reference gas concentration cn ref and a plurality of calibrated isotope ratios and a plurality of sample gas concentration measurements are determined, each being for a different time.