Stable Isotope Calibration . Any stable isotope measurement instrument requires periodic calibration, whether based on mass spectrometry or optical absorption. In the case of isotopic water, an independent comparative study confirmed that the calibration curve for an LGR instrument was constant and stable over time. In contrast, the calibration curve of a competitive CRDS-based isotopic water vapor analyzer kept changing, necessitating frequent re-measurement of this curve. The stability of the OA-ICOS isotope calibration curve is another consequence of true wavelength scanning, enabling the instrument to automatically correct for changes in sampling conditions and for variations in other non-measured gases.