The invention relates to a method for calibrating non-dispersive infrared spectrometers for measuring 13C/12C ratios in respiratory gases. The aim of the invention is to provide a simple and reliable method for eliminating the influence of the CO¿2? concentration on the measuring signals of non-dispersive infrared spectrometers for determining ?13CO¿2/12CO2 ratios in respiratory gases. The method should also guarantee that essentially the same physical conditions are present during calibration and measurement. According to the invention, the two chambers of the measuring cuvette of the non-dispersive infrared spectrometer are first filled with a CO2-free gas, the zero point is set and a cuvette with a known CO2 content and 13CO2/12CO2 ratio is moved into the path of the rays. The measuring signal that is then established for 12CO2 is reproduced in the measuring phase by adding the necessary quantity of measured gas to the CO2-free gas in one of the two chambers of the measuring cuvette. The inventive method is particularly suitable for the 13C breath test, in the field of medical research and diagnosis.
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