A SENSITIVE ISOTOPE SELECTIVE NONDISPERSIVE INFRARED SPECTROMETER FOR (CO2)-C-13 AND (CO2)-C-12 CONCENTRATION MEASUREMENTS IN BREATH SAMPLES

We present a nondispersive infrared spectrometer (NDIRS) for the measurement of the (CO2)-C-13/(CO2)-C-12-ratio in breath samples. A commercial NDIR spectrometer for CO2 concentration measurements in industrial process control was modified using two separate optical channels for the (CO2)-C-13 and (CO2)-C-12 detection. Cross interference due to overlapping absorption lines of both isotopic gases was successfully eliminated. The sensitivity of this device is +/- 0.4 parts per thousand of the (CO2)-C-13/(CO2)-C-12-ratio in a range of 2.5 to 5% of total CO2. This is sufficient for biomedical applications. Our spectrometer is small in size, cheap and simple to operate and thus a true alternative to isotope ratio mass spectrometers (IRMS). Several biomedical applications with breath samples were demonstrated and were compared in very good agreement with IRMS.