Nondispersive infrared spectroscopy (NDIR) allows the continuous analysis of respiratory gases. Due to its high selectivity, simple and robust setup, and small footprint, it is also used to support13CO2breath tests to assess bacterial growth in the stomach, gut, or liver function. CO2NDIR signals, however, are biased by oxygen in the gas matrix. This complicates NDIR-based breath tests, if the inspired oxygen concentration has to be adjusted to the subject's requirements, or hyperoxia-induced effects were studied. To avoid the oxygen-induced bias, a “dilution” approach was developed: expired gas is mixed with N2to lower the oxygen content down to the usual range of 15–20%. Accuracy and precision were tested using synthetic gas mixtures with increasing13CO2-to-12CO2ratios (13CO2/12CO2), either based on synthetic air with ∼20% volume O2or on pure O2. For samples with δ13C values smaller than 300 (or13CO2/12CO2smaller than 0.003), the dilution does not significantly increase the bias in the13CO2/12CO2determination, and the within-run imprecision is smaller than 1 δ13C. The practical use of this approach was validated in a pig study using a sepsis model reflecting a clinical situation that requires an increased oxygen concentration for respiration. The N2dilution eliminated the high bias in NDIR measurement, thus allowing the determination of the impact of oxygenation on glucose oxidation in patients ventilated with increased oxygen.

We present a nondispersive infrared spectrometer (NDIRS) for the measurement of the '3C0,/'2C0,-ratio in breath samples. A commercial NDIR spectrometer for CO, concentration measurements in industrial process control was modified using two separate optical channels for the 13C0, and "CO, detection. Cross interference due to overlapping absorption lines of both isotopic gases was successfully eliminated. The sensitivity of this device is & 0.4% of the '3C0,/'2C0,-ratio in a range of 2.5 to 5%of total CO,. This is sufficient for biomedical applications. Our spectrometer is small in size, cheap and simple to operateand 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.

A mass spectrometer (MS) and a novel UV analyzer were coupled in the experimental study of the transient Selective Catalytic Reduction (SCR) reactivity, aimed at the development of a dynamic numerical model of SCR converters for the selective reduction of NO x in Diesel exhausts. Their parallel use revealed an effective method for the understanding of key elements in the SCR reaction mechanism under real operating conditions.

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