Development of an infrared diagnostic instrument for the measurement of CO₂ isotope ratios in breath

Abstract

Stable CO₂ isotope breath tests are established as a valuable tool in diagnostic and investigative medicine. The instrument conventionally used is an Isotope Ratio Mass Spectrometer, however, the expense and complexity of such an instrument severely restricts the widespread and routine use of isotope tests. To realize their full potential an alternative instrument which is reliable, uncomplicated, insensitive to environmental and component fluctuations and affordable is required. We present a system that satisfies these criteria using broadband non-dispersive infrared spectroscopy. Two isotopically distinct channel pathlengths are recorded for a basal and ¹³CO₂ enriched breath sample, the change in channel pathlength ratios required to restore transmitted intensity equilibrium between channels being related to the change in the ¹³CO₂/¹²CO₂ concentration ratio. The novelty of the system lies in this negative feedback loop balancing the signal by means of adjusting one of the channels pathlength. Results are presented proving the validity of using infrared spectroscopy for making such measurements without interference from breath trace compounds. Theoretical computer simulations using detailed modeling of CO₂ spectroscopy have been used to evaluate and quantify the risk posed to reliable measurement of ¹³CO₂/¹²CO₂ due to various spectral effects. From this we have established operating tolerances necessary to avoid or limit the generation of a spurious result.

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