Infrared laser spectroscopy for online recording of exhaled carbon monoxide—a progress report

Despite the growing number of reports on breath CO measurements, the development of rapid and sensitive analysis techniques for measurements of this breath constituent still remains a challenge. We demonstrate the application of infrared laser spectroscopy for exhaled CO analysis. The breath samples are analyzed in real-time during single exhalations by means of cavity ring-down spectroscopy. This is an ultra-sensitive laser-based method for the analysis of trace gases with precision on the ppb level (parts per billion). The noise-equivalent CO level of this method is 7 ppb Hz(-1/2); the time resolution is around 1 s. The expirograms were recorded with exhalation flow rates varying from 4 l min(-1) up to 50 l min(-1). Alveolar phase (phase III) of expiration shows a remarkable flow-rate dependence. Also, expirograms were recorded after a breath holding time between 0 s and 60 s. The normalized slope of the alveolar plateau (S(n)) was determined, which is between 0.004 l(-1) and 0.15 l(-1).