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).