Laser-based absorption spectroscopy as a technique for rapid in-line analysis of respired gas concentrations of O2 and CO2

Abstract

The use of sidestream analyzers for respired gas analysis is almost universal. However, they are not ideal for measurements of respiratory gas exchange because the analyses are both temporally dissociated from measurements of respiratory flow and also not generally conducted under the same physical conditions. This study explores the possibility of constructing an all optical, fast response, in-line breath analyzer for oxygen and carbon dioxide. Using direct absorption spectroscopy with a diode laser operating at a wavelength near 2 μm, measurements of expired carbon dioxide concentrations were obtained with an absolute limit of detection of 0.04% at a time resolution of 10 ms. Simultaneously, cavity enhanced absorption spectroscopy at a wavelength near 760 nm was employed to obtain measurements of expired oxygen concentrations with an absolute limit of detection of 0.26% at a time resolution of 10 ms. We conclude that laser-based absorption spectroscopy is a promising technology for in-line analysis of respired carbon dioxide and oxygen concentrations.

Figures

Fig. 1.

Schematic of overall experimental system. Gas flow is shown in green. The path for radiation at 760 nm (O2 measurement) is shown in red. The path for radiation at 2 μm (CO2 measurement) is shown in blue. DFB, distributed feedback diode laser (for measurement of O2); VCSEL, vertical cavity surface emitting laser (for measurement of CO2); APD, Si avalanche photodiode (for detection of radiation at 760 nm). The 2 μm radiation is detected with a biased InGaAs photodiode. Radiation at 760 nm passes through an optical isolator to prevent feedback to the DFB.

Fig. 2.

Example calibration data. Top, CO2 calibration; bottom, O2 calibration. Left, spectra obtained in the presence of pure background gas (N2) and calibration gas (containing fixed quantities of either CO2 or O2). Right, absorption profiles calculated from the calibration spectra.

Fig. 3.

Example data recorded from a subject breathing air under resting conditions. Top, O2 concentration record; bottom, CO2 concentration record.