Simultaneous Measurements of CO and CO 2 Employing Wavelength Modulation Spectroscopy Using a Signal Averaging Technique at 1.578 μm

Abstract

A resolved line pair was selected for simultaneous measurement of carbon monoxide (CO) and carbon dioxide (CO₂) in the near-infrared (NIR) region. The spectral lines of CO and CO₂ at 1.578 µm were measured by wavelength modulation spectroscopy (WMS)-2f and the absorption was enhanced with a multipass absorption cell. The white noise was further reduced by averaging technology. The detection sensitivity (1σ) for the system is estimated at 2.63 × 10⁻⁷cm⁻¹ for direct absorption spectroscopy. The ultimate detection limits of CO₂ and CO mixed with pure N₂ at 75 Torr are 29 parts per million (ppm) and 47 ppm, respectively. It is demonstrated that the signal is highly linear with the concentration in the range of 100–800 ppm. Based on an Allan variation analysis, the minimum detectable limit of CO₂ and CO is 7.5 and 14 ppm, respectively. The response time of the system is about 30 s and a relationship of temperature dependence was obtained. As an example, an in situ measurement of exhaust of alkane combustion emission is presented.