Characterization of a mid-infrared hollow waveguide gas cell for the analysis of carbon monoxide and nitric oxide.

Infrared spectroscopy is commonly applied to the analysis of small gas-phase molecules. One of the limitations of using Fourier transform infrared (FT-IR) spectroscopy for these applications is the time response of long path length gas cells. Hollow waveguides (HW) that transmit in the mid-infrared spectral range have higher optical efficiencies compared to long path length cells due to smaller cell volumes. This study characterizes a silver coated, 2 mm inner diameter HW for the analysis of carbon monoxide (CO) and nitric oxide (NO) and compares the performance to a 3 m gas cell and traditional gas analyzers. The HW was found to have a CO response time less than the NDIR analyzer and approximately one-tenth of the response time on the FT-IR system equipped with a 3 m gas cell. The utility of the increased response time was demonstrated by measuring CO concentrations in sidestream cigarette smoke at the same temporal resolution as an NDIR analyzer. A 10 to 60% increase in sensitivity using various frequencies for both CO and NO was observed using the HW compared to the 3 m multipass gas cell. However, cost savings for gas-sensing applications can be achieved on a per analyte basis by using FT-IR spectroscopy, especially in combination with a HW gas-sensing module, which is significantly less expensive than a multipass gas cell.