In recent years, a number of studies have examined the use of hollow waveguides in infrared (IR) spectroscopy. Hollow waveguides generally exhibit efficient transmission over a wide spectral range in the mid-IR at frequencies where most molecules have their fundamental vibrations. These waveguides are particularly effective for IR gas spectroscopy where absorption pathlengths of > 10 cm are required. While conventional IR absorption cells rely on proper alignment of a series of mirrors to compactly yield pathlengths in excess of 15 cm, hollow waveguides may serve as IR absorption cells by admitting gas into the waveguide bore. Use of hollow waveguides minimizes alignment errors while facilitating increased pathlengths and decreased sample volumes and may thus provide improved sensitivity and decreased response time over conventional sample cells.
Rebecca L. Kozodoy, Ronald H. Micheels, and James A. Harrington, "Small-Bore Hollow Waveguide Infrared Absorption Cells for Gas Sensing," Appl. Spectrosc. 50, 415-417 (1996)