Infrared hollow waveguides were studied for use as a capillary flow cell in spectroscopic gas sensing. Explosive gases (CH4, CH4H10) were detected successfully by using a ZnS-coated Ag hollow waveguide, which has a high transmittance over a wide infrared spectral region. The response time was reduced to 0.1 s owing to the decrease in the cell volume. Pollution gases (SO2, NO2) were measured by a glass hollow waveguide, which is durable for such corrosive gases. By using these hollow waveguides, we can extend the optical pathlength easily, and consequently we can expect enhancement of the sensitivity.
S. Sato, M. Saito, and M. Miyagi, "Infrared Hollow Waveguides for Capillary Flow Cells," Appl. Spectrosc. 47, 1665-1669 (1993)