Optical, remote sensing, sub parts-per-billion (ppb) to percent level gas analyzer. The Cerex Air Sentry was the first open path FTIR to be commercialized and has continued to lead the industry with reliable performance and innovative features. It performs chemical detection and quantification of compounds using open-path, Fourier Transform InfraRed (FTIR) Differential Optical Absorption Spectroscopy (DOAS).