The design of a portable open-path quantum cascade laser (QSL)-based tunable laser differential optical absorption spectroscopy (DOAS) system that operates with a center wavelength of 9.6 μm for multiple species trace gas detection is presented. This system is designed for highly sensitive measurements of O3 and NH3. QSL open-path system has a potential resolution fifty times greater than conventional open-path-FTIR. While the advantage of FTIR and NIR-differential optical absorption spectroscopy was broadband measurement of multi-species or ease of material sourcing, an external cavity QSL operating in turn-key products can now harness the advantage of both designs. This is an abstract of a paper presented at the 101st AWMA Annual Conference and Exhibition (Portland, OR 6/24-27/2008).