Imaging spectrometry enables passive, stand-off detection and analysis of the chemical composition of gas plumes and surfaces over wide geographic areas. This paper describes the use of a long-wavelength infrared imaging spectroradiometer, comprised of a low-order tunable Fabry-Perot etalon coupled to an HgCdTe detector array, to perform multispectral detection of chemical vapor plumes. The tunable Fabry-Perot etalon used in this work provides coverage of the 9.5 to 14 µm spectral region with a resolution of 7 to 9 cm^-1. The etalon-based imaging system provides the opportunity to image a scene at only those wavelengths needed for chemical species identification and quantification and thereby minimize the data volume necessary for selective species detection. We present initial results using a brassboard imaging system for stand-off detection and quantification of chemical vapor plumes against near ambient temperature backgrounds. These data shows detections limits of 22 ppmv x m and 0.6 ppmv x m for DMMP and SF₆ respectively for a gas/background ΔT of 6 K. The system noise-equivalent spectral radiance is approximately 2 µW cm^-2 sr^-1 µm^-1. Model calculations are presented comparing the measured sensitivity of the sensor to the anticipated signal levels for two chemical release scenarios.