A portable spectrophotometric
system for detecting one or more target substances. In a representative embodiment, a
system of the invention has an optical
grating, an array of photo-detectors, and a MEMS device having a movable plate positioned between the
grating and the array. Light transmitted through a gaseous sample is dispersed by the
grating and is imaged onto the movable plate, which has a plurality of openings corresponding to selected
infrared absorption lines of the target substance. A small-amplitude oscillation is imparted onto the plate such that the openings periodically move in and out of alignment with the corresponding intensity features in the image, which modulates electrical signals generated by the corresponding photo-detectors. A lock-in
signal processor analyzes the
modulation pattern by comparing it to the pattern expected in the presence of the target substance. When a
positive correlation between the patterns is established, the
system warns the user about the presence of the target substance.