We have developed a compact instrument for sensitive, rapid and continuous measurement of trace gases in air, with results presented here for methane (CH 4 ), nitric oxide (NO), nitrous oxide (N 2 O) and ammonia (NH 3 ). This instrument takes advantage of recent technology in quantum cascade (QC) lasers and infrared detectors, which allows high sensitivity without cryogenic liquids, e.g., 0.2 ppb (0.2×10 -9 ) of NH 3 in air in 1 s. One may substitute a QC laser operating at a different wavelength to measure other gases. The instrument operates continuously, requiring little operator attention, and web-based remote access is provided for instrument control, calibration and data retrieval. The instrument design includes a thermoelectrically (TE) cooled pulsed distributed feedback (DFB) QC laser, a low volume (0.5 l) multipass cell offering 76 m absorption path length and a TE cooled detector. Integrated software for laser control and data analysis using direct absorption provides quantitative trace gas measurements without calibration gases. The instrument may be applied to field measurements of gases of environmental concern.