We describe a prototype trace gas sensor designed for real-time detection of multiple chemicals. The sensor uses an
external cavity quantum cascade laser (ECQCL) swept over its tuning range of 940-1075 cm-1 (9.30-10.7 μm) at a 10 Hz
repetition rate. The sensor was designed for operation in multiple modes, including gas sensing within a multi-pass
Heriott cell and intracavity absorption sensing using the ECQCL compliance voltage. In addition, the ECQCL
compliance voltage was used to reduce effects of long-term drifts in the ECQCL output power. The sensor was
characterized for noise, drift, and detection of chemicals including ammonia, methanol, ethanol, isopropanol, Freon-
134a, Freon-152a, and diisopropyl methylphosphonate (DIMP). We also present use of the sensor for mobile detection
of ammonia downwind of cattle facilities, in which concentrations were recorded at 1-s intervals.