Photoacoustic spectra of ammonia and water vapor were recorded by use of a continuous-wave quantum-cascade distributed-feedback (QC-DFB) laser at with a 16-mW power output. The gases were flowed through a cell that was resonant at 1.6 kHz, and the QC-DFB source was temperature tuned over 35 nm for generation of spectra or was temperature stabilized on an absorption feature peak to permit real-time concentration measurements. A detection limit of 100 parts in by volume ammonia at standard temperature and pressure was obtained for a 1-Hz bandwidth in a measurement time of 10 min.
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