Detection of NH3 impurities in H2 environment exploiting quartz-enhanced photoacoustic spectroscopy with an optimized spectrophone

We report on a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for ammonia impurities detection in hydrogen. A quantum cascade laser with a central emission wavelength of 9.062 μm was used to excite the NH3 absorption line at 1103.44 cm−1, with a linestrength of 1.51ꞏ10−19 cm/molecule. Compared to detecting contaminants in ambient air, the distinct properties of hydrogen required the design of a dedicated QEPAS spectrophone optimized for operation in hydrogen-based mixtures. The custom-made spectrophone was composed by a QTF excited at the first overtone mode at 44 kHz, acoustically coupled with an easy-to-align, dual-tube amplification system. The custom-made spectrophone was implemented in a QEPAS sensor, achieving a minimum detection limit of 95 ppb with a lock-in integration time of 0.1 s. Furthermore, the Allan-Werle deviation analysis returned a detection limit as low as 1.5 ppb when for an integration time of 30 s.