Abstract
In this report, an ultra-high sensitive quartz-enhanced photoacoustic spectroscopy (QEPAS) based ammonia (NH3) sensor using a power amplified diode laser and a low resonance frequency quartz tuning fork (QTF) was demonstrated for the first time. A fiber-coupled, continuous wave (CW), distributed feedback (DFB) diode laser with a watt level output power boosted by an erbium-doped fiber amplifier (EDFA) was used as the QEPAS excitation source. A QTF with a resonance frequency of 30.72 kHz was employed as an acoustic wave transducer. The modulation depth in the wavelength modulation spectroscopy (WMS) based QEPAS system was optimized theoretically and validated by experimental measurements. For the reported NH3 sensor system, a 418.4 ppbv (parts per billion by volume) minimum detection limit at a NH3 absorption line of 6533.4 cm−1 was achieved when the modulation depth was set to the optimum value of 0.188 cm−1. The ppb-level detection sensitivity verified the design of the reported QEPAS method and makes it suitable for use in environmental monitoring and other applications.
Highlights
Ammonia (NH3) detection is widely used for industrial and environmental pollution monitoring, automotive exhaust analysis and medical diagnostics
The laser wavelength modulation depth coefficient should be optimized in order to improve the 2f quartz-enhanced photoacoustic spectroscopy (QEPAS) signal amplitude
An ultra-high sensitive QEPAS based NH3 sensor was demonstrated in this paper
Summary
Ammonia (NH3) detection is widely used for industrial and environmental pollution monitoring, automotive exhaust analysis and medical diagnostics. As one of the most widely used LAS methods, tunable diode laser absorption spectroscopy (TDLAS) employing a multi-pass cell (MPC), where effective optical path length is extended to tens or even to hundreds of meters, typically allows to reach a detection limits of the analyte species at ppm or ppb levels [6,7]. This type of sensor is usually bulky due to the large size of a MPC and the increased number of optical components that are needed for laser beam alignment. The Q-factor of typical PAS cell is low (
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