Light intensity correction for quartz-enhanced photoacoustic spectroscopy using photothermal baseline

Abstract

A convenient method of light intensity correction for quartz-enhanced photoacoustic spectroscopy (QEPAS) using photothermal baseline is demonstrated. The laser beam passes through the prongs of the quartz tuning fork (QTF) and then focused on the root of the prongs. First harmonic (1f) analysis is utilized to process the simultaneously induced photoacoustic and photothermal signals. The optical path length for photothermal spectroscopy is minimized to millimeter level, yielding negligible gas absorption. The demodulated 1f signal can be regarded as the superposition of the photoacoustic signal and the non-absorption photothermal baseline. A good linear relationship (R2 = 0.999) is observed between amplitude of photothermal baseline and light intensity. QEPAS signal normalized by photothermal baseline shows a good immunity to light intensity variation. An excellent linear response between normalized QEPAS signal and gas concentration is achieved. According to the Allan deviation analysis, the minimum detection limit for CH4 is 0.31 ppm at an integration time of 1,200 s. With this strategy, the precise gas concentration and accurate light intensity of a QEPAS system can be simultaneously obtained with only a single QTF. Compared with the light intensity correction using a photodetector or a power meter, this method entails a low cost and small footprint. It is promising to mitigate the influence from light intensity drift in long-term field measurement of QEPAS systems.

Simultaneous Detection of Gas Concentration and Light Intensity Based on Dual-Quartz-Enhanced Photoacoustic-Photothermal Spectroscopy

“…By using this technology, it is possible to monitor the laser light intensity in addition to detecting dangerous chemicals at a distance [24,25]. By applying the method of scanning wavelength modulation, the amplitude of the demodulated 1f component is proportional to the light intensity and can be used to accurately track fluctuations in light intensity [26]. As a result, several academics paid close attention to QEPTS [27,28].…”

In Situ High-Precision Measurement of Deep-Sea Dissolved Methane by Quartz-Enhanced Photoacoustic and Light-Induced Thermoelastic Spectroscopy

“…However, application of the QEPAS technology in the field of deep-sea dissolved gas measurement is rarely reported. Light-induced thermoelastic spectroscopy (LITES) is the direct detection spectroscopy technique, which utilizes the light-induced thermoelastic effect of a quartz tuning fork (QTF) as the basic principle of light detection. , The mechanical vibration of QTF is caused by elastic deformation through light-thermo-elastic conversion. , Due to the piezoelectric effect of QTF, the mechanical energy is converted into an electrical signal. , Combining the QEPAS and the LITES enables simultaneous and accurate measurement of gas concentration and laser light intensity …”

“…39,40 Combining the QEPAS and the LITES enables simultaneous and accurate measurement of gas concentration and laser light intensity. 41 By aiming at field application of dissolved gas analysis in the deep-sea cold seep, we report a QEPAS-based CH 4 sensor assisted with a gas−liquid separator system. The spectrometer is equipped with frequency division multiplexing technology, which allows simultaneous measurement of photoacoustic signals and light-induced thermoelastic signals.…”