Load capacitance matching for resonant frequency adjusting-based multi-quartz tuning fork-enhanced laser spectroscopic sensing.

What we believe to be a novel load capacitance matching method is proposed for adjusting the resonant frequency of a quartz tuning fork (QTF) in a multi-QTF-based laser spectroscopy gas sensor for the first time. A sensing system integrating quartz-enhanced photoacoustic spectroscopy (QEPAS) and light-induced thermoelastic spectroscopy (LITES) was constructed to validate the proposed method. In this system, the QEPAS module consists of QTF1, while the LITES module is based on QTF2. Methane was chosen as the target gas to evaluate both the reliability of the method and the sensor's performance. Without frequency matching, the difference in resonant frequencies between QTF1 and QTF2 was 6.67 Hz, resulting in a superposition efficiency of only 33.7% for the QEPAS-LITES sensor. When the load capacitance matching method was applied, the load capacitance of QTF2 was adjusted to bring its resonant frequency closer to that of QTF1. This successfully matched the resonant frequencies of both QTFs, improving the superposition efficiency to 98.7%. The QEPAS-LITES sensor utilizing the load capacitance matching method demonstrated a strong linear concentration response. Additionally, long-term stability was evaluated through Allan deviation analysis. With an average time of 100 s, the minimum detection limit of the QEPAS-LITES sensor system was improved to 5.91 ppm.