A Quartz-Enhanced Photothermal Spectroscopy for C₂H₂ and CH₄ Detection Based on Time Division Multiplexing Technology

A quartz-enhanced photothermal spectroscopy (QEPTS) sensor for acetylene (C2H2) and methane (CH4) detection based on time division multiplexing (TDM) technology was developed. The QEPTS based on internal excitation inside tuning fork arm was implemented for the signal improvement. In the excitation structure, the part silver layer on the surface of quartz tuning fork (QTF) is chemically etched to guarantee the laser beam was injected to another side of the QTF arm at a certain angle for achieving roundtrips between both sides of the QTF single arm. The photothermal excitation presents a continuous linear form. The photothermal excitation presents a continuous linear form. Compared with the traditional excitation, the proposed detection scheme has the longer excitation path length of the photothermal signal, the better response of QTF to the laser in the refraction process inside the tuning fork arm, and the higher photothermal signal intensity. Moreover, the C2H2 and CH4 detection were achieved by the TDM of the corresponding distributed feedback (DFB) laser driver current. The experimental results showed that the minimum detection limits (MDL) of C2H2 and CH4 were 1.1689 and 1.17 ppmv, respectively. Moreover, the normalized noise-equivalent absorption (NNEA)of C2H2 is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4.534\times 10^{-9}~{\mathrm {cm}}^{-1}\,\cdot \,{\mathrm {W/Hz}}^{{1}/{2}}$ </tex-math></inline-formula> and that of CH4 is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.283\times 10^{-9}~{\mathrm {cm}}^{-1}\,\cdot\,{\mathrm {W/Hz}}^{{1}/{2}}$ </tex-math></inline-formula>. In addition, C2H2 and CH4 samples with varying concentrations ranging from 400 to 2000 ppmv and 400–2500 ppmv were generated and used for linearity measurement, respectively, and the linear regression analysis resulted in an R-squared value of 0.99884 for CH4 and 0.99826 for C2H2.