ppb-Level SO₂ Photoacoustic Sensors with a Suppressed Absorption–Desorption Effect by Using a 7.41 μm External-Cavity Quantum Cascade Laser

A sensitive photoacoustic sensor system for the detection of ppb-level sulfur dioxide (SO₂) was developed by the use of a continuous-wave room-temperature, high-power quantum cascade laser (QCL) with an external diffraction grating cavity geometry. The excitation wavelength of the QCL was set to 7.41 μm for the strongest SO₂ absorption line strength. A custom-made differential photoacoustic cell (PAC) with two identical resonators was designed to allow a gas flow rate up to 1200 sccm. A qualitative theoretical model was employed in order to understand the dynamic adsorption and desorption processes of SO₂ in the PAC walls. A 1σ detection limit of 2.45 ppb, corresponding to a normalized noise equivalent absorption value of 3.32 × 10⁻⁹ cm⁻¹ W/Hz1/2, was achieved after measures for suppressing the absorption-desorption effect were taken.