Sub-ppb detection of formaldehyde with cantilever enhanced photoacoustic spectroscopy using quantum cascade laser source

A novel cantilever enhanced photoacoustic spectrometer with mid-infrared quantum cascade laser was applied for selective and sensitive formaldehyde (CH2O) gas measurement. The spectrum of formaldehyde was measured from 1,772 to 1,777 cm⁻¹ by tuning the laser with a spectral resolution of 0.018 cm⁻¹. The band at 1,773.959 cm⁻¹ was selected for data analysis, at which position the laser emitted 47 mW. In univariate measurement, the detection limit (3σ, 0.951 s) and the normalized noise equivalent absorption coefficient (3σ) for amplitude modulation (AM) were 1.6 ppbv and 7.32 × 10⁻¹⁰ W cm⁻¹ (Hz)^−1/2 and for wavelength modulation (WM) 1.3 ppbv and 6.04 × 10⁻¹⁰ W cm⁻¹ (Hz)^−1/2. In multivariate measurement, the detection limit (3σ) can be as low as 901 pptv (1,773.833–1,774.085 cm⁻¹, 15 spectral points each 0.951 s) for AM and 623 pptv (1,773.743–1,774.265 cm⁻¹, 30 spectral points each 0.951 s) for WM. Because measurement time increases in multivariate measurement, its application is justified only when interferents need to be resolved. Potential improvements of the system are discussed.