Abstract
The photo-acoustic (PA) methane gas analyzer based on a quantum cascade laser (QCL; ~7.7 μm/1800 Hz/24 mW), a resonant differential PA detector, and a sealed gas-filled cell was investigated. The measurement of methane concentration below the background value in the air (~0.3 ppm CH4) is shown, the standard dispersion was (1σ) ≈ (10–11) ppb CH4 with an integration time of 10 s. Under conditions of temperature instability (or emission wavelength) of QCL when normalized to a gas-filled cell, the relative measurement error of the CH4 concentration does not exceed 3%. A decrease in the average QCL radiation power (~24–12–6 mW) leads to a noticeable deterioration in the threshold sensitivity of the PA gas analyzer. It is recommended to increase the average QCL power level to ~50 mW. The dynamic range of measuring the concentration of methane of the PA gas analyzer in the linear mode was ~4 decades (from 0.3 ppm to 2000–3000 ppm CH4).
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