Methanol detection under variable humidity conditions using an improved wavelength modulation spectroscopy method based on first harmonic signal analysis.

Methanol is a hazardous volatile organic compound, and its reliable detection under varying ambient conditions remains challenging, particularly due to interference from water vapor. In this work, an improved wavelength modulation spectroscopy (WMS) method based on first-harmonic (1f) signal analysis is proposed for methanol measurement under variable humidity. By applying a logarithmic transformation to the transmitted intensity, a linear relationship between the 1f signal and methanol concentration is established without relying on the absorption line shape. Furthermore, a point-selection strategy is introduced to suppress water-vapor interference, by selecting a wavelength where the first-harmonic (1f) contribution of water vapor is nearly zero-corresponding to its absorption peak-thereby enabling direct concentration retrieval of methanol. The proposed method was validated via calibration experiments, exhibiting excellent linearity with a coefficient of determination R2 = 0.999. Allan deviation analysis yielded a minimum detectable concentration of 182 ppb at an optimal integration time of 8.2 s. Measurements under different humidity levels showed stable retrieved methanol concentrations, with a maximum relative error of 3.1%. A comparative study confirmed that the proposed 1f-based method significantly outperforms the conventional 2f-based approach in accuracy, detection limit, and robustness against humidity variations. These results demonstrate that the proposed method offers a reliable and effective solution for methanol detection in environments with fluctuating humidity.