Relationship between harmonic line shape and temperature and pressure for wavelength modulation spectroscopy

The real-time online detection of oxygen concentrations in medicine glass vials has attracted much attention in the field of pharmaceutical manufacturing. Based on wavelength modulation spectroscopy theory, harmonic line shape is an important factor affecting detection accuracy and speed in the process of gas concentration detection. The gas harmonic line shape is determined by the Voigt profile, which is the convolution of the Gauss profile and the Lorentz profile. However, the high computational complexity of the Voigt harmonic line shape makes its computation speed slow. Therefore, in practical applications, the Gauss or Lorentz harmonic line shape is usually used to replace the Voigt harmonic line shape under certain conditions. Based on the HITRAN database, the difference in the positive and negative peak values (namely, the peak-to-peak value) and the error function are introduced to analyze the approximating effect between the Gauss and Lorentz harmonic line shapes and the Voigt harmonic line shape. Taking the second harmonic of oxygen as the example, the peak-to-peak value of the Gauss and Lorentz harmonic line shapes is simulated at different temperatures and pressures. Then, based on the approximate linear relationship between the modulation depth and peak width of the second harmonic line shape, a method of fitting the actual second harmonic using the Lorentz line shape is presented. Experimental results indicate that the proposed strategy improves the accuracy and stability of oxygen concentration detection in medicine glass vials and can meet the actual detection speed.