Wavelength modulation absorption spectroscopy with 2f signal detection: application to molecular oxygen detection

Abstract

Wavelength modulation spectroscopy using second harmonic (2f) signals is one of the modulation techniques in absorption spectroscopy used to improve the absorption signal by noise reduction. This second harmonic (2f) signal has been simulated using MATLAB, a new approach based on numerical integration, to investigate the influence of normalized modulation amplitude, m and the optimal 2f signal was found to occur at m = 2.1. Experimentally, free molecular oxygen gas was measured at three different temperatures and at different optical pathlength using the 2f signal of the wavelength modulation absorption spectroscopy technique. The technique made it possible to verify the fact that, the longer the pathlength, the greater the intensity of the absorption signal. However the intensities of the 2f signal, which depict the absorption of molecular oxygen gas, were found to decrease with increasing temperatures. Using gas in scattering medium absorption spectroscopy (GASMAS) technique which is a modification of the wavelength modulation absorption spectroscopy set-up, molecular oxygen gas was probed in highly scattering media of locally branded polyurethane foams; Latex foam and Ash foam and polystyrene foam. The experiment revealed that for maximum 2f signal detection in the scattering media, the width of the scattering sample should be at least equal to the detection area of the detector. The thicknesses of the scattering samples were found to be inversely related to the intensities of their signals.