Pathlength Determination for Gas in Scattering Media Absorption Spectroscopy

• Main Authors: Liang Mei, Gabriel Somesfalean, Sune Svanberg
• Format: Article
• Language: English
• Published: MDPI AG 2014-02-01
• Series: Sensors
• Subjects: gas in scattering media absorption spectroscopy; GASMAS; TDLAS; time-of-flight spectroscopy; frequency modulated light scattering interferometry; frequency domain photon migration; Beer-Lambert law; wavelength modulation spectroscopy; pathlength; scattering; oxygen; water vapor; pores; porous media; porosity; line shape
• Online Access: http://www.mdpi.com/1424-8220/14/3/3871

Gas in scattering media absorption spectroscopy (GASMAS) has been extensively studied and applied during recent years in, e.g., food packaging, human sinus monitoring, gas diffusion studies, and pharmaceutical tablet characterization. The focus has been on the evaluation of the gas absorption pathlength in porous media, which a priori is unknown due to heavy light scattering. In this paper, three different approaches are summarized. One possibility is to simultaneously monitor another gas with known concentration (e.g., water vapor), the pathlength of which can then be obtained and used for the target gas (e.g., oxygen) to retrieve its concentration. The second approach is to measure the mean optical pathlength or physical pathlength with other methods, including time-of-flight spectroscopy, frequency-modulated light scattering interferometry and the frequency domain photon migration method. By utilizing these methods, an average concentration can be obtained and the porosities of the material are studied. The last method retrieves the gas concentration without knowing its pathlength by analyzing the gas absorption line shape, which depends upon the concentration of buffer gases due to intermolecular collisions. The pathlength enhancement effect due to multiple scattering enables also the use of porous media as multipass gas cells for trace gas monitoring. All these efforts open up a multitude of different applications for the GASMAS technique.