Abstract
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.
Highlights
Tunable diode laser absorption spectroscopy (TDLAS) has been widely utilized to monitor natural and anthropogenic gas emissions during recent decades [1,2,3]
A typical Gas in scattering media absorption spectroscopy (GASMAS) setup is shown in Figure 2, which shows a great similarity with a traditional
Lgas can be deduced if the gas concentration is known. From this point of view, GASMAS is a method which can give the mean pathlength through the embedded gas in the porous media
Summary
Tunable diode laser absorption spectroscopy (TDLAS) has been widely utilized to monitor natural and anthropogenic gas emissions during recent decades [1,2,3]. In 2001, a variety of the TDLAS technique, referred to as gas in scattering media absorption spectroscopy (GASMAS) [14], was developed to study the gases (mainly O2 and H2O) embedded in open pores of porous scattering media, e.g., in wood materials [15,16], fruits [17], food packages [18,19], human sinus or mastoid cavities [20,21], ceramics [22], and pharmaceutical tablets [23]. As known from the Beer-Lambert law, the pathlength through the gas must be determined independently in order to obtain the gas concentration. This is difficult to achieve in a scattering medium, where light is heavily diffused. Before going into the details with the different methods, we will first discuss the physics of light propagation in porous media
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