An overview of the new field of Gas in Scattering Media Absorption Spectroscopy (GASMAS) is presented. GASMAS
combines narrow-band diode-laser spectroscopy with diffuse media optical propagation. While solids and liquids have
broad absorption features, free gas in pores and cavities in the material is characterized by sharp spectral signatures,
typically 10,000 times sharper than those of the host material. Many applications in materials science, food packaging,
pharmaceutics and medicine have been demonstrated. So far molecular oxygen and water vapour have been studied
around 760 and 935 nm, respectively. Liquid water, an important constituent in many natural materials, such as tissue,
has a low absorption at such wavelengths, allowing propagation. Polystyrene foam, wood, fruits, food-stuffs,
pharmaceutical tablets, and human sinus cavities have been studied. Transport of gas in porous media can readily be
studied by first immersing the material in, e.g., pure nitrogen, and then observing the rate at which normal air,
containing oxygen, reinvades the material. The conductance of the sinus connective passages can be measured in this
way by flushing the nasal cavity with nitrogen. Also other dynamic processes such as drying of materials can be studied.
The techniques have also been extended to remote-sensing applications (LIDAR-GASMAS).