A new aspect of gas spectroscopy, called Gas in Scattering Media Absorption Spectroscopy (GASMAS) has been proposed and demonstrated by us for the unique analysis of gas enclosures in turbid solids and liquids. Many substances, frequently of organic origin, are porous and contain free gas distributed through the material. Many examples can be given: living tissue, paper, powder, sintered materials, catalysts, foams and liquids containing gas bubbles are some examples.
The free gas molecules dispersed in porous materials have unique, sharp absorptive signatures, typically ten thousand times narrower than the spectral features of the bulk material. The small absorption imprint in the emerging, multiply scattered, diffuse light can be detected sensitively by the use of wavelength modulation techniques, easily achievable using diode laser spectroscopy. In the case of strong scattering, which is the most interesting regime of the proposed gas detection technique, a long effective path length is achieved, giving rise to strong gas signatures. The technique can be applied to any gas having narrow absorption features that are reachable with tuneable diode laser radiation.
Non-destructive gas investigations
New opportunities for non-destructive studies of gas in highly scattering, natural and man-made porous materials are provided by the GASMAS technique. In experiments on wood, anisotropy related to the fibre structure was studied. This kind of measurement may be of fundamental interest in the understanding of light transport in inhomogeneous porous materials. The new possibility to observe free gas in scattering media not only allows static gas assessment but also the study of dynamic processes, i.e. how gas is exchanged with the environment. In our experiments, the porous samples were flushed with nitrogen and the successive penetration of the atmospheric oxygen from the surrounding air was studied. One practical application is the assessment of the resistance to gas penetration of various protective covers, such as paint layers on wood.
Food packaging applications
Recently, studies of different kinds of fruit, in particular apples have been performed. The possibility for non-invasive monitoring of oxygen gas exchange inside apples in modified atmosphere packages was demonstrated. The initial measurements were recorded using a transilluminating geometry whereas subsequent measurements were acquired using a newly developed reflection probe, which certainly could be more practical in certain applications. The technology might develop into a useful tool in the packaging industry.
Another application area is the pharmaceutical one, which is explored in collaboration with the Astra Zeneca. It was shown, that the GASMAS signal from strongly scattering tablets can be related to important characteristics of the tablets .
Studies of drying processes
Drying of wood and other materials is a quite complex process. We have demonstrated that by simultaneously monitoring the oxygen and the water vapour signals from drying wood a detailed insight could be obtained. The ratio between the water and oxygen signals was shown to be a useful characteristics to determine the progress of the process.