Gas in Scattering Media Absorption Spectroscopy

Abstract

Gas in scattering media absorption spectroscopy (GASMAS) is a new variety of tunable diode laser spectroscopy (TDLS). It combines concepts from atmospheric trace‐gas monitoring with those pertinent to biological tissue optics. The former field deals with high‐resolution spectroscopy in nonscattering media, whereas the latter area is characterized by broad absorption structures in strongly scattering media. GASMAS provides novel applications in, e.g. the material science and biophotonics fields. The absorptive imprints of free gases inside pores or cavities in surrounding solid or liquid matter are typically many orders of magnitude narrower than those of the host material, a fact that is critically utilized. The gas signals are detected in the weak, multiply scattered light emerging from the illuminated sample. Wavelength‐modulation and phase‐sensitive detection techniques are employed, typically in connection with single‐mode CW lasers. Any gas with useful absorption at wavelengths where the host material does not absorb strongly can be detected. For biological tissue, containing liquid water and blood, this limits the useful region to the “tissue optical window” –650–1400 nm – where, however, the interesting gases oxygen and water vapor absorb at around 760 and 950 nm, respectively. This limitation does not pertain to other materials, particularly not to those that do not contain liquid water. GASMAS experiments, which relate to basic physics, include studies of nanoporous ceramics, where wall collisions influence the line shape and provide information on pore‐size distribution. A small piece of strongly scattering ceramic can serve as an alignment‐free multipass cell with effective path length hundreds of times longer than the physical dimension. Porosity and gas transport studies in construction materials such as polystyrene foams, wood, ceramics, and paper are examples of applications in the material science field. The technique is further very powerful for studying gas in the human body and products that humans eat, such as packaged foods, fruits, and pharmaceutical preparations. A key aspect of food packaging is to prevent oxygen to influence the food. Frequently, modified atmospheres with nitrogen or carbon dioxide as filling gases are used. Applications include monitoring the performance of packaging machines and measurements of the product on the shelf. Porosity in pharmaceutical tablets is important to determine, as it has bearing on controlled release. Following initial work on healthy volunteers, a clinical trial concerning human sinus cavities has been performed. Gas filling and composition can be used as a diagnostic tool in connection with sinusitis, a very common disorder, also related to the problem with heavy overprescription of antibiotics and associated growing bacterial resistance. Following realistic phantom studies, it was shown that it is possible to detect gas in the lungs and intestines of a newborn baby, which could be of considerable interest for future care of prematurely born children. Gas diffusion and transport can be dynamically studied in response to an abrupt change in gas composition in medicine as well as in material science. The GASMAS technique is fully nonintrusive and nondestructive.