..For the special case of human sinuses, where the turbid medium involved is dominated by a few large cavities, the discrepancy in optical properties for these two wavelengths could be comparatively small. On the other hand, water vapor actually has quite a few strong absorption lines around the oxygen absorption region, e.g., at 819.151 nm, which has the same level of absorption cross section as the one for oxygen at 760 nm. By utilizing a more close-lying absorption line, the discrepancy would be much smaller. However, the absorption cross section at 819 nm is much smaller than the one at 935 nm, which will decrease the signal-to-noise ratio and might increase the error in pathlength for this wavelength.If the saturation condition for water vapor is not fully satisfied for some reason the pathlength of water vapor would be underestimated.

However, for those cases with fully saturated water vapor concentration, the present method is a very powerful tool to evaluate the oxygen concentration, especially considering the use of a closer water vapor absorption line, i.e., 819.151 nm.5.?Pathlength Resolved GASMAS5.1. Time-of-Flight Spectroscopy and Optical PorosityWhen facing the pathlength problems in GASMAS, one could simply go around it and measure the pathlength by using other methods. For a turbid medium, the MOPL (Lm) can be obtained by using TOFS, as has been discussed above. A picosecond TOFS system and the typical time-of-flight distribution through a 10.2-mm slab of polystyrene foam are given in Figure 4.

The first combination of TOFS and GASMAS measurements was already demonstrated in the very early development stage of the GASMAS Dacomitinib technique [40], where polystyrene foams with physical porosity of around 98% were studied. The measured oxygen concentration (CO2) in the polystyrene foam can be deduced by using the mean physical pathlength (Lpm) through the medium as the gas absorption pathlength, given as:CO2=CO2airLeqO2/Lpm(4)Here CO2air is the oxygen concentration in ambient air, LeqO2 is the mean equivalent pathlength of oxygen in the polystyrene foam, obtained from Equation (2). Lpm can be given by Lpm = Lm/neff, where neff, is the effective refractive index of the turbid media. In the case of unknown neff,, Lm could also be used in Equation (4). Clearly, Lpm is not the true pathlength through the pores (Lgas), but also includes the pathlength through the matrix material (Ls), i.e., Lpm = Ls + Lgas, and Lm = nsLs + Lgas. Here ns is the refractive index of the matrix material. Thus, Equation (3) only gives an average gas concentration in the porous medium. The value of Lpm can be used as a good approximation of Lgas for extremely high porosity media, e.g., polystyrene foam, as shown in [40].