The main applications of GASMAS for now involve mainly oxygen and water vapor, with absorption lines around 760 nm and 935 nm, respectively. The oxygen absorption lines are due to the transitions from the ground state X3��g? to the excited state b1��g+, with the strongest absorption cross-section in the order of 6 �� 10?23 cm?2/molecule. The water vapor absorption lines used are in the transition bands of (301)��(000). The strongest absorption cross section of water vapor in this region is about 2 �� 10?21 cm?2/molecule, which is about 30 times stronger than that for oxygen at 760 nm; however, to be noted, water vapor generally has much lower concentration in the context of human tissues and food packages. In the following discussions, we will use these two gases as sample gases in the following sections.

However, these applications can also be explored to other gases.4.?Pathlength Calibrated GASMASTraditional TDLAS applications normally measure the gas concentration using a well-defined gas cell with a known pathlength. On the other hand, if the gas concentration is known, the pathlength can be retrieved, as can be seen from Equation (3b). Based on this simple idea, one way is to simultaneously monitor another gas with known concentration, e.g., water vapor, the saturated concentration of which depends upon the temperature according to the Arden-Buck equation [44]. In this case, the pathlength for a water vapor absorption line can be obtained and is assumed to be the same as the pathlength for the gas of interest, e.g., oxygen. Thus, the target gas (oxygen) concentration can be retrieved.

This method was first demonstrated in [45] by using the LeqH2O of water vapor to calibrate LeqO2 of oxygen, and later utilized for oxygen and water vapor diagnosis in human sinuses and food packages [18,20,46]. Figure 3 shows GASMAS data for the right and left maxillary sinuses of a healthy volunteer. Although the values of the LeqO2 are different for the two sinuses, the same ratios are obtained after normalization on the corresponding values of LeqH2O, indicating the same oxygen concentration in both well ventilated sinuses. We note that data from a clinical study involving 40 patients, after being evaluated in this way, were found to be well correlated with the results obtained by X-ray computed tomography (CT) [47].

The method assumes that the scattering cross-sections involved in determining the path length for the light probing the different Cilengitide species are the same for both wavelengths used. This is only approximately true, in view of Mie and Rayleigh scattering theory, and for certain geometries large differences can occur.Figure 3.Experimental results of pathlength calibrated GASMAS with water vapor and oxygen for human left and right sinuses (Modified from [45]).