I saw a request for help with differential peak sensing CO2 concentration equipment. I put this short response together, but I can't find the original relay of the request from some other group... so for what it's worth.
Warning: this treatment may be naive......
Comparing your expression with an expression for Beer-Lambert (below) it looks like the amplitudes of the absorption peaks are proportional to the number of gas molecules involved. The ideal gas law you have in mind is probably PV = nRT where n is the number of molecules (moles) involved. Expressing IGL in terms of number of molecules gives n = PV/rT
On the face of it, both reference peak and measurement peak amplitude should vary similarly with PV/T for an ideal gas leaving their ratio much the same so it seems like the variations with pressure and temperature you are concerned about are non ideal gas consequences and might best be handled with measuring three points of varying temp and three points of varying pressure experimentally. (3X3 test-points?)
Brian Whatcott
Appendices:
Beer-Lambert Optical absorption spectroscopy is based on the Beer-Lambert law, namely: transmission will decay exponentially as exp(-Sg(f)nL) where S is the absorption line strength, g(f) is the line shape function, n is the concentration of absorbing gas molecules, and L is the optical path length.
Your Expression