are you interested in the IR spectrum of the surface or the adsorbed gas? if you are interested in the condensed phase only, you can use a gas that is not IR-active (N2 for example); if the gas is absorbing in the IR you will certainly see a bulk effect; in the latter case a thoughtful systematic variation of the conditions may help (e.g. testing different conditions under which the adsorption behavior is the same but the bulk gas is different) 

Prof. Kiefer,

Thank you for your valuable contribution. The exposed gas is IR active. My aim is to investigate this sample-gas interaction upon adsorption by observing the changes in the bands (i.e. splitting, shifting, etc..) according to the known IR-active vibrational modes of that gas.

The "mechanism" term within my question statement implies to the following:

The incident IR beam on the rough surface of the sample is diffusely reflected; before the incidence and after the reflection of the IR beam, the IR beam passes through the gaseous environment above the sample. Hence, the bulk gas might absorb photons of IR radiation and affect the IR that is detected by the detector, and therefore affecting the resulting spectrum. 

By principle, do you think that this free-gas contribution is significant and/or can it be detected by looking at the resulted spectrum?

in influence of the bulk gas depends on the pathlength the IR radiation is travelling through the gas. The shorter the pathlength the less effect. Can you vary the pathlength? A systematic variation may allow you to correct for the bulk gas contribution.

Mrs/Miss/Mr Anaya,

Please find an answer of your question in the following paper:

1. A revised Kubelka–Munk theory for spectral simulation of phosphor-based white light-emitting diodes, Kunio Ishida, Iwao Mitsuishi, Yasushi Hattori, and Shinya Nunoue, Applied Physics Letters 93, 241910 (2008)