Background Correction in Transmission Measurements

The two main sources of background absorption (i.e., absorption from material other than the sample) are the solvent used for liquid solutions and the air in the optical light path. In a conventional double-beam dispersive system, comparing the sample beam to the reference beam and recording the difference spectrum in real time automatically eliminate absorption from air and solvent. If the sample is a liquid solution, a matching liquid cell with pure solvent is placed in the reference beam. The absorption from the solvent and from the air is measured simultaneously and subtracted from the sample beam signal. 

However, FTIR is a single-beam system and both air and solvent contribute to the signal, so corrections must be made in several steps. 

The solvent absorption spectrum is measured by putting pure solvent in the hquid sample cell and recording its spectrum. This spectmm is stored by the computer under an assigned hie name (e.g.. Spectrum A). The cell (or an identical cell) is then hlled with the sample solution in that solvent, its spectmm taken, recorded, and stored under a hie name (e.g., Spectmm B). Spectmm A is then subtracted from Spectrum B, giving the net spectmm of the sample. Of course, in this approach, any absorphon by the air is also measured in both Spectmm A and Spectmm B, so the absorption by air is also corrected for. 

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