Experimental Proof of the Relation between Thickness of the Probed Surface Layer and Absorbance in FT-IR/ATR Spectroscopy

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Abstract
Experimental investigation has been made of the relation of absorbance of a band in FT-IR/ATR spectroscopy to the thickness of the probed surface layer from the measurements of two-layered samples. The two-layered samples are composed of a polystyrene surface layer varying in thickness from 0.01 to 3.64 μm and a polyurethane base layer of 100 *im thickness. The component spectrum of the surface or base layer was separated by spectral subtraction. FT-IR/ATR spectroscopic measurements were mostly measured by nonpolarized incident light. The relation of observed absorbances of several selected bands of both the surface and base layers to the thickness of the surface layer was compared with the theoretical calculations from Harrick's equation. In the comparison, the effect of “nonpolarization” of the incident light was corrected for the observed absorbances. The agreement was excellent between the observation of and the theoretical calculation for both the surface and base layers. A method was proposed to determine the thickness of the spectrally separable surface layer on a film. The usefulness of this method was demonstrated by application to the redetermination of the thickness of the surface layer of the present two-layered samples. An expression was proposed to give the contribution of a surface layer of a certain thickness to the total absorbance of a band in ATR spectroscopy.