4. Plastic-coated paper

Plastic-coated paper is another example of a layered material. The material used in this example has a 35 micrometer thick coating and was analyzed by both FTIR-PAS and DRIFTS in order to probe deeper into this rather thickly coated material than can be achieved by FTIR-PAS alone. An MTEC multisampling option pictured in Fig. 28 was used to measure the DRIFTS spectrum. This option allows DRIFTS, PAS, and transmittance spectra to be obtained rapidly by interchanging sampling heads shown in the foreground of Fig. 28 without changing FTIR accessories.

Fig. 28. MTEC multisampling options with interchangeable sampling heads shown in the fore- ground (left to right) for diffuse reflectance (DRIFTS), photoacoustic (PAS), and transmission sampling. The sampling heads for DRIFTS and transmission operation have carbon black absorber elements incorporated within to sense the fraction of the infrared beam that is diffusely reflected or transmitted, respectively.

Spectra of the paper obtained with shallow- and deep-sampling PAS and DRIFTS are shown in sequence going from bottom to top in Fig. 29. The shallow-sampling photoacoustic spectrum measured at an OPD velocity of 0.5 cm/s is dominated by the absorbance bands of the 35 micrometer thick coating. It is possible in this case to essentially isolate the plastic spectrum by shallow-sampling alone without spectral subtraction due to the coating's thickness and thermal properties.