There are two basic mechanisms that lead to baseline slope with strongly absorbing samples. The first is due to light scattering within the sample and causes the baseline to slope upwards with increasing wavenumber. The second is due to differences in thermal response as a function of infrared beam modulation frequency between the sample and the reference standard used to ratio spectra. For instance, there is usually less baseline slope with a solid slab sample geometry if the reference standard has the same geometry. Consequently, a slab of glassy carbon or graphite is a preferable reference standard to carbon black in either a thin coating or powder form if a slab of material is being analyzed.

1. Composite material prepreg

Carbon fiber/epoxy prepregs are too opaque for transmission spectroscopy but yield a good FTIR-PAS spectrum of the epoxy component as is shown in Fig. 31. This spectrum was obtained on a 7 mm x 7 mm square of material cut with scissors. The plot shows a baseline that slopes upward with increasing wavenumber which is characteristic of light scattering within the sample.

Fig. 31. FTIR-PAS spectrum of a carbon fiber/epoxy prepreg used in composite materials. The spectrum has a sloping baseline due to light scattering within the sample. Absorbance bands appear in the spectrum between 1400-1900 cm^-1 and 3500-3900 cm^-1 due to moisture in the sample chamber. If more time is allowed for purging and for desiccant to work, these bands would be eliminated.