Observation of a Penetration Depth Gradient in Attenuated Total Reflection Fourier Transform Infrared Spectroscopic Imaging Applications

摘要

Fourier transform infrared (FT-IR) spectroscopic imaging offers the possibility of combining spectral and spatial information, thereby enabling a spatial chemical visualization of samples. Thus, sample areas can be analyzed by infrared spectroscopy with high lateral resolution (up to 3-4 (mu)m). The performance of FT-IR imaging equipment has been discussed by many authors, and numerous applications have been described. An important application area is the analysis of polymers, including diffusion and dissolution processes. Another field of research is the analysis of skin sections, biological tissues, and biomaterials. Pharmaceutical issues such as drug release or dissolution of tablets have also been analyzed by FT-IR imaging. Besides transmission measurements, attenuated total reflection (ATR) imaging is often applied. In this note we report on investigations of homogeneous samples by ATR imaging measurements with the FastIR~(TM) accessory. In order to check the illumination uniformity of the ATR crystal, we chose a homogeneous liquid sample (paraffin oil) because of its perfect contact to the ATR crystal. However, despite this perfect contact, the integrated absorbance for a particular band is not constant for the imaged area. Instead, an absorbance gradient in the Z direction of the image (direction of the IR beam from the source to the detector) is revealed. These findings were also analyzed for two solid samples (adhesive tapes) and an emulsion. The influence of this absorbance gradient on ATR imaging results will be discussed.