Chemical Imaging with Variable Angles of Incidence Using a Diamond Attenuated Total Reflection Accessory

A new development in Fourier transform infrared (FT-IR) imaging using a diamond attenuated total reflection (ATR) imaging accessory in a novel manner that allows the angle of incidence to be varied in order to obtain images from subsurface layers of different thickness is introduced. Chemical images of samples from the same area but with different depths of penetration are obtained by changing the angle of incidence as well as using different spectral bands at different wavenumbers. Changes in the angle of incidence with this accessory were made possible by taking advantage of the relatively large numerical aperture employed by the original imaging optics. This arrangement allowed us to introduce an additional movable aperture in the optical design to restrict the angle of incidence to certain values. Two samples have been studied, one for the calibration of the angle of incidence while the other demonstrates the capability of obtaining three-dimensional (3D) information using this approach. Advantages of this new approach include the relatively high spatial resolution (it can spatially resolve features as small as 12 mu m without a microscope) and no change in the imaging area and sampling area during manipulation of the angle of incidence.