Perkin-Elmer MultiScope System, User Reference

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Applications
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Reflectance FT-IR Microspectroscopy
The MultiScope System Microscope can be used with samples that display any of
the three types of reflectance: diffuse reflectance, specular reflectance, or
reflection-absorption.
Figure 32 Three Types of Reflectance
Figure 32 shows how the incident radiation (I
0
) is reflected in each type of
reflectance. It is not uncommon for two or more of these processes to occur
simultaneously, depending on the structure of the sample.
Diffuse Reflectance
In diffuse reflectance, the incident radiation is reflected in all directions from the
surface of the sample. This type of reflectance is seen in samples with matt
surfaces, such as paper. The broad collection angle of the microscope enables it to
capture a large proportion of the diffusely reflected radiation and send it to the
detector.
A problem often encountered when using the diffuse reflectance technique is that
there is a large specular component in the reflected radiation. Figure 33 shows
three diffuse reflectance spectra of polymethyl methacrylate (PMMA) shavings.
In the top spectrum, the shavings were neat. In the next two spectra, the PMMA
was diluted with successively larger amounts of KBr.
In the neat sample, the presence of interfering specular reflectance introduces
nonlinearities to the spectral data. For example, the relative intensities of the strong
C=O and C-O stretching absorptions are not as expected (arrows on the top
spectrum).
As the sample is diluted in a non-absorbing matrix (KBr), specular reflectance is
minimized. The bottom spectrum in more closely matches an absorption spectrum.