PerkinElmer Spotlight 150, User Manual

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Techniques for Collecting Spectra . 91
Reflectance FT-IR Microspectroscopy
The Spotlight 150 enables you to collect images and spectra from samples that display any of
the three types of reflectance: diffuse reflectance, specular reflectance, or reflection-
absorption.
Figure 40 Three types of reflectance
Figure 40 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 and
powders. 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 41 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
non linearities 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).