PerkinElmer Spotlight 400 Series, User Manual

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Techniques for Collecting IR Images and Spectra . 91
Reflectance FT-IR Microspectroscopy
The Spotlight imaging system 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 35 Three types of reflectance
Figure 35 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 imager enables it to capture a large proportion of the
diffusely reflected radiation and send it to the detector. In addition, the diffuse reflectance
sampling mode compensates for any loss of signal by increasing the gain of 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 35 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).
As the sample is diluted in a non-absorbing matrix (KBr), specular reflectance is minimized.
The bottom spectrum more closely matches an absorption spectrum.
NOTE: Sample cups for obtaining diffuse reflectance spectra of powders are available from
PerkinElmer: regular cup (part number 01862760) and micro cup (01862761).