PerkinElmer Frontier Optica FT-IR, Руководство пользователя

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56 . Frontier Optica FT-IR User's Guide
Sample thickness
The sample thickness impacts on the image focus at the detector, and therefore may affect
the accuracy of the measurement. You may find reduced accuracy when, for example,
measuring germanium (
n
≈ 4) samples with sample thicknesses greater than 6 mm. Sample
thickness effects can also be significant when tilting samples (see
Sample tilt
on page 55).
Non-planar samples
You should be aware that taking measurements from non-planar samples will introduce
errors. You may be able to improve the accuracy of such measurements by, for example,
comparing a coated component with a similar uncoated component.
Small samples
We advise that you mount small samples over an aperture in an opaque mask to prevent the
beam from passing round the edge of the sample. However, the mask
must
be in the same
position for both background and sample measurements.
Samples with near-parallel reflecting surfaces
An optical substrate with parallel reflecting surfaces can act as an etalon. If the free spectral
range of the etalon (the fringe spacing) is finer than the resolution selected on the Frontier
Optica FT-IR, the fringes will not be seen directly.
However, in spectral regions with atmospheric absorption, the spectral filtering due to the
etalon fringes can interfere with the atmospheric absorption lines, resulting in apparent
excess noise in the atmospheric absorption region. This is shown in Figure 20, in the region
2000–1300 cm
− 1
(water vapor) for a 2 mm thick sample of uncoated germanium with near-
parallel surfaces.
This can be reduced by purging the sample area with dry nitrogen (see
Purging the
Spectrometer
on page 66), and by ensuring that the instrument desiccant is changed
regularly (see
Changing the Desiccant
on page 63).
Figure 20 Water vapor interference in an FT-IR spectrum of a 2 mm thick sample
of uncoated germanium