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78 . Spotlight 200 User's Guide
Collecting Spectra With the ATR Objective
The ATR (attenuated total reflectance) technique enables the collection of spectra from
materials that are too opaque for transmission measurements, and too strongly absorbing for
good reflectance measurements. Spectra can be collected with little sample preparation.
Spectrum software enables you to automatically map a sample using the ATR objective.
Spectra are collected by touching the ATR objective on the sample and collecting the
spectrum generated from the surface layer of the sample. See the on-screen Help for further
information.
The ATR objective uses a crystal made from a material that transmits infrared radiation, and
has a high refractive index. An infrared beam enters the crystal and is internally reflected
within the crystal, creating an evanescent wave. At each reflection inside the crystal, the
wave continues beyond the crystal surface into a sample that is held in close contact. The
penetration depth depends on the refractive indices of the crystal and the sample. For a
germanium crystal, the penetration depth for most samples is less than 1 µm. The
penetration depth also varies with the wavenumber of the infrared radiation:
Where:
n
is the refractive index of the ATR crystal (for germanium 4.0)
1
λ
is the wavelength of the radiation
θ
is the angle of incidence of the beam
n
is the ratio of the refractive indices of the sample and the ATR crystal.
21
ATR objective
The ATR objective enables the microscopic examination of samples in order to locate the
exact area of interest. The ATR crystal is moved vertically out of the beam path to enable
you to view the sample, and place the area of interest in the center of the field of view. The
crystal is then placed onto the sample under positive pressure to collect a spectrum.
The crystal has a small contact area formed by the flattened point of a cone; this ensures a
100 µm diameter contact area.
Two versions of the ATR objective are available. The manual ATR objective requires you
to raise and lower the crystal using a toggle bar on the ATR assembly. The automated ATR
objective not only controls the crystal automatically, but also includes a weighbridge to
measure the force applied to the sample by the crystal, which helps you obtain more
repeatable spectra.
NOTE: The weighbridge communicates with the microscope using a small infrared
transmitter located behind a window at the rear of the platform. Take care not to
cover this window with any part of your sample as this will prevent the weighbridge
from transmitting the value of the force applied by the crystal.
λ
d
=
p
p
n
2
θ
n
2
2
(sin
−
1
21
1
2 /
)
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