PerkinElmer Spotlight 150, Руководство пользователя

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70 . Spotlight 150 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.
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:
2 / 1 2
21
2
1
) (sin 2 n n
d
p
−
=
θ p
λ
Where:
1
n
is the refractive index of the ATR crystal (for germanium 4.0)
λ
is the wavelength of the radiation
θ
is the angle of incidence of the beam
21
n
is the ratio of the refractive indices of the sample and the ATR crystal.
ATR objective
The ATR objective enables the microscopic examination of samples 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 an
approximately 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 controls the crystal automatically.