PerkinElmer Spotlight 400 Series, User Manual

Page: 77 / 120

Techniques for Collecting IR Images and Spectra . 77
Collecting Spectra with the ATR Objective
NOTE: The ATR objective is optional and mainly applicable to mid-infrared applications.
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.
SpectrumIMAGE software enables you to automatically image a sample using the optional
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.
NOTE: You must be in Point mode in the SpectrumIMAGE software when using the ATR
objective, as the ATR objective cannot be used in Image mode. Point mode is not
available if you are using the Spotlight with Long Wavelength option.
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 π
λ
Where:
1
n
is the refractive index of the ATR crystal (4.0 for germanium)
λ
is the wavelength of the radiation
θ
is the angle on 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 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.