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System Component Description
FTIR
Interferometer
7-10
Omni FTIR Multi Gas CEMS Instruction Manual
This assembly modulates all the IR frequencies down to frequencies that
can be sensed by the detector. This is achieved by using a beamsplitter
which is located in the center of the assembly and secured at an angle of
45°. The divided light is directed to the fixed mirror at the top of the
assembly and to the moving mirror located within the cylinder to the left.
This mirror causes one of the beams to travel a continually changing
distance; so when the beams recombine, the waves interfere with each other
constructively and destructively, creating an interferogram. The center
burst of an interferogram occurs when all the frequencies are in phase. This
happens only when the two mirrors are the same distance from the
beamsplitter. The information outside the center burst provides the fine
details of the absorbance spectrum. The longer the moving mirror strokes,
the higher the resolution.
Note The KBr beamsplitter material is hygroscopic, or absorbs water from
the air, so the FTIR instrument must be continuously purged to remove
water.
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Laser
The FTIR analyzer uses a helium neon (He-Ne) laser, equipped with a
neutral-density filter to help reduce power and reflections. The laser has a
life expectancy of at least four years and can be replaced as needed. It is pre-
aligned and needs no adjustment when it is installed.
Mirrors
The analyzer uses a number of precision mirrors to focus and direct the
infrared beam for optimum performance.
Aperture
The aperture is a fixed-diameter opening that optimizes spectral line shape
(resolution) by defining the number and direction of the infrared rays
reaching the sample. Using an aperture has these advantages: it improves
wave number accuracy by acting as a point source of infrared radiation, and
it helps prevent infrared energy saturation, so the response of the detector is
more linear.
The proper setting of the fixed aperture has been selected based on the type
of detector installed and the resolution setting. In general, you will find
that the larger the aperture, the better the signal-to-noise ratio of the
collected data. The DTGS detector can accommodate most of the energy
from the source, which means the software selects a large aperture size.
Thermo Fisher Scientific
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