Thermo Scientific MIRAN 205B Series, Инструкция по эксплуатации

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Chapter 13 Infrared Spectroscopy Tutorial
13-20
In the cell, the infrared light is reflected back and forth through the sample by
gold-coated mirrors(s). This folded-pathlength design allows for adjustment of pathlength
between 0.5 and 12.5 meters. The greater number of reflections, the more energy is
absorbed by the sample before the infrared radiation reaches the detector (e). When the
infrared radiation finally impinges on the detector, the energy is converted to an electric
signal, which, after amplification and translation into a digital form, appears on the
readout display. Concentration values can be presented in concentration or absorbance
units.
Gases are brought into the analyzer by means of an air sampling pump at the rate of 14
liters per minute. The need to change the pathlength is prompted by the microprocessor
and then the pathlength mechanism must be adjusted manually to 0.5 or 12.5 meters. The
adjustment mechanism causes a small rod to press on a mirror inside the cell. By pivoting
the angle of the mirror, the number of reflections of light is increased or decreased
depending on the direction the mirror is moved.
All analyzers must have a source of infrared radiation. The source in the SapphIRe
Analyzer is a metal alloy filament, which, when heated to 825°C, emits a broad spectrum
of radiation. The filament's operation is similar to the one used in a regular light bulb
except that it produces primarily wavelengths of infrared energy or heat instead of light.
Radiation from the filament passes through revolving blades which interrupt (or chop) the
beam and synchronize the electronic circuits with the IR energy reaching the
photodetector cell. The beam of energy must be interrupted so that the intensity of IR
energy reaching the photodetector constantly varies. Otherwise the detector would
rapidly become saturated and insensitive to the IR radiation.
Next, the modulated beam passes through a monochromatic filter made of special glass
that allows a narrow band of IR radiation to pass through at any one moment. This
infrared linear variable filter is capable of selecting one of seven fixed bandpass filters or
individual wavelengths in the range of 7.7 to 14.1 microns (Figure 13-19). This is
accomplished by a servo mechanism automatically moving the filter to the correct
position. The filter moves between fixed and variable wavelength positions.
Figure 13-19 . Linear Variable Filter
Resolution is the ability of the analyzer to use only that wavelength called for by the
analysis. The variable filter used in the SapphIRe Analyzer has varying resolution
depending on the wavelength range selected (Table 13-1).