Thermo Scientific MIRAN 205B Series, Инструкция по эксплуатации

Страница: 139 / 214

Chapter 13 Infrared Spectroscopy Tutorial
13-19
unknown sample. By knowing at what wavelengths energy is absorbed, the types
of molecules in a compound can be identified.
2.
Selecting a good fingerprint for a compound involves finding an intense
absorption peak in its IR spectrum at a wavelength where:
a. it can easily be identified from among other similar compounds, and
b.
interference from water and carbon dioxide is minimal.
These practical considerations often mean selecting an alternate analytical wavelength for
identifying and measuring the target compound.
3. We can also determine how much of the chemical compound is present using
quantitative techniques based on Beer's Law.
When a certain quantity of IR energy is beamed through the sample cell containing
molecules of a chemical compound, the molecules absorb a certain percentage of that
energy. The amount of energy absorbed can be expressed in % Transmittance or in
Absorbance Units.
By knowing how much IR energy is absorbed, we can figure out how many molecules
are in the chamber. Very low concentration levels of gases can thus be measured and
expressed in parts per million (ppm).
HOW THE SAPPHIRE ANALYZER WORKS
The SapphIRe Analyzer is a single-beam, microprocessor-controlled, infrared analyzer
with a variable pathlength gas cell and an optical filter (Figure 13-18). In the analyzer,
infrared light is generated by a glowing metal alloy filament (a) heated to a dull orange
glow. A rotating chopper blade (b) interrupts and synchronizes the beam so that the
intensity of the signal reaching the detector is modulated. The beam then passes through a
variable filter (c), to remove extraneous wavelengths of light, into the sample cell (d)
containing the gas sample.
Figure 13-18. Internal Components of the SapphIRe Analyzer
Source
Optical
Filter
Sample
Cell
Detector
Chopper