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The IR Spectrophotometer The IR Spectrophotometer (or sometimes called the "IR spectrometer") comes in many forms (Modern Fourier Transform devices can be purchased for around £11,500, ($18,500 USD)). The workings of the device are shown below, and although it looks fairly complex, it is very similar to the internal set-up of a UV-Vis spectrophotometer.
Fig.1: An IR Spectrophotometer schematic, where array E and F are shown below in greater detail. Although it looks confusing, its a relatively simple instrument, with A being a source of IR radiation (typically a "Nernst Glower" (mixed Zr, Th and Ce oxides), or a "Globar filament" (silicon carbide)). The IR radiation is bounded off of a planar mirror and is redirect using curved mirror array B, which sends radiation through both the reference sample (the "blank", C), and the sample (D). The beam is then fed through a chopping mirror (E, the same as in a UV-Vis spectrophotometer), which then allows 50 % of each beam through the monochromator array in pulses equal to the frequency of rotation of the mirror. A deeper look at the semi-mirror array is shown below (Fig. 2). Once the beam is through the semi-mirror array, it is sent via a series of curved and planar mirrors to a monochromator (such as as eshelle grating), F. This grating is a simple block that sends off different frequencies of light at different angles (like a prism). A more detailed look at the monochromator array is shown below (Fig. 3). A motor rotates the grating, so that the frequency you want is refracted down to the rest of the system, so when a scan is performed the grating is simply rotated smoothly to diffract the separate wavelengths through the rest of the system one by one, so a full frequency sweep can be recorded. The IR radiation is captured by the detector, G, which is very often a simple thermocouple. As with UV-Vis spectrophotometry, a spectrum of absorbance is usually denoted in percent. transmission spectra are just reciprocal absorbance .
Fig.2: The IR Semi-Mirror Array. The two beams have been falsely coloured so you can see how it works. As the semi mirror rotates, it divides the amount of IR radiation going to the monochromator into 50 % from the reference, and 50 % from the sample. The computer attached to the detector knows the frequency at which the semi-mirror is rotating, and simply rebuilds data at the end
Fig.3:
The Monochromator Array. One of the beams has been
coloured blue so you can follow it more easily.
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