High-resolution FT-IR spectrometers

The most common sample compartment in high-resolution FT-IR spectrometers is the White cell, where the beam passes a path of 1-1000 m. In a long-path gas cell, however, a small f/number (focal length of the mirror/diameter of the pupil) and a large size of the circular image may lead to a loss of effectivity. In the Oulu interferometer this problem was solved by an alternative multiple-path gas cell, where the images at the focal planes occur on a circle around the principal axis of the cell. The aberrations remain small in spite of a small f/number, because the successive images are relatively near to the principal axis of the optics. 

The instrumental resolution of high-resolution FT-IR spectrometers is, in practice, of the order of 0.001 cm L In addition to high resolution, a good absolute accuracy of the line positions in the spectrum is... 

The main field of use of high-resolution FT-IR spectrometers is molecular spectroscopy. The... 

An example of a typical rotation-vibration band, measured with a high-resolution FT-IR spectrometer, is shown in Figure 11. It is the bending band V2 of CO2 near 667 cm . This spectrum was recorded by the Oulu spectrometer with an instrumental resolution of 0.002 cm The relative errors of the wavenumber compared to model equations are of the order of 10 . The vibration energy levels of different normal modes are sometimes very close to each other. In this case these normal modes can be coupled so that the energy levels are shifted apart. Now the observed wavenumber cannot be expressed in a closed form and the rotational lines of the spectrum are shifted. With the proper quantum... 

Traditionally, FT-IR spectrometers used to be divided into two groups, routine and research spectrometers. Both have an affiliated PC for the data processing and handling. Routine spectrometers usually have a resolution limit of ca. 1 cm. Research spectrometers can achieve resolution as high as 0.001 cm . Sources, beamsplitters and detectors are exchangeable in research spectrometers, so one could use these spectrometers from 40000 down to 20 cm (from the UV to far-IR range). In some spectrometers different sources and detectors are installed permanently. They can be switched on or off by means of flip mirrors. Nowadays there are no designated limits between routine and research instraments. 

Left A Nicolet 560 E.S.P.FT-IR spectrometer.fl/ghf A high-resolution FTIR spectrum of carbon monoxide. 

An example of a real system where these factors have been brought into consideration may be found in commercial FT-IR spectrometers that collect high-resolution... 

One of us (P.R.G.) has incorporated neural network software into a low-resolution OP/FT-IR spectrometer. Once the components had been identified, an online PLS calibration and prediction was carried out Fheliminary results are promising. As an example, a mixture of acetone, methyl ethyl ketone (MEK), and diisobutyl ketone was allowed to evaporate close to the beam of an OP/FT-IR instrument. The software recognized the presence of these three compounds and allowed their quantification. The data obtained in this way are shown in Figure 22.8. Acetone and MEK are seen at relatively high concentrations at the start... 

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