Interferometer, Fourier transform instruments

Figure 7.13 (a) Infrared interferometer Fourier transform instrument (b) GC infrared lightpipe sample cell (c) HPLC infrared mieroliquid sample cell. 

A schematic diagram of a Fourier transform instrument is given in Fig. 1. The simplest form of the Michelson interferometer consists of two mutually perpendicular mirrors, one of which can move in the direction of the beam. Between both mirrors there is a beam-splitter where the radiation is partially reflected (to the moving mirror) and partially transmitted (to the fixed mirror). Both parts of the beam return to the beam-splitter where, because of the difference in path ([Pg.127]

Fourier transform instrument Instruments based on interferometers for wavelength separation. Near infrared light passes through a scanning interferometer and Fourier transformation gives intensity as a function of frequency. When samples are placed in the beam (before or after the interferometer), the sample absorbs at some frequencies, and the intensities are reduced into an interferogram. The mathematical FT function is then used to convert the interferogram to an absorption spectrum of the sample. 

The maximum optical retardation, then, determines the ultimate resolution which is possible with a particular Fourier transform instrument. However, other design features must also be considered when deciding whether that ultimate resolution is in fact achievable. In particular, the size of the aperture through the Michel son interferometer must also be considered. 

Dispersive elements and interferometers are widely used in vibrational microspectroscopy. As in bulk measurements, microscopic Raman studies are carried out with grating monochromators, spectrographs, or Fourier transform spectrometers, although Fourier transform instruments are usually limited to applications in the near-infrared spectral region. Infrared microspectroscopy, by contrast, is almost exclusively a Fourier transform technique. 

In between the source and detector, the spectrometer must have some means of analyzing the radiation so that an intensity can be deduced for each wavelength resolution element. Two completely different types of devices are used, namely, monochromators and interferometers. Monochromators with gratings or prisms are used in dispersive instruments, and interferometers are used in Fourier transform instruments. 

The methodology that involves instruments that utilize the interferometer and Fourier transformation mentioned in Section 8.6 has come to be known as Fourier transform infrared spectrometry (FTIR). 

We have seen that one of the key aspects of Fourier transform NIR analyzers is their control of frequency accuracy and long-term reproducibility of instrument line shape through the use of an internal optical reference, normally provided as a HeNe gas laser. Such lasers are reasonably compact, and have acceptable lifetimes of around 3 to 4 years before requiring replacement. However, we also saw how the reduction in overall interferometer dimensions can be one of the main drivers towards achieving the improved mechanical and thermal stability which allows FT-NIR devices to be deployed routinely and reliably in process applications. 

In a Fourier transform IR instrument the principles are the same except that the monochromator is replaced by an interferometer. An interferometer uses a moving mirror to displace part of the radiation produced by a source (Fig. 5.4) thus producing an interferogram which can be transformed using an equation called the Fourier transform in order to extract the spectrum from a series of overlapping frequencies. The advantage of this technique is that a full spectral scan can be acquired in about 1 s compared to the 2-3 min required for a dispersive instrument... 

Fourier transform infrared spectrometers first appeared in the 1970s. These single beam instruments, which differ from scanning spectrometers, have an interferometer of the Michelson type placed between the source and the sample, replacing the monochromator (Figs 10.9c and 10.11). 

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