Advantages of Fourier Transform Spectroscopy

The mathematics of the conversion of an interferogram into a spectrum is the Fourier transformation (FT). Accordingly, the spectrum is given by 

Equations (5.8) and (5.9) are interconvertible and are known as a Fourier transform pair. 

Do not worry if your knowledge of calculus is not up to these equations. Fortunately, it is not necessary to have a detailed knowledge of the mathematics involved in order to carry out experiments using an FT spectrometer. Based on fully developed software the computer performs these transformations. 

The essential steps for obtaining an FT-IR spectrum are to produce an interferogram with and without a sample in the beam and then transform these interferograms into spectra of the source with sample absorption and the source without sample absorption (see for instance MIR files GLYCIN, WATER, and VASELINE). The ratio of the former and the latter is the IR transmission spectrum of the sample. 

In the case of FT-IR spectroscopy, the sample is usually placed between the interferometer and the detector. For FT Raman spectroscopy the scattering volume of the sample itself is the radiation source. 

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From Table 2 it is observed that the dispersive NIR ensembles (NIR and NIR R) result in the best cross validated models. The potential advantages of Fourier transform spectroscopy [5] are in practice outnumbered by a more reproducible setup and saimpling procedures. 

An advantage of Fourier transform spectroscopy is that the entire interferogram is recorded in a few seconds and stored in a computer. The signal-to-noise ratio can be improved by collecting tens or hundreds of interferograms and averaging them. 

Advantages of Fourier transform infrared spectrometers are so great that it is nearly impossible to purchase a dispersive infrared spectrometer. Fourier transform visible and ultraviolet spectrometers are not commercially available, because of the requirement to sample the interferometer at intervals of S = l/(2Av). For visible spectroscopy, Av could be 25 000 cm 1 (corresponding to 400 nm), giving S = 0.2 im and a mirror movement of 0.1 xm between data points. Such fine control over significant ranges of mirror motion is not feasible. 

The classical approach to the analysis of mixtures by use of infrared spectroscopy consists in identifying specific, strong bands that belong to a suspected component, obtain a pure spectrum of the suspected component, and then remove those in the spectrum of the mixture that are due to the identified compound. The process is repeated for the remaining bands in the mixture spectra. Once the component spectra are known for a mixture, a series of calibration curves is produced. These curves relate concentration to absorbance, using Beer s law. The concentration of the components of the mixture are then obtained by interpolation. The advantage of Fourier-transform, infrared spectroscopy is that components of a mixture may be... 

Fourier transform methods have come into their own as a means of studying the optical spectra of gas-phase radicals. Both infrared (FTIR) and ultraviolet/visible spectroscopy (FTUV/VIS) are now used to scrutinize these reactive molecules. We discuss the underlying principles of Fourier transform spectroscopy (FTS) with particular emphasis on the advantages and drawbacks of FTIR and FTUV/VIS measurements. Extensive tables are presented of metastable molecules that have been studied by Fourier transform methods. 

As mentioned in the introduction, a major advantage that Fourier transform spectroscopy has over laser spectroscopy is that it is straightforward to record the entire spectrum of a species at once. Diode lasers in the infrared are not continuously tunable and have mode gaps which can only be filled by switching diodes. Many ultraviolet lasers are not continuously tunable either. Tunable difference frequency methods and diode lasers involve much longer scan times than are necessary with a Fourier transform device. For example, the Bomem DA3.002 can scan a bandwidth of 100 or more wavenumbers in the mid-IR at a resolution of 0.005 cm-1 in less than 3 minutes. A diode laser which scans in 20 MHz steps may require more than a day to scan the same spectral region. 

For this reason, the reviewer proposes to introduce the reader to Fourier transform spectroscopy in the hope that he will make use of it. The basic physical principles of spectroscopy and the theory and practice of Fourier transform spectroscopy are described. Its advantages and disadvantages are discussed relative to spectroscopic problems and always with reference to the grating spectrometer as representing conventional spectroscopy. 

Summarizing the results of our discussion of the practice of Fourier transform spectroscopy, we start with the presumption that the equipment for most routine spectroscopic investigations consists of a Fourier spectrometer with a Michelson interferometer and a digital computer. In other words, the advantages of the lamellar grating used as a two-beam interferometer, and of phase modulation, for example, have been utilized only for certain special applications in the extreme far-infrared. All commercial Fourier spectrometers are available with a computer attached, which in most cases not only performs the Fourier transform but is also programmed to control the instrument. Commercial instruments have a remote switch for the selection of the different spectral ranges, and the filters and beams... 

In recent times, various modifications of the basic DSC method have been reported. They include the so-called modulated DSC, in which a sinusoidally changing amplitude that is governed, as in standard DSC, by the temperature measured at the sample position.The modification resembles in several aspects the features of Fourier transform spectroscopy. Its main advantage over standard DSC resides in its ability to differentiate between reversible and irreversible thermal effects. "... 

The advantages of Fourier transform near-infrared Raman spectroscopy are ... 

To summarize, twenty years ago it was well established that Fourier transform spectrometers possessed a great advantage in sensitivity over more conventional spectrometers, but that the practice of Fourier transform spectroscopy was severely restricted because of the need for powerful computers to generate the spectral result and because the optical devices were very sensitive to perturbations. 

The photoacoustic effect was first discovered by Alexander Graham Bell in the early 1880s [18], but it was not applied to Fourier transform infrared (FTIR) spectroscopy until a century later [19,20], Significant advantages of FTIR photoacoustic spectroscopy (PAS) include the following (1) Spectra may be... 

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function ... 

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