Fourier transform multiplex advantage

As in all Fourier transform methods in spectroscopy, the FTIR spectrometer benefits greatly from the multiplex, or Fellgett, advantage of detecting a broad band of radiation (a wide wavenumber range) all the time. By comparison, a spectrometer that disperses the radiation with a prism or diffraction grating detects, at any instant, only that narrow band of radiation that the orientation of the prism or grating allows to fall on the detector, as in the type of infrared spectrometer described in Section 3.6. 

In the infrared spectral range in general Fourier transform (FT) interferometers are used. In comparison with dispersive spectrometers FTIR enables higher optical throughput and the multiplex advantage at equivalent high spectral resolution. In... 

The transform from the interferogram to the spectrum is carried out by the dedicated minicomputer on the instrument. The theory of Fourier-transform infrared spectroscopy has been treated, and is readily available in the literature.21,22,166 Consequently, the advantages of F.t.-i.r. dispersive spectroscopy will only be outlined in a qualitative sense (i) The Fellgett or multiplex advantage arises from the fact that the F.t.-i.r. spectrometer examines the entire spectrum in the same period of time as that required... 

In the IR spectral region the noise characteristics, typical of the measurement system, have led to the development of multiplex detectors whose operation is based on the use of mathematical transformations such as the Fourier and the Hadamad. With these detection systems, an appropriate encoding (transformation) of the raw spectral signal is performed, which permits simultaneous monitoring of a wide spectral window with a single-channel sensor. A SNR (multiplex) advantage is achieved that is typically proportional to the square root of either the observation time or the number of spectral-resolution elements contained in that spectral window. 

We shall conclude this chapter with a few speculative remarks on possible future developments of nonlinear IR spectroscopy on peptides and proteins. Up to now, we have demonstrated a detailed relationship between the known structure of a few model peptides and the excitonic system of coupled amide I vibrations and have proven the correctness of the excitonic coupling model (at least in principle). We have demonstrated two realizations of 2D-IR spectroscopy a frequency domain (incoherent) technique (Section IV.C) and a form of semi-impulsive method (Section IV.E), which from the experimental viewpoint is extremely simple. Other 2D methods, proposed recently by Mukamel and coworkers (47), would not pose any additional experimental difficulty. In the case of NMR, time domain Fourier transform (FT) methods have proven to be more sensitive by far as a result of the multiplex advantage, which compensates for the small population differences of spin transitions at room temperature. It was recently demonstrated that FT methods are just as advantageous in the infrared regime, although one has to measure electric fields rather than intensities, which cannot be done directly by an electric field detector but requires heterodyned echoes or spectral interferometry (146). Future work will have to explore which experimental technique is most powerful and reliable. 

In infrared spectroscopy, the detector noise is usually much higher than the noise from other sources. In this case, multiplex recording provides an additional advantage. An interferogram contains the detector noise only once, independently of the number of spectral channels. Fourier transformation produces a spectrum where the SNR) of each spectral element is related to that of one line, SNR by ... 

The second advantage of Fourier transform instruments is the so-called multiplex advantage. This is related to the fact that in a Fourier transform spectrometer, in con-... 

The other principal advantage which applies to Fourier transform spectroscopy is the multiplex or "Fellgett" advantage 21,64) n yas P. Fellgett who first pointed out that there is an advantage when the data in all elements of a spectrum are obtained simultaneously instead of being measured for each element successively. In Fourier transform spectroscopy, the radiation in the Michelson interferometer is not separated into spectral elements. The interferogram contains... 

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