Dispersion instrumentation

This assumes that both spectra have the same resolution, and that it takes the same amount of time to collect the whole interferogram as is required to obtain one wavelength on the dispersive instrument (which is usually a reasonable assumption). Thus, interferograms can be obtained and averaged together in the same... 

Hadamard transform [17], For example the IR spectrum (512 data points) shown in Fig. 40.31a is reconstructed by the first 2, 4, 8,. .. 256 Hadamard coefficients (Fig. 40.38). In analogy to spectrometers which directly measure in the Fourier domain, there are also spectrometers which directly measure in the Hadamard domain. Fourier and Hadamard spectrometers are called non-dispersive. The advantage of these spectrometers is that all radiation reaches the detector whereas in dispersive instruments (using a monochromator) radiation of a certain wavelength (and thus with a lower intensity) sequentially reaches the detector. 

Most modem IR facilities will use a Fourier Transform IR Spectrometer (FTIR), rather than a dispersive instrument. The essential feature is that all of the light from the source falls on to the detector at any instant, which thus leads to increased signal levels, thereby automatically improving the signal-to-noise ratio at all points on the spectrum. 

Electrochemically Modulated Infra-Red Spectroscopy (EMIRS) [23] consists of applying a square-wave potential modulation to the working electrode and analyzing the modulated part of the IR detector response using a dispersive instrument. 

Fig. 1. Comparison of amide V VCD for an identical sample of poly-L-lysine in D20 as measured on the UIC dispersive instrument (top) and on the ChirallRFT-VCD instrument (at Vanderbilt University, kindly made available by Prof. Prasad Polavarapu). Sample spectra were run at the same resolution for the same total time ( 1 h) in each case. The FTIR absorbance spectrum of the sample is shown below. VCD spectra are offset for sake of comparison. Each ideal baseline is indicated by a thin line, the scale providing a measure of amplitude. Noise can be estimated as the fluctuation in the baseline before and after the amide V, which indicates the S/N advantage of the single band dispersive measurement.

In-situ Fourier transform infrared spectroscopy. The final technique in this section concerns the FTIR approach which is based quite simply on the far greater throughput and speed of an FTIR spectrometer compared to a dispersive instrument. In situ FTIR has several acronyms depending on the exact method used. In general, as in the EMIRS technique, the FTIR-... 

They are especially attractive for analyzing weak light signals. This is because in an FTIR spectrometer all of the frequencies arrive at the detector simultaneously, so that the energy throughput is much greater than for dispersive instruments (the... 

FTIR spectrometers provide a high resolution compared to dispersive instruments. Moreover, this resolution is constant over the full spectral range. 

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... 

Barbillat, J. and Da Silva, E., Near infrared Raman spectroscopy with dispersive instruments and multichannel detection, Spectrochim. Acta A, 53, 2411, 1997. 

By using a Fourier Transform I.R. including the computer supported dispersion instruments, spectral substraction can effectively cancel out unchanging bands and expose only those participating in the reaction. Likewise, the spectrum of the substrate (e.g., the wafer) can also be cancelled out. We believe that data obtained this way has increased the sensitivity of this method. 

It is noteworthy that this was one of the first published chemical studies where IR spectra were taken with a Michelson-Interferometer rather than with a dispersive instrument. In fact, Masamune claims that it was only by virtue of this new technology that he and his co-workers were able to pinpoint the missing bands of CB (S. Masamune, personal communication). 

Let us cite an example to help us judge the equivalence between Fourier and dispersive instruments. A grating spectrometer employing a four-passed 8 x 104-line grating in the first order has a resolving power of 4 x 8 x 104 = 3.2 x 105. At 3200 cm -1 in the near infrared, this instrument has a Rayleigh resolution of 10" 2 cm- L The same resolution can be achieved by a Fourier... 

For a more accurate representation of the instrumental distortion effects see Section II.G of Chapter 2. The approximation given by Eq. (14) is convenient and for high-dispersion instruments observing weakly absorbing spectral lines it is often accurate enough for a number of experimental uses. 

This multiplex advantage in FT-IR is easiest to understand with respect to the speed of the measurement. For a resolution of 1 cm-1 and frequency range of 400 to 4,000, the FT-IR measurement time is 3,600 times faster than with a dispersive instrument. Consequently, when the time of measurement is limited, the FT-IR has an advantage of two orders of magnitude in speed. For studies of the curing of epoxies this time advantage can be extremely valuable 9). 

All of the usual sampling techniques used in infrared spectroscopy can be used with FT-IR instrumentation. The optics of the sampling chamber of commercial FT-IR instruments are the same as the traditional dispersive instruments so the accessories can be used without modification for the most part. To make full use of the larger aperature of the FT-IR instrument, some accessories should be modified to accomodate the larger beam. The instrumental advantages of FT-IR allow one to use a number of sampling techniques which are not effective using dispersive instrumentation. Transmission, diffuse reflectance and internal reflectance techniques are most often used in the study of epoxy resins. 

Because, to date, no IRS accessories have been designed to benefit from the larger beam diameter of FT-IR, the spectral improvement achieved with FT-IR IRS is not as great as observed with FT-IR transmission compared to dispersive instruments. However, the signal averaging capability and speed make FT-IR a very useful tool. 

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