Attenuated linear spectroscopy

Figure 6.16 Attenuated total reflection surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS) spectra for the oxidation of 0.1 M HCOOH in 0.5 M H2SO4 on a polycrystaUine electrode. The bands at 2055 -2075 and 1800-1850 cm are assigned to linear- and bridge-bonded CO, whereas the band at 1323 cm corresponds to adsorbed formate. (Reproduced from Samjeske et al. [2006].)...

We see that the amplitude-transfer characteristic is given by 27r[l + (coRC)2] -1/2. The power-transfer characteristic is given by the square of this quantity. It has the form of a Cauchy function and attenuates high frequencies. Brodersen (1953) and Stewart (1967) have analyzed in detail the performance of other linear electrical filters applied in spectroscopy. 

Torano and Van Hattum [27] used Fourier transform mid-infrared (FTMIR) spectroscopy with an attenuated total reflection (RTR) module for determination of vigabatrin. The drug was extracted from the capsule content after addition of sodium thiosulfate IS solution. The extract was concentrated and applied to the FTMIR-ART module. The ratio of the area of the drug peak to that of the IS was linear over the concentration range of 90-110 mg/ml. The accuracy of the method in this range was 99.7-100.5% with a variability of 0.4-1.3%. 

In the experimental study of surface excitons various optical methods have been used successfully, including the methods of linear and nonlinear spectroscopy of surface polaritons. A particularly large body of information has been obtained by the method of attenuated total reflection of light (ATR), introduced by Otto (1 2) (Fig. 12.1) to study surface plasmons in metals. Later the useful modification of ATR method also was introduced by Kretschmann (3) (the so-called Kretschmann configuration, see Fig. 12.2). The different modification of ATR method has opened the way to an important development in the optical studies of surface waves and later was used by numerous authors for investigations of various surface excitations. 

Attenuated total reflection (ATR) FTIR is one of the most useful tools for characterising the chemical composition and physical characteristics of polymer surfaces [53]. One useful application is the measurement of molecular orientation using polarised infrared ATR spectroscopy [54,55]. The polarised infrared ATR spectra normally include three-dimensional (e.g., machine, transverse, and thickness direction) orientational information in contrast to the polarised transmission infrared linear dichroism. In addition, band absorbance of less than 0.7 au is easily achieved, even with the strong absorption bands, because the penetration depth of ATR from sample surfaces can be adjusted to a few micrometers by changing the internal reflection element and/or the angle of incidence. If successful combination of the dynamic infrared spectroscopy and the ATR methods can be achieved, more useful dynamic orientational information can be obtained. 

Several properties of polymers complicate their analysis via VS. First, a problem unique to transmission IR spectroscopy is that polymers are very strong absorbers of IR radiation. Therefore, in order to be within the linear region of Beer s law, an extremely thin polymer film must be used in transmission. A good rule of thumb is to keep the thickness below 5 J,m. While the production of such thin films is possible in the laboratory, it must be remembered that most commonly encountered polymer systems are much thicker than this. As a result, the most commonly used industrial IR techniques are reflectance techniques such as attenuated total reflectance (ATR) or reflection-absorption spectroscopy (RAS), which have much smaller effective optical path lengths, typically on the order 1 lm or below. 

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