Attenuated total reflection correction

IR spectra of starch can be obtained with an IR spectrometer such as a Digilab FTS 7000 spectrometer, Digilab USA, Randolph, MA, equipped with a thermoelectrically cooled deuterated tri-glycine sulfate (DTGS) detector using an attenuated total reflectance (ATR) accessory at a resolution of 4 cm by 128 scans. Spectra are baseline-corrected, and then deconvoluted between wavenumbers 1200 to 800 cm . A half-band width of 15 cm and a resolution enhancement factor of 1.5 with Bessel apodization are employed. Intensity measurements are performed on the deconvoluted spectra by recording the height of the absorbance bands from the baseline. 

Many techniques are based on this principle and can be used for the analysis of all types of samples. The spectrum obtained from reflected light is not identical to that obtained by transmittance. The spectral composition of the reflected beam depends on the variation of the refractive index of the compound with wavelength. This can lead to specular reflection, diffuse reflection or attenuated total reflection. Each device is designed to favour only one of the above. The recorded spectrum must be corrected using computer software. 

Figure 10.20—Devices allowing the study of samples by reflection, a) Diffuse reflection device b) attenuated total reflection (ATR) device c) comparison of the spectra of benzoic acid obtained by transmission (KBr disc) and by diffuse reflection using the Kubelka Munk correction. The depth of penetration of the IR beam depends on the wavelength. The absorbance for longer wavelengths would be overestimated if no correction was applied.

Figure for Exercise 18-E. Infrared absorption spectra ot 10-50 vol% acetone in water. Vertical arrow shows corrected absorbance for 50 vol% acetone, which is obtained by subtracting baseline absorbance from peak absorbance. [Spectra from A. Atran, ftir Absorbance linearity ol Square Column Attenuated Total Reflectance, Am. Leri). February 1993, p. 40MMM.]... 

Specific spectroscopic techniques are used for the analysis of polymer surface (or more correctly of a thin layer at the surface of the polymer). They are applied for the study of surface coatings, surface oxidation, surface morphology, etc. These techniques are typically done by irradiating the polymer surface with photons, electrons or ions that penetrate only a thin layer of the polymer surface. This irradiation is followed by the absorption of a part of the incident radiation or by the emission of specific radiation, which is subsequently analyzed providing information about the polymer surface. One of the most common techniques used for the study of polymer surfaces is attenuated total reflectance in IR (ATR), also known as internal reflection spectroscopy. Other techniques include scanning electron microscopy, photoacoustic spectroscopy, electron spectroscopy for chemical analysis (ESCA), Auger electron spectroscopy, secondary ion mass spectroscopy (SIMS), etc. 

The internal reflectance technique is usually called attenuated total reflection (ATR) spectroscopy. It is especially useful for studying strongly absorbing media, for example, aqueous solutions. When the infrared radiation is absorbed in the test medium, one obtains a spectrum similar to that from a transmission experiment. However, there are distortions in the ATR spectrum, especially in the region of intense bands. One reason for distortion is the fact that the depth of penetration varies with wavelength. The other effect is due to the change of the refractive index of the solution in the region of the intense band. ATR spectra should be corrected for these effects so that they may be compared to normal transmission spectra. 

Obtaining spectra by reflection is an alternative to the procedures described above (see Figure 10.17). The corresponding devices are based upon attenuated total reflection, specular reflection or diffused reflection and are only usable with FTIR as the spectra obtained by reflected light must be corrected by computer software to render them comparable with transmission spectra. 

If medium 1 is absorbing, the evanescent field wiU be absorbed and less intensity can be reflected (attenuated total reflection (ATR)). An ATR spectrum is similar to the conventional absorption spectrum except for the band intensities at longer wavelengths. At longer wavelengths the evanescent field penetrates ever deeper into the sample, equivalent to an increasing sample thickness. Sometimes an empirical so-called ATR correction is appHed in order to compensate across the spectrum for the linear wavelength increase in Eq. (10) ... 

B.4.2.4.2 Liquid-Liquid Inter ce and Attenuated Total Reflection Total internal reflection at a hquid-hquid interface can be used to monitor ion transfer across the interface. The kinetics of the reduction of TCNQ and the oxidation of l,l -dimethylferrocene by [Fe(CN)6] in the aqueous phase has been considered [163]. The kinetics of these reactions were studied by chronoabsorptom-etry, assuming diffusion control [Eq. 23, allowing for the reflection correction, (2/COS0)]. Ion-transfer kinetics across interfaces has been treated theoretically and apphed to the study of indicator transfer [179]. Chronoabsorptometric studies show that hgand-assisted Cu(II) transfer is controlled by the diffusion of metal... 

FT-Raman spectra were carried out using Perkin-Elmer spectrometer equipped with Nd YAG laser source. Spectra were accumulated from 64 scans at a resolution of 4cm. An optical bench alignment was performed before each Raman measurements to ensure that e spectrometer was fine-tuned and the detector signal maximized. FT-IR spectra were recorded using Perkin-Elmer spectrometer equipped with diamante crystal for attenuated total reflection (ATR). The FT-IR or Raman spectra were smoothed and their baselines were corrected using the automatic smooth and the automatic baseline correct functions of the built-in software of the spectrophotometer. Then, the intensities of the interested peaks were measured. 

An alternative reflection setup makes use of single (or multiple) internal reflection within an OTE (Figure 4). However, at each internal reflection, a small portion of the intensity leaks out (it is correctly known as an evanescent wave) into the thin film electrode layer and beyond into the solution and can be used to detect any absorbing species. The method is also known as attenuated total reflection (ATR). The evanescent wave intensity decays exponentially as exp( — bix) with distance x from the interface. The penetration depth, 5, depends on the wavelength and the optical properties of the substrate, electrode film, and solution d = /l/(4/i Im... 

Bruun, S.W. et al. (2006) Correcting attenuated total reflection-Fourier transform infrared spectra for water vapor and carbon dioxide. Appl. Spectrosc., 60 (9), 1029-1039. 

Abstract This chapter describes recent breakthroughs in the instrumentation for far-ultraviolet (FUV) spectroscopy. The key technique is attenuated total reflection (ATR) that is frequently used in the infrared region. ATR technique decreases the absorbance of samples with strong absorptivity because of the penetration depth of the evanescent wave less than 100 nm. Therefore, ATR-FUV spectroscopy realizes the measurement of FUV spectra of samples in liquid and solid states. Some applications (in-line monitoring, characterization of polymers and time-resolved spectroscopy in sub-microsecond) are introduced in terms of instrumentation. This chapter explains not only the detail of the instruments but also the mathematical correction for ATR spectra to separate the absorption and refraction indices. 

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