ATR correction

The ATR technique is a commonly used infrared internal reflection sampling technique. It samples only the surface layer in contact with the ATR element the sampling depth probed is typically of the order of 0.3-3 pm [1]. Unless software corrected, compared with a transmission spectrum, the relative intensity of bands within an ATR spectrum increase in intensity with decreasing wavenumber. Several FTIR instrument companies now supply "ATR-correction" software developed to correct for the different relative intensities of bands observed between ATR and transmission spectra, so that ATR spectra can be more easily compared to and searched against transmission spectra. 

Figure 5 ATR-corrected FTIR spectrum of the white residue that resides in the tiny defects shown in Figure 4.

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) ... 

Sometimes an empirical so-called ATR correction is applied to compensate across the spectrum for linear wavelength increase, which is termed as Eq.(3) ... 

I n the case of the ATR spectra that were to be compared witii transmission spectra (only Figure 4), an ATR correction routine was used to allow for the variation in penetration depHi by multiplying the sample spectrum by a wavelength-dependent factor to correct the relative peak intensities. The standard ZnDTP ATR spectra collected on an iron-coated germanium crystal were corrected for tiie baseline after the ATR correction (on ly Figure 4). All the oilier ATR spectra presented in this work (Figures 5—7) are reported wiHiout ATR correction, as acquired. 

Although the relative band intensities of ATR spectra that have been subjected to the ATR correction are similar to those of the corresponding spectra measured in transmission, they are not the same because of the effect of anomalous dispersion (i.e., the variation of refractive index across an absorption band see Section 1.5.2). Let the refractive index, 2, of the optically rare material (the sample ) in spectral regions where there is no absorption be ave. Because 2 is smaller than Wave at higher wavenumbers than the band center, Vq (see, e.g.. Figure 13.19), the depth of penetration will be less than the value calculated by Eq. 15.4 when ave... 

In many commercially available FT-IR spectrometer systems, software (called the ATR correction) for correcting the band intensities of an observed ATR spectrum for the wavelength-dependent depth of penetration expressed in Equation (13.4) is installed, in order to make the observed ATR spectrum more closely resemble a transmission spectrum. In ATR spectra, peak positions, particularly those of intense bands, tend to have wavenumbers lower than those in corresponding transmission spectra. As described earlier, this is related to the anomalous dispersion of the refractive index in the vicinity of the absorption band. The effect has been discussed and software for correcting ATR spectra to take account of this effect is available [9]. The software for this purpose is also installed in some commercially available FT-IR spectrometers. Input data necessary for this software are the refractive index of the sample and the IRE, the angle of incidence, and the number of internal reflections. 

Nunn, S., and Nishikida, K. (2008) Advanced ATR Correction Algorithm. Thermo Scientific Application Note 50581. 

FIGURE 4.51 Bottom The ATR spectrum of sucrose. Top The ATR corrected spectrum of sucrose. The dashed lines show that in the uncorrected spectrum the peaks at high and low wavenumber are different in size, and that in the corrected spectrum they are similar. 

If you must search ATR spectra against non-ATR libraries there is help. Some FTIR software packages contain an ATR Correction function. This function adjusts the relative intensities in an ATR spectrum so it looks more like a spectrum measured using a transmission experiment the net effect is to make the peaks at higher wavennmber bigger than they were in the original ATR spectrum. A comparison of the ATR spectrum and ATR corrected spectrum of sucrose is seen in Figure 4.51. 

From this equation it can be seen that the depth of penetration depends on the angle of incidence of the infrared radiation, the refractive indices of the ATR element and the sample, and the wavelength of the radiation. As a consequence of lower penetration at higher wavenumber (shorter wavelength), bands are relatively weaker compared to a transmission spectrum, but surface specificity is higher. It has to be kept in mind that the refractive index of a medium may change in the vicinity of an absorption band. This is especially the case for strong bands for which this variation (anomalous dispersion) can distort the band shape and shift the peak maxima, but mathematical models can be applied that correct for this effect, and these are made available as software commands by some instrument manufacturers. 

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. 

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.

As noted above, this correlation and that of Fig. 1 are deficient in not recognizing that the product of the nucleophilic reaction is not the alcohol, as implied by the correlation with pATR, but the protonated alcohol. However, it is reasonable to suppose that variation of the pATas for O-protonation of the alcohols, which are required to correct values of ATR, are small compared with variations in pifR itself (and thus pATH2o) and would not significantly affect the quality of the correlation. It is also true that the correlation is dominated by the large and variable values of pA n.o for aromatic products of deprotonation. These tend to obscure variations in product ratios for tertiary alkyl and secondary benzylic cations which are the focus of a previous discussion of this partitioning by Richard.5... 

The cross section area of the collinear IR beam is -1 cm2 and thus sufficient to cover the entire area of the C face of the ATR crystal, and as a result, a complete coverage of faces A by the IR radiation is achieved. Hence, the measured absorbance should be proportional to the fraction of the total area of faces A and C covered by the monolayer. Of course, all six faces of the crystal are covered with the monolayer film. However, only the A faces contribute to the measured signal via internal reflection. This is because the area of the C faces is only 7% of the total area (A + C) in a typical ATR crystal, and the differences between transition mode (in the C faces) and ATR mode (in the A faces) are not very large. Therefore, it was suggested by Maoz and Sagiv that no corrections for this effect are needed (1). 

Fig. 7. Baseline-corrected ATR-FTIR spectra through the NHS activation sequence discussed in the text (a) freshly prepared H/Si(lll), (b) after functionalization with undecylenic acid, (c) surface (b) reacted with NHS/EDC for 1 hour at room temperature and (d) surface (c) after reaction with TEGamine. The background used is the spectrum of a clean oxidized ATR Si(lll) crystal for (a) and the spectrum of a Si(lll)-H surface for (b) and (c). Reprinted from [53].

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. 

Figure 10.18 Spectra by reflection, (a) From a sample of plexiglass, three types of reflection are displayed. Left, crude spectra and right, spectra after correction. Above, crude signal of specular reflection and the result in units of K following application of the Kramers-Kronig (transformation of the reflectance) calculation middle, spectrum obtained by diffused hght comparison of the crude spectrum with the Kubelka-Munk correction below, spectrum obtained by ATR, the latter requiring a fine correction to reduce the absorbance at higher wavelengths which would be overestimated (b) comparison of two spectra of benzoic acid, one obtained through transmission, the other by diffused reflection and subsequent K-M correction.

Similarly the two terms needed to calculate the correction factor, fro, for a specified value of specific heat ratio, y, take the form of a point value, fro(ATr, 0.95) ... 

Fig. 13. ATR-FTIR spectra recorded during the oxidation of a 3-methylthiophene polymer in contact with an electrolyte solution containing tetrabutylammonium perchlorate. The spectrum numbers correspond to the numbers in Fig. 15. The spectra are smoothed but not baseline corrected. The spectrum of the electrolyte solution was subtracted.

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