ATR crystal

Figure 52.6. Single beam IR spectra of the Nafion-Ti02 film coated on the ATR crystal before and after introducing gas phase ethanol, and during UV illumination for (a) 2 1 and G ) hi Nafion-Ti02 films.

Fixed pathlength transmission flow-cells for aqueous solution analysis are easily clogged. Attenuated total reflectance (ATR) provides an alternative method for aqueous solution analysis that avoids this problem. Sabo et al. [493] have reported the first application of an ATR flow-cell for both NPLC and RPLC-FUR. In micro-ATR-IR spectroscopy coupled to HPLC, the trapped effluent of the HPLC separation is added dropwise to the ATR crystal, where the chromatographic solvent is evaporated and the sample is enriched relative to the solution [494], Detection limits are not optimal. The ATR flow-cell is clearly inferior to other interfaces. 

ATR is one of the most useful and versatile sampling modes in IR spectroscopy. When radiation is internally reflected at the interface between a high-refractive index ATR crystal (usually Ge, ZnSe, Si, or diamond) and the sample, an evanescent wave penetrates inside the sample to a depth that depends on the wavelength, the refractive indices, and the incidence angle. Because the penetration depth is typically less than 2 pm, ATR provides surface specific information, which can be seen as an advantage or not if surface orientation differs from that of the bulk. It also allows one to study thick samples without preparation and can be used to characterize highly absorbing bands that are saturated in transmission measurements. 

Simply visualised, the infrared beam penetrates (of the order 0.3-3 pm, dependent on its wavelength) just beyond the ATR crystal-specimen boundary before it is reflected back and makes its way through the crystal to the detector. On this short path (of the evanescent wave) into the sample surface layer, light is absorbed, and the reflected beam carries characteristic spectral information of the sample. The decaying amplitude of the evanescent wave and the depth of penetration dp at which it has decreased to a proportion of 1 /e is defined by the Harrick equation (Equation (2)), where X is the wavelength of the incoming... 

Careful attention should be paid to sample preparation, however. The materials investigated may be changed by the preparation procedure (e.g., smearing during microtomy of soft materials at room temperature is avoided when using cryo-microtomy) or during the measurement (radiation damage, contact with ATR crystal, etc.). 

The yellow contamination was removed from the cups using a clean needle and the contaminant was transferred to a glass slide. The contaminant was partially liquid with solid portions also present. When the ATR crystal was allowed to very slightly contact the sample, the liquid portion of the contaminant was detected and the spectrum recorded is shown in Figure 33. When higher contact pressure between the ATR crystal and the contaminant was used, a completely different spectrum was produced (see Figure 34) from the solid portion of the contamination. 

Following this, attempts were made to remove a particle from the extruded plastic sheeting. When a particle was finally isolated, applying pressure with the ATR crystal resulted in cracking of the particle, so a representative spectrum could not be obtained. This brittle behavior usually indicates that the sample is inorganic in nature. 

Finally, both samples were analyzed using ATR-FTIR spectroscopy. The Si ATR crystal was brought into contact with the sample, and spectra recorded. Figure 51 compares the spectra. Based on the excellent match factor between the spectra of the two samples (99.8%) and visual comparison of these two spectra,... 

ATR FT-IR spectroscopy allows for analysis of the polymer surface, rather than the bulk of the sample. Whereas the spectra obtained from the FT-IR microscope and BCA are recorded in transmittance mode and are used to analyze the entire bead, an ATR objective can be brought into direct contact with the sample in order to yield information about the chemistry taking place mainly on the periphery of the bead. Some FT-IR microscopes are equipped with an ATR crystal, and ATR analysis can be achieved on a single bead [172], but there have been no reports of automated ATR instruments. This technique has been used in kinetic studies, to prove that the esterification of Wang resin (4) (Scheme 1.4) to give the corre-... 

The ATR technique is now routinely used for IR spectroscopy as it allows measurement of spectra for a variety of sample types with minimal preparation. The crystals employed are generally prismatic in shape, allowing contact of a flat surface with the sample. The ATR method was first adapted for HP IR spectroscopy by Moser [29-33], who realised that a conventional autoclave could easily be adapted for in situ IR spectroscopy by fitting an ATR crystal of cylindrical cross section. The technique developed by Moser is thus known as cylindrical internal reflectance (CIR) spectroscopy and high pressure cells based upon the CIR method have been commercialised by Spectra-Tech. A typical CIR cell is illustrated in Figure 3.8. 

As well as the CIR-type cells discussed above, the ATR technique has also been employed in other types of HP IR cell. An alternative arrangement to a cylindrical crystal inserted through the autoclave body (Figure 3.8) is to embed the ATR crystal in the base or wall of the autoclave. This approach was used in the HP IR cell developed by Wolf et al., illustrated in Figure 3.9 [43]. An ATR crystal is mounted in the bottom of the reactor, which can operate at pressures up to 200 bar and can be adapted to give a high pressure flow-cell. 

Attenuated total reflection (ATR) is the most common reflectance measurement modahty. ATR spectra cannot be compared to absorption spectra. While the same peaks are observed, their relative intensities differ considerably. The absorbances depend on the angle of incidence, not on sample thickness, since the radiation penetrates only a few micrometers into the sample. The major advantage of ATR spectroscopy is ease of use with a wide variety of solid samples. The spectra are readily obtainable with a minimum of preparation Samples are simply pressed against the dense ATR crystal. Plastics, rubbers, packaging materials, pastes, powders, solids, and dosage forms such as tablets can all be handled directly in a similar way. 

Fig. 17. Cross-section of the view cell at the spectroscopic level. The IR beam is directed either through the ATR crystal (bottom, solid lines) or through the transmission windows (upper part, dashed lines). The four mirrors are mounted on a motor-driven mobile attachment (76).

Attenuated Total Reflection (ATR).4c A sample brought in contact with the totally reflecting surface of a high-refractive-index material (the ATR crystal), will, on IR irradiation, give an evanescent wave in the less dense medium that extends beyond the reflecting interface. This wave will be attenuated in regions of the IR spectrum where the sample absorbs energy. Observation of such waves constitute ATR measurements. Only the small amounts of beads necessary to cover the area of the ATR crystal are required. 

Clean the ATR crystal by thoroughly wiping it with low-lint tissue papers as many times as needed. To ensure absence of cross-contamination, apply the next test sample... 

When melted fat or oil is placed on the surface of an ATR crystal like diamond, the infrared light penetrates a distance of only a few micrometers into the test sample, when the conditions of total internal reflection apply... 

In recent years, more efficient ATR accessories that are suitable for quantitative analyses were developed. In particular, small diamond ATR crystals that have a capacity of — I pi have been successfully tested and are recommended for trans fat analysis. 

It is essential to ensure that the test portion of the fat being analyzed completely covers the horizontal surface of the ATR crystal for the quantitative determination to succeed. In some cases, after placing a fat test portion on the ATR crystal, the melted or liquid fat beads up and partially rolls off the surface of the ATR crystal. The only recourse the analyst would have is to try again. 

Thoroughly wiping the horizontal surface of the ATR crystal with low-lint paper has been the method of choice for cleaning without the use of any solvent. However, to ensure the complete removal of a test sample the analyst should also apply the subsequent test portion and clean the crystal once again. Additionally, the analyst could make an infrared measurement after cleaning the crystal the absence of a spectrum would confirm that the crystal is clean. 

Figure 8.4 The ASI ReactIR 1000 reaction analysis system with a stainless-steel Parr autoclave modified with permanently mounted ATR crystal (SiComp) at the bottom of the reactor (purchased from Mettler Toledo).

Some important functional groups and their region of absorbance are highlighted. Cheddar cheese was scanned by pressing 0.5 g of cheese on a diamond attenuated total reflectance (ATR) crystal. Extracts was scanned by drying 10 pi of the extract on a zinc selenide ATR crystal. 

The ATR crystal also absorbs light over a certain interval of the infrared range which, therefore, will not be available for measurements. 

An interesting technique recently published uses FTIR spectroscopy to detect anions [149]. Thin films of tetraalkylated ferrocenium salts were evaporated onto ATR crystals. Exposure to anion-containing solutions caused incorporation of the anions, and they could then be detected via their IR absorbances. Limits of detection for 10-min analyses varied from 10 6 to 10 s M for a variety of anions however, this technique cannot be used to detect anions that do not have IR absorbances, such as halides. 

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