ATR cell

Attenuated total reflectance (ATR) cell for use In Infrared spectroscopy. 

In order to select the instmmental conditions for carrying out the ATR measurements several parameters including the number of accumulated scans per spectra or nominal resolution were tested. To avoid the crosscontamination and to establish an appropriate strategy for cleaning the ATR cell between samples, several procedures were tested using background and blank controls. Moreover, the possible sample sedimentation on the ATR plate cell due to the complexity of the sample matrix during the spectra acquisition was also checked. 

Figure 2.39 (a) Schematic representation of the experimental arrangement for attenuated total reflection of infrared radiation in an electrochemical cell, (b) Schematic representation of the ATR cell design commonly employed in in situ 1R ATR experiments. SS = stainless steel cell body, usually coated with teflon P — Ge or Si prism WE = working electrode, evaporated or sputtered onto prism CE = platinum counter electrode RE = reference electrode T = teflon or viton O ring seals E = electrolyte. 

Recently, polyethylene and Teflon mesh sample holders have been used. A drop of sample is placed on the mesh and spread to a relatively uniform thickness for analysis. These holders can often be rinsed and reused. A very convenient alternative to liquid sample holders is the technique called attenuated total reflection or ATR. The ATR cell is a crystal of gallium arsenide, GaAs and the infrared radiation enters one end of the trapezoidal crystal. With the angles adjusted to obtain total internal reflection, all of the IR radiation passes through the crystal and exits the other end as shown in Fig. 5.14. 

The last ATR cell described here in detail was designed for the study of catalytic reactions at high pressures and in particular in supercritical fluids. A schematic representation of the design is shown in Fig. 17 (76). An important issue in this type of reaction is the phase behavior of the system, which can have a large influence on the catalytic reaction 77,IS). The cell consists of a horizontal stainless-steel cylinder. It is designed to allow monitoring of the phase behavior via a video camera. For this purpose, one end of the cylinder is sealed with a sapphire window, behind... 

The equipment for the combination of spectroscopies is shown schematically in Fig. 31 96). The ATR cell was equipped with a fused silica window for UV-vis... 

FT-IR has been applied for determining the sucrose content of sugar cane juice [21]. In place of the more familiar transmission cell, an attenuated total reflectance (ATR) cell and clarified sugar cane juice were used to record FT-IR spectra from 800 to 1250 cm"1. In the spectra, significant wavenumbers (927.59, 997.02, 1054.87, 1116.51, and 1137.80 cm"1) have been identified for sucrose. The application of PCR has been proposed for the development of a calibration equation for sucrose content. PCR is basically a MLR applied to scores assessed by PCA. On the basis of FT-IR spectra and sucrose content, an accurate calibration equation could be obtained by the application of PCR. The root mean square difference between predicted FT-IR values and the actual values were 0.12 % (w/v) with a bias of -0.03 % (w/v). The accuracy of FT-IR for determining sugar cane sucrose is almost equal to that of NIR [25]. 

The most commonly used technique for obtaining a spectrum is the attenuated total reflectance (ATR) method in the multiple internal reflectance (MIR) mode. In this method, the infrared beam is passed into a special crystal of a selenide (KRS-5). The angle of incidence is such that the beam will bounce along the crystal. A sample of the polyurethane is placed hard up against the crystal. The infrared just penetrates into the material before it continues down the crystal. A number of internal bounces are obtained along the crystal. Up to 25 reflectances are obtained from a 2 mm crystal. Figure 9.8 illustrates the infrared path in an ATR cell. 

DCP and uncured films were deposited on a multi-reflectance prism. Next, the ATR cell was heated to 140 °C and spectra were recorded as a function of curing time, while the angle of incidence was fixed at 45°, which minimised loss of radiation. The decrease of the absorption relating to the third monomer pendent unsaturation was monitored at 1685, 3045, 966 and 870 cm"1 for EPDM polymers containing ENB, DCPD, HD and MNB, respectively. The absorption was normalised with the methyl absorption at 1380 cm 1 which was not affected during curing. 

Braue and Pannella (10) used a flow-through ATR cell for quantitative measurement of tabun, sarin, and soman in water. The sample change and the measurement could be performed safely due to quite dilute samples and airtight fittings. The method they developed has a useful range of 0.5-2.0 mg/ml. The quantitation is based on the P=0 stretching near 1240 cm-1. 

On-line MIR ZnSe ATR analysis of microbial cultures has been used primarily for non-invasive monitoring of alcoholic or lactic fermentations. Alberti et al. [76] reported the use of a ZnSe cylindrical ATR crystal to monitor accurately substrate and product concentrations from a fed-batch fermentation of Saccharomyces cerevisiae. Picque et al. [77] also used a ZnSe ATR cell for monitoring fermentations and found that whereas NIR spectra obtained from alcoholic or lactic fermentation samples contained no peaks or zones whose absorbance varied significantly, both transmission and ATR MIR could be used successfully to measure products. Fayolle et al. [78] have employed MIR for online analysis of substrate, major metabolites and lactic acid bacteria in a fermentation process (using a germanium window flow-through cell), and... 

A slurry of silica particles with adsorbed BSA was placed in a horizontal ATR cell equipped with a ZnSe crystal (5 reflection). The spectrum of a slurry of bare silica was recorded and subtracted as a background. 

Figure 6.5-1 Example of an ATR cell with temperature control (Wyzgol, 1989).

Wilks Enterprises (South Norwalk, MA) [4] recently introduced a device incorporating a miniature attenuated total reflection (ATR) cell as its interface. The device resembles an old-fashioned electrical fuse for a home fuse box. In place of the metal contact at the base, a small ATR cell protrudes from the base. Through this, monochromatic light courses. 

Figure 5. (A2) Various spectral quantities related to liquid water in the mid-IR region. Experimental values of log(/Q(i )//(i )) are shown in the case of an absorption set-np with a 1 p,m thick sample and of an ATR cell with an immersed portion of the crystal about 3 cm long (41). The optical constants n and k are also displayed together with the imaginary dielectric constants e".

A typical multiple-reflection spectroelectrochemical ATR cell is shown in Fig. 7 and Fig. 8 shows a schematic diagram of a variable-angle single-reflection ATR cell for phase-sensitive detection. 

To attain this goal, we used ATR techniques which have been described previously (6). A liquid ATR cell can be used to circulate protein solutions (or blood) through the cell while spectrally monitoring the adsorption of proteins onto the surface of the ATR crystal. In addition, the ATR crystal can be coated with a thin layer of polymer, permitting us to follow the adsorption of pro-... 

The crystals used in ATR cells are made from materials which have low solubility in water and are of very high refractive index. Such materials include zinc selenide (ZnSe), germanium (Ge) and thallium/iodide (KRS-5). The properties of these commonly used materials for ATR crystals are summarised in Table 3.3. Different designs of ATR cells allow both liquid and solid samples to be examined. 

It is also possible to set up a flow-through ATR cell by including both an inlet and an outlet arrangement in the apparatus. This allows for the continuous flow of solutions through the cell and thus permits spectral changes with time to be monitored. 

The spectrum of a piotein. film (refractive index of 15> v as produced by using-an ATR cell made of KRS 5 (reiracti ve indent of 2 4) If the madent radiation enters the cell crvstal at an angle of 60°C what IS the depth of penetration mto the sample... 

Variable temperature cells are available which can be controlled to within 0.1°C over the range from -180 to 250°C. An electrical heating system is used for temperatures above ambient, with liquid nitrogen and a heater for low temperatures. These cells can be used to study phase transitions and the kinetics of reactions. As well as transmission temperature cells, variable temperature ATR cells and cells for microsampling are also available. 

However, because we have used an ATR cell in this current study we cannot use the Beer-Lambert law directly, as we do not have a pathlength for the calculation. However, as this variable does not change during the experiment we can ignore this factor and exploit the fact that absorbance is directly proportional to the sample concentration. Therefore, we can write the following ... 

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