Internal-reflection element materials used

Atr—ftir can be readily performed on most commercial ftir spectrometers through the use of an attachment for atr spectroscopy. These devices provide ir-transparent internal reflection elements that are typically made of Ge, KRS-5, ZnSe, or ZnS. These internal reflection elements are made of materials that are of extremely high purity to avoid losses from absorption by impurities in these devices. Coupling of a thin film or surface sample to one of these reflection elements is accompHshed by pressing the sample against the element while acquiring the spectmm. 

Table 6.3 Common IR transmitting materials used for IR optics and ATR Internal reflectance elements (IREs)...

This is achieved by a polymer film covering the internal reflection element (IRE) it acts continuously as a reversibly extracting medium. This effect may be used for a quantitative determination of organic samples. Because the polymer material is hydrophobic, only weak absorption bands due to water appear in the spectra. 

Attenuated total-reflectance (ATR) spectroscopy is a widely used sampling technique, in which a sample is placed in contact with a reflecting medium (a plate or prism shaped material called an internal reflectance element). A beam of radiation entering the prism is reflected internally if the angle of incidence at the interface between sample and prism is greater than the critical angle (a function of the refractive index of the sample and the prism). 

The use of infrared spectroscopy in the Earth and environmental sciences has been widespread for decades however, until development of the attenuated total reflectance (ATR) technique, the primary use was ex situ material characterization (Chen and Gardella, 1998 Tejedor-Tejedor et al., 1998 Degenhardt and McQuillan, 1999 Peak et al., 1999 Wijnja and Schulthess, 1999 Aral and Sparks, 2001 Kirwan et al., 2003). For the study of environmental systems, the strength of the ATR-Fourier transform infrared (FTIR) technique lies in its intrinsic surface sensitivity. Spectra are collected only from absorptions of an evanescent wave with a maximum penetration depth of several micrometers from the internal reflection element into the solution phase (Harrick, 1967). This short optical path length allows one to overcome any absorption due to an aqueous phase associated with the sample while maintaining a high sensitivity to species at the mineral-water interface (McQuillan, 2001). Therefore, ATR—FTIR represents a technique capable of performing in situ spectroscopic studies in real time. 

ATR or internal reflectance uses an optical element of high RI. This optical element is called the internal reflection element (IRE) or the ATR crystal. Light traveling in a high-RI material is... 

Coarse or hard powders are not well served by either the compressed pellet or mull technique, mainly because of difficulties associated with grinding. In such situations, the best approaches require the use of an accessory, such as a diffuse reflectance or photoacoustic detector. Both diffuse reflectance and photoacoustic methods [99,100] may be applied to most forms of powdered solids. As a rule, photoacoustic measurements, which are the only form of true absorption measurement, are not significantly influenced by sample morphology. An alternative procedure for powders is ATR, especially a horizontal accessory, preferably equipped with a pressure applicator. Note that the use of pressure is recommended to ensure intimate contact between the sample and the IRE (internal reflectance element) surface. Normally, the sample must conform to the surface of the IRE, and because the strength of the IRE is typically limited, the procedure is recommended only for soft powders. However, with the introduction of diamond-based ATR accessories [101-103], it is possible to handle most types of powdered material. 

Common IR Transmitting Materials used for ATR Internal Reflectance Elements... 

For any material, n( is determined by Snell s law. A few materials have no significant absorption in the mid- and near infrared. Those materials with low refractive index (1.45 high refractive index (2.4 internal reflection elements (see Chapter 15). For organic and inorganic molecules whose spectra exhibit typical absorption bands, the refractive index changes across the absorption band. A typical refractive index spectrum has the appearance shown in Figure. Aa. This... 

The chalcogenides are all insoluble in water and other common solvents. ZnSe and CdTe have excellent transmission characteristics. The only problem with these materials is their high refractive index, which leads to high front-surface reflectance (see Section 13.2.2), so that transmission spectra of liquids held in cells fabricated from these materials often give rise to interference fringes (see Section 11.1.3). These materials aU make excellent internal reflection elements. AMTIR (amorphous material that transmits infrared radiation) is a mixture of several chalcogenides. Many optical fibers used for mid-infrared spectrometry are made from this material (see Section 15.4). 

Table 15.1. Common Materials Used as Mid-Infrared Internal Reflection Elements with Critical Angle when ni = 1.5...

As implied by Eq. 15.2, all the materials in Table 15.1 must have higher refractive indices than the material with which they are in contact. Materials that are commonly used as windows in the mid-infrared [e.g., KBr n = 1.53) and KCl ( i = 1.45)] are not included in this list, as their refractive indices are too low for use as IREs. Because the refractive indices of KBr and KCl are roughly equal to the refractive index of organic compounds, total internal reflection will not be observed. In this case, radiation passes directly through the IRE and sample without regard to the angle of incidence. In other words, these materials are better suited for use as infrared-transmitting windows than as internal reflection elements. 

Attenuated Total Reflection Measurements 183 Table 13.1 High refractive-index materials used for the internal reflection element (IRE)... 

In order to enhance spatial resolution, it is necessary to make the NA of the objective larger, as is clear from Equation (17.3) that is, either n or 6, or both of them should be increased. Due to the optical geometry of a microscope, there is an upper limit for 0. On the other hand, it is possible practically to increase n by introducing an attenuated total reflection (ATR) accessory into a microscope (see Chapter 13 for the ATR method this is a frequently used accessory for many recent infrared microspectroscopy absorption measurements). The refractive index n of Ge, which is a commonly used material for an internal reflection element (IRE) in the ATR method, is about 4, and the NA when using a Ge IRE exceeds 2. This means that, if an ATR accessory with a Ge IRE is combined with a microscope, the theoretical spatial resolution is enhanced about four times that of the conventional reflection measurement. In fact, in the FT-IR microspectroscopic imaging measurement with a Ge ATR accessory, it has been confirmed that a spatial resolution comparable to the infrared wavelength used for the measurement is realized, and thus a higher spatial resolution may be attainable. 

There are many applications for internal reflectance spectroscopy, and only a few will be mentioned here. Internal reflectance spectroscopy can be used to obtain the spectra of rubbery materials that are hard to grind. The rubbery material is simply pressed against the internal reflectance plate, and it is ready to run. Carbon-fllled rubber or other polymers may be run using a high index of refraction germanium as the internal reflectance element. Internal reflectance is used to obtain selectively the top few micrometers of a sample surface where the composition may be different than that further down. It is also good for water solutions because the controlled penetration keeps the effective sample thickness small. 

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