Attenuated total internal reflection ATR

Infrared spectra of zeolitic samples can be measured in several different modes. These include transmission, diffuse reflectance, attenuated total internal reflection (ATR) and emission. Transmission and diffuse reflectance are by far the most widely used of these techniques. In the transmission mode, the sample is placed directly in the infrared beam of the instrument and the light passing through or transmitted is measured by the detector. This transmitted signal (T) is ratioed to the open beam (no sample) signal (To) to get the transmission spectrum of the sample. The transmission spectrum is converted to an absorbance spectrum ... 

Short path length flow cells may be used in lower volume, or lower flow situations. The extremely small gap between the optics of fibers limits them to these types of applications. It is possible to make flow cells with sample gaps as small as 25 um, so that they may be used with highly absorbing species. With even higher absorbing species, attenuated total internal reflection (ATR) probes may be appropriate (see below). 

Attenuated total internal reflection (ATR) probes offer several advantages over other probe types. ATR is a phenomenon that relies on a difference in the index of refraction of a crystal and that of the solution with which it is in contact to prevent light from escaping the crystal. Only the evanescent wave of the light interacts with the solution layer at the crystal face. The result is an optical pathlength of only a few microns. Typical designs make use of faceted crystals or hemispheres (see Figure 6.1). The most common ATR material in the UV-vis is sapphire. In rare cases, fused silica may be used. ATR allows spectra to be taken of neat samples with optical density (OD) of 500-1000... 

Infrared spectra may be obtained from surfaces using a variety of techniques. These included attenuated total internal reflectance (ATR) and specular and diffuse reflectance. These techniques involve the infrared beam passing through only the outer few micrometers of the sample. The most widely applicable is ATR. A typical two-layer plastic material used for lidding on plastic food trays consists of polyethylene and poly(ethylene terephthalate) bound together with an adhesive. ATR infrared spectra of the two surfaces will easily identify one surface as... 

Sample preparation was greatly simplified in the late 1990s by the introduction of commercial diamond attenuated total internal reflection (ATR) systems. These use a small diamond as a single-pass ATR element held in a metal plate. The sample is placed on the diamond, a plate is then brought down onto it and clamped in position. This ensures excellent contact between the ATR element and the sample, so immediately eliminates the problem of poor contact that made ATR so problematical for use with solids, particularly powders. The accessory can be permanently installed in the purged sample compartment of the spectrometer, so there is only a very short pathlength that is in the open air. This means that there is no need to wait for the sample compartment to purge to reduce the atmospheric water and carbon dioxide absorption. The result is that the sample can be placed in the accessory and scanned immediately. 

In the case of linear spectroscopic methods such as FTIR, attenuated total internal reflection (ATR) has been used to enhance the sensitivity of these methods [38-40]. In a similar fashion, a total internal reflection geometry has been used here to enhance the otherwise weak sum-frequency (SF) response from the liquid/liquid interface. In addition to a strong resonant contribution from the C-H stretching vibrations within the molecules, the intensity is also dependent upon the Fresnel factors for the input fields, /i, and the outgoing SF, fsp, as shown in Eq. (1). When the incident beams are directed on the interface at their respective critical angles, an enhancement of several orders of magnitude in the SF response is achievable over that of an external reflection geometry [25,35,36,41-43]. 

The surfaces of infrared-transparent materials that are available in the form of shaped and polished crystals, such as silicon or germanium, can be studied with good sensitivity by using attenuated total internal reflection (ATR) in conjunction with multiple reflection procedures. 

Infrared spectra can also be obtained by reflecting the IR beam on the surface of a sample. This technique is applied when it is not possible to obtain an IR spectrum by a transmission technique. Attenuated total reflectance (ATR) also known as ATIR (attenuated total internal reflectance) is based on multiple internal reflectance of the IR beam on the sample surface using a high refractive index crystal (e.g.. 

Attenuated Total Internal Reflection Infrared Spectroscopy (ATR)... 

Note ATR - attenuated total internal reflection IR spectroscopy. 

Attenuated total reflection (ATR), also called internal reflection, is based on the phenomenon of total internal reflection. In ATR the infrared beam is directed into an infrared-transmitting crystal so that it strikes the crystal surface at less than the critical angle and undergoes total internal reflection. 

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. 

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