Attenuated total reflectance ATR

Generally each ray is forced to make several internal reflections before exiting the crystal. As the penetration depth at each reflection is around a micron, a crystal allowing 10 internal reflections will generate a spectrum that, in transmission, would require a cell 10 microns thick. 

The simplicity of sample presentation in such a cell has generated a rapid rise in the popularity of the technique. It is ideal for quantitative 

The appearance of an ATR spectrum is very subtly different to a transmission spectrum. Individual band shapes are slightly unsymmetric-al, but, more importantly, the intensity of the high-wavenumber spectrum (O-H and C-H stretches for instance) is significantly reduced. This cannot easily be avoided, as the ATR effect (equation (10.4)) shows that sampling depth is proportional to wavelength. However, this is not a 

ATR may also be used to study solids which cannot easily be examined using mulls or KBr discs. The sample should ideally be quite plastic so that contact with the crystal is better effected. Carbon-filled rubber is an example of a material with a spectrum which is almost imposssible to record using conventional transmission. Fibres and films, such as wrappers, are usually easier to examine with ATR than with transmission. A weight, or sprung pressure pad attached to the ATR assembly, will help ensure good contact. 

In this method, as indicated above, the light is passed through an IR-trans-parent crystal at such an angle that it suffers one or more internal reflections. Either the crystal itself or, more usually, a thin metallic layer deposited upon it, is the electrode. In the early experiments, where the IR-transparent crystal was also the working electrode, a compromise had to be reached between the electrical conductivity of the crystal and its IR transparency since good conductors are also good absorbers thus, Ge prisms were used [18]. Subsequently, very thin layers of metal (Au, Pt, or Fe) were evaporated on to the IR crystal to form the working electrode. 

However, there remains a problem with this arrangement. If too thin a layer of a metal such as gold is deposited, a purple layer of high resistance is formed owing to the deposition of the gold as islands rather than as a continuous film [19] if too thick a layer of gold is deposited, the evanescent 

LARGE BACK PIECE SMALL FRONT PIECE 

Reference Electrode Counter Electrode Working Electrode 

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. 

= Critical angle n, = Refractive index of sample = Refractive index of crystal 

FIG U RE 4.47 The optical processes that take place when an infrared beam in a crystal of high refractive index, n, encounters a sample of lower refractive index n. Gj is the angle of incidence, 0r is the angle of refraction, 0 is the angle of reflectance, and 0 is the critical angle above which total internal reflection takes place. Total internal reflectances to take place when n n and 0j 0.  

FIG U RE 4.48 An illustration of the evanescent wave, or hot spot, that forms when an infrared beam undergoes total internal reflectance. 

In theory any sample that can be brought into contact with a hot spot will have its spectrum measured. For solids, polymers, and powders a clamp should be used to apply pressure to the sample to obtain good sample/hot spot contact. 

FIGURE 4.49 An example of an ATR crystal with two hot spots on its top surface. Pressure is applied to some solid samples using a clamp to facilitate sample/hot spot contact. 

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Figure Bl.22.4. Differential IR absorption spectra from a metal-oxide silicon field-effect transistor (MOSFET) as a fiinction of gate voltage (or inversion layer density, n, which is the parameter reported in the figure). Clear peaks are seen in these spectra for the 0-1, 0-2 and 0-3 inter-electric-field subband transitions that develop for charge carriers when confined to a narrow (<100 A) region near the oxide-semiconductor interface. The inset shows a schematic representation of the attenuated total reflection (ATR) arrangement used in these experiments. These data provide an example of the use of ATR IR spectroscopy for the probing of electronic states in semiconductor surfaces [44]-...

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

The attenuated total reflectance (ATR) technique is used commonly in the near-infrared for obtaining absorption spectra of thin Aims and opaque materials. The sample, of refractive index i, is placed in direct contact with a material which is transparent in the region of interest, such as thallium bromide/thallium iodide (known as KRS-5), silver chloride or germanium, of relatively high refractive index so that Then, as Figure 3.f8... 

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. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

In order to characterize the surface regions of a sample that has been modified in some way, as is usually the case in adhesion-related investigations, some sort of a reflection experiment is required. Two types of experiments, attenuated total reflection (ATR) and reflection-absorption infrared spectroscopy (RAIR),... 

Surface composition and morphology of copolymeric systems and blends are usually studied by contact angle (wettability) and surface tension measurements and more recently by x-ray photoelectron spectroscopy (XPS or ESCA). Other techniques that are also used include surface sensitive FT-IR (e.g., Attenuated Total Reflectance, ATR, and Diffuse Reflectance, DR) and EDAX. Due to the nature of each of these techniques, they provide information on varying surface thicknesses, ranging from 5 to 50 A (contact angle and ESCA) to 20,000-30,000 A (ATR-IR and EDAX). Therefore, they can be used together to complement each other in studying the depth profiles of polymer surfaces. 

A number of techniques have been employed that are capable of giving information about amorphous phases. These include infrared spectroscopy, especially the use of the attenuated total reflection (ATR) or Fourier transform (FT) techniques. They also include electron probe microanalysis, scanning electron microscopy, and nuclear magnetic resonance (NMR) spectroscopy. Nor are wet chemical methods to be neglected for they, too, form part of the armoury of methods that have been used to elucidate the chemistry and microstructure of these materials. 

The experimental apparatus consists of a gas flow system with a four-port valve, a multi-reflection Attenuated Total Reflection (ATR) accessory (Pike Technologies), and a custom reactor manifold mounted to the ATR top plate, shown in Fig. 45.1. The ATR reactor manifold consists of (i) a Cap2 window for UV irradiation, (ii) an inlet and outlet port, and (iii) an injection port for the liquid phase reactant. 

The photocatalytic oxidation of alcohols constitutes a novel approach for the synthesis of aldehydes and acid from alcohols. Modification of Ti02 catalyst with Pt and Nafion could block the catalyst active sites for the oxidation of ethanol to CO2. Incorporation of Pt resulted in enhanced selectivity towards formate (HCOO ad)-Blocking of active sites by Nafion resulted in formation of significantly smaller amounts of intermediate species, CO2 and H2O, and accumulation of photogenerated electrons. The IR experimental teclmique has been extended to Attenuated Total Reflectance (ATR), enabling the study of liquid phase photocatalytic systems. 

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. 

For solid-phase samples or for membrane interaction studies, it can be very useful to use the alternative sampling technique of attenuated total reflectance (ATR). Films or solutions can be placed on a specially... 

In materials investigations surface-sensitive techniques are of special interest. The major contribution of infrared spectroscopy to this field is internal reflection spectroscopy (IRS), often called the "attenuated total reflection" (ATR) technique. To describe theory and principle, electromagnetic wave theory must be apphed [33]. 

FTIR spectroscopy was conducted on the white residue using an attenuated total reflection (ATR) attachment [l].1 After the blister was opened, the ATR... 

Internal reflectance (attenuated total reflectance ATR ). The internal reflectance or, more usually, attenuated total reflectance (ATR), technique depends on the total reflectance of an IR beam at the internal face of an IR-transparent crystal of high IR refractive index, as shown in Figure 2.38. Medium 1 is a prism of such a material (for example, Si, Ge or KRS-5 [thallous bromide- iodide]), medium 2 is a thin coating of a metal (Au, Pt, Fe) which forms the working electrode and medium 3 is the electrolyte. The... 

For IR sensing, three transducer principles are standard classical transmission for (sufficiently) transparent samples, (diffuse) reflection for opaque samples, in particular solids and strongly turbid liquids and attenuated total reflection (ATR), in particular for strongly absorbing samples and fluids with varying amounts of suspended solids or gas bubbles. 

ATR spectroscopy devices, 24 114. See also Attenuated total reflection (ATR) Attachment materials... 

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