Attenuated total reflectance sampling depth

While electron or ion beam techniques can only be applied under ultra-high vacuum, optical techniques have no specific requirements concerning sample environment and are generally easier to use. The surface information which can be obtained is, however, quite different and mostly does not contain direct chemical information. While with infra-red attenuated total reflection spectroscopy (IR-ATR) a deep surface area with a typical depth of some micrometers is investigated, other techniques like phase-measurement interference microscopy (PMIM) have, due to interference effects, a much better surface sensitivity. PMIM is a very quick technique for surface roughness and homogeneity inspection with subnanometer resolution. 

Attenuated total reflectance infrared (ATR-IR) is used to study films, coatings, threads, powders, interfaces, and solutions. (It also serves as the basis for much of the communication systems based on fiber optics.) ATR occurs when radiation enters from a more-dense material (i.e., a material with a higher refractive index) into a material that is less dense (i.e., with a lower refractive index). The fraction of the incident radiation reflected increases when the angle of incidence increases. The incident radiation is reflected at the interface when the angle of incidence is greater than the critical angle. The radiation penetrates a short depth into the interface before complete reflection occurs. This penetration is called the evanescent wave. Its intensity is reduced by the sample which absorbs. 

Figure 10.20—Devices allowing the study of samples by reflection, a) Diffuse reflection device b) attenuated total reflection (ATR) device c) comparison of the spectra of benzoic acid obtained by transmission (KBr disc) and by diffuse reflection using the Kubelka Munk correction. The depth of penetration of the IR beam depends on the wavelength. The absorbance for longer wavelengths would be overestimated if no correction was applied.

The primary techniques used in this study include X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIR), and attenuated total reflectance infrared spectroscopy (ATR). XPS is the most surface-sensitive technique of the three. It provides quantitative information about the elemental composition of near-surface regions (< ca. 50 A sampling depth), but gives the least specific information about chemical structure. RAIR is restricted to the study of thin films on reflective substrates and is ideal for film thicknesses of the order of a few tens of angstroms. As a vibrational spectroscopy, it provides the type of structure-specific information that is difficult to obtain from XPS. The... 

Attenuated total reflection FTIR is a well-established technique for obtaining absorbance spectra of opaque samples. The mode of interaction is unique because the probing radiation is propagated in a high index-of-refraction internal reflection element (IRE). The radiation interacts with the material of interest, which is in close contact with the IRE, forming an interface across which a nonpropagating evanescent field penetrates the surface of the material of interest to a depth in the order of one wavelength of the radiation. The electric field at the interface penetrates the rarer medium in the form of an evanescent field whose amplitude decays exponentially with distance into the rarer medium. 

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. 

The polymer properties in thin films are then compared with the bulk as measured by FTIR attenuated total reflection spectroscopy (ATR). A modest refractive index of the internal reflection element (IRE made of ZnSe, n=2.43 at 2000 cm ) and an incident angle of 65° - still low but well above the critical angle of total reflection - are chosen for the p-polarized light in order to obtain an information depth of several microns. Hence, the ATR spectra provide the bulk properties of the polymer sample. 

The combination of infrared spectroscopy with the theories of reflection has made advances in surface analysis possible. Attenuated Total Reflectance (ATR) spectroscopy is an innovative technique for proving chemical information of a sample surface and the ability to quantify newly formed species, based upon Pick s second law. The fundamentals of attenuated total reflection (ATR) spectroscopy date back to the initial work of Jacques Fahrenfort and N.J. Harrick, both of whom independently devised the theories of ATR spectroscopy and suggested a wide range of applications. The schematic showing ATR-FTIR configuration is illustrated in Fig. 1 (KS. Kwan, 1998). The penetration depth, d, can be estimated as ... 

Attenuated Total Reflection infrared (ATR) spectra were obtained on a Bruker Equinox 55 FT-IR spectrometer using a SpectraTech Model 300 ATR. By varying the types of crystals and the angles of incidence, information from different depths within the sample can be obtained. Transmission FT-IR spectroscopy was also performed on the samples. 

Vibrational spectroscopy represents two physically different, yet complementary spectroscopic techniques IR and Raman spectroscopy. Although both methods have been utilised for many years, recent advances in electronics, computer technologies and sampling made Fourier transform infrared (FTIR) and Raman (FT-Raman) one of the most powerful and versatile analytical tools. Enhanced sensitivity and surface selectivity allows non-invasive, no-vacuum molecular level analysis of surface and interfaces. Emphasis is placed on recent advances in attenuated total reflectance (ATR), step-scan photoacoustic (SS-PA), Fourier transform infrared (FTIR) and FT-Raman microscopies, as utilised to the analysis of polymeric surfaces and interfaces. A combination of these probes allows detection of molecular level changes responsible for macroscopic changes in three dimensions from various depths. 7 refs. 

The most common infrared sampling techniques used to examine paint samples are attenuated total reflectance and photoacoustic spectroscopies. Liquid paints, dried films and paint chips may all be investigated in this way. Depth profiling can be useful when examining paint, as the surface properties will vary importantly from the bulk properties. In addition, most paint films contain two or more layers with different compositions, and so reflectance techniques are necessary for the characterization of individual layers. 

In order to determine the composition and structure of a biomaterial surface different methods which provide varying degrees of information are commonly used (Fig. 6). Surface-sensitive infrared spectroscopy suppHes the characteristic absorption bands of functional groups with an informational depth of 0.1-10 pm by measurement in attenuated total reflectance (IR-ATR). In the case of samples with rough surfaces photoacoustic spectroscopy (PAS), which allows an informational depth of approximately 20 pm, can be used [72]. The achieved informational depths are usually larger than the thickness of the modified interface, so that the spectra include information on the bulk composition as well. As a consequence, surface-sensitive infrared spectroscopy is often not sensitive enough for the characterization of the modified surfaces. 

For opaque samples, such as fibers or paint chips, attenuated total reflectance-infrared (ATR-IR) spectroscopy is more commonly used. The sample is positioned over a crystal, and pressure is applied to ensure good contact between the sample and crystal. Infiared radiation is passed through the crystal, and because of the close contact, the radiation penetrates a small depth into the sample. Certain energies are absorbed depending on the chemical bonds within the sample, resulting in the characteristic spectrum of the sample. 

Attenuated total reflection (ATR) spectroscopy is an internal reflection spectroscopy sampling technique. It is limited to sampling the surface layer of a sample, to a depth of between about 0.3 and 3 pm, but offers the advantage of not requiring a thin section to be prepared from such as a tissue sample. This... 

Attenuated total reflection (ATR) FTIR is one of the most useful tools for characterising the chemical composition and physical characteristics of polymer surfaces [53]. One useful application is the measurement of molecular orientation using polarised infrared ATR spectroscopy [54,55]. The polarised infrared ATR spectra normally include three-dimensional (e.g., machine, transverse, and thickness direction) orientational information in contrast to the polarised transmission infrared linear dichroism. In addition, band absorbance of less than 0.7 au is easily achieved, even with the strong absorption bands, because the penetration depth of ATR from sample surfaces can be adjusted to a few micrometers by changing the internal reflection element and/or the angle of incidence. If successful combination of the dynamic infrared spectroscopy and the ATR methods can be achieved, more useful dynamic orientational information can be obtained. 

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