Attenuated total reflectance

ATR infrared spectroscopy ( from English- attenuated total reflection, dt attenuated total reflection) is a sampling technique of infrared spectroscopy (IR ) spectroscopy for studying the surfaces of opaque materials such as paint layers or polymer films and liquid samples such as solvent mixtures. The method was first introduced by Harrick in 1960 and Fort Drive 1961. The intensity of the reflected light is measured, it can draw conclusions about the absorbent medium.

Design and function

The core of this method is an optical waveguide, is performed in the radiation total reflection, a so-called reflection element (English: internal reflection element, IRE). The optical waveguide is a prism, a fiber without a special coat or ATR crystal in which multiple reflections are usually possible.

In total reflection form behind the reflecting interface evanescent waves. These have about the range of a wavelength. A sample is then brought close to the surface of the optical waveguide, it can interact with the evanescent wave. The guided in the waveguide light is attenuated.

A guideline for the necessary approximation of the sample to the interface is the penetration depth dp of the evanescent wave. This is defined as the distance from the interface, wherein the amplitude of the electric field only 1/e-tel ( ≈ 37%) corresponds to the amplitude at the interface. For light of wavelength λ and incident angle Θ (e.g. ATR element with the refractive index n1 ) is obtained at the transition from an optically denser medium to an optically thinner medium (e.g. a liquid medium with the refractive index n2):

A decaying ( evanescent ) wave having the wavelength 500 nm, this results in total internal reflection ( at 60 °) at the interface of glass (n1 = nGlas = 1.5 ) and air ( n2 = nair = 1.0), a penetration depth of 95 nm for typical angle in the range of 45 ° and typical refractive index ratios can be said that the penetration depth is about 1 /5 to 1 /4 of the wavelength of the incident light, it increases the depth of penetration decreases with increasing ratio n1/n2.

Application

Infrared spectroscopy

In infrared spectroscopy, solid and liquid samples are placed in the evanescent field and measured the wavelength-dependent absorption. Solid samples are thereby pressed against the surface of the optical waveguide in order to obtain the strongest possible measurement signal. To increase the sensitivity of optical fibers are used, in which the measuring beam is reflected multiple times.

For the recording of IR spectra of workpieces and material samples ATR today is one of the most common methods used. As materials for IR radiation is chosen depending on the wave number range usually ZnSe, Ge, Thalliumbromidiodid (KRS -5), Si, AMTIR (of English. Amorphous material Transmitting infrared radiation, eg GeAsSe = AMTIR -1) or diamond.

In the investigation of samples is to ensure that the refractive index of the ATR element used is sufficiently large, such that upon contact between the medium and ATR element satisfy the conditions for total internal reflection are met.

Surface plasmon resonance spectroscopy

In the surface plasmon resonance spectroscopy ( SPRS ), the angle-dependent absorption is measured at fixed wavelengths. The evanescent field excites under certain conditions of surface plasmons on a boundary surface of the sample and absorbed there. Taking the reflection angle dependent on, one can calculate from the course, the complex refractive index of the sample.

Pros and Cons

• Even non-soluble substances are measurable ( eg polymers).
• The production of the KBr compacts is omitted.
• The sensitivity at the ATR - infrared spectroscopy is adjusted by the penetration depth and the number of reflections. Due to the small penetration depth of this method is especially suitable for highly absorbing samples.
• The transparent material in the medium infrared range are generally mechanically and / or chemically unstable. Here except in expensive ATR elements of diamond.

Comparability of ATR and transmission spectra

As the transmission technology for a long time was the dominant IR measurement method, there are large collections and databases of transmission spectra. The relative similarity of the ATR and transmission spectra suggests, the transmission spectra may be used for the identification of materials by means of ATR spectra. However, a direct comparison of the spectra not result in more complex spectra generally to reliable results. The reason for this lies in the wavelength and strong absorption centers ( high extinction coefficient ) -dependent penetration depth of the surface wave, and thus the depth of information. The result is dependent on the wavelength relative band intensity. The absorption bands to longer wavelengths (smaller wavenumbers ) getting broader and more intense than the corresponding transmission spectra. The position of the absorption bands is, however, identical in transmission and ATR spectra.

Nevertheless, to improve the comparability of ATR and transmission spectra, so that older spectra can be used, there are mathematical methods, the so-called ATR correction. In addition to simple ATR correction, which makes only one wave speed scaling of the spectrum, some manufacturers of spectroscopy software also offer so-called advanced ATR correction method. This use, inter alia, the angle of incidence and the refractive indices of the crystal and the sample for correction.

For this reason, some providers of spectral databases now run next to Databases with transmission spectra that also with ATR spectra. This is an important aspect, since the ATR technique in many areas is the dominant IR technique now, for example, in process analysis.