ATR infrared spectroscopy

ATR infrared spectroscopy (from English attenuated total reflection 'attenuated total reflection ' ) is a measurement technique of infrared spectroscopy (IR spectroscopy) for the surface investigation of opaque substances such as e.g. B. lacquer layers or polymer films and also liquid samples such. B. Mixtures of solvents. The method was first introduced by Harrick in 1960 and Fahrenfort in 1961. The intensity of the reflected light is measured, which allows conclusions to be drawn about the absorbing medium.

Layout and function

The core of this method is an optical fiber in which radiation is guided in total reflection, a so-called internal reflection element (IRE). This optical waveguide is usually a prism, a fiber without a cladding ( FEWS for fiber evanescent wave spectroscopy ) or a special differently shaped ATR crystal (= ATR element) in which multiple reflections are possible.

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

Penetration depth of the evanescent wave with the angle of incidence in the case of the refractive index ratio of 1.4 to 2.4

A guideline for the necessary approach of the sample to the interface is the penetration depth d _{p of} the evanescent wave. This is defined as the distance from the interface at which the amplitude of the electric field only corresponds to 1 / e-th (≈ 37%) of the amplitude at the interface. For light of wavelength λ and the angle of incidence Θ, the transition from the optically denser medium (e.g. ATR element with the refractive index n ₁ ) to an optically thinner medium (e.g. a liquid medium with the refractive index n ₂ ) :

{\ displaystyle d _ {\ text {p}} = {\ frac {\ lambda} {2 \ pi n_ {1} {\ sqrt {\ sin ^ {2} (\ Theta) - (n_ {2} / n_ { 1}) ^ {2}}}}}}

For a decaying (evanescent) wave with a wavelength of 500 nm, total reflection (below 60 °) at the interface between glass ( n ₁ = n _glass = 1.5) and air ( n ₂ = n _air = 1, 0) a penetration depth of 95 nm. For typical angles in the range around 45 ° and typical refractive index ratios, it can be said that the penetration depth is approx. 1/5 to 1/4 of the wavelength of the incident light, the penetration depth increases with increasing ratio n ₁ / n ₂ from.

application

Infrared spectroscopy

Holder with KRS5 crystal (pink) for ATR infrared spectroscopy

Multi-reflection crystals; Above: silicon crystal with the base of a parallelogram, underneath crystals with a trapezoid base made of zinc selenide (center) and silicon (below).

In infrared spectroscopy , solid and liquid samples are brought into the evanescent field and the wavelength-dependent absorption is measured. Solid samples are pressed against the surface of the optical waveguide in order to obtain the strongest possible measurement signal. To increase the sensitivity, fiber optic cables are used in which the measuring beam is reflected several times and the signal strength is added up.

For the recording of IR spectra of workpieces and material samples, ATR is one of the most frequently used methods today. Depending on the wave number range, materials for IR radiation are usually ZnSe , Ge , thallium bromide iodide (KRS-5), Si , AMTIR (from English amorphous material transmitting infrared radiation , e.g. GeAsSe = AMTIR-1) or diamond .

When examining samples, care must be taken that the refractive index of the ATR element used is sufficiently large so that the conditions for total reflection are still met when the medium and ATR element come into contact.

Chalcogenide glass fibers are used to investigate the smallest amounts of liquid with FEWS ( fiber evanescent wave spectroscopy ) in the mid-infrared .

Surface plasmon resonance spectroscopy

In the surface plasmon resonance (SPRS) the angle-dependent absorption is measured at fixed wavelengths. Under certain conditions, the evanescent field excites surface plasmons at an interface of the sample and is absorbed in the process. If the reflection is recorded as a function of the angle, the complex refractive index of the sample can be calculated from the curve.

Advantages and disadvantages

Insoluble substances can also be measured (e.g. thermosetting plastics ).
There is no need to produce the KBr pressed parts.
The sensitivity of ATR infrared spectroscopy is set via the penetration depth and the number of reflections. Due to the low penetration depth, this method is particularly suitable for strongly absorbing samples.
The materials that are transparent in the mid-infrared range are mostly mechanically and / or chemically unstable. The exception here are expensive ATR elements made of diamond.

Comparability of ATR and transmission spectra

Since transmission technology was the dominant IR measurement method for a long time, there are large collections and databases of transmission spectra. The relative similarity of the ATR and transmission spectra suggests that the transmission spectra can be used to identify materials by means of ATR spectra. However, a direct comparison of the spectra generally does not lead to reliable results for more complex spectra. The reason for this lies in the depth of penetration of the surface wave, which is dependent on the wavelength and strong absorption centers (high extinction coefficient), and thus the depth of information. The result is a relative band intensity that is dependent on the wavelength. The absorption bands become wider and more intense towards larger wavelengths (smaller wavenumber) than with corresponding transmission spectra. However, the position of the absorption bands is identical for transmission and ATR spectra.

In order to improve the comparability of ATR and transmission spectra, so that older spectra can also be used, there are mathematical methods known as ATR correction . In addition to the simple ATR correction, which only carries out a wavenumber weighting of the spectrum, some manufacturers of spectroscopy software also offer so-called advanced ATR correction methods . These also use the angle of incidence and the refractive indices of the crystal and the sample (mostly a fixed value and not a spectrum) for the correction. Another reason is that when measuring in transmission, the spectrum results from the absorption when the sample passes through and the reflection losses at the two interfaces. Therefore, the extended correction procedures are only suitable for qualitative but not quantitative comparison.

For some time now, some providers of spectra databases have been running databases with transmission spectra as well as databases with ATR spectra. This is an important aspect, since ATR technology is now the dominant IR technology in many areas, for example in process analysis .

literature

NJ Harrick: Internal Reflection Spectroscopy . John Wiley & Sons Inc, 1967, ISBN 0-470-35250-7 .
Francis M. Mirabella: Internal Reflection Spectroscopy: Methods and Techniques: Theory and Applications . Marcel Dekker Ltd., 1992, ISBN 0-8247-8730-7 .
Francis M. Mirabella: Modern Techniques in Applied Molecular Spectroscopy . Wiley & Sons, 1998, ISBN 0-471-12359-5 .

Web links

FT-IR Spectroscopy - Attenuated Total Reflectance (ATR) (PDF, 226 kB) , Technical Information from Perkin Elmer, a manufacturer of spectrometers.
Online monitoring of reactions using ATR-FTIR spectroscopy. (No longer available online.) HiTec Zang GmbH, 2007, archived from the original on November 25, 2007 ; Retrieved August 5, 2012 .

Footnotes and individual references

↑ NJ Harrick: Surface chemistry from spectral analysis of totally internally reflected radiation . In: J. Phys. Chem. Band 64 , no. 9 , 1960, pp. 1110-1114 , doi : 10.1021 / j100838a005 . , quoted in Helmut Günzler, Harald Böck: IR Spectroscopy: An Introduction . 2nd Edition. Wiley-VCH, 1983, ISBN 3-527-21089-X , pp. 308 .
↑ J. Fahrenfort: Attenuated total reflection A new principle for the production of useful infra-red reflection spectra of organic compounds . In: Spectrochimica Acta . tape 17 , 1961, pp. 698-709 , doi : 10.1016 / 0371-1951 (61) 80136-7 . , quoted in Helmut Günzler, Hans-Ulrich Gremlich: IR - Spectroscopy: An Introduction . 4th edition. Wiley-VCH, 2003, ISBN 3-527-30801-6 , pp. 123 .
↑ ^a ^b Maëna Le Corvec u. a .: Mid-infrared spectroscopy of serum, a promising non-invasive method to assess prognosis in patients with ascites and cirrhosis . In: PLOS ONE . tape 12 , no. 10 , October 11, 2017, p. e0185997 , doi : 10.1371 / journal.pone.0185997 .
↑ NJ Harrick: Internal Reflection Spectroscopy . John Wiley & Sons Inc, 1967, ISBN 0-470-35250-7 .
↑ Frank L. Pedrotti: Optics for engineers: Fundamentals. Springer-Verlag, Berlin / Heidelberg / New York 2005, ISBN 3-540-22813-6 .
↑ Transmission Curves. (No longer available online.) RMI Laser Optics and Coatings Prototype Through Production, archived from the original on December 3, 2013 ; accessed on March 3, 2008 (English, transmission window of some important IR materials).

[1] NJ Harrick: Surface chemistry from spectral analysis of totally internally reflected radiation . In: J. Phys. Chem. Band 64 , no. 9 , 1960, pp. 1110-1114 , doi : 10.1021 / j100838a005 . , quoted in Helmut Günzler, Harald Böck: IR Spectroscopy: An Introduction . 2nd Edition. Wiley-VCH, 1983, ISBN 3-527-21089-X , pp. 308 .

[2] J. Fahrenfort: Attenuated total reflection A new principle for the production of useful infra-red reflection spectra of organic compounds . In: Spectrochimica Acta . tape 17 , 1961, pp. 698-709 , doi : 10.1016 / 0371-1951 (61) 80136-7 . , quoted in Helmut Günzler, Hans-Ulrich Gremlich: IR - Spectroscopy: An Introduction . 4th edition. Wiley-VCH, 2003, ISBN 3-527-30801-6 , pp. 123 .

[plos-3] Maëna Le Corvec u. a .: Mid-infrared spectroscopy of serum, a promising non-invasive method to assess prognosis in patients with ascites and cirrhosis . In: PLOS ONE . tape 12 , no. 10 , October 11, 2017, p. e0185997 , doi : 10.1371 / journal.pone.0185997 .

[4] NJ Harrick: Internal Reflection Spectroscopy . John Wiley & Sons Inc, 1967, ISBN 0-470-35250-7 .

[5] Frank L. Pedrotti: Optics for engineers: Fundamentals. Springer-Verlag, Berlin / Heidelberg / New York 2005, ISBN 3-540-22813-6 .

[6] Transmission Curves. (No longer available online.) RMI Laser Optics and Coatings Prototype Through Production, archived from the original on December 3, 2013 ; accessed on March 3, 2008 (English, transmission window of some important IR materials).