ATR-FT-IR spectroscopy, general aspects

Attenuated total reflectance (ATR) has been developed since 1959 [24] and is now probably the most common sampling technique in FT-IR spectroscopy.

ATR generally allows qualitative and quantitative analysis of samples with little or no sample preparation. In principle it is a non-destructive technique. ATR is easily miniaturized so that high-quality spectra can be scanned of samples with a diameter far less than a millimetre.

Generally, in this technique the sample is placed in contact with the internal reflection element (IRE), and IR radiation from the source is directed into IRE at a certain angle (θ) that the light is totally reflected (see Figure 1) [27]. In order to observe total internal reflection the angle of the incident radiation θ must exceed the critical angle θc. This angle is a function of the real parts of the refractive indices of both the sample and the ATR crystal. The critical angle is defined as [24]:

where, n1 is the refractive index of the ATR crystal (or internal reflection element) and n2 is the refractive index of the sample.

On internal reflection a part of the IR beam (so-called evanescent wave) penetrates into the sample to a depth of a few microns (dp) and is partially (depending on the composition of the sample) absorbed by the sample. The result is a selective attenuation of the radiation at those wavelengths at which the sample absorbs [24, 30, 31]. So generally, due to a sample interaction with the penetrating beam, the beam loses energy at those wavelengths where the sample absorbs and thus an infrared spectrum is obtained.

Fig. 1. Schematic representation of total internal reflection

Only the sample surface is analysed since the beam penetrates just a few micrometers into the sample [22]. The intensity of evanescent wave (or the radiation that penetrates into the sample) decays exponentially with distance from the surface of the ATR crystal. As the effective penetration depth is usually a fraction of wavelength, total internal reflectance is generally insensitive to sample thickness and permits thick samples to be analyzed [24].

Due to the low depth of penetration there must be good contact between the sample and the crystal surface. The internal reflection element (IRE) is also called ATR crystal.

High refractive index materials are chosen for the ATR crystal to minimize the critical angle. As the angle of incidence approaches the critical angle, the bands tend to broaden for lower wavenumbers and the minima are displaced to lower wavenumbers. The hardness of the material is also an important characteristic, because in order to obtain good contact with the sample the sample has to be strongly pressed against the crystal.

Table 1. Some ATR crystal materials [22, 27]

A hard and brittle crystal, water insoluble, resistant to most solvents, slightly soluble in acids

KRS-5 15 000–250 2,37 Soft crystal, water insoluble, soluble in bases, deforms under pressure

AgCl 25 000–400 2,0 very soft crystal that is insoluble in water, sensitive to light

Ge 20 000–600 4,0 A hard and brittle crystal, it is chemically inert

Diamond 40 000–200 2,4 Hard crystal, insoluble, inert, expensive.

Very useful for high-pressure or corrosive work.

The ATR accessory consist of a mirror system that sends the source radiation through the attachment and a second mirror system that directs the radiation into the detector [27].

The depth to which the evanescent wave (or IR radiation) extends into the sample is defined as the depth of penetration (d_p). The depth of penetration (of the evanescent wave) is the distance from the crystal-sample interface where the intensity of the evanescent wave decays to 1/e of its original value. It can be estimated by the following eq [32, 33]:

2 θ – angle of incidence of the IR radiation

Changes in the angle of incidence of the infrared radiation have an effect on the depth of penetration and thus the ATR spectrum of a sample. In most devices it is not possible to vary the angle continuously, usually 30°, 45° and 60° are employed [34]. The angle of incidence must be chosen to exceed the critical angle in order to have internal reflection and produce an ATR spectrum.

ATR-IR spectra are similar, but not identical, to those obtained in the transmission mode [30]. It follows from equation (2) that in order to observe any ATR effect at all the refractive index of the sample (n₂) must be lower than that of the ATR crystal (n₁). If this condition is not fulfilled then internal reflectance will not occur – the light will be transmitted rather than internally

reflected in the ATR crystal. The depth of penetration (and thus the effective path length) is dependent on the wavelength and refractive index of the sample (given that n₁ and θ are constant).

The depth of penetration is proportional to wavelength, and therefore, an increasing depth of penetration is observed at higher wavelengths (lower wavenumbers) [22]. This leads to the increase of the intensity of the bands at low wavenumbers relative to high wavenumbers and is one cause of differences between the ATR-IR spectra and transmission IR spectra.

The refractive index also affects the depth of penetration of the evanescent wave into the sample. By increasing the refractive index of the ATR crystal (or IRE), the depth of penetration will decrease. This will decrease the effective pathlength and therefore decrease the absorbance of the spectrum. [32]

Refractive index of a material is also a function of wavelength. In the IR spectral region the change of refractive index with wavelength is not monotonous. At certain wavelengths large nonlinearities are observed [35, 36].

Absorption bands that occur near such nonlinearities can be significantly different from those observed in the transmission mode. This is another cause of differences between IR spectra obtained using transmission and ATR modes.

In order to make the spectra obtained using ATR comparable to the transmission spectra, correction algorithms (so-called ATR correction) have been developed [35].

The ATR technique can be used for solids, liquids, powders, pastes, rubbers, textiles, paper etc. As mentioned above, the efficient depth to which the radiation penetrates into the sample is only few micrometers and is independent of sample thickness. So, ATR spectra can be obtained from many samples that cannot be studied in the transmission mode. These include for example samples that show very strong absorption, resist preparation of thin film, are characteristic only as a thick layers [27].

2.4.4. IR spectroscopy of inorganic pigments, general aspects