Internal reflection, sampling

The ATR technique is a commonly used infrared internal reflection sampling technique. It samples only the surface layer in contact with the ATR element the sampling depth probed is typically of the order of 0.3-3 pm [1]. Unless software corrected, compared with a transmission spectrum, the relative intensity of bands within an ATR spectrum increase in intensity with decreasing wavenumber. Several FTIR instrument companies now supply "ATR-correction" software developed to correct for the different relative intensities of bands observed between ATR and transmission spectra, so that ATR spectra can be more easily compared to and searched against transmission spectra. 

One disadvantage of films is that they may cause excessive light-scattering intransmittance spectrometry. This shortcoming can be overcome by using attenuated total-reflection or internal-reflection sampling techniques. 

Pattacini, S. Solving Analytical Problems Using Infrared Spectroscopy Internal Reflectance Sampling Techniques, Pattacini Associates, LLC Danbury, CT. 

Figure 2.13 Schematic representation of an ATR prism showing the principle of the internal reflection sampling technique. See text for detaUs.

In 1960, Harrick demonstrated that, for transparent substrates, absorption spectra of adsorbed layers could be obtained using internal reflection [42]. By cutting the sample in a specific trapezoidal shape, the IR beam can be made to enter tlirough one end, bounce internally a number of times from the flat parallel edges, and exit the other end without any losses, leading to high adsorption coeflScients for the species adsorbed on the external surfaces of the plate (Irigher than in the case of external reflection) [24]. This is the basis for the ATR teclmique. 

Atr—ftir can be readily performed on most commercial ftir spectrometers through the use of an attachment for atr spectroscopy. These devices provide ir-transparent internal reflection elements that are typically made of Ge, KRS-5, ZnSe, or ZnS. These internal reflection elements are made of materials that are of extremely high purity to avoid losses from absorption by impurities in these devices. Coupling of a thin film or surface sample to one of these reflection elements is accompHshed by pressing the sample against the element while acquiring the spectmm. 

Multiple Internal Reflection Spectroscopy (MIR) is an alternative approach in which the IR beam is passed through a thin, IR transmitting sample such that it undergoes total internal reflection alternately from the front and rear faces of the sample (Figure 3.1(b)). At each reflection, some of the IT radiation may be absorbed by species adsorbed on the solid surface. 

ATR is one of the most useful and versatile sampling modes in IR spectroscopy. When radiation is internally reflected at the interface between a high-refractive index ATR crystal (usually Ge, ZnSe, Si, or diamond) and the sample, an evanescent wave penetrates inside the sample to a depth that depends on the wavelength, the refractive indices, and the incidence angle. Because the penetration depth is typically less than 2 pm, ATR provides surface specific information, which can be seen as an advantage or not if surface orientation differs from that of the bulk. It also allows one to study thick samples without preparation and can be used to characterize highly absorbing bands that are saturated in transmission measurements. 

Antiblock additives can be seen on the surface of films using optical microscopy or SEM. Identification can normally be achieved with internal reflection IR spectroscopy (e.g., with a germanium crystal to minimise sampling depth) or using an X-ray attachment with the electron microscope. 

The methods of choice for the determination of acrylonitrile in environmental samples are GC/NPD (Page 1985), GC/FID (EPA 1982a), and GC/MS (EPA 1982b). Multiple internal reflectance infrared spectrometry (Jacobs and Syrjala 1978) is useful for monitoring low levels of acrylonitrile in air. 

This layer is then analysed directly by internal reflectance infra-red spectroscopy. Since there is no handling of the sample, contamination is reduced to a minimum. However, only infra-red spectral analysis is possible with this system since the material absorbed on the germanium prism is always a mixture of compounds, and since the spectrophotometer used for the production of the spectra is not a high-precision unit, the information coming from this technique is limited. While identification of specific compounds is not usually possible, changes in spectra, which can be related to the time of day, season, or to singular events, can be observed. 

Mattson and Mark [55,56] reported some criticism of Kawahara s technique. They claim that evaporation of the solvent chloroform by infrared heating removes volatiles and causes large changes in the ratios. An oil sample was shown to suffer such alteration by the infrared during repeated analysis. The absorption of all bands decreased nonuniformly between 20 and 100% over a period of 30 min. They propose the application of internal reflection spectrometry as a rapid, direct qualitative technique requiring no sample pretreatment. 

Kawahara and Ballinger [53,57] has used their method to characterise a number of known and unknown petroleum samples. All of these studies used the normal transmission method to obtain infrared spectra however, the feasibility of using internal reflection to obtain infrared spectra has been demonstrated by several groups (Mattson and Mark [55], Mark et al. [58],... 

Recently, polyethylene and Teflon mesh sample holders have been used. A drop of sample is placed on the mesh and spread to a relatively uniform thickness for analysis. These holders can often be rinsed and reused. A very convenient alternative to liquid sample holders is the technique called attenuated total reflection or ATR. The ATR cell is a crystal of gallium arsenide, GaAs and the infrared radiation enters one end of the trapezoidal crystal. With the angles adjusted to obtain total internal reflection, all of the IR radiation passes through the crystal and exits the other end as shown in Fig. 5.14. 

Fig. 5.14. Schematic diagram of an ATR gallium arsenide crystal and the total internal reflection of a light ray. The sample is placed on top of the crystal and interacts with the evanescent wave producing the spectrum.

In this method, the sample (such as a polymer film) is pressed against a transparent material having a high refractive index (called the internal reflection element (IRE)). The IR light beam passes through this material, rather than air, before and after reflecting from the sample, hence the reason for describing... 

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