Attenuated total reflection transmission

Fixed pathlength transmission flow-cells for aqueous solution analysis are easily clogged. Attenuated total reflectance (ATR) provides an alternative method for aqueous solution analysis that avoids this problem. Sabo et al. [493] have reported the first application of an ATR flow-cell for both NPLC and RPLC-FUR. In micro-ATR-IR spectroscopy coupled to HPLC, the trapped effluent of the HPLC separation is added dropwise to the ATR crystal, where the chromatographic solvent is evaporated and the sample is enriched relative to the solution [494], Detection limits are not optimal. The ATR flow-cell is clearly inferior to other interfaces. 

For IR sensing, three transducer principles are standard classical transmission for (sufficiently) transparent samples, (diffuse) reflection for opaque samples, in particular solids and strongly turbid liquids and attenuated total reflection (ATR), in particular for strongly absorbing samples and fluids with varying amounts of suspended solids or gas bubbles. 

A majority of traditional NIR measurements are made on solid materials and these involve reflectance measurements, notably via diffuse reflectance. Likewise, in the mid-IR not all spectral measurements involve the transmission of radiation. Such measurements include internal reflectance (also known as attenuated total reflectance, ATR), external reflectance (front surface, mirror -style or specular reflectance), bulk diffuse reflectance (less common in the mid-IR compared to NIR), and photoacoustic determinations. Photoacoustic detection has been applied to trace-level gas measurements and commercial instruments are available based on this mode of detection. It is important to note that the photoacoustic spectrum is a direct measurement of infrared absorption. While most infrared spectra are either directly or indirectly correlated... 

Solid state spectra can be measured by the Attenuated Total Reflectance or Multiple Internal Reflection methods as well as by simple transmission techniques (222). Brooker (223) has reported the discovery of the translational vibrations of NaN02, NaNOs, and CaCOs (calcite). The orientation of the bonds in (U02)(N0s)2. 6 H2O has been ascertained from the spectra obtained in this manner (223). 

Many polymers are too tough to be ground even at liquid nitrogen temperatures. Consequently, surface techniques are often used. Internal reflectance or attenuated total reflectance (ATR) is the second most commonly used infrared technique [38-40]. For soft or pliable polymers or solutions, ATR is an extremely versatile technique and the spectrum is similar to a transmission spectrum. Unlike transmission, the spectrum obtained is independent of sample thickness. 

FT Spectrometers FT spectrometers (Figure 3) differ from scanning spectrometers by the fact that the recorded signal is an interferogram [14] (see Chapter 6.2). They can be coupled to a microscope or macrochamber with an FPA detector. FT chemical imaging systems (CISs) are available for Raman, NIR, and IR spectroscopy. However, they can only be considered as research instruments. For example, most IR imaging systems are FT spectrometers coupled to microscopes. This type of spectrometer allows the acquisition of spectra in reflection, attenuated total reflection (ATR), or transmission mode. 

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.

Using internal reflection, also known as attenuated total reflection (ATR), an official ATR-FTIR method (AOCS, 1999a AOAC International, 2000) was recently developed (Mossoba et al., 1996, 2001b Adam et al., 2000) to rapidly (5 min) measure the 966 cm-1 trans band as a symmetric feature on a horizontal baseline (Fig. Dl.7.1 A). The experimental aspects of this ATR infrared official method are far less complex than those involving the conventional transmission measurements. This approach entails (1) ratioing the trans test sam-... 

The extent of the interaction between the evanescent field and the absorbing medium is formally described by the effective thickness, effective thickness is the thickness of the absorbing phase that would have to be passed through by the incident beam in a transmission experiment to give the same energy loss as in the attenuated total reflection experiment. The exact expressions for effective thickness can be very complex. However, for a single attenuated total reflection of an incident beam of radiation of electric field E that occurs at an interface between two bulk phases (ie, phase 2 is not a thin film), d is given by... 

FT-IR has been applied for determining the sucrose content of sugar cane juice [21]. In place of the more familiar transmission cell, an attenuated total reflectance (ATR) cell and clarified sugar cane juice were used to record FT-IR spectra from 800 to 1250 cm"1. In the spectra, significant wavenumbers (927.59, 997.02, 1054.87, 1116.51, and 1137.80 cm"1) have been identified for sucrose. The application of PCR has been proposed for the development of a calibration equation for sucrose content. PCR is basically a MLR applied to scores assessed by PCA. On the basis of FT-IR spectra and sucrose content, an accurate calibration equation could be obtained by the application of PCR. The root mean square difference between predicted FT-IR values and the actual values were 0.12 % (w/v) with a bias of -0.03 % (w/v). The accuracy of FT-IR for determining sugar cane sucrose is almost equal to that of NIR [25]. 

FTIR reflectance and transmission spectroscopy is used for analysis of thin films. Nevertheless, due to the high absorptivities of mid-IR bands, the film thickness must be limited (up to 100 pm, depending on the specific bands chosen) in order to perform an accurate qualitative analysis. Other IR methods, such as attenuated total reflectance (ATR) and photoacoustic methods provide IR spectra of thick material, because they penetrate a very thin layer at the surface of a sample. However, is important to point out that the effective pathlength for the ATR and the photo-acoustic methods depends on the refractive index and thermal diffusivity, respectively. Therefore, the use of these techniques for the quantitative analysis of non-homo-geneous materials can be difficult. 

The sample thickness in the DIT cells (25 pm) is very large in comparison with the usual thickness of samples for infrared transmission studies, and the cell itself is also thick (2.8 mm). Studies in the attenuated total reflectance (ATR) mode do not fit the experiment. For all these reasons, it was by no means certain that infrared studies could in fact be performed using the existing DIT cells. 

Fourier-transformed infrared spectroscopy (FT1R), either in the transmission mode(70), the grazing incidence reflection (GI) mode(7,5) or the attenuated total reflection (ATR) mode(7,2), has been the most widely used experimental tool for the characterization and structure determination of SA monolayers. GI-IR is especially useful in determining the molecular orientation in the film structures because it senses only the vibrational component perpendicular to the substrate surface(7,5). Polarized ATR-IR can also be used to study molecular orientation(7,77). McKeigue and Gula-ri(72) have used ATR-IR to quantitatively study the adsorption of the surfactant Aerosol-OT. 

Infrared spectroscopy Transmission infrared spectroscopy Diffuse reflectance infrared spectroscopy (DRIFTS) Attenuated total reflection (ATR)... 

In the case of infrared spectroscopy, various types of infrared measurement methods (transmission method, attenuated total reflection (ATR) method, reflection-absorption (RA) method, surface-enhanced method, etc.) may be applied. 

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