ATR-IR spectroscopy

Figure Bl.22.4. Differential IR absorption spectra from a metal-oxide silicon field-effect transistor (MOSFET) as a fiinction of gate voltage (or inversion layer density, n, which is the parameter reported in the figure). Clear peaks are seen in these spectra for the 0-1, 0-2 and 0-3 inter-electric-field subband transitions that develop for charge carriers when confined to a narrow (<100 A) region near the oxide-semiconductor interface. The inset shows a schematic representation of the attenuated total reflection (ATR) arrangement used in these experiments. These data provide an example of the use of ATR IR spectroscopy for the probing of electronic states in semiconductor surfaces [44]-...

Recently, we [13,14] evidenced by ATR-IR spectroscopy that the membrane potential of ionophore-incorporated, PVC-based liquid membranes is governed by permselective transport of primary cations into the ATR-active layer of the membrane surface. More recently, we [14 16] observed optical second harmonic generation (SHG) for ionophore-incorporated PVC-based liquid membranes, and confirmed that the membrane potential is primarily governed by the SHG active, oriented complexed cations at the... 

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. 

Fourier transform-infrared (FT-IR) spectroscopic studies on SO-SA complexation provides information that may be complementary to that of NMR and other techniques, namely, in particular, on the involvement of functional groups in intermolecular and intramolecular interactions. Attenuated total reflectance (ATR) IR spectroscopy has been used for the study of binding modes of cinchona alkaloid selectors either in solution [95] or in solid state [94], or directly on the CSP [96]. 

Because of the relatively high loading of functional groups on these hyperbranched PE powders, it was feasible to characterize the products and intermediates in this catalysts synthesis by P CP-MAS NMR spectroscopy, ATR-IR spectroscopy, and XPS analysis. P CP-MAS NMR spectroscopy was especially useful for in distinguishing the phosphinated powder, phosphine-palladium complex, and any adventitiously formed phosphine oxide. Similar NMR analyses were not successfully carried out on hyperbranched grafts on PE films. However, when this same phosphine ligand synthesis and introduction of Pd was carried out on a PE film sample, it was possible to analyze... 

Scanning electron microscopy, scanning probe microscopy, ATR-IR spectroscopy, contact angle measurements... 

The aim of this review is to provide an assessment of the state of this field. After a summary of some basic theoretical results, the focus is on experimental aspects, ranging from cell design to specialized techniques such as modulation excitation spectroscopy (MES). We emphasize the opportunities and limitations of ATR-IR spectroscopy in catalysis research. 

Fig. 1. Experimental geometry for attenuated total reflection (ATR) IR spectroscopy. Iq and /are the intensities of the incident and transmitted (reflected) radiation, respectively d is the angle of incidence, and and 112 are the refractive indices of the IRE and sample, respectively.

Fig. 18. In situ ATR-IR spectroscopy of heterogeneous solid-liquid catalytic reactions gives simultaneous information about dissolved species and species adsorbed on the catalyst.

In our experience, the principal challenges in the application of ATR IR spectroscopy for investigations of functioning solid catalysts are associated with the sensitivity of the measurement and the complexity of the samples. The former is an issue common to most surface spectroscopies. The latter has to do with the simultaneous presence of many species at a catalytic solid-liquid interface these species include dissolved reactants, adsorbed intermediates, spectators, and products. The spectra are a superposition of the spectra of the individual species. The question of whether a species is a spectator or instead involved in the catalytic cycle is not easily answered and represents a challenge for in situ spectroscopy in general. Thus, there is a need for specialized techniques to be used in combination with ATR spectroscopy to enhance sensitivity and introduce selectivity. 

In the following we discuss examples that further illustrate the range of possible applications of ATR-IR spectroscopy in catalysis research. 

Fio. 33. CD adsorption on alumina-supported platinum (40) and palladium (42) characterized by ATR-IR spectroscopy during catalysis. Top spectra taken at different CD concentrations in hydrogen-saturated CH2CI2. The CD concentration increases from bottom to top (0-3 x 10 M for platinum and 0 4.3 X 10 M for palladium). Vibrational bands indicative of various adsorption modes of CD (bottom) are marked. Bands indicative of species 2 are absent from spectra observed for CD adsorption on palladium (40,42). 

In many catalytic reactions, solid, liquid, and gas phases are involved, and the phase behavior often has a strong influence on mixing and mass transfer and consequently on the catalytic performance. Supercritical fluids, especially supercritical CO2, have gained considerable attention as environmentally benign solvents (e.g., (94y). The combined use of in situ transmission and ATR-IR spectroscopy together with video monitoring is a promising approach for elucidation of the behavior of a... 

Fig. 35. Effect of phase behavior on palladium-catalyzed oxidation of benzyl alcohol to benzaldehyde in supercritical CO2 characterized by transmission- and ATR-IR spectroscopy combined with video monitoring of the reaction mixture (102). The figure at the top shows the pressure dependence of the reaction rate. Note the strong increase of the oxidation rate between 140 and 150 bar. The in situ ATR spectra (middle) taken at 145 and 150 bar, respectively, indicate that a change from a biphasic (region A) to a monophasic (B) reaction mixture occurred in the catalyst surface region in this pressure range. This change in the phase behavior was corroborated by the simultaneous video monitoring, as shown at the bottom of the figure.

ATR-IR spectroscopy can be used as a spy inside a reactor for on-line monitoring and control of a reaction. The emphasis in this kind of application of ATR spectroscopy is on the detection of reactants and products in the bulk fluid phase. Such applications benefit from the excellent time resolution of FTIR instruments compared to other analytical tools, such as chromatographs. The method can be used in investigations of kinetics of reactions in batch reactors instrumentation has been developed and even commercialized that allows measurements at elevated temperatures and pressures. 

The fact that ATR-IR spectroscopy uses an evanescent field and therefore probes only the volume very close to the IRE has important consequences for its application in heterogeneous catalysis, in investigations of films of powder catalysts. The catalyst particle size and packing affect the size of the detectable signals from the catalyst and bulk phase. Furthermore, if the catalyst layer is much thicker than the penetration depth of the evanescent field, diffusion of reactants and products may influence the observed signals. In fast reactions, gradients may exist within the catalyst layer, and ATR probes only the slice closest to the IRE. 

Weighing the potential and limitations of the technique as elaborated in this article—and furthermore on the basis of the fact that the technique and instrumentation are easily accessible, we predict that ATR-IR spectroscopy will become a powerful and widely used tool for the investigation of heterogeneous catalytic reactions occurring in the presence of liquid-phase reactants. 

ATR spectroscopy in the infrared has been used extensively in protein adsorption studies. Transmission IR spectra of a protein contain a wealth of conformational information. ATR-IR spectroscopy has been used to study protein adsorption from whole, flowing blood ex vivo 164). Fourier transform (FT) infrared spectra (ATR-FTIR) can be collected each 5-10 seconds165), thus making kinetic study of protein adsorption by IR possible 166). Interaction of protein with soft contact lens materials has been studied by ATR-FTIR 167). The ATR-IR method suffers from problems similar to TIRF there is no direct quantitation of the amount of protein adsorbed, although a scheme similar to the one used for intrinsic TIRF has been proposed 168) the depth of penetration is usually much larger than in any other evanescent method, i.e. up to 1000 nm water absorbs strongly in the infrared and can overwhelm the protein signal, even with spectral subtraction applied. 

More recently, Markovich et al. (91) utilized a combination of solid-state infra-red (IR) and NMR methods to study the amorphous to crystalline API transition of SCH 48461 in solid dispersion capsule formulations. In this illustrative study, dissolution testing initially revealed inter-and intralot variations of capsules stored under accelerated stability conditions (25°C/60% RH, 30°C/60% RH, and 40°C/80% RH). PXRD analysis could not explain the dissolution data being collected on lots stored at accelerated conditions and revealed no differences from original diffraction patterns. Two additional analytical techniques, attenuated total reflectance IR (ATR-IR) spectroscopy and solid-state 13C NMR spectroscopy, were employed to study the physical form in the actual solid dispersion formulations. 

Only two spectroscopic studies on sulfur vulcanisation of EPDM by Fujimoto and coworkers are available [73-74], Using attenuated total reflectance (ATR) IR spectroscopy they showed that during sulfur/TMTD/MBT/ZnO/stearic acid vulcanization, the C=C bands at 3035, 966 and 870 cm 1 of the residual unsaturations of the EPDM third monomers, DCPD, 1,4-hexadiene (HD) and 5-methylidene-2-norbornene (MNB), respectively, decreased in intensity as a function of time at 140 and 150 °C. The relative decrease in intensity was shown to correlate with the increase in crosslink density. In Sections 6.2.2.2 and 6.2.2.3 it will be shown that this decrease of intensity should not be interpreted as a loss of unsaturation during sulfur vulcanisation of EPDM. 

Concerning the latter, PEM composed of PEI and PMA-MS were described, which were deposited on silicon substrates and the water uptake from humid air (90% rel. hum.) of the dry sample was measured by ATR-IR spectroscopy. In Fig. 9, band areas of the v(OH) band, which were normal-... 

Bayada, A., Lawrance, G.A., Maeder, M., and Molloy, K.J., ATR-IR spectroscopy for the investigation of solution reaction kinetics — hydrolysis of trimethyl phosphate, Appl. Spectrosc., 1995, 49, 1789-1792. 

The effect of adriamycin, an anti-tumor chemical, on cardiolipin (CL), a phospholipid specific to the inner mitochondrial membrane, was studied by ATR-IR spectroscopy. It was shown that in the very stable complex the structures of both adriamycin and cardiolipin are different from those of the pure substances (Goormaghtigh et al., 1987). 

The only piece of experimental evidence for the orientation of apoA-I helices on DMPC discoidal complexes has come from the use of polarized attenuated total reflection infrared (ATR-IR) spectroscopy (Brasseur et al. (1990 Wald et al., 1990). Although internal reflection spectroscopy is not new (Harrick, 1967), its combination with polarized IR measurements of oriented biological membranes is a more recent application, which allows the spectrum to be taken in the presence of water. The technique has been used most frequently to study the orientation of lipid functional groups relative to the membrane plane in pure lipid bilayers (Fringeli and Gunthard, 1981 Holmgre et al., 1987 Okamura et al., 1990 Hubner and Mantsch, 1991), but has also been applied to studies of... 

Ex situ IR data are collected on dried, diluted powder films in a low vacuum enviromnent or one purged with a dry gas such as N2. Attenuated total reflectance (ATR)-IR spectroscopy provides surface-sensitive IR measurements and can be used for in situ studies of sorption phenomena. Raman spectroscopy is a related vibrational spectroscopy that provides complimentary information to IR. It can also be used to collect vibrational spectra of aqueous samples. Typical data reduction for vibrational spectra involves subtraction of a background spectmm collected under identical conditions from the raw, averaged sample spectrum. Data analysis usually consists of an examination of changes in peak position and shape and peak fitting (Smith, 1996). These and other spectral parameters are tracked as a function of maaoscopic variables such as pH, adsorption density, and ionic strength. 

T. Burgi, ATR-IR Spectroscopy at the Metal-Liquid Interface Influence of Fihn Properties on Anomalous Band-Shape, P/ryi. Chem. Chem. Phys. 3, 2124-2130 (2001). 

McQuillan, A. J. (2001). Probing solid-solution interfacial chemistry with ATR-IR spectroscopy of particle films. Adv. Mater. 13, 1034-1038. 

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