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Quantitative approach to the sample contact problem in attenuated total reflection spectroscopy: Theoretical considerations
Abstract
The problem of non-ideal contact between an internal reflection element (IRE) and a rough sample under study is considered. The influence of the width of the gap on the intensity of attenuated total reflection (ATR) spectra is quantitatively analysed and equations for the absorbance (A) are derived. It is shown that in many instances surface roughness reduces A as compared with the true (i.e., corresponding to ideal contact between IRE and sample) value (Aid) by a factor of 2–5. Quantitative ATR analysis is evidently impossible under such circumstances without regard to the influence of the air gap. To solve this problem, two approaches are developed. Both methods are based on the measurement of the A values followed by calculation of Aid quantities. The first approach is based on the measurement of spectra with two different polarizations and the second utilizes the ratio of A values obtained for the band under study and for the “internal standard” band. It is shown that relative errors in Aid determination are less than 5% for both methods.
A method for measuring the optical functions of a flat sample made of homogeneous and isotropic material, using attenuated total reflection spectroscopy when there is poor contact between the sample and the internal reflection element is presented. The approach consists in treating the spacing between the internal reflection element and the sample as an adjustable parameter, along with the dispersion model parameters, in the simultaneous fitting of s- and p-polarized spectra obtained when the gap distance is unknown. The method is tested with both synthetic and experimental (polystyrene) spectra. The results demonstrate the method's ability to accurately determine the optical functions even in the presence of a contact problem.
Fluoropolymers have unique properties, which are desired in many applications, e.g. high thermal stability, chemical inertness, low surface tension and low dielectric constants. For some engineering aspects their poor adhesion to other materials is a disadvantage. One possibility to improve adhesion is the surface modification by plasmas. In this work the microwave plasma treatment of a copolymer of tetrafluoroethylene and perfluoroalkoxyvinyl ether (PFA) was investigated.In a slot antenna 2.45 GHz microwave plasma source (SLAN) the plasmas of oxygen, nitrogen, argon and their mixtures are generated. Changes in the ATR-infrared spectra of PFA after different plasma treatments are studied.The ATR-infrared spectra of the plasma modified PFA show a new absorption peak at 1884 cm−1, which is related to the acid fluoride end group −(C=O)−F. This peak is observed for each plasma treatment, but a pure oxygen plasma yields a maximum peak. Different formation mechanisms of the acid fluoride group are discussed.An argon plasma treatment results in a highly branched and crosslinked structure located in a thin surface layer. The addition of 10% oxygen or nitrogen to the argon reduces branching and crosslinking. Pure oxygen plasma treatment causes no crosslinking.
The molecular orientation at surfaces plays a key role in many of the interesting optical properties of organic thin films. Accurate determination of molecular “tilt” angles and the distribution in tilt angles is essential for an improved understanding of performance characteristics, i.e., the structure−function relationship. While there are several spectroscopic techniques available for measurement of molecular orientation at solid surfaces, little effort has been dedicated to understanding the contributions of surface roughness on measured orientation values. In this paper, following an overview of spectroscopic techniques and previous approximations of surface roughness effects, a quantitative theory for treating surface roughness contributions to molecular orientation measurements made by linear dichroism is presented. The theory is sufficiently general that it may be extended to other spectroscopic techniques, such as second-harmonic generation. It is anticipated that knowledge of the molecular orientation with respect to the local surface (i.e., accounting for surface roughness) will provide a better understanding of the orientation in organic thin films.
This article deals with recent developments in the use of mid-infrared (MIR) spectroscopy for the analysis of foods. It has become apparent that MIR can be used to address a wide range of issues and provide solutions for rapid analysis and on-line control. In parallel with the new applications to food, which include new qualitative and quantitative applications and discriminant (classification) methods, there have been several technical advances in other fields that are set to impact on the food sector. New applications of transmission methods are described, which have been particularly successful for the analysis of oils and fats. Despite new advances in other sampling techniques, transmission methods have been quite widely employed. Diffuse reflectance has also been used with some considerable success, with new accessory designs and applications in food authentication using chemometric methods. However, the largest number of new applications and technical developments have used attenuated total reflectance (ATR). Novel ATR cells have been designed for high-temperature, high-pressure and a range of on-line applications. ATR has been used for a wide range of analytical application. The analysis of sugars in various systems has been particularly well studied. The authors predict that the use of MIR spectroscopy is likely to continue to increase and develop in the near future.
The attenuated total reflection (ATR) technique is shown to be promising in studying the molecular structure of polymer surfaces by IR spectroscopy. The problems connected with obtaining quantitative data from ATR spectra of films and fibers are considered. The influence of instrument errors on measurement accuracy is discussed in detail. Orientation of macromolecular chains, conformation structure, intensity of intermolecular interaction, average molecular weight, and other molecular characteristics obtained for surfaces and bulk of polymer films and fibers are presented.
Explicit formulas are derived for the mean-square electric fields induced by plane electromagnetic radiation in a two-phase, three-phase, and N-phase stratified medium. The first (incident) and last phases are semi-infinite in extent. Boundaries separating phases are plane and parallel. Phases are isotropic with arbitrary optical constants. Simple relationships follow for special cases such as at the critical angle for a two-phase system. Equations for reflectance, transmittance, and phase changes on reflectance and transmittance are given. Details are given concerning the energy absorption process, especially in the two- and three-layer cases. Equations for the N-layer case are in terms of characteristic matrices which can be readily programmed for a computer.
A method for quantitatively studying the absorption gradients in films of a thickness of an incident radiation wavelength, by means of attenuated total reflection (ATR) spectroscopy is proposed. The possibilities and limitations of the method are discussed.
A series of polypropylene-copolymer laminates produced by biaxial orientation in the 120–155°C range has been characterized by refractometry and infrared (IR) spectroscopy. The copolymer was a random ethylene-propylene resin and the IR techniques included tilted-film transmission as well as internal reflection, both using polarized radiation. The IR techniques yielded spectra corresponding to the machine, transverse, and through directions. Similarly, the refractometry gave refractive indices of both components of each laminate along all three axes. Results from the three techniques were compared and contrasted and the validity of the calculated internal reflectance spectra discussed. Practical methods to estimate the source and extent of errors in the latter technique from false radiation and contact problems were explored.