Attenuated total reflection spectroscopy sample cells

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

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. 

An infrared spectrum is a plot of percent radiation absorbed versus the frequency of the incident radiation given in wavenumbers (cm ) or in wave length ( xm). A variation of this method, diffuse reflectance spectroscopy, is used for samples with poor transmittance, e.g. cubic hematite crystals. Increased resolution and sensitivity as well as more rapid collection of data is provided by Fourier-transform-IR (FTIR), which averages a large number of spectra. Another IR technique makes use of attenuated total reflectance FTIR (ATR-FTIR) often using a cylindrical internal reflectance cell (CIR) (e.g. Tejedor-Tejedor Anderson, 1986). ATR enables wet systems and adsorbing species to be studied in situ. 

Numerous books cover the topic of sampling methods in infrared spectroscopy (see, e.g., references [10-12]), and a detailed description of all the various alternatives is beyond the scope of this chapter. Instead, we will focus on the two sampling methods that are most commonly employed in food analysis applications, namely, the use of transmission cells for recording the spectra of solutions and the total internal reflection technique, also known as attenuated total reflectance (ATR). Readers who wish to learn about the techniques not covered here may consult the references cited above. 

First, we will examine the various ways of examining samples using the traditional transmission methods of infrared spectroscopy. In the second part of this ch ter we will examine the more modem reflectance methods that arh now available, such as the attenuated total reflectance, diffuse and specular reflectance methods. We ml also look at a number of more specialist techniques which you might encounter, such as photoacoustic spectroscopy, gas chromatography-infrared spectroscopy, and the use of temperature and raicrosampling cells. 

A cell for characterising the diffusion of small molecules through thin polymer films using attenuated total reflectance (ATR) FTIR spectroscopy was described. The cell was designed to be used with precast (commercially extruded) polymer films, thus enabling the as-processed transport properties of the film to be studied. The cell was used to measure the diffusion of carbon dioxide, amyl acetate and limonene, and simultaneous diffusion of the individual components from a 50/50 mixture of amyl acetate and limonene through the thin polymer films (HDPE, LDPE and PS). Diffusion coefficients measured with the ATR technique compared favourably with values obtained from gravimetric measurements with the same penetrants and polymer samples. 20 refs. 

One of the most common uses of IR spectroscopy in the analysis of fatty acids and fats is the determination of fmn -unsaturation. An interesting procedure for tran5-unsaturation using IR attenuated total reflectance (ATR) spectroscopy has been described (Sipos and Ackman, 1978). The samples do not have to be weighed or diluted. A special ATR cell is used to make readings at 10.3 / m. The system can be used to monitor a continuous flow system, and the presence of catalyst does not affect the readings. 

The methods of presenting samples such as a tissue or isolated single cell for study in an FT-IR microscope have to date been predominantly confined to transmission and, the so-called, transflection sampling techniques. The latter is actually a reflection-absorption technique vide infra). Of increasing recent interest is use of the so-called ATR sampling technique for the analysis of tissue samples. ATR is an abbreviation for attenuated total reflection and is an internal reflection spectroscopy technique. On the horizon are perhaps nearfield techniques. Each of these will now be considered in turn. 

Although IR spectroscopy is most useful for qualitative analysis, many quantitative tests have been developed to satisfy special needs. Fuller and coworkers describe a technique suitable for quality control analysis of formulated liquid detergents by FTIR using a trough liquid attenuated total reflectance cell (7-9). The method has the capability of simultaneously measuring the concentration of the surfactant and of several solvents and adjuvants. ATR spectroscopy, with FT or dispersive instrumentation, allows direct, qualitative analysis of surfactant-containing samples, such as toothpaste, bar soap, and shampoos, and minimizes interference from water (10). Reflectance IR spectroscopy has even... 

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