Attenuated total reflectance, sample

FIGURE 3 Infrared absorption spectra (obtained using attenuated total reflectance sampling) of lactose monohydrate (solid trace), lactose anhydrate (broken trace), and a 1 1 blend of the monohydrate and anhydrate (dashed trace) (H. G. Brittain, unpublished results). 

The attenuated total reflectance (ATR) technique is used commonly in the near-infrared for obtaining absorption spectra of thin Aims and opaque materials. The sample, of refractive index i, is placed in direct contact with a material which is transparent in the region of interest, such as thallium bromide/thallium iodide (known as KRS-5), silver chloride or germanium, of relatively high refractive index so that Then, as Figure 3.f8... 

DETERMINATION OF NUTRITIONAL PARAMETERS OF YOGHURT SAMPLES THROUGH PARTIAL-LEAST-SQUARES ATTENUATED TOTAL REFLECTANCE FOURIER TRANSFORM INFRARED SPECTROMETRY... 

The aim of this work is the determination of several nutritional parameters, such as Energetic Value, Protein, Fat, and Carbohydrates content, in commercially available yoghurt samples by using Attenuated Total Reflectance Fourier Transform Infrared (ATR-FT-IR) spectrometry and a partial least square approach. 

In order to characterize the surface regions of a sample that has been modified in some way, as is usually the case in adhesion-related investigations, some sort of a reflection experiment is required. Two types of experiments, attenuated total reflection (ATR) and reflection-absorption infrared spectroscopy (RAIR),... 

For on-bead analysis vibrational spectroscopy (IR-spectroscopy) can be employed attenuated total reflection is a method allowing fast and nondestructive on-bead analysis of small samples (single bead analysis) without significant sample preparation. Solid phase NMR is the method of choice if complex structural analysis is intended on the support. Spatially resolved analysis on the resin is possible with microscopic techniques. 

While electron or ion beam techniques can only be applied under ultra-high vacuum, optical techniques have no specific requirements concerning sample environment and are generally easier to use. The surface information which can be obtained is, however, quite different and mostly does not contain direct chemical information. While with infra-red attenuated total reflection spectroscopy (IR-ATR) a deep surface area with a typical depth of some micrometers is investigated, other techniques like phase-measurement interference microscopy (PMIM) have, due to interference effects, a much better surface sensitivity. PMIM is a very quick technique for surface roughness and homogeneity inspection with subnanometer resolution. 

It is generally assumed the fluorescence and Fourier transform mid-infrared (FT-IR) spectroscopies do not suffer from the above-mentioned inconveniences and may be applied to turbid samples. Front-face (fluorescence) and attenuated total reflection (FT-IR) techniques may provide information on the structure of adsorbed proteins. 

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. 

For solid-phase samples or for membrane interaction studies, it can be very useful to use the alternative sampling technique of attenuated total reflectance (ATR). Films or solutions can be placed on a specially... 

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. 

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. 

Attenuated total reflectance infrared (ATR-IR) is used to study films, coatings, threads, powders, interfaces, and solutions. (It also serves as the basis for much of the communication systems based on fiber optics.) ATR occurs when radiation enters from a more-dense material (i.e., a material with a higher refractive index) into a material that is less dense (i.e., with a lower refractive index). The fraction of the incident radiation reflected increases when the angle of incidence increases. The incident radiation is reflected at the interface when the angle of incidence is greater than the critical angle. The radiation penetrates a short depth into the interface before complete reflection occurs. This penetration is called the evanescent wave. Its intensity is reduced by the sample which absorbs. 

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. 

Attenuated total reflectance (ATR) is another recent development in sample handling (Fig. 5.8). In this case the sample may be run in a gel or cream and this method may be used to characterise both formulation matrices and their interactions with the drugs present in them. If the active ingredient is relatively concentrated and if a blank of the matrix is run using the same technique it may be subtracted from the sample to yield a spectrum of the active ingredient. ATR also provides another technique which can be used for the characterisation of polymorphs. 

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. 

Attenuated total reflection (ATR) is the most common reflectance measurement modahty. ATR spectra cannot be compared to absorption spectra. While the same peaks are observed, their relative intensities differ considerably. The absorbances depend on the angle of incidence, not on sample thickness, since the radiation penetrates only a few micrometers into the sample. The major advantage of ATR spectroscopy is ease of use with a wide variety of solid samples. The spectra are readily obtainable with a minimum of preparation Samples are simply pressed against the dense ATR crystal. Plastics, rubbers, packaging materials, pastes, powders, solids, and dosage forms such as tablets can all be handled directly in a similar way. 

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.

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