Reflectance methods

Reflectance techniques can be used for samples which are difficult to analyse by normal transmittance methods. Reflectance methods can be divided into two categories internal reflectance measurements can be made by using an attenuated total reflectance (ATR) ceil in contact with the sample, while there are also external reflectance measurements which involve an infrared beam being reflected directly from the sample surface. 

Rather than measure the light transmitted through a sample, as in all of the IR sampling methods discussed thus far, some methods measure the light reflected from a sample. There are various methods employed to measure this reflectance, depending on the properties of the sample. 

In this method, the sample (such as a polymer film) is pressed against a transparent material having a high refractive index (called the internal reflection element (IRE)). The IR light beam passes through this material, rather than air, before and after reflecting from the sample, hence the reason for describing 

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Ruck-strahler. m. reflector, -strahlung, /. reflection. -strahltmgsmesser, m. reflectome-ter. -strahlverfahren, n. reflection method, -streuung, /. scattering (of rays). -Strom, m. back flow (Elec.) reverse current, return current, -strfimung, /. backflow, reflux. -titrieren, n. back titration, rlicktreiben, v.t. drive back repel. 

Phosphoric acid ester was used as a model for the estimation of concentration of a reagent in an adsorbed layer by optical measurements of the intensity of a beam reflecting externally from the liquid-liquid interface. The refractive index of an adsorbed layer between water and organic solution phases was measured through an external reflection method with a polarized incident laser beam to estimate the concentration of a surfactant at the interface. Variation of the interfacial concentration with the bulk concentration estimated on phosphoric acid ester in heptane and water system from the optical method agreed with the results determined from the interfacial tension measurements... 

Since solid-state reactions can easily be monitored by continuous measurement of spectra, it is easy to study the mechanism of the reactions. For this purpose, IR spectroscopy is the most useful, because IR spectra can be measured simply as Nujol mulls or directly for any mixture of solid-solid, solid-liquid, or liquid-liquid by using the ATP (attenuated total reflection) method. Some such examples of the mechanistic study are described. 

Afanasyeva and Bruch (1999) have applied the FTIP reflection method to investigate the surfaces of materials used for intraocular lenses (IOLs) in the production of implants that are non-toxic to the eye. In particular they studied passivated lens surfaces, which is a treatment which impedes the growth of cells upon the lens surface. Lenses made of PMMA and of sapphire were examined, and Figure 3.5 shows a typical IR spectrum in the 3000-2800 cm-1 wavenumber region. 

Several means are employed to detect X-rays with ion chambers and scintillation detectors sufficient for use in the adsorption and reflection methods where only the intensity of the X-ray beam is of interest. However,... 

The in-sifu FTIR approach can be divided into the transmittance, external reflectance and internal reflectance methods. 

The authors wanted to select indicators that specifically tap melancholic depression. To evaluate this construct, a principal components analysis of the joint pool of K-SADS and BDI items was performed. Two independent statistical tests suggested a two-component solution, but the resulting components appeared to reflect method factors, rather than substantive factors. Specifically, all of the BDI items loaded on the first component (except for three items that did not load on either component) and nearly all of the K-SADS items loaded on the second component. In fact, the first component correlated. 98 with the BDI and the second component correlated. 93 with the K-SADS. Ambrosini et al., however, concluded that the first component reflected depression severity and the second component reflected melancholic depression. This interpretation was somewhat at odds with the data. Specifically, the second component included some K-SADS items that did not tap symptoms of melancholia (e.g., irritability and anger) and did not include some BDI items that measure symptoms of melancholia (e.g., loss of appetite). 

Figure 23. A total reflection method for measurements of Raman spectra of thin LB films.

Nonresonance Raman spectra of the alternating LB films were measured by a total reflection method shown in Figure 23. The films were deposited on quartz prisms. The s-polarized beam of 647.1 nm from a Kr laser was incident upon the interface between the quartz and film at an angle of 45° from the quarz side, and totally reflected. Raman line scattered from the film in the direction of 45° from the surface was measured through a Spex Triplemate by a Photometries PM512 CCD detector with 512x512 pixels operated at -125 °C. The spectral resolution was about 5 cm 1. 

Raman spectroscopy, because of its versatility and wide applicability, has been used for a wide range of art historical and conservation science (Edwards 2000) and archaeological applications (Smith and Clark 2004). Fourier transform Raman spectroscopy (FTRS) in particular has the advantage of being a reflective method which allows direct, nondestructive analysis. It can also be used through a microscope to allow the characterization of small samples. 

What advantage do the KBr pellet method and the reflectance methods have over the solution and mineral oil mull methods for solids ... 

Name three reflectance methods for solid sampling and tell what the differences are. 

Briefly describe the diffuse reflectance method for solids. 

The diffuse reflectance method is a method for solids in which the powdered solid, held in a cup, is irradiated with the IR beam. The scattered reflected light is captured by the detector and the spectrum displayed. 

Figure 4.9 The Laue back-reflection method. The direction of the incident beam and the reciprocal lattice are fixed in space, and wavelengths are selected out of the beam. The Ewald sphere may be any diameter between the short and long wavelength cutoffs. The larger circle shows an intermediate wavelength and the diffracted beams that result at this wavelength...

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). 

Figure 3.1 Schematic representation of transmission and reflectance methods.

Schneider et al. have reported a cell for HP IR spectroscopy which combines both transmission and reflectance methods for in situ investigations of multiphase reactions [47]. The upper part of the cell can be monitored by variable pathlength... 

Figure 9. Average elemental C concentrations by the reflectance method in the Los Angeles Basin 1958-1972, in y-gm ((Q) air monitoring station). Error bounds represent a 95% confidence interval on the long-term means.

V.S. Ilyukhin et al, "Measurement of Adiabatic Shock Waves of Cast TNT, Crystalline Hexogen and Nitromethane , Dokl-AkadN 131, 793-96(1960) CA 55, 24013 (1961) Engl Transln in SovietPhysics Doklady, No 5, 337-40(1960). The reflection method was used, with 5-mm Cu plates... 

The thickness of the compression zone is about 10" cm at atmospheric pressure thus the time for compression is about lO" sec. Since the wave length of visible light is of this order of magnitude, the optical reflectivity method was developed. Experiments were carried out in which the angle of incidence on the shock front was kept constant, and the angle of observation varied. In the case of simple air shock... 

More detailed description of the refraction method is given on pp 353—54 and of reflection method on p 354... 

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