Multiple-reflection accessories

Determination of Respirable Quartz by Infrared Spectroscopy with a Multiple Internal Reflectance Accessory... 

Experimental work has shown that the analysis of quartz in respirable dust by Infrared spectroscopy using a Multiple Internal Reflectance Accessory is a viable technique that is sensitive, accurate and simple to perform. Linearity of a calibration curve from 0 to 200 micrograms has been demonstrated. A detection limit of approximately ten micrograms of quartz was obtained. An accuracy of + 35% at a 95% confidence level was demonstrated by data obtained from participation in the NIOSH PAT Program. 

MAIR spectroscopy at IR wavelengths was used to identify the polymers, monitor any induced structural transformations after the various treatments, and verify the freedom of the thin film from residual trapped solvent or adventitious contaminants 1, 2). A model 9 internal reflection accessory (Wilks Scientific Corp.) was used in conjunction with Beckman IR-12 and IR-7 and Perkin-Elmer 21, 257, and 457 IR spectrometers to record reflection spectra of the immediate interface of the sample which was clamped against the multiple internal reflection prisms made from the thallium bromide salt KRS-5. 

Figure 4.21 Schematic ATR sampling accessory. The internal reflection crystal permits multiple reflections. At each reflection a small amount of IR energy penetrates the sample and absorption occurs at the vibrational frequencies for the sample. (Courtesy of Pattacini, Pattacini Associates, LLC, Danbury, CT.)...

Figure 4.6. Schematic arrangement of conventionai diffuse-reflectance accessory for measuring multiple-transmission spectra of microsamples. (1,2) focusing optical system of accessory (3) sample holder (4) plate under study (5) metallic mirrors, (a) Plate with polished surfaces placed between ground metallic surfaces. (6) Plate with ground surfaces placed between polished mirrors. Reprinted, by permission, from V. P. Tolstoy, Methodsof UV-Vis and IR Spectroscopy of Nanolayers, St. Petersburg University Press, 1998, p. 130, Fig. 4.10. Copyright 1998 St. Petersburg University Press.

Figure 3 Photograph of horizontal (overhead) multiple internal reflection accessory. (Reproduced by kind permission of Specac Ltd., Orpington, Kent, UK.)...

More recent multiple internal reflection accessories, such as the FI-ATR (see the section Liquids ), have enabled mid-infrared measurements to be made routinely and practically on a wide range of pastelike and latex-like samples and soft waxes. With dispersions, though, there may be a tendency for the solid phase to migrate preferentially to the surface of the internal reflection element. In some circumstances a capillary layer of the material between a pair of transparent infrared windows may suffice. The solute phase of a latex sample is most frequently presented for study as a thin film deposit on a transparent infrared support after elimination (evaporation) of the solvent (see the section Films cast from solution ). 

Figure 4.21 Schematic ATR sampling accessory. The internal reflection crystal permits multiple reflections.

With most solid materials, sample-crystal contact is a big problem. On the basis that several poor contacts will give the same path length as one good contact, the accessory was changed from a single- to a multiple-reflection unit. There are FMIR crystals in use that have several hundred contact points and which can be used for very thin layers. Unfortunately, these are not commercially available at present. 

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... 

The sample absorbs the evanescent field so that the totally reflected wave is attenuated accordingly the reflectance spectrum closely resembles a transmission spectrum. Usually such spectra can actually be inteipreted and evaluated as transmittance, and commercially available accessories are designed to support this. With angles of reflection well above the critical one, some sensitivity is lost which however is regained by multiple internal reflections. How pronounced the actual spectrum is, can be adjusted by the number of reflections used, i.e. the sample-coated area of the slab whether both surfaces, one surface, or just a fraction. In any case the area of a given reflection should fully be covered to avoid spectral dilution. 

If a microscope facility is not available, there are other special sampling accessories available to allow examination of microgram or microlitre amounts. This is accomplished by using a beam condenser so that as much as possible of the beam passes through the sample. Microcells are available with volumes of around 4 til and pathlengths up to 1 mm. A diamond anvil cell (DAC) uses two diamonds to compress a sample to a thickness suitable for measurement and increase the surface area. This technique can be used at normal atmospheric pressures, but it may also be employed to study samples under high pressures and improve the quality of the spectrum of trace samples. Alternatively, a multiple internal reflectance ceU may also be used as this technique can produce stronger spectra. 

ATP Accessory. Modern FT-IR instruments now offer an attenuated total reflectance (ATR) accessory along with the typical transmittance module. The ATR method provides a powerful sampling technique that virtually eliminates sample preparation with both liquids and solids, thus leading to a dramatic improvement in throughput in the teaching laboratory. Although manufacturers offer multiple crystal options, the diamond ATR offers the best option for maximum durability in... 

Current data suggest that cellulose biosynthesis is a bacterial invention and that eukaryotes acquired the process via multiple lateral gene transfers. Bacteria and eukaryota have independently evolved regulatory mechanisms and molecular structures to utilize the p-1,4-homopolymer synthesized by the catalytic activity of homologous cellulose synthase enzymes. The differences in accessory enzymes probably reflect not only convergent evolution to produce a cellulose I crystalline allomorph, but also inventions of alternative products such as cellulose II, noncrystalline cellulose, or nematic ordered cellulose. 

A Figure 5.34 Attenuated total reflection (ATR) as used as a sampling accessory for FTIR. A series of internal reflections provides multiple absorptive events similar to the multiple absorptions that occur off of different particles in DRIFTS. 

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