Fully nondestructive techniques

Nondestructive methods such instrumental methods in fact only require simple pretreatment which does not require the extemporaneous physical destruction of the test sample. In fact, some of so-called nondestructive techniques do not leave the sample unaffected. Instrumental Neutron Activation Analysis affects the sample in so far that the elements are radioactively transformed. The sample after analysis becomes radioactive and cannot be considered as unaffected Fully nondestructive techniques are limited to a few number of techniques applied in certain situations they are particularly rare for the quantitative analysis of solid materials H NMR, NIR, Raman, XRF and related techniques, etc. 

As discussed in the previous chapter a preparation procedure leads to several sets of samples, often produced in batches. It is necessary to assess that no difference exists within each of the sets, between sets and between batches. Consequently, homogeneity testing will try to measure differences between sub-samples within or between vials of materials. As it is not possible to measure all samples produced (unless a nondestructive technique is available) a strategy for the selection of representative samples is necessary. To demonstrate the absence or the existence of differences between samples, it is necessary that the analytical procedure is fully reproducible. If differences between measurements are too large due to the measurement method, inhomogeneity cannot be detected. In order to reveal presence of spot contamination, the measurement must be done on the substance of interest or any other substance known to present exactly the same properties and showing the same behaviour or distribution pattern (tracer). 

Fully nondestractive techniques do not touch the membrane surface and may be used during operation. These requirements make it that such techniques are likely limited to in-line transport measurements and optical reflection measurements. The transport measurements are discussed under quasi nondestructive techniques. Spectroscopic ellipsometry provides the thickness and composition for layers with 0p < 50 nm and X < 5 p,m, mutually independent, for the optically smooth supports and membranes. The composition is in that case obtained from interpretation of the refractive index and may include information about 4>p and the amount of adsorbed molecules. The use of this method was demonstrated first in Benes et al. (2001) for the layer thickness and CO2 sorption in supported amorphous silica membranes, as shown in Figure 34.10, left. Spectroscopic ellipsometry is now used routinely in our laboratory for layer thickness of supported (-y-alumina) membranes. This analysis involves determination of the optical constants of uncoated macroporous AKP15 and AKP30 a-Al203 supports, described in Figure 34.11. As the -y-alumina membranes are optically transparent, the dispersion in refractive index n can be described as a Cauchy type material with the form... 

This chapter presents new information about the physical properties of humic acid fractions from the Okefenokee Swamp, Georgia. Specialized techniques of fluorescence depolarization spectroscopy and phase-shift fluorometry allow the nondestructive determination of molar volume and shape in aqueous solutions. The techniques also provide sufficient data to make a reliable estimate of the number of different fluorophores in the molecule their respective excitation and emission spectra, and their phase-resolved emission spectra. These measurements are possible even in instances where two fluorophores have nearly identical emission specta. The general theoretical background of each method is presented first, followed by the specific results of our measurements. Parts of the theoretical treatment of depolarization and phase-shift fluorometry given here are more fully expanded upon in (5,9-ll). Recent work and reviews of these techniques are given by Warner and McGown (72). 

Manufacturers of TLC materials and accessories are well prepared to satisfy the needs for professionally performed PLC. High-quality precoated preparative plates are available from a number of eommercial sources. Alternatively, less expensive or specialty preparative plates ean be homemade in the laboratory, and loose sorbents and coating devices ean be purehased for this purpose. More-or-less-automated devices can also be purehased for band application of higher quantities of sample solutions to preparative layers. At least for some users, sophisticated densitometric and other instrumental techniques are available as nondestructive tools for preliminary detention and identification of separated compounds in order to enhance the effieiency of their isolation. The only aid still missing, and maybe the most important of all, is a comprehensive monograph on PLC that might encourage and instruct many potential users on how to fully benefit from this very versatile, efficient, relatively inexpensive, and rather easy to use isolation and purification technique. This book was planned to fill that void. 

Magnetic Methods. The preceding methods are destructive tests in that the restoration technique permanently alters the speciman. If improper conditions are applied in destructive tests, there is often no second chance to recover the number. Nondestructive methods are therefore especially attractive. Several promising, nondestructive approaches for serial number recovery from ferromagnetic alloys are based on the magnetization behavior of the metal. The potential of this method has been realized (15) but appears not to have been fully exploited. 

A recording technique of holograms and the nondestructive readout in a photorefractive polymer utilizes two-photon absorption. The holograms are formed through the photorefractive effect. The technique uses the excitation of the electroactive chromophore with femtosecond pulses, followed by charge injection into a PVK matrix. The holograms can be fully erased with a pulsed laser beam. However, they are insensitive to continuous-wave laser beams with the same wavelength [184]. 

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