Physical fractionations

Isomer separation beyond physical fractional crystallization has been accompHshed by derivatization using methyl formate to make /V-formyl derivatives and acetic anhydride to prepare the corresponding acetamides (1). Alkaline hydrolysis regenerates the analytically pure amine configurational isomers. 

While it is expected that the source rocks for the radionuclides of interest in many environments were deposited more than a million years ago and that the isotopes of uranium would be in a state of radioactive equilibrium, physical fractionation of " U from U during water-rock interaction results in disequilibrium conditions in the fluid phase. This is a result of (1) preferential leaching of " U from damaged sites of the crystal lattice upon alpha decay of U, (2) oxidation of insoluble tetravalent " U to soluble hexavalent " U during alpha decay, and (3) alpha recoil of " Th (and its daughter " U) into the solute phase. If initial ( " U/ U).4 in the waters can be reasonably estimated a priori, the following relationship can be used to establish the time T since deposition,... 

Using the newer methods, such as gas chromatography, liquid-liquid chromatography, fluorometry, and mass spectrometry, it is possible to measure many compounds at the parts-per-billion level, and a few selected compounds with special characteristics at the parts-per-trillion level. Even with these sensitivities, however, a considerable concentration must usually be undertaken to permit the chemical or physical fractionation necessary to render the final analyses interpretable. A major effort has therefore been expended on the study of methods of separation and concentration, and this is discussed further in Chap. 8. 

Although the investigations of both Raunkjaer et al. (1995) and Almeida (1999) showed that removal of COD — measured as a dissolved fraction — took place in aerobic sewers, a total COD removal was more difficult to identify. From a process point of view, it is clear that total COD is a parameter with fundamental limitations, because it does not reflect the transformation of dissolved organic fractions of substrates into particulate biomass. The dissolved organic fractions (i.e., VFAs and part of the carbohydrates and proteins) are, from an analytical point of view and under aerobic conditions, considered to be useful indicators of microbial activity and substrate removal in a sewer. The kinetics of the removal or transformations of these components can, however, not clearly be expressed. Removal of dissolved carbohydrates can be empirically described in terms of 1 -order kinetics, but a conceptual formulation of a theory of the microbial activity in a sewer in this way is not possible. The conclusion is that theoretical limitations and methodological problems are major obstacles for characterization of microbial processes in sewers based on bulk parameters like COD, even when these parameters are determined as specific chemical or physical fractions. 

Physical fractionation, of oils, 10 813-814 Physical materials standards, 15 742 Physical metallurgy, 16 127 Physical models, for process control, 20 687 Physical netpoints, in shape-memory polymers, 22 356, 358... 

Physical processes that separate one type of material from another can also produce chemical and isotopic fractionations. Several important chemical properties of chondrites probably reflect some sort of physical fractionation process. 

Physical fractionation of silicate components (refractory CAIs and an olivine-rich component, perhaps chondrules) are suggested by trends in Si/Al and Mg/Al in bulk chondrites. 

Planetary differentiation is a fractionation event of the first order, and it involves both chemical fractionation and physical fractionation processes. Planetary crusts are enriched in elements that occur in silicate minerals that melt at relatively low temperatures. Recall from Chapter 4 that the high solar system abundances of magnesium, silicon, and iron mean that the silicate portions of planetesimals and planets will be dominated by olivine and pyroxenes. Partial melting of sources dominated by olivine and pyroxene ( ultramafic rocks ) produces basaltic liquids that ascend buoyantly and erupt on the surface. It is thus no surprise that most crusts are made of basalts. Remelting of basaltic crust produces magmas richer in silica, eventually resulting in granites, as on the Earth. 

The formation of the Moon s crust, composed primarily of feldspar (the rock is called anorthosite) illustrates how physical fractionation can occur during differentiation. Early in its history, a significant portion of the Moon was melted to form a magma ocean. The first minerals to crystallize, olivine and pyroxene, sank because of their high densities and formed an ultramafic mantle. Once feldspar began to crystallize, it floated and accumulated near the surface to produce the crust. 

The 14C content of SOM decreases with depth in the soil profile (Martin et al., 1990). An estimate of the passive pool could be obtained by measurements of the 14C of SOM in deeper layers (Harrison and Broecker, 1993). Physical fractionation and sequential extraction have also been used, and they have shown progressively lower 14C/12C ratios in decomposing residues. 

Christensen, B. T. (1992). Physical fractionation of soil and organic matter in primary particle size and density separates. Adv. Soil Sci. 20,1-90. 

Table 2.4 Element speciation by analysis of different physical fractions of a soil or sediment using a sensitive analytical technique such as AAS or neutron activation analysis...

Quideau and co-workers have compared the soil carbon cycling dynamics of the oak and pine lysimeters using physical fractionation (Quideau et al., 1998), radiocarbon (Quideau et al., 2001),... 

Christensen B. T. (2001) Physical fractionation of soil and structural and functional complexity in organic matter turnover. Euro. J. Soil Sci. 52(3), 345-353. 

Thus, bitumen from a specific deposit is not a uniform material. The chemical and physical (fractional) composition can vary not only with the location and age of the deposit but also with the depth. 

Most concentration and isolation techniques, except for evaporative and freeze-concentration techniques, are also the first steps in chemicfd and physical fractionation of aquatic humic substances. This chapter will concentrate primarily on techniques used to subfractionate and chromatographi-cally separate aquatic humic substances previously isolated as crude fractions. 

The purposes of this chapter are (1) to present an overview of fractionation methods that have been successfully applied to aquatic humic substances (2) to examine chemical and physical fractionation mechanisms in the light of what is known about aquatic humic substance properties and structure (3) to postulate new fractionation approaches that hopefully will result in more homogeneous fractions and, ultimately, pure compounds that comprise aquatic humic substances. 

Ultrafiltration, gel permeation chromatography, and ultracentrifugation are also discussed by Swift in Chapter 15 of this book, and most of the physical fractionation procedures applied to soil humic substances can also be applied to aquatic humic substances. Dissolved aquatic humic substances are already dissolved in water in contrast to soil systems, and physical fractionations can be performed with or without preconcentration. If possible, these physical fractionation procedures need to be carried out at ambient concentrations to avoid intermolecular aggregation after concentration. 

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