Multistage sampling techniques

Zone refining, as outlined above, is a multistage batch technique in which successive operations are performed on a single batch of sample. The main limitation of such a procedure is the decrease in efficiency of zone passes as their number increases. Thus, ultimate purity is obtainable only at a prohibitive cost of time and money. 

The main limitation of the technique is the problem of achieving radiochemical, as distinct from chemical, purification of the final labeled product. Purification of the irradiated sample is necessary to remove radiation-induced degradation products and labile tritium. Many of the degradation products are not only chemically similar to the parent compound, but also possess much higher specific activities. For complete purification it is necessary to use multistage processes, such as gas and column chromatography, countercurrent distribution, and fractional distillation. Distribution of isotope within a molecule is generally random and nonuniform however, in some circumstances useful specificity can be achieved.17... 

See also Air Analysis Sampling. Extraction Solvent Extraction Multistage Countercurrent Distribution Supercritical Fluid Extraction. Gas Chromatography Multidimensional Techniques. Mass Spectrometry Time-of-Flight. Quality Assurance Quality Control Reference Materials. Water Analysis Overview. 

To circumvent the various limitations of the basic technique, several continuous zone refining methods have been developed in which feed enters at one point in the sample while the product and waste leave at other points (Figure 4). The effect of countercurrent movement of solid and liquid phases is achieved by the movement of molten zones. In addition to the horizontal continuous refiner, the zone void vertical refiner and zone transport refiner are other modifications of this class. Cross-flow zone refiners and rotating drum multistage crystallizers based on the above principle are mainly used in growing single crystals rather than in purification of materials. 

There have been many efforts to synthesize size selected ZnS with a very narrow size distribution, however only a few were successfiil. Most of the techniques follow a capping route and a wet chemical synthesis with solvents like mercaptoethanol, ethanol, methanol, acetonitril, dimethylformamide (DMF) etc. Verities of physical and chemical techniques are used for the synthesis of nanoparticle polymer composite, so that the film can be cast directly, thus avoiding the loading of the particles via multistage processes. In present studies the nanociystaline ZnS samples in powder form and in nanocomposite form were prepared by chemical technique. 

Typically, two approaches are taken to measure the presence and concentration of odorants on textiles direct analysis of a textile substrate and analysis of the headspace above a sample. Direct analysis of the textile substrate usually involves a multistage process of extraction and clean-up before sample analysis using gas chromatography-mass spectrometry analytical techniques. Coupling gas chromatography techniques with olfactometry (GC-0) is a useful way to determine odor-active compounds in a substance. GC-0 has been applied successfully for many years, albeit the still relies on a human as the odor-detecting instrument. 

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