Volatilization separations based

Separations based upon differences in the physical properties of the components. When procedures (1) or (2) are unsatisfactory for the separation of a mixture of organic compounds, purely physical methods may be employed. Thus a mixture of volatile liquids may be fractionally distilled (compare Sections 11,15 and 11,17) the degree of separation may be determined by the range of boiling points and/or the refractive indices and densities of the different fractions that are collected. A mixture of non-volatile sohds may frequently be separated by making use of the differences in solubilities in inert solvents the separation is usually controlled by m.p. determinations. Sometimes one of the components of the mixture is volatile and can be separated by sublimation (see Section 11,45). 

Examples of the formation of volatile metal compounds or complexes leading to separations based on gas-liquid and gas-solid distribution are much rarer. 

We have seen that elements can be separated based on their volatility, either through gas-solid or gas-liquid reactions. There are many types of reactions that form a continuum between equilibrium condensation (or its inverse, evaporation) on the one hand and purely kinetically controlled reactions, such as Rayleigh distillation, on the other. In some cases, isotopic fractionation can assist in identifying the processes involved. 

For highly volatile aromas of fruits or wine, the single- or double step separation based on pressure is not sufficient, and needs expensive precipitation at very low temperatures. About minus 50°C are necessary for sufficient recovery of aroma components. For this application an aroma rectification, with multiple withdrawal at different temperatures, is appropriate. 

Solvation in supercritical fluids depends on the interactions between the solute molecules and die supercritical fluid medium. For example, in pure supercritical fluids, solute solubility depends upon density (1-3). Moreover, because the density of supercritical fluids may be increased significantly by small pressure increases, one may employ pressure to control solubility. Thus, this density-dependent solubility enhancement may be used to effect separations based on differences in solute volatilities (4,5). Enhancements in both solute solubility and separation selectivity have also been realized by addition of cosolvents (sometimes called entrainers or modifiers) (6-9). From these studies, it is thought that the solubility enhancements are due to the increased local density of the solvent mixtures, as well as specific interactions (e.g., hydrogen bonding) between the solute and the cosolvent (10). 

SEPARATIONS BASED UPON DIFFERENCES IN THE VOLATILITIES OF THE COMPONENTS IN AQUEOUS SOLUTION... 

Irvine, G. B. and Shaw, C. (1986) Highperformance gel permeation chromatography of proteins and peptides on columns of TSKG2000SW and TSKG3000SW a volatile solvent giving separation based on charge and size of polypeptides. Anal. Biochem. 155, 141-148. 

As seen in Table II, the four asphaltene-acid fractions exhibit different volatilities. However, the MS data in Tables V-VII and the IR data reveal no significant differences in the group types present in the volatiles from these four fractions. Consequently, the production of four acid fractions represents the use of an inadequate quantity of A-29 resin in the asphaltene separation rather than a separation based upon compound type. Therefore, the individual asphaltene-acid fractions could have been combined prior to analysis. 

An evaporator provides one stage of separation based on relative volatility. It is typically used with systems having a large relative volatility, such as salts and solvents. When water is being removed as an overhead vapor, multieffect operation may be used to provide improved energy efficiency. Figure 3.14 shows two alternative direct material balance evaporator control schemes. 

In simple distillation, a feed is separated into a distillate and bottoms product with multiple stages of separation based on relative volatility. Both distillate and bottom composition may be... 

A large number of methods for the separation of triethylaluminum and the product n-a-olefins have been investigated. Room or low temperature separations based on solvent extraction, absorption, or crystallization have been uniformly unsuccessful. Therefore it seemed that the only hope for success lay in devising some scheme employing volatility differences in order to effect a separation, or partial separation, of triethylaluminum from the product a-olefin. 

Sample dissolution can be performed with liquid or solid reagents at elevated temperatures causing a chemical reaction or melting of the mixture. The dissolution aids may introduce considerable contamination, as they are present in large excess. In addition, the vessels and also the laboratory atmosphere may constitute a source of contamination, whereas analyte losses particularly due to volatilization or adsorption onto the vessel walls may occur. Dissolution procedures for many types of materials can be found in the literature. They are used together with methodology for trace matrix separations based on classical chemical principles. 

Distillation is a method of separation based on the difference in composition between a liquid mixture and the vapor formed from it. The composition difference is due to differing effective vapor pressures, or volatilities, of the components of the liquid. When such a difference does not exist, as at an azeotropic point, separation by distillation is not possible. The most elementaiy form of the method is simpledistillation in which the liquid mixture is brought to boiling and the vapor formed is separated and condensed to form a product if the process is continuous, it is called flash distillation or an equilibrium flash, and if the feed mixture is available as an isolated batch of maleiial. the process is a form of batch distillation and the compositions of the collected vapor and residua] liquid are thus time dependent. 

Most of the present-day techniques allow for assessment of As species in aqueous samples, without the requirement of any preconcentration or pretreatment. This is particularly true when coupled to hydride generation, which has shown to significantly increase the sensitivity of the analytical method, due to high selectivity of hydride formation and separation of the analytical compound from the matrix. This method is based on the very first report of As species separation, based on the ability of As species to form volatile hydrides and on their differences in boiling points (Figure 20.3)." ... 

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