Secondary solvent

The reactions of the radicals (whether primary, secondary, solvent-derived, etc.) with monomer may not be entirely regio- or ehemoseleetive. Reactions, such as head addition, abstraction or aromatic substitution, often compete with tail... 

Reaction detectors are a convenient means of performing online postcolumn derivatization in HPLC. The derivative reaction is performed after the separation of the sample by the column and prior to detection in a continuous reactor. The mobile phase flow is not interrupted during the analysis and reaction, although it may be augmented by the addition of a secondary solvent to aid the reaction or to conform to the requirements of the detector. Reaction detectors are finding increasing application for the analysis of trace components in complex matrices where both high detection sensitivity and selectivity are needed. Many suitable reaction techniques have been published for this purpose [641-650]. 

For polar solutes and solvents, particularly those capable of hydrogen bonding, secondary solvent effects due to the specific nature of solute-solvent interactions may also have to be included in the model, since the ass imption that they are identical in the adsorbed and mobile phases, and therefore self-canceling, is no longer necessarily true. The addition of a secondary solvent term... 

Various additional (secondary solvent cleaning) procedures have been proposed, including vacuum distillation [30], treatment with macroreticular anion-exchange resins [31], and treatment with activated alumina [32,33]. These secondary cleanup operations increase solvent quality, and continue to be studied to better evaluate their process and economic advantages. In the French UP3 Purex plant, vacuum distillation is used to regulate TBP concentration and solvent quality with beneficial effects. The latter include... 

An alternative to adjusting process conditions is to have a secondary solvent precipitation stage. Toluene was found to be the best solvent tried and, again, low levels of trace elements were obtained. Using this method, however, there is a loss of product, but this is mainly high molecular weight species, which are difficult to hydrocrack and are probably responsible for deposition of carbonaceous material on the catalyst. 

Retention on these supports is adaquetely described by the adsorption displacement model. Nevertheless, the adsorption sites are delocalized due to the flexible moiety of the ligand, and secondary solvent effects play a significant role. The cyano phase behaves much like a deactivated silica toward nonpolar and moderately polar solutes and solvents. Cyano propyl columns appear to have basic tendencies in chloroform and acidic tendencies in methyl tertiobutyl ether (MTBE)... 

To determine the effect of mobile-phase composition on the sorption behavior of TGs on reverse-phase columns, two mixtures were employed acetonitrile/ethanol (80 20) and aceto-nitrile/methanol (80 20). A very rapid analysis resulted, with excellent peak shape and adequate resolution, when ethanol was used as the secondary solvent. Substituting an equal amount of methanol for ethanol resulted in increased solute retention, poor detector response, and asymmetrical peaks. Methanol forms a monomolecular layer on octadecyl-derived silica, which may explain the increase in solute retention caused by methanol. Also, the use of methanol would... 

Modified Mobile Phases. In addition to pure supercritical fluids, much research has been performed on the use of modifiers with supercritical fluids. That is, rather than switching to a completely different supercritical fluid for the mobile phase, a small percentage of a secondary solvent can be added to modify the mobile phase while (hopefully) maintaining the mild critical parameters of the primary fluid. 

All experiments using lipid membranes employed equal weight ratios of the phospholipid or oxidized phospholipid, and one of the steroids. For the trough experiments, 2 mg of phospholipid and 2 mg of steroid were dissolved In a total of 5 ml of solvent. Hexane was used as the solvent In most Instances, but sma1 1 quantities of chloroform were necessary as a secondary solvent for complete dissolution of the steroid dlol and trlol species. Approximately 0.1 ml of solution was slowly spread on the aqueous sub-phase (consisting of pure water or 0.1 M KC1) In the trough, and the solvent was allowed to completely evaporate before compression experiments were Initiated. Compression was performed slowly In all cases to allow surface equilibration and each sample solution was Investigated at least four times to ensure reproducibility. 

Solubility can also be enhanced by the presence of other compounds. This phenomenon is caused by one or more compounds acting as solubility enhancers for other compounds present on a surface. This phenomenon is sometimes called the local cosolvent effect. A typical method of enhancing contaminant solubility is through the addition of a small amount of secondary solvent to the SCF cleaning system. Alcohols are commonly used in this manner to increase solubilities of more polar contaminants. However, more subtle local cosolvent effects have been observed. Perhaps a classic example was first reported by Kumik and Reid. In their study, they observed that the solubilities of both naphthalene and benzoic acid in supercritical CO2 were enhanced by 107% and 280%, respectively, when both species were present. It has also been shown that there needs to be enough of a secondary component present in solution about the local contaminant environment to enhance the solubility of another compound, This example demonstrated that an excess of phenanthrene promoted the solubility of anthracene in supercritical COj, but since anthracene was only present in very small quantities, it did not help to enhance the overall solubility of phenanthrene. A... 

The results for the removal of PAHs from the 5 surface substrates are summarized in Table 16. In general, the 23 PAHs listed in the table averaged removal rates around 90% from the smooth surfaces and over 80% for the porous cast magnesium surface. In contrast, supercritical fluid extraction studies using CO2 for the removal of PAHs from soils for environmental applications have shown relatively poor removal efficiencies for many of the compounds listed in the table often requiring the addition of secondary solvents to the C02. However, it appears that from the results on the removal of the PAHs shown in Table 16, surface contamination is... 

A suitable mixture for toluene-soluble materials is toluene containing 5 g PPO/1 or 7 g butyl-PBD/1 0.1 g POPOP or dimethyl-POPOP/1. This mixture cannot be used for measuring the radioactivity in aqueous samples since water is immiscible with toluene. A further solvent (secondary solvent) miscible with both water and primary solvent must be added to this mixture to enable water to be incorporated. This amount of aqueous solution which can be incorporated in a particular mixture without phase separation depends on the identity of the secondary solvent, the relative proportions of primary and secondary solvents and the temperature. 

The secondary solvents used for this purpose are a surface-active agent or emulsifiers. Triton X-100 and Triton N-101 are nonionic surfactants commonly used in laboratory-prepared scintillation cocktails. Anionic surfactants give better sampleholding capacities than the nonionic surfactants for certain types of salt solutions. 

As a general rule, the greater the ratio of secondary to primary solvent, the greater the amount of water which can be incorporated, but the extra secondary solvents and the extra water increase the quenching. 

A secondary solvent such as dioxane to improve solubility of aqueous samples or surfactants such as sodium dodecylbenzenesulfonate as emulsifier. 

Xanthone, flavone and similar compounds. A difference in chloroform specificity from 2-methoxyethanol is again demonstrated In Figure 11. Xanthydrol is eluted before xanthone and flavone with chloroform as the modifier. Perhaps this shows the useful coupling of a proton donor solvent and proton acceptor solute (a large secondary solvent effect in an adsorption system (43)), the Interaction that was hoped for with the selection of chloroform as one of the selectivity triangle modifiers and apparent here because of a less strong adsorption of the xanthydrol on the fully active silica than some of the other basic solutes used in the preliminary studies. 

The chemical description of this interaction is still to be determined. It appears that there exists some threshold solvent power (defined either by the pure carbon dioxide density or the modifier identity and concentration in a modifier/carbon dioxide mixture) at which the solvent can begin to compete successfully with a particular stationary phase for a particular solute. Whether this involves a deactivation of active sites amenable to specific solute adsorption on the silica surface or a secondary solvent effect (43) where the mobile phase interacts with the solute as well as with the adsorption surface is unknown. 

Figure 4.3 Strategies for crystallization directly from solvent (Left), use of a secondary solvent (centre) and in a controlled atmosphere using a secondary solvent (right)...

Equation (8-3) can be corrected for secondary solvent effects by addition of a correction term A,... 

Some interesting secondary solvent effects have been noted for adsorption of the polycyclic aromatic hydrocarbons and certain of their derivatives on alumina 14). The more nearly linear isomers (e.g., anthracene relative to phenanthrene, 2-bromonaphthalene relative to 1-bromonaph-thalene) are preferentially adsorbed from most solvents, owing to the apparent weak localization of these compounds on linear site complexes see Section 11-2B, There is also a tendency for the preferential adsorption of strong solvent molecules on these same linear site complexes, with the result that strong solvents (or their solutions in weaker solvents) behave as selectively stronger solvents toward the preferentially adsorbed linear aromatics, relative to less linear isomers. As a consequence the ratio of values for two such isomers varies sharply with the solvent used, despite the fact that Eq. (8-3) predicts that this ratio should remain constant for all solvents i.e., A. is generally constant for two or more isomers. Jn extreme cases the ratio of K" values for two isomers of this type can be varied by a factor of 10 or more, depending upon the solvent used (see Table 11-4). 

D. Some Practical Aspects of Secondary Solvent Effects... 

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