Organic modifier effect solvents

Some advice can be formulated for the choice of organic modifier, (i) Acetonitrile as an aprotic solvent cannot interact with residual silanols, whereas the protic methanol can. Thus, when measuring retention factors, methanol is the cosolvent of choice, as it reduces the secondary interactions between the solutes and the free silanol groups, (ii) For the study of the performance of new stationary phases one should use acetonitrile, as the effects of free silanol groups are fuUy expressed [35]. (iri) Acetonitrile with its better elution capacity can be considered as the best organic modifier for Hpophilicity measurements of highly Hpophihc compounds with adequate stationary phases [36]. 

Note that the interfacing of LC techniques with MS puts significant constraints on the solvents that can be used i.e., they must be volatile, with a low salt concentration, for MS compatibility. Narrow-bore columns, which use much smaller amounts of salt and organic modifier, appear to have potential for facilitating IEC-MS applications.40 Despite the excellent sensitivity of MS detection for most elements, however, there are cases where matrix effects can interfere. In this situation, combination of IEC with atomic emission spectrometry (AES) or atomic absorption spectrometry (AAS) may be preferable, and can also provide better precision.21 32 4142 Other types of... 

Silica stationary phases for exclusion can be used with either organic or aqueous solvents. Some types are bonded phases, others are unmodified. When aqueous phases are used with silica exclusion columns, small amounts of polar mobile phase modifiers (inorganic salts or polar organic solvents) often have to be used to reduce adsorption effects. 

Solvents regularly used in organic reactions are used in heterogeneous catalysis of organic reactions. When solvent information is known, it accompanies other reaction information in each chapter. It must be remembered, however, that the solvent may interact with the catalyst surface and be converted into something undesirable or may combine with or modify one or more of the reactants. The example in Table 1.351 shows the rather minor effect of solvents on the stereochemistry of hydrogenation of the exo double bond in a spatane precursor. 

Organic modifier Solvent strength Solvent strength generally increases with the volume percent of organic modifier. Its effect is most important when hydrophobic mechanisms contribute significantly to retention. In this case, changing... 

Miscible organic solutes modify the solvent properties of the solution to decrease the interfacial tension and give rise to an enhanced solubility of organic chemicals in a phenomenon often called cosolvency . According to theory, a miscible organic chemical such as a short chain alcohol will have the effect of modifying the structure of the water in which it is dissolved. On the macroscopic scale, this will manifest itself as a decrease in the surface tension of the solution [238,246]. 

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