Physicochemical parameters relationships with retention

It should be stressed that Equation 12.13 is approximated and it should be used cautiously to calculate physicochemical parameters from the retention data because of the associated error. However, it can be useful to obtain simplified, approximated relationships between retention and field-particle interaction as will be detailed below for the specific FEE techniques. The physicochemical parameters of the analytes can be calculated when Equation 12.11 is used in association with the pertinent X expression. Table 12.1 reports the explicit expressions of X for different subtechniques. Examples of... 

Much effort has been devoted to the development of reliable calculation methods for the prediction of the retention behaviour of analyses with well-known chemical structure and physicochemical parameters. Calculations can facilitate the rapid optimization of the separation process, reducing the number of preliminary experiments required for optimization. It has been earlier recognized that only one physicochemical parameter is not sufficient for the prediction of the retention of analyte in any RP-HPLC system. One of the most popular multivariate models for the calculation of the retention parameters of analyte is the linear solvation energy relationship (LSER) ... 

The chromatographic characteristics of 14 peptides have been determined on a PGC column, using acetonitrile-water mixtures as eluents.The majority of peptides showed irregular retention behavior their retention decreased with increasing concentration of acetonitrile, reached a minimum, and increased again with increasing concentration of acetonitrile. For the elucidation of the relationship between the various physicochemical parameters and retention characteristics, principal component analysis (PCA) was employed. Calculations indicated that... 

Polymers have also been used for the coating of alumina. Thus, maleic acid adsorbed onto the alumina surface was, in situ, polymerized with 1-octadecene and cross-linked with l,4-divinylbenzene. ° It was assumed that the polymer forms a monolayer on the alumina, forming a reversed-phase surface. This assumption was substantiated by results showing that the retention order of model compounds was the same as on an ODS column. The lower separation capacity of the new stationary phase was tentatively explained by the lower surface porosity of alumina. Principal component analysis was employed for the elucidation of the relationship between the retention behavior of non-homologous series of solutes on polybutadiene (PBD)-coated alumina and their physicochemical parameters.Calculations revealed significant relationships... 

I = 21 y = 0.394 r = 0.9476 F = 49.8 where log P is the hydrophobicity, bondrefr is the molecular refractivity, delta is the submolecular polarity parameter, ind indicator variable (0 for heterocyclics and 1 for benzene derivatives). Calculations indicated that PBD-coated alumina behaves as an RP stationary phase, the bulkiness and the polarity of the solute significantly influencing the retention. The separation efficiency of PBD-coated alumina was compared with those of other stationary phases for the analysis of Catharanthus alkaloids. It was established that the pH of the mobile phase, the concentration and type of the organic modifier, and the presence of salt simultaneously influence the retention. In this special case, the efficiency of PBD-coated alumina was inferior to that of ODS. The retention characteristics of polyethylene-coated alumina (PE-Alu) have been studied in detail using various nonionic surfactants as model compounds.It was found that PE-Alu behaves as an RP stationary phase and separates the surfactants according to the character of the hydrophobic moiety. The relationship between the physicochemical descriptors of 25 aromatic solutes and their retention on PE-coated silica (PE-Si) and PE-Alu was elucidated by stepwise regression analysis. 

In spite of very diverse successful practical applications, the mechanism of com-plexation and the relationship between structure and selectivity are still at best only partly solved and remain open for discussion. Thermodynamic studies could supply some valuable information facilitating an understanding of the physicochemical basis of the complexation processes. The GC modified with CyDs is one of a variety of experimental methods employed in the determination of thermodynamic quantities for the formation of CyD inclusion complexes (see Chapters 8-10). The thermodynamic parameters for separation of the enantiomers were determined for various derivatives of CyDs dissolved in various stationary phases [63-65] or as a Uquid derivatized form [66]. Interesting observations were made by Armstrong et al. [66]. The authors postulated two different retention mechanisms. The first involved classical formation of the inclusion complex with high thermodynamic values of AH, AAH, and AAS and a relatively low column capacity and the second loose, probably external, multiple association with the CyD characterized by lower AH, AAH, and AAS values. The thermodynamic parameters of complexation processes obtained from liquid and gas chromatography measurements are collected in Table 5.2. It is clear from those data that for all the compounds presented the complexation processes are enthalpy-driven since in all cases AH is more negative than TAS. 

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