Retention polar contribution

Rohrschneider further assumed that the polar contribution to the retention index can be described by a simple linear equation, involving a series of constants which are solely dependent on the solute (a to e) and a series of constants which depend solely on the stationary phase (x, y, z, u and s). The following equation may now be applied ... 

W-C4H9) [106, 107]. Ab initio MO calculations have shown that the [2 - - 2] cycloaddition between cis- or Jraw-alkenes and isocyanates to yield y9-lactams takes place by a concerted mechanism in the gas phase, with retention of the alkene configuration in the product. However, increasing solvent polarity contributes to the asynchronicity of the reaction to such an extent that the mechanism changes from a concerted to a two-step process involving a zwitterionic intermediate, with a consequent loss of alkene stereospecificity, as observed in the reaction between chlorosulfonyl isocyanates and vinyl ethers [794]. 

Although the exact mechanism is not very clear, the following factors may contribute to the modifier effect on chromatographic retention. Polar modifiers may cover the active sites of the stationary phase (deactivation) so that solute retention is reduced. This can be explained by the differences in retention change between packed and open-tubular columns when small amounts of modifiers were used. Open-tubular columns normally do not show the drastic changes in retention or efficiency upon the addition of small amounts (<2%) of modifier as most packed columns do. These less drastic differences were caused by the differences in the degree of deactivation of the packed column stationary phase as compared with the open-tubular-column stationary phase. An open-tubular column has fewer active sites present and, thus, fewer active sites are present for the modifier to deactivate. 

Uptake of water and polar solvents by the column From mixed mobile phases, polar solvent(s) are preferentially adsorbed on the surface of polar adsorbents, sometimes giving rise to multilayer solvent adsorption on the adsorbent support. In such a case, the retention is contributed to by a liquid-liquid partition mechanism between the adsorbed liquid layer and the bulk mobile phase, in addition to the adsorption. Such a mixed-mode mechanism can be intentionally utilized for separation of strongly polar or even ionic compounds. 

Although the exact mechanism is not very clear, the following factors may contribute to the modifier effect on chromatographic retention. Polar modifiers may cover the... 

Interactive LC systems are those where solute retention is predominantly controlled by the relative strengths of the molecular interactions between solute molecules with those of the two phases. In such systems, exclusion and entropically driven interactions will be minor contributions to retention. The three basically different types of molecular interaction, dispersive, polar and ionic give rise to three subgroups, each subgroup representing a separation where one specific type of interaction dominates in the stationary phase and thus governs solute retention. The subgroups are as follows ... 

The mobile phase was an aqueous solution containing 50 mM sodium phosphate and 150 mM sodium chloride at a pH 7.0. Although ionic interactions are likely to constitute the major contribution to retention and selectivity, there would also be significant polar interactions and some dispersive interactions between the aromatic nuclei of the solutes and the aromatic nuclei and the aliphatic side chains of resin respectively. Under these circumstances, without considerable experimental work, it is impossible to identify the relative magnitude of the different contributions from each type of interaction. 

There are surprisingly few studies of the retention mechanism for open tubular columns but the theory presented for packed columns should be equally applicable. For normal film thicknesses open tubular columns have a large surface area/volume ratio and the contribution of interfacial adsorption to retention should be significant for those solutes that exhibit adsorption tendencies. Interfacial adsorption has been shown to affect the reproducibility of retention for columns prepared with nonpolar phases of different film thicknesses [322-324]. The poor reproducibility of retention index values for columns prepared from polar phases was demonstrated to be c(ue to interfacial... 

Variations in retention and selectivity have been studied in cyano, phenyl, and octyl reversed bonded phase HPLC columns. The retention of toluene, phenol, aniline, and nitrobenzene in these columns has been measured using binary mixtures of water and methanol, acetonitrile, or tetrahydrofuran mobile phases in order to determine the relative contributions of proton donor-proton acceptor and dipole-dipole interactions in the retention process. Retention and selectivity in these columns were correlated with polar group selectivities of mobile-phase organic modifiers and the polarity of the bonded stationary phases. In spite of the prominent role of bonded phase volume and residual silanols in the retention process, each column exhibited some unique selectivities when used with different organic modifiers [84],... 

---