Retention Mechanism 551 Solvatochromic Analysis

An extensive review of the mechanism of the retention process in RPLC has been published in 1993 in a special issue of the Journal of Chromatography [50].

Quantitative structure-retention relationships have given some insight in the most important properties in retention [51]. For instance, in the case of polyaromatic hydrocarbons a combination of Van der Waals volume and kinetic energy from molecular dynamics studies can be used to predict log k values [51]. Valuable insight in the descriptors relevant for retention can be obtained by socalled solvatochromic analysis. This is based on the linear solvation-energy relationships (LSER) methodology developed by Taft, Kamlet and others. This approach has been applied to chromatographic retention, and can be written as [52, 53]:

log k = log k„ + m(dj - Ss2) V2/100 + s(jim* - jt *)712*

where log k0 is an independent term, m, s, a and b are the coefficients of the regression, V2/100 is the scaled volume of the solute (to bring them to the same order of magnitude as the hydrogen-bonding descriptors a and /?), 8 is the Hildebrand solubility parameter, and jt*, a and /3 are the Kamlet-Taft solvatochromic polarity (ji*), hydrogen bond acidity (a) and hydrogen bond basicity (/3) parameters (see Chapter 18). The subscripts s and m denotes the stationary and mobile phases, respectively, while subscripts 1 and 2 are the solvent and solute, respectively.

With a fixed mobile phase and a fixed stationary phase, Eq. (11) simplifies to:

For the same solute in the same column, but with different mobile phase compositions, Eq. (11) can be rewritten as:

Assuming that there is a linear relationship between <f and ji*, this equation can be further simplified to:

Solvatochromic analysis is a suitable procedure to compare stationary phases to each other [26, 29, 31, 54-51). For example, from these analyses it was inferred that the stationary phase of ODS is considerably more polar than bulk alkane solutions [57]. In some studies, it was concluded that the hydrogen-bonding parameter a is of greater importance for retention behaviour then (i [29], while in others the reverse was found [52, 53], This prompted several groups to explore to possibility to measure hydrogen bond basicity by RPLC [59],

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