Physicochemical parameters, retention

Discussed below is the impact of various other physicochemical parameters excluding solubility (see Section 15.8.4) on retention in HPLC. 

As the logarithm of 1-octanol-water partition coefficient (log P) describes the hydrophobicity of molecules and the retention of solutes in RP-HPLC depends on the hydrophobicity, a strong correlation can be expected between the log V value and the retention of solutes in RP-HPLC. Besides log P, a considerable number of physicochemical parameters have been tested for their capacity to predict retention in RP-HPLC. Thus, Snyder s polarity index, fraction of positively and negatively charged surface area, molecular bulkiness, nonpolar surface area, electron donor and acceptor capacity, various ster-ical parameters, and the energy of highest occupied molecular orbit have all been included in QSRR calculations. 

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) ... 

T. Cserhati, E. Forgacs and S. Balogh, Relationship between the reversed-phase retention of monotetrazolium and ditetrazolium salts and their physicochemical parameters. J. Planar Chromatogr.-Mod. TLC, 12 (1999) 446 451. 

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... 

The classical FEE retention equation (see Equation 12.11) does not apply to ThEEE since relevant physicochemical parameters—affecting both flow profile and analyte concentration distribution in the channel cross section—are temperature dependent and thus not constant in the channel cross-sectional area. Inside the channel, the flow of solvent carrier follows a distorted, parabolic flow profile because of the changing values of the carrier properties along the channel thickness (density, viscosity, and thermal conductivity). Under these conditions, the concentration profile differs from the exponential profile since the velocity profile is strongly distorted with respect to the parabolic profile. By taking into account these effects, the ThEEE retention equation (see Equation 12.11) becomes ... 

The choice of the FFF technique dictates which physicochemical parameters of the analyte govern its retention in the channel FIFFF separates solely by size, SdFFF by both size and density, ThFFF by size and chanical composition, and EIFFF by mass and charge. The dependence of retention on factors other than size can be advantageous in some applications, and different information can be obtained by employing different techniques in combination or in sequence. On the other hand, the properties that can be characterized by FFF include analyte mass, density, volume, diffusion coefficient, charge, electrophoretic mobility, p/ (isoelectric point), molecular weight, and particle diameter. 

Quinones et al. (2000) reported the successful use of neural networks to predict the half-life of a series of 30 antihistamines. The input for the network was derived from the output of CODES, a routine that generates descriptors for a structure based on atom nature, bonding, and connectivity. Attempts to correlate the half-life with the physicochemical parameters log Kow, pKa, molecular weight, molar refractivity, molar volume, parachor, and polarity were unsuccessful. In a subsequent study by Quinones-Torrelo et al. (2001), the authors correlated the half-life of 18 antihistamines with their retention in a biopartitioning micellar chromatography system with a resultant correlation coefficient (R2adj) value of 0.89. The correlation is explained in that the retention in this system is dependent on hydrophobic, electronic, and steric properties, which are also important in determining half-life. 

The simple linear form of Eq. 9.30 (corresponding to the linear region on the left-hand side of Figure 9.11) is convenient for relating observed retention to the physicochemical parameters making up A. 

Structure-based prediction software predicts retention times or important physicochemical processes based on chemical structures. Application databases store chromatographic methods for later retrieval and adaptation to new samples with similar structures and physicochemical parameters. 

A second approach is based on physicochemical parameters, used in ACD/LC Simulator. The prediction of RP retention times based on physicochemical parameters assumes that the primary retention mechanism is hydrophobicity of the compound as a function of its ionic form at a given pH. The general approach is given as... 

M. McBrien, E. Kolovanov, and R. Taylor, Highly accurate prediction of retention times in standard gradient chromatographic systems based on physicochemical parameters, March 9-14, 2003, Pittsburgh conference, Orlando, FL. 

The empirical physicochemical parameters have a good informative value for determining the mechanism of retention operating in a given chromatographic sy.stem. There are exhaustive compilations of such parameters like >/-octanol-water partition coefficients [45,46] or the LSER-based analyte parameters [47,48]. The problem is. however, that there is a lack of such descriptors for many analytes of interest in actual QSRR studies. 

Although the relationship between ordinary difiusion and molecular weight is well understood, we are still struggling to understand how composition affects thermal diffusion, and therefore ThFFF retention. Although we can relate retention to the phenomenological coefficient for thermal diffusion (Dt), Dt has not been successfully related to physicochemical parameters of the polymer and solvent. As a result, retention cannot be related to polymer composition unless Dt is first determined empirically. 

These methods allow not only the classification and clustering of any set of chromatographic systems but also exact determination of the relationship between the characteristics (physicochemical parameters or molecular substructures) of solutes and their retention behavior. It can be further concluded that chemometry considerably... 

The retention mechanism is very simple. The only physicochemical parameter responsible for solute retention is the liquid-liquid partition coefficient, P. The retention volume, is simply... 

In field-flow fractionation (FFF), retention can be related through a well-defined equation to the applied held and governing physicochemical parameters of the analyte. Therefore, in principle, FFF is a primary measurement technique that does not require calibration, but only if the governing physiochemical parameters are either the analyte parameters of interest or their relationship to the parameter of interest (such a molecular weight) is well deflned. 

In thermal FFF, the applied held is a temperature drop (AT) across the channel, and the physicochemical parameter that governs retention is the Soret coefficient, which is the ratio of the thermodiffusion coefficient Dj) to the ordinary (mass) diffusion coefficient D). Because AT is set by the user, retention in a thermal FFF channel can be used to calculate the Soret coeffident of a polymer-solvent system. 

The decisive advantage of countercurrent chromatography (CCC) in Po/w measurement is that there is no correlation at all. Water, saturated with octanol, is the mobile phase. Octanol saturated with water is the stationary phase. The octanol-water partition coefficient of a given solute is the only physicochemical parameter responsible for the solute retention. If the solute is not highly pure, it is likely that the impurities will have differing Po/w values. This means that if the impurities have differing retention volumes, they are separated during the measurement from the solute of interest. The Po/w value is easily derived from the CCC retention equation ... 

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... 

Eorgacs, E. Csethati, T. Relationship between retention characteristics and physicochemical parameters of solutes on porous graphitized carbon column. J. Pharm. Biomed. Anal. 1998, 18, 505 -510. 

Besides MS detection, identification of unknown peaks in GC routine analysis of environmental samples can be aided by the use of correlations between physicochemical parameters and structure of the analytes to predict the retention times. The correlation between the boiling points and the retention times of chloro- and bromo-benzenes and of some chloro- and nitro-substituted phenols was investigated for nonpolar capillary columns and allowed tentative identification of many compounds belonging to these analogous series. ... 

---