Chromatographic behavior of solutes

The different organic modifiers used to derive the most suitable mobile phases lead to different parameters namely isocratic logfe and extrapolated logkw. The extrapolation method has no reality in terms of chromatographic behavior of solutes. However, mainly by correlation with log Pod (Eqs. 2 and 3) several studies have demonstrated the interest of these extrapolated retention factors as predictors of the lipophilicity of solutes. 

The model, therefore, predicts the elution behavior of solutes during a chromatographic process over a swollen gel as the stationary phase as a function of solute size and of the gel nanomorphology. On the reverse, from the elution behavior of solutes of known molecular size it is possible to extract the polymer chain concentration from chromatographic experiments, where an unknown swollen gel is the stationary phase. This is the basis of the ISEC, which is so often mentioned through this chapter [16,17,105,106]. 

The mobile phase should not produce chemical transformations of the separated components because it can modify the chromatographic behavior of the system. Solvents having weak reversible bonds with the solute are recommended. 

To identify isomeric thienothiophenes, the chromatographic behavior of mono- and dialkyl-substituted thienothiophenes 1 and 2 was studied.Thienothiophene 1 and its alkylated derivatives were shown to be characterized by greater retention volumes than the corresponding thienothiophenes 2. The linearity of the retention volume vs. boiling point relationship allowed the thienothiophene isomers to be identified. Studies on solution thermodynamics of thienothiophenes in the stationary phase showed that isomeric thienothiophenes 1 and 2 do not differ appreciably in their heats of solution. For example, the calculated heats of solution of 5-ethyl-3-methylthieno[2,3-h]thiophene (26) and 5-ethyl-3-methylthieno[3,2-b]thiophene (27) in polyethyleneglycol adipate are both about 16 kcWmole. ... 

Adsorption. Hydrophobic interactions, which may occur using aqueous mobile phases, usually can be eliminated by the addition of an organic modifier to the aqueous mobile phase (30,33) or by a reduction of ionic strength (3A 25.)- Recently, Haglund and Marsden (36-AO) have undertaken a systematic study on the chromatographic behavior of low molecular weight solutes on Sephadex packings and explained these results in terms of hydrophobic interactions. 

Relative humidity and temperature are two variables which have influence on the chromatographic behavior of the solutes [2] but which can not always be set at desired levels. The relative humidity is expected to have a large influence, while temperature has a small influence. In reference [2] it is stated that a temperature change of 5 degrees seldom exceeds reproducibility limits of the standard working techniques. It is most feasible to discuss the effect of variation in relative humidity and temperature in terms of activity. Therefore in the following paragraphs first the concept of activity will be introduced. Then the concept will be applied in a short examination of the effect of relative humidity and temperature on the retention. 

The sample diluent affects the solute dispersion. If we consider the effects of three different diluents (hexane, chloroform, and acetone) on the chromatographic behavior of a TG mixture on RP columns using, for example, acetonitrile and ethanol as the mobile phase, we can see that the TGs dissolved in hexane provided only a minute chromatographic trace, whereas dissolution in chloroform yielded excellent detection and resolution. These results can best be explained by invoking the solvophobic theory of Horvath and Melander (85). 

With binary and ternary supercritical mixtures as chromatographic mobile phases, solute retention mechanisms are unclear. Polar modifiers produce a nonlinear relationship between the log of solute partition ratios (k ) and the percentage of modifier in the mobile phase. The only form of liquid chromatography (LC) that produces non-linear retention is liquid-solid adsorption chromatography (LSC) where the retention of solutes follows the adsorption isotherm of the polar modifier (6). Recent measurements confirm that extensive adsorption of both carbon dioxide (7,8) and methanol (8,9) occurs from supercritical methanol/carbon dioxide mixtures. Although extensive adsorption of mobile phase components clearly occurs, a classic adsorption mechanism does not appear to describe chromatographic behavior of polar solutes in packed column SFC. 

The problem with using surfactant-modified stationary phases in LC is that the surfactant will usually slowly elute (bleed) from the support thus resulting in different retention behavior of solutes with time. This is why most applications are in the area of GC or GLC. An exciting recent advance has been reported by Okahata, et al (181). Namely, a procedure has been developed for immobilizing a stable surfactant vesicle bilayer as the stationary phase in GC. A bilayer polyion complex composed of DODAB vesicles and sodium poly(styrene sulfonate) was deposited on Uniport HP and its properties as a GC stationary phase evaluated. Unlike previous lipid bilayers which exhibited poor physical stability, the DODAB polyion phase was stable. Additionally, the temperature-retention behavior of test solutes exhibited a phase transition inflection point. The work demonstrates that immobilized surfactant vesicle bilayer stationary phases can be employed in GC separations (181). Further work in this direction will likely lead to many such unique gas chromatographic supports and novel separations. 

The study of the chromatographic behavior of natural indole alkaloids on cyclodextrin polymers was different, and unexpectedly high retentions were observed in mildly acidic buffer solutions at room temperature, which permitted their separation by inclusion chromatography (25) (Table IV). Figure 7 shows the separation of two Vinca-alkaloids of very similar structure, the (+)-vincamine and (+)-apovincamine. 

A1 Rifai et al. studied the chromatographic behavior of macroporous particles in reversed-phase electrochromatography and compared the results with pressure-driven LC with capillary columns as well as with standard-bore 4.6-mm-i.d. columns [23]. Using 400-nm-pore particles they obtained 650,000 plates/m in the CEC mode, corresponding to a reduced plate height of only 0.2. These high efficiencies were obtained for low retained solutes and provided a five-fold improvement of the optimal efficiency obtained in pressure-driven LC on the same column, and a 10-fold improvement compared to the 4.6-mm-i.d. column. 

Standards commonly employed [5] to calibrate SEC columns do not have a well-defined size. Carefully characterized spherical solutes in the appropriate size range are therefore of considerable interest. The chromatographic behavior of carboxylated starburst dendrimers — characterized by quasi-elastic light scatter-... 

Zhang, Q.H. Feng, Y.Q. Yan, L. Da, S.L. Wang, Z.H. Retention behavior of solutes on liquid chromatographic column packed with dynamically modified zirconia. Chin. J. Chromatogr. 1999, 17, 229-231. 

Moreover, this method can quickly become laborious because an internal standard elution which is compatible with the analysis conditions must be found. Conditions that must be fulfilled by the internal standard are its purity must be known and it must be chemically inert toward solutes and mobile phase on the one hand, its retention time must be different from those of all the constituents present in the sample and, on the other hand, it should be as close as possible as the retention time(s) of the product(s) to be determined. It has also been demonstrated that a necessary correlation was required between chromatographic behaviors of the internal standard and the product to quantify [3]. Otherwise, the use of an internal standard can even degrade the precision of the results. A comparison of the precision of internal and external standard has also been carried out through a liquid chromatography collaborative study [4]. 

General rate model Chromatographic separation of solutes with complex mass transfer and adsorption behavior (e.g. bio separations or ion exchange chromatography)... 

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