Eluent organic modifier concentration

The maximum retention factor (kQ) is related to the log P value and k and k are the retention factors of the cationic and anionic forms, respectively. The pKa values are known, and the retention factor in a given eluent can therefore be predicted in reversed-phase liquid chromatography using an alkyl-bonded silica gel or polystyrene gel column. The separation conditions can be adjusted according to their logP and pKa values by the selection of a suitable organic modifier concentration and the pH of the eluent.3,4... 

Thus, in conclusion, it can be stated that retention studies such as the ones outlined above permit the selection of operation conditions (type and concentration of the organic modifier, concentration and pH of the buffer, temperature of the eluent) which lead to sufficient initial solute retention (k >10) and maximum separation selectivity necessary for a successful displacement chromatographic separation. 

Variation of solute retention and separation selectivity as a function of the eluent temperature at constant organic modifier concentration, pH and ionic strength. Solutes tryptophane enantiomers column alpha-... 

The reduction of the capacity factors with increasing organic modifier concentration in the eluent was weaker when methanol was used compared to acetonitrile, and this was attribnted to its lower polarity [5]. Even if methanol or acetonitrile are the most common organic modifiers, an unusual solvent, tetramethylene oxide [16], was recently tested in the IPC of sulfides and aromatic sulfonated compounds and proved to play an important role in adjusting retention. 

General dependence of the analyte retention on the eluent composition in reversed-phase FIPLC shows an exponential decay with the increase of the organic modifier concentration. This is usually described in the following form ... 

Figure 7 illustrates how an isocratic method was developed for the resolution of the two compounds. The compounds coeluted after 2.4 min using an eluent of 65% v/v acetonitrile/water (Fig. 7A). As the acetonitrile concentration was progressively decreased, resolution was improved and retention was increased. It can be seen that this also results in much broader peaks. Partial resolution was observed when the organic modifier concentration was 55% v/v (Fig. 7B), near-baseline resolution was seen at 50% v/v (Fig. 7C), and complete resolution was achieved when the organic modifier concentration was 45% v/v (Fig. 7D). It can be seen from the structures that these metabolites bear an abundance of ionizable phenolic hydroxyls, and this feature necessitated the use of a buffered eluent for preparative work (see Subheading 2.3.2.).

Global LSER calculations have also been applied to the study of the retention of ioniz-able analyses in RP-HPLC. While the retention of neutral analyses does not depend on the pH of the mobile phase the retention of analyses with one or more ionizable substructures considerably depends on the pH even at the same concentration of organic modifier in the eluent. The relationship between the retention and pH of the mobile phase and pK value of the analyte can be described by... 

Moreover, in various experiments it was found that at a constant total counterion concentration in the eluent the dependence of the retention factors on the organic modifier content tp largely follows the linear solvent strength theory (LSS) (Equation 1.5)... 

The first results of optimization in chromatography were published in 1975 Since then a growing number of optimization experiments in HPLC using the Simplex procedure has been reported (table 9). The examples are mainly reversed-phase separations, in which the composition of the ternary or binary mobile phase composition is optimized. The factors optimized are usually a selection from flow rate, column temperature and length, the eluents constitution (e.g. organic modifier content, buffer concentration and pH), the gradient shape. Seven years after the first applications of Simplex optimization had appeared, the first fully automated optimization of HPLC separations was published by Berridge in 1982. This development coincid-... 

In MLC, the mobile phase consists of surfactants at concentrations above their critical micelle concentration (CMC) in an aqueous solvent with an alkyl-bonded phase (52). Retention behavior in MLC is controlled by solute partitioning from the bulk solvent into micelles and into stationary phase as well as on direct transfer from the micelles in the mobile phase into the stationary phase. Eluent strength in MLC is inversely related to micelle concentration. A linear relationship exists between the inverse of retention factor and micelle concentration. Similar to what is observed in RPLC, a linear relationship exists between retention in MLC and , the volume fraction of the organic modifier. Modeling retention in MLC is much more complicated than in RPLC. The number of parameters is important. Micelles are obviously a new domain in both liquid chromatography and electrophoresis. Readers interested in the topic will appreciate Ref. 53, a special volume on it. 

The chiral resolutions on re-acidic and re-basic CSPs were carried out under the normal phase mode. However, some reports are also available dealing with the use of reversed-phase eluents, but the prolonged use of the reversed-phase mobile phase is not recommended. With the development of the more stable and new CSPs, the use of the reversed-phase mode came into existence on these CSPs. Currently, both modes of mobile phases (i.e., normal and reversed) are in use. Therefore, the optimization of the chiral resolution on these phases can be achieved by varying the concentration of the mobile phases, including the use of organic modifiers. In addition, the temperature, structures of solutes, and CSPs are also important parameters that control the chiral resolution on these CSPs. 

Generally, cyclodextrin-silicas are used in the reversed-phase mode. The most important retention controlling parameters to consider are the tope and cxncentration of the organic modifier, and the pH, concentration and type of the buffer in the eluent. 

Concentration of the Organic Solvent in the Eluent. As in other reversed-phase systems, the solute retention curves (log k vs. % organic modifier) on cyclodextrin-silicas are quasi-linear, at least over a limited concentration range. Such curves are shown in Figs. 2-4 for the positional isomers of nitrophenol (Fig. 2) and chloroaniline (Fig.3), and the enantiomers of Ibuprofen (Fig.4). Similar retention curves were observed and published for other positional iscmers (67), cis/trans isomers (68) and enantiomers (69). ... 

---