Mobile phase organic modifier, function

P. Zhuang, R. Thompson, and T. O Brien, A retention model for polar selectivity in reversed phase chromatography as a function of mobile phase organic modifier type, /. Liq. Chrom. Rel. Technol. 28 (2005), 1345-1356. 

This problem was resolved by Nakae et al. [7] using non-polar octadecylsilica as the stationary phase and a solution of 0.1 M of sodium perchlorate in methanol/water (80 20) as the mobile phase. The ternary system (water-alcohol-salt), previously used by Fudano and Konishi [8] as an eluent for the separation of ionic surfactants at higher concentrations, induced the so-called salting out effect . The addition of the organic solvent to the water modified the polarity of the eluent and produced a good separation within a short period of time [9]. It also has the function of dissociating the surfactant micelles in individual molecules that are dissolved in the eluent [8], The presence of the salt (NaC104) in the mobile phase has a considerable influence on... 

For ESP ionization, the analytes must be ionic, or have an ionizable functional group, or be able to form an ionic adduct in solution the analytes are commonly detected as deprotonated species or as cation adducts of a proton or an alkali metal ion. When using positive ion ESP ionization, use of ammonium acetate as a mobile-phase modifier is generally unsuitable. Instead, organic modi-... 

Most stationary phases used in bonded-phase chromatography in its reversed-phase mode are based on octadecylsilane functionality (C18 columns). The mobile phases typically used in this context are water, aqueous buffers of a given pH and ionic strength, and mixtures of water and a miscible organic modifier, such as methanol or acetonitrile. 

Figure 4-14. Selectivity for steroids as a function of organic mobile-phase component. Chromatograms showing the elution order of ail eight congeners as a function of organic modifier with 0.1% formic acid as the buffer phase on YMC ODS-AQ column at ambient temperature. (Top panel) 25% acetonitrile, 1.5-mL/min flow rate (middle panel) 45% methanol, 1.2-mL/min flow rate (bottom panel) 20% tetrahydrofuran, 1.5-mL/min flow rate. (Reprinted from reference 51, with permission.)...

The change in the mobile-phase pH of a particular buffer as a function of the organic compositions will be referred to as the pH shift in the following sections in this book. For acidic buffers/modifiers, the relative increase in the pH will be dependent upon the type and concentration of acidic modifier and... 

Enantiomeric forms of proteins are not accessible so that a conceptual reversal of the elution order by switching to the enantiomeric CSP is not feasible. However, in some special ca.ses, the elution order can be influenced by the mobile phase conditions. For example, on an ovomucoid CSP a reversal in elution order of the enantiomers of the ketal tosylate intermediate of azalanstat was observed when the organic modifier was changed from ethanol to acetonitrile. This unusual effect has been attributed to a change in binding domains or recognition sites on the ovomucoid protein as a function of the organic modifier of the mobile phase [ 149. ... 

For solutes meeting the requirements for use with a standard or derivatized p-CD CSP, the NEC-p-CD CSP can be used with aqueous mobile phases modified with acetonitrile or another organic modifier. Since chiral recognition is a function of the interaction of the solute with the chirality of the CD as a whole, the configuration about the stereogenic... 

The net retention of analyte is a function of all the solute-stationary phase, solute-mobile phase and mobile phase-stationary phase interactions that contribute to retention. It follows that if the interactions with the stationary phase are constant then retention and selectivity are only functions of the composition of the mobile phase. The simplest mobile phase will generally be composed of a mixture of two solvents a weak solvent A and a strong solvent B. For example a typical reversed-phase eluant will consist of a mixture of water, the weakest solvent in reversed-phase systems, and a stronger organic modifier such as methanol or acetonitrile. Similarly, a normal-phase eluant will typically comprise a mixture of a weak solvent such as a -heptane and a stronger modifier such as chloroform or ethyl acetate. 

Fig. 7 Retention behavior of a typical analyte in IIC as a function of HR mobile-phase concentration and organic modifier percentage in the eluent, according to Eq. 12.

It is general. From two equations, quantitative predictions can be made for the retention behavior of charged, multiply charged, neutral, and zwitterion-ic solutes in IIC as a function of both the mobile-phase and the stationary-phase concentrations of the HR. Retention equations can also quantitatively take into account the influence of the organic modifier concentration and the ionic strength in the absence of HR, they reduce to the well-known relationships of RP-HPLC. 

The relationships between capacity factor, k , and organic modifier concentration in the mobile phase, and the effect of the column temperature on k for the antibiotics studied have been used to define k as a function of T and V (volume fraction) on the basis of a small number of experimental measurements for a given combination of column, organic solvent, and type of antibiotic. From calculated values of k, resolution values, Rs, may be estimated for adjacent band-pairs under all conditions. The method developed enables the optimization of RP-HPLC separations of the p-lactam antibiotics in the absence of difficult theoretical calculations, using a small number of experimental data, including the influence of the organic solvent in the mobile phase (isopropanol) and the column temperature. 

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