Methanol/acetonitrile organic modulator

FIGURE 2.14 Influence of organic modulator concentration on solute retention. Comparison of methanol and acetonitrile demonstration of isoeluotropic mixtures. 

FIGURE 2.16 Influence of an organic modulator on selectivity. Comparison between acetonitrile and methanol. Isoelutropic conditions for ethyl benzene. Column Symmetry C8, 150 x4mm. Mobile phase methanol-water. 55-45, v-v. Acetonitrile-water, 42-58, v-v. Solntes 1, thiourea 2, aniline 3, phenol 4, o-toluidine 5, m-toluidine 6, -tolnidine 7, V,V-dimethyl aniline 8, benzoic acid ethylester 9, tolnene 10, ethyl benzene. 

The modulator (in NPLC, a strong, polar solvent, or in IXC, a buffer) or the organic modifier (in RPLC, methanol, acetonitrile, or THE) may affect the retention of the components of the sample in different possible ways. In the most classical case, such as in ion-exchange chromatography or in normal phase HPLC, the... 

Reversed-phase separations currently dominate in CEC. As a result, the vast majority of the mobile phases are mixtures of water and an organic solvent, typically acetonitrile or methanol. In addition to the modulation of the retention, the mobile phase in CEC also conducts electricity and must contain mobile ions. This is achieved by using aqueous mixtures of salts instead of pure water. The discussion in Sect. 2 of this chapter indicated that the electro osmotic flow is created by ionized functionalities. The extent of ionization of these functionalities that directly affects the flow rate depends on the pH value of the mobile phase. Therefore, the mobile phase must be buffered to a pH that is desired to achieve the optimal flow velocity. Obviously there are at least three parameters of the mobile phase that have to be controlled (i) percentage of the organic solvent, (ii) the ionic strength of the aqueous component, and (iii) its pH value. 

Common surfactants that have been used in MEKC, are listed in Table 3.1 with the respective critical micelle concentrations the most popular are SDS, bile salts, and hydrophobic chain quaternary ammonium salts. Selectivity can also be modulated by the addition to the aqueous buffer of organic solvents (methanol, isopropanol, acetonitrile, tetrahydrofuran, up to a concentration of 50%). These agents will reduce the hydrophobic interactions between analytes and micelles in a way similar to reversed-phase chromatography. Organic modifiers also reduce the cohesion of the hydrophobic core of the micelles, increasing the mass transfer kinetics and, consequently, efficiency. Nonionic... 

The organic solvents often used as modifiers in the aqueous mobile phase consist of n-propanol, isopropanol, methanol, and acetonitrile. They are efficient agents for modulating the hydrophobic interaction between the analytes and the protein stationary phase [144]. An increase in the organic modifier in the aqueous organic mobile phase will decrease the retention, but will have minimal effect on the overall enantioselectivity [141]. 

Type IV includes chiral phases that usually interact with the enantiomeric analytes through the formation of metal complexes. There are usually used to separate amino acid enantiomers. These types of phases are also called ligand exchange phases. The transient diastereomeric complexes are ternary metal complexes between a transitional metal (usually Cu +), an amino acid enantiomeric analyte, and another compound immobilized on the CSP which is able to undergo complexation with the transitional metal (see also the ligand exchange section. Section 22.5). The two enantiomers are separated based on the difference in the stability constant of the two diastereomeric species. The mobile phases used to separate such enantiomeric analytes are usually aqueous solutions of copper (II) salts such as copper sulfate or copper acetate. To modulate the retention, several parameters—such as the pH of the mobile phase, the concentration of the copper ion, or the addition of an organic modifier such as acetonitrile or methanol in the mobile phase—can be varied. 

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