Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Retention of the Organic Modifier or Modulator

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 [Pg.705]

Gradient Elution Chromatography under Nonlinear Conditions [Pg.706]

however, that there are more complex cases. For example, it has been shown that, although the adsorption of acetonitrile on Cig silica is not very much stronger than that of methanol, the behavior of the bonded layer is quite different in both cases [18,19]. Methanol and water molecules participate in hydrogen bonds in which they can be either donor or acceptor of hydrogen. This is not the case of acetonitrile, which can be only acceptor and is more hydrophobic. Agglomerates of acetonitrile molecules form in aqueous solutions and a layer of pure acetonitrile forms over or mixes into the layer of alkyl chains bonded to silica and against this layer. As a consequence, the nature of the adsorption isotherm of solutes may be quite different from methanol or acetonitrile solutions [18]. All the consequences of this phenomenon have not been clarified yet. [Pg.706]

Although the primary restilt is the same, i.e., the retention time of the component band decreases with increasing mobile phase concentration of the strong solvent, there are important differences in the behavior of overloaded columns in normal and in reversed phase chromatography. The competitive retention mechanism, its behavior in nonlinear chromatography, and its consequences when the strong solvent is adsorbed are discussed in Qiapter 13. The solubility mechanism is often much simpler. The isotherm coefficients merely depend on the concentration of the strong solvent in the mobile phase. [Pg.706]

The competitive interactions between the feed components and the strong sol- [Pg.706]


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. [Pg.1039]

The co-adsorption of neutral and/or ionic species plays a dominant role in the retention characteristics of solutes in electrochemically modulated chromatographic columns. The electrochemically modulated liquid chromatography is a new and promising technique, which uses conductive stationary phases and the whole column is configured as an electrochemical cell. ° If the mobile phase consists of the polar solvent S, an inert electrolyte, the organic modifier B and the eluite A, the capacity factor, k, for eluite A is given by ° ° ... [Pg.160]


See other pages where Retention of the Organic Modifier or Modulator is mentioned: [Pg.699]    [Pg.705]    [Pg.705]    [Pg.707]    [Pg.709]    [Pg.699]    [Pg.705]    [Pg.705]    [Pg.707]    [Pg.709]    [Pg.606]    [Pg.53]    [Pg.701]    [Pg.81]    [Pg.1109]    [Pg.166]    [Pg.150]    [Pg.254]   


SEARCH



Organic modifiers

Organically modified

Retention of

Retention of the

© 2024 chempedia.info