Nature of Organic Modifiers

The polarity of an organic modifier is inversely proportional to its impact on retention attenuation different selectivities can be obtained using diverse organic modifiers 

Two other reasons ILs have not attracted wider interest as putative organic modifier replacers are (1) their poorer UV transparency and (2) higher cost. Conversely, the successful use of these ionic compounds in IPC relies on the multiplicity of roles they can play simultaneously. The use of ILs as silanol suppressors (described in Chapter 5) and as IPRs (discussed in Chapter 7) demonstrates that they are versatile reagents in IPC as confirmed by their ability to control retention of ionized sample without including the organic modifier in the eluent. For this reason, they were proposed as environmentally friendly mobile phases [20], 

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The nature of the modifier and the modifier concentration impact both retention and selectivity in packed column SFC. SFC offers considerable flexibility in modifier selection because nearly all commonly used organic modifiers, including methanol and acetonitrile, are miscible with CO,. In contrast, methanol and acetonitrile are rarely used as modifiers in normal phase LC because they are immiscible with hexane [68]. 

The CHI parameter approximates the percentage of organic modifier in the mobile phase for eluting the compounds and can be used for high-throughput determination of physicochemical properties (50-100 compounds per day). CHI is a system property index, and depends on the nature of the stationary phase and the organic modifier as well as the pH of the mobile phase for ionizable compounds. 

The aqueous mobile phase conditions, e.g., buffer pH, type of buffer, ionic strength, nature and concentration of competing ligands, type and amount of organic modifier and temperature, also influence the overall observed enantioselectivity of CLEC systems. 

In sub-FC, a detailed study of the influence of mobile phase additives on the chiral resolution of isoxazoline-based Ilb/IIIb receptor antagonists was carried out by Blackwell [145] on Chiralcel OD-H CSPs. The different mobile phase additives used were acetic acid, trifluoroacetic acid, formic acid, water, triethylamine, triethanolamine, n-hexylamine, trimethyl phosphate, and tri-w-butyl phosphate. In general, n-hexylamine and tri-/ -butyl phosphate mobile phase additives resulted in better resolution. The chiral separation of four 1,3-dioxolane derivatives on an amylose-based column has been described [151]. The effects of mobile phase composition, temperature, and pressure have been investigated. The nature of the modifier is the main parameter it has the highest impact on chiral resolution and is more important than the polarity of the mobile phase. Therefore, the organic modifier that gave the best enantiomeric separation was different for each compound. 

Mobile phases are usually binary or ternary mixtures of solvents. Selectivity is affected mostly by mobile phase composition rather than strength, and peak shape and retention are both influenced by the addition of organic modifiers.101 Some compounds naturally have 77-donor or 77-acceptor groups and can be resolved directly. In many cases, however, introduction of 77-donating groups by derivatization steps is necessary. Figure 2.20 shows the proposed three-point interaction of 3-aminobenzo[a]pyrene, a polycyclic aromatic hydrocarbon (PAH), with a Pirkle-type stationary phase.111 Two possible interactions are illustrated, showing the best orientations for maximum interaction. 

The rate of change in retention at varying surfactant and organic solvent concentration depends on the solute charge and polarity, as well as on the nature of both modifiers. The existence of different intermolecular forces governing the retention, the magnitude of which is altered by each modifier, explains this behavior. The more hydrophobic the solute, the more intense is the... 

Enantiomers are distinguished on the basis of their interaction with a chiral selector. Development of chiral selectors or chiral stationary phases (CSPs) for GC, HPLC, and CE has rapidly opened a new dimension in the area of chiral drug separation techniques. There are different chiral selectors available for enantiomeric separation of drugs and pharmaceuticals. Finding a suitable chiral selector, whether immobilized on a solid support (GC, HPLC) or added to a running buffer (HPLC, CE), is still often based on trial and error. A few predictions can be made, however, if common structural elements are present. After a selector has been chosen, variables, such as the nature, ionic strength, and pH of buffer, can be varied, as can presence of organic modifiers, temperature, and so on. 

Peptide separations have been easier to optimize without severe problems of irreversible solute/surface interactions, demonstrating that in nature, the whole is not necessarily equivalent to the sum of its parts. An example of a peptide separation illustrating the selectivity which can be introduced into the CZE experiment by the judicious choice of organic modifiers was done by Oda et al. 

Retention and separation selectivity are controlled by adjusting the composition of the mobile phase by making changes in the nature and concentration of the ion-pair reagent, the buffer composition and pH value, and the type and amount of organic modifier in the mobile phase. 

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