Normal-phase chromatography advantages

Hydrophilic interaction chromatography (HB IC) or aqueous normal-phase chromatography (ANP) refers to the use of polar stationary phases (e.g., bare silica, silica, or polymeric phases with bonded zwitterionic ligands, diol phases) in combination with a mobile phase rich in organic solvent but containing a significant amount of water (typically at least 3%). Bell [97] summarized the advantages of this technique as follows ... 

It is also possible to employ detectors with solutions flowing over a static mercury drop electrode or a carbon fiber microelectrode, or to use flow-through electrodes, with the electrode simply an open tube or porous matrix. The latter can offer complete electrolysis, namely, coulometric detection. The extremely small dimensions of ultramicroelectrodes (discussed in Section 4.5.4) offer the advantages of flow-rate independence (and hence a low noise level) and operation in nonconductive mobile phases (such as those of normal-phase chromatography or supercritical fluid chromatography). 

The great advantage of APPI is that it can be used to ionize nonpolar classes of compounds such as alkanes, alkenes, and aromatics that are not ionized by ESI or APCI and it can be interfaced with normal-phase chromatography,49 51 where the corona discharge (APCI) and the high-voltage discharge (ESI) present a potential explosion hazard. 

Due to the complex stmcture of these antibiotics, most of them function equally well in reversed, normal and modified polar ionic phases. All three solvent modes generally show different selectivities with different analytes. Sometimes, equivalent separations are obtained in both the normal and the reversed phase mode. This ability to operate in two different solvent modes is an advantage in determining the best preparative methodology where sample solubility is a key issue. In normal phase chromatography, the most commonly used solvents are typically hexane, ethanol, methanol and so on. The optimization of chiral resolution is achieved by adding some other organic solvent, such as acetic acid, tetrahydrofuran (THF), diethylamine (DBA) or triethylamine (TEA) [50, 51]. 

Generally, better chromatographic performance is found with chiral separations in the normal phase for most column manufacturers. It is also likely that the easier solvent removal after collecting the isolated enantiomer, is what drove the industry to normal phase chromatography for chiral applications. It is advantageous to the chiral chromatographer that the majority of the commercially available normal phase LC CSPs and modifiers can be used on both LC and SFC instrumentation. This flexibility allows methods developed using one mode to be transferred to the other... 

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