Reversed-phase chromatography eluent strength

Adsorption chromatography on bare silica is an example of normal-phase chromatography, in which we use a polar stationary phase and a less polar solvent. A more polar solvent has a higher eluent strength. Reversed-phase chromatography is the more common... 

The data in Table 5-1 also demonstrate several other behaviors typical of reverse-phase chromatography. First, the elution order of the sample components is from the most polar, eluting first, to the most nonpolar, eluting last. Second, as the eluent is made more polar (more water), the sample compounds are less soluble in the mobile phase, and hence more soluble in the stationary phase. Therefore, retention is increased. The third behavior is that for every compound and stationary phase combination, a variety of solvent mixtures can be used as mobile phases to elute a compound with approximately the same k value. Mobile phases that have identical eluent strengths are referred to as isoeluotropic or equieluotropic for that compound. The general trend is for similar retention to exist in isoeluotropic mobile phases. However, isoeluotropic mobile phases sometimes have a different selectivity (a) for a pair of compounds due to secondary interactions between the solvent and analyte. 

Why does eluent strength increase in reversed-phase chromatography when the fraction of organic solvent is increased ... 

As opposed to the usual polar stationary phase and a less polar or non-polar mobile phase, the stationary phase in reverse phase chromatography is hydrophobic (hydrophobic bonded phase usually possessing Cjg or Cg functional groups) and the mobile phase is polar (fully or partially aqueous). In this case, polar substances will interact more with the polar mobile phase and elute first. As the non-polarity of the solute components increases, their retention times will also increase since they will interact more with the non-polar stationary phase.The reverse phase system is therefore very useful for separation of non-polar solutes. Water, an extremely polar solvent becomes the weakest eluent here. Methemol and acetonitrile are stronger eluents than water. Solvents of intermediate eluting strength cem be obtained by mixing one of these solvents with water. 

An alternative to normal phase chromatography is reversed phase LC (RPLC). This method is the most popular today owing to its unmatched simplicity, versatility and scope. In RPLC, the stationary phase is nonpolar, while the mobile phase is polar and usually contains water. The strength of the eluent increases as the polarity of the mobile phase decreases. This reversal of the properties of the stationary and the mobile phases compared to normal-phase chromatography... 

There are two different modes in which on-line extraction/clean-up by turbulent flow chromatography is performed (Zhou 2005). In the simpler mode, single column extraction, the analytical sample or primary extract is injected directly onto the extraction column and extracted directly onto the analytical column by the turbulent flow mobile phase that has high eluent strength (Section 4.4.2a), i.e., high organic content in the case of analytes to be analyzed by reverse phase HPLC. This... 

The setnp for polymer HPLC is qnite similar to SEC systems with a few modifications. Liqnid Adsorption Chromatography (LAC) requires the adsorption and desorption on a stationary phase. Therefore, in most cases isocratic separation is not sufficient. Gradients with respect to pH valne, ionic strength, eluent composition, or temperature are applied. The most common approach is to use eluent composition gradients. In contrast to SEC where polymeric phases dominate, silica-based column packings are the most important stationary phase. Both normal phase and reversed phase separations have been described. A summary of different applications in copolymer separation is offered by Pasch [27]. Detectors used in gradient LAC are mainly UV/DAD detectors and ELSD. 

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