Chromatography, general selectivity

RP chromatography generally offers a better selectivity than NPLC for the separation of molecules differing in hydrophobic parts of the molecules, but there are some practical reasons for selecting NP chromatography methods in specific cases, as follows ... 

Ion-suppression chromatography is used for the separation of weak acids and bases by reversed-phase chromatography [296-299]. The pH of the mobile phase is generally selected to either optimize the band spacing of a series of ionizable substances with different dissociation constants or set to a value to completely inhibit the dissociation of all ionizable compounds. Neutral and ionic substances can be separated simultaneously... 

Another important instrument required in modem LC is a sensitive or selective detector for continuous monitoring of the column effluent. In GC. the differences in physical properties of the mobile phase (carrier gas) and the sample are great enough for universal detectors with good sensitivity to be used (e.g., flame ionization detector, thermal conductivity detector, - Gas Chromatography). The problem in LC is that the physical properties of the mobile phase and the sample are often very similar, which makes the use of a universal detector impossible. Nevertheless, presently available LC detectors are very sensitive, are generally selective, and have a relatively wide range of applications (see Table 1). 

To select a column for a particular analytical problem, the first step is to make a choice about the pore size(s) to be used for the separation. In general, one cannot expect that a single pore size will fulfill the needs of a separation. In size exclusion chromatography, it is more common that columns of different types are combined with each other to deliver the separation range needed for a particular analysis. Therefore, column banks with different pore sizes are frequently combined with each other to maximize the separation power for... 

---