Phase System Dependent on Solubility

The most important step in developing a chromatographic system is the choice of mobile phase. Although the mobile phase influences the separation process in many ways, the main decision parameters for the choice of the mobile phase are the maximum solubilities of the target components and the selectivity of the separation. Solubility is essential for the productivity of the chromatographic separation process, because it is a precondition for exploiting the whole range of column loadability. 

An old alchemist maxim, similia similibus solvuntur ( like dissolves like ), is the oldest rule for selecting suitable solvents, meaning that the nature of the solute determines the nature of the solvent. Due to the classification of phase systems in Section 3.2.3, organic and aqueous solvents are distinguished. Non-polar to medium-polar substances show best solubility in typical organic solvents while medium-polar to polar substances show best solubility in aqueous solvents. 

NP phase systems are used for the separation of nonionic, apolar to medium-polar samples. If the sample is soluble in organic solvents an NP phase system should be applied as a first test system (e.g., silica and organic mobile phase). FIow-ever, if the sample shows best solubility in aqueous solvents the sample is quite polar. In this case, RP phase systems with semi-polar (cyano, amino or diol) or apolar packings such as alkylsilica should be used. 

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