The use of mobile phases with a low content of organic modifier (< 5 %) generally leads to what is known as "phase collapse". The latter is best described as a dewetting phenomenon, in which the highly aqueous mobile phase is excluded from the hydrophobic pore system due to surface tension. The effect is observed predominantly with reversed phase packing materials with high ligand density (> 3.2 μmol/m2). The loss of wetted surface results in a decrease in accessible interaction sites for the solute, and leads to a loss in retention and / or reduced loading capacity. The process can easily be reversed by purging the packed bed with a high content of organic modifier (> 50%) The phenomenon is most dominant for C18 and C8 packing materials with pores < 150 Å, but even with C4 modified packing materials, 100% aqueous mobile phases should be avoided in order to assure robust chromatography.
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