Mobile phases

In HPLC type and composition of the mobile phase (eluent) is one of the variable influencing the separation. Despite of the large variety of solvents used in HPLC, there are several common properties:
Purity
Detector compatibility
Solubility of the sample
Low viscosity
Chemical inertness
Reasonable price

Each mode of HPLC has its own requirements. For normal phase mode solvents are mainly nonpolar, for reversed-phase eluents are usually a mixture of water with some polar organic solvent such as acetonitrile.

Size-exclusion HPLC has a special reqirements, SEC eluents has to dissolve polymers, but the most important is that SEC eluent has to supress all possible interactions of the sample molecule with the surface of the packing material.