Subject eluent strength

Separation is dependent on three different mechanims Donnan exclusion, steric exclusion, and adsorption/partitioning. Donnan exclusion causes strong acids to elute in the void volume of the column. Weak acids that are partially ionized in the eluent are not subject to the Donnan exclusion and can penetrate into the pores of the packing. Separation is achieved through differences in acid strength, size, and hydrophobicity. The major advantage of ion exclusion is the ability to handle samples that contain both strong and weak acids. 

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