Technique in which trace amounts of compounds from a weak mobile phase or solution are retained on an HPLC packing and then are eluted by the addition of a stronger mobile phase in a concentrated form. The technique has been most successfully applied in the concentration of trace amounts of hydrophobic compounds (e.g.. poly-nuclear aromatic hydrocarbons) out of water using a reversed-phase column. A strong Solvent such as acetonitrile serves to elute the enriched compounds.