“…The resin, which is made of styrene-divinylbenzene copolymer, exhibits a great tendency to adsorb hydrophobic substances on the surface. It has been demonstrated that the XAD resins act as reversed stationary supports in HPLC (4) and that enhanced retention is observed for various organic ions when suitable counterions such as tetraalkylammonium ions are present in the mobile phase (5). The mechanism, though not yet well elucidated, is explained on the basis of an electrical double layer, an ion exchange, or an ion-pair model (5,6).…”