An screening method and apparatus for online identification of a reactive compound bonding with a target compound includes a first dimension analysis step whereby a sample including a target compound and a reactive compound are injected together into a sample injection port. The sample is led to a first dimension column using a first dimension mobile phase and is separated into components. An isolation step isolates the sample portion into a loop passage. A reactive compound concentration step includes leading the reaction products into a trap column that selectively traps the reactive compound while dissociating it from the reaction products isolated by the isolation step. A second dimension analysis step leads the sample trapped in the trap column to a second dimension column using a second dimension mobile phase.