The invention relates to a method of characterizing interaction between two species in a liquid environment, wherein a liquid comprising said at least one species is passed as a flow through a measurement system, and wherein the interaction takes place within said measurement system. The method comprises generating a concentration gradient of at least a first one of said species or of at least one other species having an influence on the interaction or on interacted components. The flow of liquid is passed through a sensor device, and a result of interaction between said at least two species is detected. The flow of liquid is intersected at least once with a further liquid before the flow is passed through said sensor, so as to create at least two separated liquid segments having different concentrations of at least one of said species forming the concentration gradient.