This BI experimental protocol begins with injection of small amount of microspheres into a carrier stream (A). Next, sample is injected and transported downstream (B) where reagent-containing beads are packed in a well-defined geometry (typical bed volume 2 to 10 mcrL). When the sample zone reaches the packed bead layer, reactive groups on the bead surfaces capture analyte molecules (C). As the sample zone moves away (D), the matrix material is washed out, leaving on the beads the captured analyte. The beads can be either discarded (E) or retained for next assay cycle.