Abstract: An apparatus for use in an assay process includes a pre-incubation chamber (10) defining a plurality of wells (14) having a porous base (18). The pre-incubation chamber (10) is attachable to a housing (12) carrying a porous membrane (32) disposed above and touching a body of absorbent material (36). A pattern of capture analyte is carried on the porous membrane (32). Pins depending from the underside (20) of the pre-incubation chamber locate in holes defined in corners on the top of the housing to locate the chamber housing in a set position on top of the housing such that capture analyte is disposed under the bases of the wells of the pre-incubation chamber.