An apparatus for performing a binding assay for an analyte of interest present in a sample based upon measurement of electrochemiluminescence at an electrode surface comprising a cell defining a sample containing volume intersecting with inlet and outlet means, an electrode having a substantially horizontally positioned surface exposed to and positioned below a portion of the sample containing volume, means for impressing electrochemical energy upon said electrode sufficient to generate luminescence, means for magnetically collecting particles along said surface and means for measuring the luminescence generated at said electrode.