Magnetic particle based electrochemiluminescent detection apparatus and method

A method and apparatus for measuring electrochemiluminescence from a sample composition are described wherein magnetically responsive electrochemiluminescent active species are captured on the electrode with the aid of a capture magnet having a configuration such that the magnetic flux lines (or the magnetic field gradient) of at least one magnetic field source therein are compressed and/or dispersed. This capture magnet improves the distribution of the magnetically responsive electrochemiluminescent active species on the electrode surface and reduces interference with the photomultiplier tube, thereby enhancing the ECL signal and improving sensitivity. The improved capture and distribution also allows for shorter assay times.