Abstract: The present invention provides compositions and methods useful for assaying binding of compounds to the hERG K+ channel. According to a method of the present invention, a compound of interest is added to the hERG K+ channel in the presence of a selenium analog of a competitive inhibitor of the hERG K+ channel. Next, the amount of the selenium analog of the competitive inhibitor that bound to the hERG K+ channel is quantified using mass spectrometry. The quantified amount can then be used to determine the amount of the compound of interest that bound to the hERG K+ channel. A selenium analog of any competitive inhibitor of the hERG K+ channel may be used according to the present invention, including but not limited to selenium analogs of the small molecule dofetilide; the peptide BeKm-1; or a combination of both.