TPEN (Tetrakis-(2-pyridylmethyl)ethylenediamine) is an excellent heavy metal chelator while it does not affect Ca++, Mg++ , Na+ or K+ concentrations. Thus, TPEN is a useful tool to identify effects of heavy metals (Zn++, Fe++/Fe+++, Cu++, and Mn++, etc.) on fluorescent indicators of Ca++, Mg++, Na+ and K+.