"M1 may be a moiety that can complex with a metal, metal ion, or organometallic compound (a chelating agent) and S1 is a metal, metal ion, or organometallic compound (a chelate) that forms a complex with M1, and M2 is a moiety on a biological molecule that binds to the M1/S1 complex (Gershon and Khilko, 1995, Journal of Immunological Methods, 7371)." . . . .