Detection of heavy metals by immunoassay: Optimization and validation of a rapid, portable assay for ionic cadmium

An immunoassay is described that measured Cd(II) in aqueous samples at concentrations from approximately 7 to 500 ppb. The assay utilized a monoclonal antibody that bound tightly to a cadmium-ethylenediaminetetraacetic acid (EDTA) complex but not to metal-free EDTA. A inhibition immunoassay format was employed for this analysis; ionic cadmium was diluted into an excess of EDTA before being incubated with the antibody in the presence of an immobilized Cd(II)-EDTA conjugate. Ca(II), Na(I), and K(I), cations commonly encountered in ambient water samples, did not interfere with the cadmium immunoassay at concentrations approaching their solubility limit. The assay reliably measured Cd(II) in the presence of a 1 mM excess of Fe(III), Mg(II), and Pb(II). Zn(II) and Ni(II) had minimal effect on the assay at levels below 100 mu M, and the immunoassay was relatively insensitive to interferences by In(III) and Mn(II) at concentrations up to 10 mu M. Hg(II) had the ability to cause a false positive in the assay, but only at concentrations higher than 1 mu M. The assay compared favorably with atomic absorption spectroscopy in its ability to measure cadmium in spiked water samples taken from a Louisiana bayou.