Simple method to reduce interference from excess magnesium in cadmium immunoassays.
In order to develop a rapid inexpensive test for cadmium in rice, we identified an antibody specific for cadmium-EDTA complexes; this antibody binds to cadmium-EDTA with a Kd of approximately 10(-8) M. Although the antibody's cross reactivity to magnesium was minimal (Kd approximately 10(-5) M), the high toxicity of cadmium coupled with the high natural occurrence of magnesium in rice resulted in a situation where magnesium interfered with cadmium determination and resulted in falsely elevated estimates of cadmium. Fortunately, the formation constant of EDTA for cadmium is approximately 5 x 10(7) times higher (at pH 7) than the formation constant of EDTA for magnesium, and we were able to eliminate the magnesium interference by judicious selection of the EDTA concentration used in the assay. The resulting equilibria are complex, but we show that a relatively simple two-step model in which cadmium and magnesium compete for EDTA followed by cadmium-EDTA and magnesium-EDTA competing for antibody provided a good fit to the measured data. These analyses enabled appropriate selection of the optimum EDTA concentration for an immunoassay with improved selectivity.
Sasaki
Kazuhiro
K
Environmental Science Research Laboratory, Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko-Shi, Chiba, Japan 270-1194. k-sasaki@criepi.denken.or.jp
Oguma
Shinichi
S
Glass
Thomas
T
Namiki
Yukie
Y
Sugiyama
Hideo
H
Ohmura
Naoya
N
Blake
Diane A
DA
D000918:Antibody Specificity; D002104:Cadmium / Q000032:analysis* / Q000276:immunology; D004492:Edetic Acid; D004785:Environmental Pollutants / Q000032:analysis; D007118:Immunoassay / Q000379:methods*; D008274:Magnesium / Q000276:immunology
D004785:Environmental Pollutants; D002104:Cadmium; D004492:Edetic Acid; D008274:Magnesium