The electron paramagnetic resonance signal of the cupric ion (its perpendicular component) at −196°C was found to be different in shape in human blood plasma and serum. On the addition of acid, substrate of ceruloplasmin, reducing agent, or denaturing agent, serum exhibited a complex electron paramagnetic resonance spectrum similar to that of a copper-diethyldithiocarbamate complex in an organic solvent, with a simultaneous disappearance of the cupric ion signal. Such an electron paramagnetic resonance spectrum occurred only from human serum but not from animal (guinea pig, mouse, rat, cattle, or monkey) serum.
T. Kakuda, H. Tanaka, E. Kimoto, and F. Morishige, "Electron Paramagnetic Resonance Spectrum of Human Serum Copper," Appl. Spectrosc. 34, 276-280 (1980)