ABSTRACT
Humic substances contain stable organic free radicals: that is, they are paramagnetic and produce electron spin resonance (ESR) signals in the solid or solution state. The basic phenomenon underlying the method of ESR spectroscopy is the Zeeman effect. This involves the interaction of an unpaired electron with an external applied magnetic field. Nuclear magnetic resonance (NMR) depends on the interaction of nuclear spins with a magnetic field. In the ENDOR experiment, the magnetic field is centered on an ESR transition and irradiated with high intensity microwave power until the signal is saturated. One of the most disappointing aspects of the ESR spectra of humic substances is the lack of hyperfine structure. The most frequently cited ESR parameter in the humic acid literature is the spin content.
