Effects of humidity and metal ions on the free radicals analysis of peat humus

The effects of humidity and metal ions in the determination of humic organic free radicals concentration (HOFRC) were studied by electron paramagnetic resonance (EPR). Increasing the sample humidity caused a decrease in both HOFRC, probably due to free radicals recombination, and in the derivative peak-to-peak line width (ΔH), probably due to a higher spin–spin relaxation time (T2) as the molecular mobility increased (shorter correlation time). Diamagnetic ions (Na+, Zn2+ and Al3+) did not affect the HOFRC, while paramagnetic ions (Fe3+, Mn2+ and Cu2+) induced an intense HOFRC decrease. This effect was associated with dipolar-relaxation enhancement at high (above 0.01%) paramagnetic ion concentrations, since the inhomogeneity parameter decreased to less than 1. Further, a possible exchange interaction and/or shift in the free radicals equilibrium reactions might have occurred at low paramagnetic ion concentrations (below 0.01%).