Effects of diverse experimental conditions on ESR spectra of humic substances

Abstract

The effects, on ESR spectra of a number of humic substances, of varying the following experimental conditions were examined: (a) no pretreatment vs. hydrolysis with 6N HCl; (b) running samples as powders vs. in 1N NaOH solution; (c) the presence and absence of oxygen; (d) analyzing samples at room temperature and at -196°C; and (e) decreasing the modulation and microwave power and increasing the time constant and scan time. Under all of these conditions the ESR spectrum of the humic materials consisted of a single symmetrical line devoid of any hyperfine splitting. While hydrolysis with 6N HCl led to substantial weight losses, the residues were richer in free radicals, more polymerized and possibly more aromatic than the original humic materials. Thus, the free radical content increased as the molecular complexity of the humic materials increased. Measurements made on solution required considerably less material, were more sensitive to oxygen but provided for a more homogeneous medium than those made on powders. Failure to observe splitting in the ESR spectra under the wide range of experimental conditions employed in this investigation, appears to limit the usefulness of ESR spectrometry for structural investigations on humic substances.