Radiolabelling of humic substances with 14C by azo coupling [14C]phenyldiazonium ions

For the first time, natural and synthetic humic substances were radiolabelled by azo coupling [U-14C]phenyldiazonium ions onto the aromatic fragments of their macromolecules under mild reaction conditions. The radiolabelling procedure was optimized with respect to pH, reaction temperature and the molar ratio of the humic substance to the labelling compound. The labelled humic substances were purified by precipitation or ultrafiltration. The chemical yields were in the range between 23% and 95%, and the specific radioactivities varied between 68 and 206 MBq 14C per gram of the humic substance, depending on the origin of the humic substance and the purification method. With the 14C-labelled humic compounds thus obtained, we were able to detect humic substances at concentrations as low as 5 μg/L. These radiolabelled compounds can be used in long-term studies because, according to size exclusion chromatography data, there are no signs of their decomposition even after 5 months of storage.