A new technique for radiolabelling of humic substances

Radioiodination of humic substances

The known IODO-GEN™-method [1] was adapted for radiolabeling of humic and fulvic acids with131I. The water insoluble oxidizing agent 1,3,4,6tetrachloro-3α,6α-diphenylglycoluril (IODO-GEN™) forms an iodous ion species (I+), which undergoes an electrophilic I/H-substitution on aromatic moieties of the humic and fulvic acids. This method offers mild conditions with a lesser extent of oxidative alterations of the target molecule, accompanied by an easy handling due to the virtual water-insolubility of the oxidizing agent. The method was optimized and different techniques were tested for the purification of the radioiodinated humic material. The yield of the labeling procedure varies between 45 and 75% depending on the provenance of the humic material and the applied purification method. A specific activity up to 40 MBq/mg was achieved. Furthermore, the known inherent photo-susceptibility of the iodinated humic substance and the influence of reducing agents were verified. An additional release of131I up to 20% and up to 35%, respectively were observed.

Radioiodination of humic substances via azocoupling with 3-[125I] iodoaniline

A new method is described for radiolabeling humic substances (HS) with iodine radioisotopes. The method radiolabels the electron-rich aromatic moieties of HS with the 3-[125I]iodobenzenediazonium ion via azocoupling. The method uses four steps: (i) 3-aminobenzenetrimethylstannane is synthesized and isolated by using a silica gel column, (ii) 3-[125I]iodoaniline is synthesized and isolated by HPLC, with radiochemical yields of up to 60%, (iii) 3-[125I]iodobenzenediazonium chloride is synthesized, and the reaction mixture from this step is used in step iv to radioiodinate HS with radiochemical yields of up to 95% (with reference to 3-[125I]iodoaniline). The advantage of this method is that it is selective radiolabeling, placing the radiolabel in a specific site (the 3-position of the phenyl ring) within HS molecules, which minimizes unwanted secondary chemical interactions. Investigations of the stability of the radiolabel and the effect of photoreductive dehalogenation showed that there was a negligible release of 125I. The production of radiolabeled HS using this method allows the sensitive detection of HS in laboratory and field studies. In addition, the method offers the possibility of using different iodine radioisotopes simultaneously in investigations using HS.

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-(14)C]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 206MBq (14)C per gram of the humic substance, depending on the origin of the humic substance and the purification method. With the (14)C-labelled humic compounds thus obtained, we were able to detect humic substances at concentrations as low as 5microg/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.