Theranostic mercury 197(m)Hg: comparing different Hg/Au separation methods

Objectives: The access to no-carrier-added 197(m)Hg for imaging and therapy research based on proton or deuteron irradiation of gold was recently reported1,2. The development of a rapid, reliable method for Hg/Au separation represents an important prerequisite for increasing yields. Ideally would be, a reversible interaction at least of one of the two metal ions, allowing for the product elution into a small volume. Besides the liquid-liquid extraction with methyl isobutyl keton (MIBK)1, the solid phase extraction using LN resin (LaNthanides) containing di(2-ethylhexyl)orthophosphoric acid as extractant was examined for this application2.
Methods: The gold target was irradiated for 120 minutes with a 25 µA beam current of 10 MeV protons resulting in 200 MBq of 197(m)Hg or with 15.6 MeV deuteron beam at 7.8 µA beam current for 180 min resulting in ca 800 MBq of of 197(m)Hg (EOB). The irradiated gold foil was dissolved after 1 h in 700 µl of aqua regia (freshly prepared 1 h before EOB from 525 µl 30% HCl + 175 µl 65% HNO3) at room temperature. The column preparation was carried out directly before use by loading 3.6 g LN resin slurried with 10 ml of 6M HCl onto the column and rinsing with additional 30 ml of 6M HCl. After dilution of the 700 µl product solution with 300 µl 6M HCl, this mixture was loaded onto the column and eluted with 6M HCl in 1 ml aliquots.
Results: Comparing with the previously described liquid-liquid extraction1, the solid phase extraction using the LN resin showed shorter performance time. After loading the mixture of chloroauric acid and n.c.a. mercury chloride, the colored gold solution was observed to rapidly distribute in the upper part of the column and then slowly proceeds down during the stepwise elution with 6M HCl. After the addition of 5–6 ml of HCl, the yellow chloroauric acid extended roughly two thirds down the column and almost stopped to move, while over 90% of n.c.a. radiomercury chloride (higher than 60-80% extracted with 4×500 µl MIBK) eluted in the following 2 ml of HCl. The separated 197(m)Hg has excellent radionuclidic purity with no detectable traces of 198Au. It is massively produced in the deuteron activation of gold and acts as a very sensitive tracer of the separation process efficiency.
Conclusions: In contrast to the liquid-liquid extraction, LN resin based method is significantly more efficient and provides product of high radionuclidic purity. Another major advantages compared to the liquid-liquid extraction are obviously 1) better handling and easy automation that shorten the separation time and minimize radiation burden, 2) negligible product losses 3) open possibility to collect and recycle the target material.