Oxygen-18 water: Recycle?

Aim
The standard method for fluorine-18 production relies on proton irradiation of oxygen-18 water. Increased demand and costs of O-18 water forced us to consider its re-use for research purposes. After irradiation and F-18 separation the target water is contaminated with radioactive metal ions and organic solvents disturbing its re-use. Therefore, thorough purification is needed before further irradiation is possible. Here, we present two methods for the removal of organic contaminants from used target water.

Materials and Methods
Two different methods were accomplished to oxidize the organic contaminants ethanol and acetone in O-18 water using photo-oxidation (Pen-Ray 254 nm, 5.4 W) [1] and a wet chemical method with KMnO₄ and NaOH. Thereafter vacuum distillation was performed and the target water was validated by gas chromatography, ICP-OES, pH value, conductivity and pycnometry. In addition artificially contaminated O-16 water was used to evaluate the potential of both methods concerning the oxidation of other potential contaminants such as methanol and acetonitrile.
Target irradiations were performed at equal parameters of bombarding (beam current on target 35 μA, 11.7 μAh) at a CYCLONE 18/9 (IBA Molecular, Nirtra^® Fluor L-target (99.9% niobium): 2 mL, target window: Havar alloy, 50 μm).
Radionuclide purity and yields were determined by gamma spectrometry (ORTEC) and an ionization chamber (MED Nuklear-Medizintechnik Dresden GmbH). Produced F-18 was used in nucleophilic substitution reactions in various radiosyntheses of ongoing projects.

Results and Conclusion
Using both purification methods a comparable decrease of organic contaminants from 400 μg mL-1 ethanol and 44 μg mL^-1 acetone to 10 μg mL^-1 – 50 μg mL^-1 ethanol was achieved. Photo-oxidation was approximately 1.5times faster. Important parameters (amount of oxidation agent, temperature, treatment time) for both methods were defined to ensure reproducibility of the radiosyntheses after comparable proton irradiation as done for the purchased target water. We observed a loss of production yield (19%) compared to purchased water due to a lower O-18 concentration. No influence on radionuclide purity or radiochemical yields was observed. Both methods enable a multiple re-cycling of target water for successful F-18 production and application for research purposes. The re-cycling process is limited by the amount of F-18 required for radiosyntheses. Starting from 87% enrichment of O-18 in target water we found 83% enrichement after one cycle allowing reasonable production yields and efficient economical usage of the purchased target water including efforts to minimize the contamination of target water.
[1] DE 29504388 U1, 1995
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