Purification Of Used Oxygen-18 Water: Quality Assessment And Re-use

Rapid development of PET and the design of larger targets cause a greater need of oxygen-18 enriched water as target material for fluorine-18 production and entail increased costs for oxygen-18 enriched water. This forced us to consider its re-use for research purposes. Used O-18 water is mainly contaminated with metal radionuclides, non-radioactive salts and organic solvents. Here, we present (i) two methods for the removal of organic contaminants, (ii) analytical methods for quality assessment and (iii) findings for irradiation and radiosyntheses using activity produced from re-cycled water. Samples of used target water (1 L containing 44 mg L>sup-1 acetone and 396 mg L>sup-1 ethanol) were treated with UV irradiation (254 nm) [1] or with KMnO₄/NaOH at 50°C to reduce the concentration of these contaminants below 50 mg L>sup-1, respectively. Both methods allowed close-to-zero decrease of acetone and decrease of ethanol to 23 ± 20 mg L>sup-1 (n = 8). After vacuum distillation pure re-cycled water was obtained as proven by gamma spectrometry, gas chromatography, ICP-OES, and ion chromatography. Loss of O-18 enrichment (83%, pycnometry) was not observed. A reference study on O-16 water, contaminated by addition of 150 to 200 mg L>sup-1 methanol, ethanol, acetone and acetonitrile, showed a comparable oxidation potential of both methods for methanol, ethanol and acetone (decrease within 48 h below 50 mg L>sup-1). Oxidation of acetonitrile was only achieved with the UV-lamp within 7 days. Re-cycled water and virgin water (Hyox 18 Enriched Water), partly diluted to 83% O-18 enrichment, were irradiated in the Nirtra^® Fluor L-target at a CYCLONE 18/9 (iba) with 35 µA and 11.7 µAh. Comparison of produced activities of F-18, N-13 and metal radionuclides confirmed the high quality of the re-cycled water. Compared to undiluted virgin water a loss of the production yield of ~19% was observed. No significant influence on the radionuclide purity or radiochemical reactivity was detected. F-18 obtained from virgin water or re-cycled water irradiation was comparably used for radiosyntheses. Results of automated radiosyntheses (TRACERLAB™ FX-FN) of our new α4β2 nicotinic receptor ligand [¹⁸F]flubatine (n=5) and σ₁ receptor ligand [¹⁸F]fluspidine (n=8) as well as radiosyntheses for development of the α7 nicotinic receptor ligand [¹⁸F]NS14490 and ¹⁸F-labelled CB₂-selective N-aryl-oxdiazolyl-propionamides revealed equivalent yields and unchanged product qualities. The described purification procedures enable a multiple re-cycling of target water for successful F-18 production and application for research purposes with reasonable production yields and efficient economical use of the target water.

[1] DE 29504388 U1, 1995