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Commercial Fractionation of Oxygen Isotopes.
The oxygen isotopes are prepared via enrichment procedures of the oxide and total global 18O water generation is about 500 kg/year. Gas diffusion is apparently not commercially viable. Commercial quantities are however, produced by fractional distillation of water or cryogenic distillation of NO (nitrous oxide) or CO (carbon monoxide). Typically operation begins with NO and enrichment proceeds to a level of 10-30% , whereupon subsequent enrichment then proceeds via fractional distillation of water using long columns. Due to the low enrichment of the 18O (and especially 17O isotopes) and small difference in vapour pressures, a large number of theoretical plates are needed during the distillation process. This in practice translates to a slow process with a long lead time (6-12 months) as well as high energy requirements. Fractional distillation of water then also must contend with the natural abundance of Hydrogen and Deuterium (99.985% and 0.015% respectively). Therefore, when the water is distilled a mixture of the H- and D-water products are isolated. Isolation of pure >95% H218O is achieved after isotopic normalisation: electrolysis of water (liberating the gases H2 and O2) and reaction of the 18O2 with pure H2 to generate the oxide again. The same process is also implemented with D2 if D218O is required. |
The difference in the physical properties (boiling points) of the 17O and 18O isotopic water are small and are the basis of the difficulty in separation. Due to the industrial scale and level of commercial investment, not a great deal of information is available concerning the practicalities of the distillation; however, Dostrovsky, I and Raviv, A. (1957) outline some of the early design considerations for sizes and packing material for the columns. |
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Table of Physical properties
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Jancso, G. & Van Hook, W. A. (1974) Chem Rev. 74, 689 |
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Staschewski, D. (1974) Angew. Chem. 13, 357. |
| Commercially then the gas is available in small cylinders and the water (oxide) available in a small ampules. |
(see pictures below) |
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Currently one of the main uses of 18O water is P.E.T. or positron emission tomography. This technique has gained a wide acceptance for medical imaging and used in particular for cancer screening. The basic approach is to take 18O water and to add an additional proton (from a nuclear accelerator) and to generate a short-lived 18F isotope, which in turn is used to label molecules. This procedure is done on-site in the hospital as the 18F has a half life of 110 min. Particular medical interest is with 18F-fluoro deoxyglucose ( 2-[18F]FDG ) which is one of the most commonly labeled small biomolecules. |
Radioisotopes
A number of radioisotopes have been synthesised for oxygen. They are generated via a cyclotron which provides high energy protons (originally from hydrogen ions). The collision energies of these high energy particles can be adjusted such that different isotopes can be isolated. The application of oxygen isotopes is for research and for nuclear medicine. Due to their half-life they are used in specialist applications. |
Atomic Mass (a.m.u.) |
Half-Life (s) |
Atomic number (i.e. Neutrons) |
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|---|---|---|---|
| 12O | 12.0344 | 4 | |
| 13O | 13.0248 | 8.58 x 10-3 | 5 |
| 14O | 14.0086 | 70.606 | 6 |
| 15O | 15.0031 | 122.24 | 7 |
| - | - | - | - |
| 19O | 19.0036 | 26.91 | 11 |
| 20O | 20.0041 | 13.51 | 12 |
| 21O | 21.0087 | 3.42 | 13 |
| 22O | 22.01 | 2.25 | 14 |
| 23O | 23.016 | 82 x 10-3 | 15 |
| 24O | 23.016 | 61 x 10-3 | 16 |
