XA0101188
11. United States of America
Supercritical Fluid Extraction of Positron-Emitting Radioisotopes From Solid Target Matrices
D. Schlyer, Brookhaven National Laboratory, Chemistry Department, Upton, Bldg. 901, New York 11973-5000, USA
Project Description
Supercritical fluids are attractive as media for both chemical reactions, as well as process extraction since their physical properties can be manipulated by small changes in pressure and temperature near the critical point of the fluid.
What is a supercritical fluid? Above a certain temperature, a vapor can no longer be liquefied regardless of pressure critical temperature - Tc
supercritical fluid r«gi on
solid a u &
temperature Fig. 1. Phase diagram depicting regions of solid, liquid, gas and supercritical fluid behavior. The critical point is defined by a critical pressure (Pc) and critical temperature (Tc) for a particular substance.
Such changes can result in drastic effects on density-dependent properties such as solubility, refractive index, dielectric constant, viscosity and diffusivity of the fluid[l,2,3]. This suggests that pressure tuning of a pure supercritical fluid may be a useful means to manipulate chemical reactions on the basis of a thermodynamic solvent effect. It also means that the solvation properties of the fluid can be precisely controlled to enable selective component extraction from a matrix. In recent years there has been a growing interest in applying supercritical fluid extraction to the selective removal of trace metals from solid samples [4-10]. Much of the work has been done on simple systems comprised of inert matrices such as silica or cellulose. Recently, this process as been expanded to environmental samples as well [11,12]. However, very little is understood about the exact mechanism of the extraction process. Of course, the widespread application of this technology is highly dependent on the ability of scientists to model and predict accurate phase equilibria in complex systems [13].
In this project, we plan to explore the feasibility of utilizing supercritical fluids as solvents for reaction and extraction of radioisotopes produced from solid enriched targets. The reason for this work is that many of these enriched target materials used for radioisotope production are expensive. Quite often, post-irradiation recovery of the radioisotope from the target matrix renders that material unusable, or necessitates performing a time-consuming recovery step. The use of supercritical fluids could allow us to extract the desired radioisotope in a significantly shorter time, and in some useable chemical form, while not altering the chemical state of the enriched target matrix.
This project will involve three levels of development. Within the first level, we will plan to assemble and test a supercritical fluid extraction station. A prototype system is already in place for this phase.
Supercritical Fluid Pump Extraction cell —
Fig. 2. Supercritical CO2 fluid extraction station
The first test case would be on iodine extraction from tellurium oxide. One radioisotope of clinical importance in Nuclear Imaging is iodine-124 (4.2 day half-life) produced by the 124Te(p,n)124I and the 124Te(d,2n)124I nuclear reactions [14,15] from enriched tellurium-124. One way that is used to separate the iodine radioisotope is by dry distillation at high temperatures. [16] This process typically takes many hours and most certainly affects the morphology of tellurium, which can impact, on subsequent irradiations using the same target material. Supercritical fluid extractions will be performed using tracer amounts of the radioisotope produced at the BNL cyclotron facility, and various pure fluids. Carbon dioxide fluid will be used in the first trials, but other fluids may prove better suited for the process. In the second phase, we plan to construct an optical cell [17] that will withstand the high system pressures of the supercritical fluid. We already have experience in this area because there is an ongoing project in the PET group exploring the utility of solvent clusters within supercritical fluids for conducting microscale tracer chemistry. Fig. 3. High pressure UV-Vis cell utilizes HPLC stainless cross with fiber optics serving as the cell windows.
We plan to utilize this optical cell in conjunction with the radioisotope extraction project for the purpose of performing direct solubility measurements by UV-Vis spectroscopy. This would not only be extremely useful for fine-tuning process parameters, but provide us with a detailed understanding of the molecular process in supercritical fluid mixtures that could allow us to model the phase equilibria and better predict general efficacy of the method. In the third phase of development, we need to explore whether the radioisotope extract exists in some usable chemical form. That is, can the extract be used in synthesis to render the radioisotope as part of a larger biomolecule of interest either for therapeutic reasons or diagnostic imaging? Initially, we plan to use atomic absorption spectroscopy and radio-HPLC to help elucidate chemical state of the extracted iodine. Once elucidated, we plan to try to alter the chemical state of the radioisotope by controlling post- irradiation chemistry that may occur as a consequence of the extraction methodology. This can be accomplished by manipulating the operating parameters of the fluid (ie. temperature and pressure), changing the nature of the fluid, and by introducing certain reactive additives to the fluid. References
Clifford, A.A. Supercritical Fluids, Fundamentals for Applications; NATO ASI Ser.Ser E, 273, Kluwer Academic Publishers: 1994; 449-479. Savage, P.E.; Gopalan, S.; Martino, L; Brock, E.E. AIChEJ. 1995, 41(7), 1723-1778. Phelps, C.L.; Smart, N.G.; Wai, CM. Past, Present and Possible Future Applications of Supercritical Fluid Extraction Technology, J. Chem. Edu., 1996, 12, 1163-1168. Wai, CM. Metal Extraction with Supercritical Fluids, in Emerging Separation Technologies for Metals II, R.G. Bautista (Ed.), TMS, the Minerals, Metals and Materials Society, Warrendale, PA., 1996, 233-248. Toews, K.L.; Smart, N.G.; Wai, CM., Radiochim. Ada, 1996, 75, 179. Furton, K.G.; Chen, L.; Jaffe, R., Anal. Chim. Ada. 1996, 304, 203. Wai, CM.; Wang, S.; Liu, Y.; Lopez-Avila, V.; Beckert, W.F. Talanta, 1996, 43, 2083. Lin, Y.; Wai, CM., Anal. Chem., 1994, 66, 1971. Wai, CM.; Lin, Y.; Brauer, R.D.; Wang, S.; Beckert, W.F., Talanta, 1993, 40, 1325. Laintz, K.E.; Wai, CM., Yonker, C.R.; Smith, R.D., Anal. Chem., 1992, 64, 2875. Lin, Y.; Wai, CM.; Jean, F.M.; Brauer, R.D., Environ. Sci. Technol., 1994, 28, 1190. Bertsch, P.M.; hunter, D.B.; Sutton, S.R., Bajt, S.; Rivers, M.L., Environ. Sci. Technol., 1994, 28, 980. Brennecke, J.F.; Eckert, C.A., Phase Equilibria for Supercritical Fluid Process Design, AIChEJ., 1989, 35(9), 1409-1427. Kondo, K.; Lambrecht, R.M.; Wolf, A.P., Appl. Radiat. hot., 1977, 28, 395-401. Firouzbahkt, MX.; Schlyer, D.J.; Finn, R.D.; Laguzzi, G.; Wolf, A.P., Nucl. Inst. Meth. 1993, B79, 909-910. Weinreich, R.; Knust, E.J., J. Radioanal. Nucl. Chem., Letters, 1996, 213(4), 253-261. Carrot, M.; Wai, CM., Anal. Chem., 1998, 70, 2421-2425.