Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing nonmetal element
Reexamination Certificate
2002-08-09
2004-12-07
Nicolas, Wesley A. (Department: 1742)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing nonmetal element
C205S628000, C205S622000, C205S630000, C205S631000, C205S632000, C205S633000, C205S635000, C205S636000, C205S637000, C205S638000, C205S639000, C205S083000
Reexamination Certificate
active
06827838
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of efficiently separating and recovering
18
F contained in
18
O water.
BACKGROUND ART
The positron emitting nuclides used in positron emission tomography (PET) include
11
C (half-life of 20 min),
13
N (10 min),
15
O (2 min), and
18
F (110 min), and of these
11
C and
18
F are most widely used. They have a short average lifetime and can be manufactured, in principle, without carriers. To utilize these short-lifetime nuclides, an accelerator is set up near the relevant facility. Since, particularly,
18
F has a relatively long-lifetime (half-life of 110 min), there are expectations for its application in research and medical facilities located away from the accelerator facility. The longer lifetime also allows to take time in synthesizing a labeled compound, and vigorous attempts are being made to synthesize a variety of labeled compounds. A labeled compound
18
FDG is used not only as a tracer in measuring saccharometabolism in the brain but also as an imaging agent for cancer tumors. Metastasis of cancer cells, for example, can be detected with higher sensitivity by PET than by X-ray CT or MRT.
18
F-DOPA is used for diagnosing Parkinson's disease.
18
F is produced through the
18
O(p, n)
18
F reaction by irradiating a liquid target
18
O water with protons. Only a minute amount of
18
O produces a nuclear reaction.
18
O water is expensive, and since a few grams of it is required for a single irradiation, efficient recovery and reuse of
18
O is strongly called for in order to reduce running costs. A conventional
18
F recovery method is based on the use of an ion exchange resin. The method consists of a two-step operation, in which
18
F is separated from
18
O water by ion exchange, and then
18
F is recovered from the ion exchange resin by using, e.g., carbon dioxide gas or potassium carbonate. The ion exchange resin must be carefully processed beforehand, and caution must be exercised so as to prevent the mixing of chlorine ions. While chemicals (carbon dioxide gas, potassium carbonate and the like) are used for recovery of
18
F adsorbed on the ion exchange resin, these impurities are not desirable from the viewpoint of having greater possibilities for the synthesis of labeled compounds. There are also the problems regarding the control of flow rate of a
18
F solvent for ion exchange and the clogging of the ion exchange resin column.
Alexoff et al have performed
18
F-electrodeposition experiments in search of a method of recovering
18
F alternative to the ion exchange resin method (Appl. Radiat. Isot. Vol. 40, No. 1, pp.1-6, 1989). They examined the time, voltage and electric field gradient dependence of the electrodeposition rate and recovery rate. The recovery rate of
18
F was 70% (rate of electrodeposition on a vitreous carbon electrode surface was 95%, and the ratio of re-emission of
18
F was 70%), which did not reach the recovery rate (95%) in the case of using an ion exchange resin. Further, when the voltage was increased, vitreous carbon powder dropped into the liquid solution. The authors conclude, therefore, that while the electrodeposition method can recover
18
F that does not contain impurities, the ion exchange resin method is superior for the purpose of recovering greater-strength
18
F required for PET.
A high recovery rate is required in recovering the
18
F that is produced through the nuclear reaction
18
O(p, n)
18
F by irradiating a liquid target
18
O water with protons accelerated by a cyclotron.
18
F used for the synthesis of labeled compounds for medical or biological experiment purposes requires a particularly high purity. The half-life of
18
F, though longer than that of, e.g.,
11
C (half-life 20 min), is only 110 min. Accordingly, the recovery of
18
F and synthesis of labeled compounds using
18
F must be finished in a short period of time. It is also important to recover the
18
O water at high purity after the separation and recovery of
18
F, so that the
18
O water can be reused and the running cost of
18
F manufacture for PET can be minimized.
The present invention takes advantage of the electrolysis method to avoid the problems of the ion exchange resin method, and has as its object the realization of highly efficient recovery of
18
F and high-purity
18
O water.
DISCLOSURE OF THE INVENTION
In accordance with the present invention,
18
F in
18
O water held in a container is electrodeposited on the surface of a solid electrode which is used as an anode. The electrodeposition liquid (
18
O water) remaining after the electrodeposition of
18
F is recycled for irradiation. By using, as a cathode, the solid electrode on which
18
F has been electrodeposited and, as an anode, a container holding pure water or an electrode disposed in the container with the pure water, a voltage of opposite polarity to the case of electrodeposition is applied. As a result, the
18
F electrodeposited on the solid electrode is desorbed into the pure water and recovered as an
18
F solution. By using high-purity graphite or platinum as the solid electrode, mixing of impurities, which blocks the synthesis and use of labeled compounds, can be prevented in the recovery process.
Further, by controlling the value of voltage or current for the electrodeposition and desorption of
18
F, the efficiency of electrodeposition and desorption are controlled. This method solves the problems of the electrodeposition methods according to the prior art, and enables a high-purity
18
F to be recovered at high efficiency while maintaining the purity of the expensive
18
O water.
Specifically, the method of separating and recovering
18
F according to the present invention comprises the steps of: applying a voltage by using a solid electrode as an anode and, as a cathode, either an electrode disposed in a container holding
18
O water containing
18
F or the container itself, such that the
18
F binds to the surface of the solid electrode; and applying a voltage by using the solid electrode to which the
18
F has bound as a cathode and, as an anode, either an electrode disposed in a container holding pure water or the pure-water holding container itself, such that the
18
F bound to the surface of the solid electrode is released into the pure water. The container for holding the
18
O water containing
18
F and the container for holding pure water may be one and the same or separate.
The solid electrode may use either carbon or platinum. For the recovery of a high-intensity
18
F, it is preferable to use graphite as a carbon member, or platinum which is meshed or made porous to increase its surface area.
During the step of having the
18
F bind to the solid electrode surface, the progress of electrodeposition of
18
F to the solid electrode surface may be monitored on the basis of an electric current (electrodeposition current) flowing between the anode and the cathode. The electrodeposition current initially exhibits a large value but this gradually decreases, and becomes constant when most of the
18
F in the solution has been electrodeposited on the surface of the solid electrode. Thus, the changes in the electrodeposition current can be measured so that the time at which the current becomes constant can be regarded as the time at which electrodeposition comes to an end.
Similarly, the step of releasing the
18
F bound to the surface of the solid electrode into the pure water may comprise monitoring the degree of release of
18
F into pure water on the basis of either the current (desorption current) flowing between the solid electrode (cathode) and the anode, or the voltage (desorption voltage) across the solid electrode (cathode) and the anode, or both. For example, the current flowing between the solid electrode and the anode increases as
18
F is released into pure water, but the rate of increase slows down and the current approaches a constant value as the release of
18
F approaches an end. Thus, the current flowing between the solid electrode and the anode can be monitored so that the step of
Goto Akira
Hyodo Toshio
Itoh Yoshiko
Kase Masayuki
Kurihara Toshikazu
Nicolas Wesley A.
Osha & May L.L.P.
Riken
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