Abstract:
The present invention relates to a method and an apparatus for separating and recovering a radioisotope from a solution. More particularly, certain embodiments of the invention relate to a method for recovering a radioisotope from a solution by electro-trapping and release using a microfluidic cell ( 10 ). The radioisotope may subsequently be used in the preparation of radiopharmaceuticals.

Description:
[0001]    The present invention relates to a method and an apparatus for separating and recovering a radioisotope from a solution. More particularly, certain embodiments of the invention relate to a method for recovering a radioisotope from a solution by electro-trapping and release using a microfluidic cell. The radioisotope may subsequently be used in the preparation of radiopharmaceuticals. 
       BACKGROUND TO THE INVENTION 
       [0002]    Positron emission tomography (PET) is a powerful molecular imaging technique that allows the study and visualisation of the molecular and cellular processes of human physiology which can provide important information about metabolism and diseases such as cancer. The most common positron-emitting radioisotopes suitable for incorporation into organic structures are fluorine-18 ( 18 F), carbon-11 ( 11 C), nitrogen-13 ( 13 N) and oxygen-15 ( 15 O). Because of its appropriate half-life (109.8 min) allowing sufficient time for multistep synthetic labelling reactions and transportation of doses to sites several hours away, and a low positron energy giving high resolution images,  18 F has become the most widely used and commonly available radioisotope for PET imaging. Currently, [ 18 F]-2-Fluoro-2-deoxy-β-D-glucopyranose (also known as [ 18 F]-Fluorodeoxyglucose or [ 18 F]-FDG) is the most frequently used radiopharmaceutical for PET investigations. 
         [0003]    Radiopharmaceuticals such as [ 18 F]-FDG can be synthesized by nucleophilic substitution of a precursor compound. The process usually starts with recovery of  18 F fluoride ions from  18 O-enriched water into an appropriate solvent using an anion exchange separation cartridge, followed by azeotropic evaporation to remove any residual water. The  18 F fluoride then reacts with a precursor through nucleophilic substitution. 
         [0004]    Historically, PET tracers were produced in large batches at centralised cyclotron or decay generator facilities and then transported as multiple doses to the imaging sites, usually a hospital, to be administered to multiple patients during pre-arranged PET clinics. However, recent technological advances mean that individual imaging sites are able to have mini-PET cyclotrons on site thus allowing small volumes of radioisotopes such as  18 F to be produced for the on-demand synthesis of a single dose of radiotracer i.e. “dose-on-demand”. 
         [0005]    Currently, the dose-on-demand approach is not achievable with traditional vessel-based techniques. In addition, current methods, based on either distillation and/or the combination of solvent-exchange on an anion exchange column and azeotropic evaporation step for the isolation of  18 F from its aqueous environment, are time-consuming, require complex automation and reduce radiochemical yield. The extremely high surface-to-volume ratio and small size of microreactors combining with flow chemistry have significant potential to facilitate such chemistry in terms of higher yields, shorter reaction times, reduced consumption and lower environmental impact. Azeotropic evaporation is, however, very difficult to implement on such miniaturized systems. Alternative methods based on an electrochemical cell have been reported in which  18 F is trapped and then directly released into organic solvent for the preparation of radiotracers without the need for an evaporation step. However, disadvantages of the known processes include low efficiency of trapping and/or release, reliance on high voltages or the use of complex electrochemical cells. 
         [0006]    The present invention has been devised with these issues in mind. 
         [0007]    Certain embodiments of the present invention aim to overcome the problems associated with the prior art. 
       SUMMARY OF CERTAIN EMBODIMENTS OF THE PRESENT INVENTION 
       [0008]    According to a first aspect of the invention there is provided a method for separating and recovering a radioisotope from an aqueous solution comprising the radioisotope, the method comprising:
       providing a flow of aqueous solution to a chamber comprising a first electrode and a second electrode;   generating a first electric field between the first and second electrodes, thereby trapping the radioisotope on the first electrode;   providing a flow of an organic-based solution through the chamber comprising the first and the second electrodes;   generating a second electric field between the first and the second electrodes, wherein the second electric field has an opposing polarity to the first electric field, thereby releasing the radioactive isotope from the first electrode into the organic based solution.       
 
         [0013]    Aptly, the chamber is comprised in a microfluidic chip. Aptly, the method comprises using a microfluidic chip. In an embodiment, the method comprises providing a microfluidic chip comprising the first electrode and the second electrode. 
         [0014]    According to a second aspect of the invention, there is provided a method for preparing a radiotracer, the method comprising:
       flowing an aqueous solution comprising a radioisotope through a chamber of a microfluidic cell, the chamber comprising a first electrode and a second electrode;   trapping the radioisotope on the first electrode by applying a voltage across the electrodes;   flowing an organic-based solution through the chamber of the microfluidic cell;   releasing the radioisotope from the first electrode into the solution by reversing the voltage; and   reacting the radioisotope with a precursor to form the radiotracer.       
 
         [0020]    According to a third aspect of the invention, there is provided an apparatus for separating and recovering a radioisotope from an aqueous solution comprising the radioisotope, the apparatus comprising:
       an inlet;   an outlet; and   a chamber in fluid communication with the inlet and the outlet to form a fluid pathway, wherein the chamber comprises a first electrode and a second electrode.       
 
         [0024]    In certain embodiments, the apparatus is a microfluidic apparatus, e.g. comprised on a microfluidic chip. 
         [0025]    The radioisotope may be any desired radioisotope, such as a radioisotope for the preparation of a radiotracer. In some embodiments, the radioisotope is selected from  18 F,  99m Tc,  11 C and  68 Ga. 
         [0026]    In some embodiments, the radioisotope is a radioactive fluoride isotope, also referred to herein as  18 F-fluoride or  18 F. 
         [0027]    In some embodiments, the aqueous solution comprises  18 O-enriched water. The  18 O-enriched water may comprise  18 F. 
         [0028]    In some embodiments the method comprises providing the aqueous solution at a flow rate of no greater than approximately 1 mL/min. In some embodiments the method comprises providing the aqueous solution at a flow rate of no greater than approximately 0.5 mL/min. In some embodiments the method comprises providing the aqueous solution at a flow rate of at least 0.1 mL/min. The flow rate of the aqueous solution may be from approximately 0.1 mL/min to approximately 0.3 mL/min, for example approximately 0.2 mL/min. 
         [0029]    The total volume of aqueous solution flowed to and through the chamber may be from 0.1 to 1 mL, e.g. from 0.1 to 0.5 mL, for example from 0.2 to 0.4 mL, e.g. 0.3 mL. 
         [0030]    In some embodiments, the chamber has a total volume capacity of no greater than approximately 50 μL, for example no greater than approximately 40 μL, for example no greater than approximately 30 μL, for example no greater than approximately 20 μL, for example no greater than approximately 15 μL, for example no greater than approximately 10 μL, for example no greater than approximately 8 μL. A high ratio of the solution volume to cell volume is believed to improve the efficiency of release of the radioisotope. 
         [0031]    The chamber may form a part of a microfluidic cell. Aptly, the use of a microfluidic cell enables the manipulation of low volumes and is thus beneficial in that it generates less waste, reduces reagent costs and requires lower sample volumes compared to known batch processes. A microfluidic cell additionally enables faster response times due to short diffusion distances, small heat capacities and better process control due to the faster response of the system. A microfluidic cell is thus particularly suited to the dose-on-demand production of radiopharmaceuticals. 
         [0032]    In one embodiment, the microfluidic cell may form a part of a microfluidic system. The microfluidic system may be configured for the synthesis, purification and/or analysis of a radiopharmaceutical. The microfluidic cell may be in fluid communication with one or more other components of the system. In some embodiments, the microfluidic system comprises the microfluidic cell and at least one of a radiotracer production module and a quality control module. 
         [0033]    In some embodiments, the microfluidic cell is comprised in or constitutes a Radioisotope Isolation Module (RIM) configured to receive an aqueous solution comprising a radioisotope. 
         [0034]    In some embodiments, the microfluidic system comprises a Radioisotope Isolation Module (RIM), wherein the microfluidic cell is comprised in or constitutes the Radioisotope Isolation Module (RIM) and is configured to receive an aqueous solution comprising a radioisotope. 
         [0035]    In some embodiments, the microfluidic system comprises:
       a) a Radioisotope Isolation Module (RIM) as described herein; and   b) a Radiopharmaceutical Production Module (RPM) configured to receive a first sample which comprises a concentrated and activated radioisotope from the RIM.       
 
         [0038]    In some embodiments, the microfluidic system comprises:
       a) a Radioisotope Isolation Module (RIM) as described herein;   b) a Radiopharmaceutical Production Module (RPM) configured to receive a first sample which comprises a concentrated and activated radioisotope from the RIM; and   c) a Purification Module (PM) configured to receive a second sample which comprises a radiopharmaceutical and one or more further components from the RPM.       
 
         [0042]    In some embodiments, the microfluidic system comprises:
       a) a Radioisotope Isolation Module (RIM) as described herein;   b) a Radiopharmaceutical Production Module (RPM) configured to receive a first sample which comprises a concentrated and activated radioisotope from the RIM;   c) a Purification Module (PM) configured to receive a second sample which comprises a radiopharmaceutical and one or more further components from the RPM; and   d) a Quality Control Module (QCM), configured to receive a third sample from the PM which comprises a purified radiopharmaceutical and which is further configured to determine one or more characteristics of the third sample, wherein each of the modules is a microfluidic component.       
 
         [0047]    Aptly, the integrated system and the RIM, RPM, PM and QCM are provided on a unitary microfluidic device. 
         [0048]    Aptly, the Radiotracer Production Module (RPM) may comprise a micromixer and/or a microreactor. 
         [0049]    The microfluidic cell and/or system may be comprised on a microfluidic chip. 
         [0050]    In one embodiment, the first electrode may also be referred to as the working electrode. Aptly, the first electrode comes into contact with the flow of aqueous solution. 
         [0051]    The first electrode may be of any suitable shape, for example circular, ovoid, rectangular, square or triangular. Aptly, a cross sectional shape of the first electrode may be of any suitable shape, for example circular, oval, rectangular, square or triangular. Aptly, circular and rectangular electrodes have been shown to have excellent performance in both trapping and release of a radioisotope. 
         [0052]    In some embodiments, the first electrode comprises carbon. In some embodiments, the first electrode is formed from graphite. In some embodiments, the first electrode is formed from glass carbon. In some embodiments, the first electrode is a carbon disk. In some embodiments, the first electrode may be formed from a carbon rod. For example, the first electrode may be formed by slicing a thin section from a commercially available carbon rod, such as those sold by Goodfellow Cambridge Ltd. Aptly, the carbon rod is made of graphite but is hard, unlike normal graphite which is soft. Thus, in some embodiments the first electrode is not formed from standard graphite e.g. a layered graphite. 
         [0053]    In some embodiments, the first electrode has a hardness of more than 1.0, for example more than 1.1, for example more than 1.2, for example more than 1.3, for example more than 1.4, for example more than 1.5, for example more than 1.6, for example more than 1.7, for example more than 1.8, for example more than 1.9 and/or for example more than 2.0 on the Mohs scale. 
         [0054]    In certain embodiments, the first electrode has a hardness of more than 2.0 on the Mohs scale e.g. 2.1 or greater, e.g. 2.2 or greater, e.g. 2.3 or greater. 
         [0055]    SEM images have shown that, unlike soft graphite which has a layered structure, a section of carbon rod has a flat surface comprising a plurality of recesses. Thus, the first electrode may comprise an irregular surface. In use in the method of certain embodiments of the present invention, this irregular surface comes into contact with the flow of aqueous solution. This surface structure of the carbon rod is believed to be favourable for trapping and release of a radioisotope. It has also been found that, unlike soft graphite, the first electrode formed from the carbon rod is not deformed after trapping of a radioisotope, thereby significantly increasing the performance of the electrode. Thus, in some embodiments, the first electrode is not deformed in use. In further embodiments, the first electrode is capable of retaining its original form when a voltage of at least 0.5 V, for example at least 1 V, for example at least 3 V, for example at least 5 V, for example at least 10 V, for example at least 15 V or for example at least 20 V is applied across the electrodes. 
         [0056]    In some embodiments, the carbon rod has a purity of at least 99%, for example at least 99.5%, for example at least 99.8% or for example at least 99.9%. The carbon rod may comprise trace amounts of other elements such as, but not limited to, Al, B, Ca, Cu, Fe, Mg, Na and/or Si. Each of these elements may be present in an amount of no greater than 1%, no greater than 0.5%, no greater than 0.1%, no greater than 0.05% or no greater than 0.01%. 
         [0057]    A circular first electrode may be formed by cutting a section of a desired thickness from a carbon rod. If a non-circular electrode is required (e.g. rectangular), the section can be shaped using sand paper. A first electrode produced in this way may be referred to as a carbon disk. 
         [0058]    The first electrode may have a polished surface layer. This polished surface layer may come into contact with the flow of aqueous solution. Polishing is thought to clean the electrode surface and reduce the emission of particulates during use which can be adversely affect the performance of the electrode. 
         [0059]    The first electrode may have a thickness of from 0.1 mm to 0.5 mm, for example, from 0.2 mm to 0.3 mm, e.g. 0.25 mm. 
         [0060]    In some embodiments, the surface area of the first electrode which comes into contact with the flow of aqueous solution is at least 10 mm 2 , at least 15 mm 2 , at least 20 mm 2 , at least 30 mm 2 , at least 40 mm 2 , at least 50 mm 2 , at least 60 mm 2  or at least 70 mm 2 . In some embodiments, the surface area of the first electrode which comes into contact with the flow of aqueous solution is no greater than 100 mm 2 , no greater than 90 mm 2 , no greater than 80 mm 2 , no greater than 75 mm 2 , no greater than 55 mm 2 , no greater than 35 mm 2  or no greater than 25 mm 2 . 
         [0061]    The second electrode may also be referred to as the counter electrode. The second electrode may be formed from a transition metal. Suitable transition metals include for example gold, silver, iron, zinc, copper and platinum. In some embodiments, the second electrode is formed from platinum, for example platinum foil. Platinum is particularly suitable due to its electrochemical stability. 
         [0062]    In some embodiments, the distance between the first and second electrodes is no greater than 0.5 mm. In some embodiments, the distance between the first and second electrodes is at least 0.1 mm. In some embodiments, the distance between the first and second electrodes is from 0.1 to 0.5 mm, for example from 0.2 mm to 0.4 mm, for example from 0.25 mm to 0.3 mm. Aptly, the smaller the gap between the electrodes, the less the resistance, allowing a relatively low voltage to be used. 
         [0063]    In some embodiments, the first electric field is generated by applying a voltage of no greater than 50 V, for example no greater than 30 V, or for example no greater than 20 V across the first and second electrodes. In some embodiments, the first electric field is generated by applying a voltage of from 5 to 20 V, for example 10 to 20 V, e.g. from 14 to 20 V, across the electrodes. When the first electric field is generated between the electrodes, the first electrode will have a positive charge (i.e. the first electrode functions as the anode) such that negatively charged radioisotope ions present in the aqueous solution are attracted to the first electrode, causing them to be trapped on the first electrode. 
         [0064]    In certain embodiments, the efficiency of trapping of the radioisotope on the first electrode may be at least 94%. 
         [0065]    In some embodiments, the method comprises removing the aqueous solution from the chamber prior to providing the flow of the organic-based solution into the chamber. 
         [0066]    The organic-based solution may comprise an organic solvent. Suitable organic solvents include acetonitrile, DMF, DMSO and N-methylformamide. Because some radioisotope ions such as  18 F are insoluble in organic solvents, the organic-based solution may further comprise a phase transfer reagent. In some embodiments, the phase transfer reagent is kryptofix 222 (K222). The concentration of K222 may be from 0.027 to 0.08 M. 
         [0067]    Aptly, the organic solution further comprises a source of counter ions. Aptly, if the radioisotope ions are negatively charged, the counter ions will be positively charged. The counter ions may be alkali metal ions, such as sodium or potassium ions. In some embodiments, the organic solution comprises a source of potassium ions, such as K 2 CO 3  or KHCO 3 . The concentration of K 2 CO 3  or KHCO 3  may be from 0.01 to 0.04 M. 
         [0068]    In some embodiments, the organic solution further comprises from 1 to 10%, e.g. from 2 to 8%, aptly from 3 to 6% H 2 O by volume. In some embodiments, the H 2 O content of the organic solution is about 4%. Aptly, a small amount of water significantly increases the solubility of the insoluble radioisotope ions in the organic-based solution, thereby facilitating the release of the radioisotope from the first electrode. A water content of greater than 10% may have a detrimental effect on subsequent reactions of the released radioisotope. 
         [0069]    In some embodiments the method comprises providing the organic-based solution at a flow rate of at least 1 mL/min. The flow rate may be from 0.05 to 1 mL/min, for example 0.1 to 1 mL/min. 
         [0070]    In some embodiments the method comprises providing the organic-based solution at a flow rate of at least 0.05 mL/min. The flow rate may be from 0.05 to 0.5 mL/min, or example from 0.08 to 0.3 mL/min, for example from 0.1 to 0.2 mL/min. 
         [0071]    In some embodiments, the method comprises providing the aqueous solution at a higher flow rate i.e. at a greater flow rate than the flow rate that the organic-based solution is provided at. 
         [0072]    Aptly, the method comprises providing the aqueous solution at a flow rate of approximately 0.2 mL/min and the organic-based solution at a flow rate of approximately 0.1 mL/min. 
         [0073]    The total volume of the organic-based solution provided to the chamber may be from about 0.1 to 0.5 mL, for example from about 0.2 to 0.4 mL, e.g. 0.3 mL. 
         [0074]    In some embodiments, the method further comprises washing the chamber prior to providing the flow of an organic-based solution through the chamber. The wash step may be carried out after trapping the radioisotope on the first electrode. Washing may be carried out by flowing an organic solvent, or a solution comprising an organic solvent, through the chamber. The organic solvent may be the same as that comprised within the organic-based solution which is subsequently flowed into the chamber before release of the trapped ions is effected. The washing step helps to remove residual aqueous solution from the chamber. In some embodiments, the chamber is washed with acetonitrile. 
         [0075]    In some embodiments the second electric field is generated by applying a voltage of no greater than 20 V for example no greater than 10 V, e.g. no greater than 5 V across the first and second electrodes. In some embodiments the second electric field is generated by applying a voltage of from about 1 to 5 V, for example from about 1.5 to 4.5 V (e.g. from 1.6 to 4.1 V) across the electrodes. It will be understood that the voltages described herein in relation to the second electric field may also be expressed as negative values (e.g. −10 V or −5 V), since the second electric field has an opposite polarity to the first electric field. When the second electric field is generated, the first electrode gains a negative charge (i.e. it becomes the cathode). This causes the trapped radioisotope ions to be repelled and released from the first electrode. 
         [0076]    The trapping of radioisotope ions from an aqueous solution on an electrode, followed by release of the ions into an organic-based solution as provided by certain embodiments of the present invention thus offers a convenient method of solvent exchange which is simpler, faster and easier to automate than conventional techniques based on anion exchange and azeotropic evaporation. 
         [0077]    Thus, the method of the certain embodiments of the invention may also be for concentrating the radioisotope. 
         [0078]    The efficiency of release may be at least 90%, for example at least 95%, for example at least 96%, for example at least 97% or for example at least 98%. 
         [0079]    In some embodiments, the method further comprises heating the chamber and/or the organic-based solution. Heating may be carried out by placing the chamber e.g. within a microfluidic cell on a hotplate. Heating may be carried out prior to generating the second electric field. It has been found that heating can facilitate the release of the radioisotope from the second electrode, particularly when using small volumes of solution. The chamber and/or organic-based solution may be heated to a temperature of from 40 to 100° C., e.g. 50 to 100° C., e.g. 40 to 80° C., for example 40 to 60° C., or for example from 60 to 90° C. or for example from 70 to 80° C. 
         [0080]    In some embodiments, the chamber may be heated indirectly. For example, where the chamber is part of an integrated system, for example as part of a microfluidic system comprised on a microfluidic chip, it may not be necessary to place the chamber directly adjacent to or in direct contact with a heat source in order to heat the chamber. Heating may be achieved indirectly through heat transfer by heating another area of the integrated system, for example by heating another area of the microfluidic chip. 
         [0081]    Aptly, the method may further comprise reacting the radioisotope released from the second electrode with a precursor to provide a radiopharmaceutical or a radiotracer or an intermediate in the synthesis of a radiopharmaceutical. 
         [0082]    It will be appreciated that the precursor will be selected in accordance with the desired radiotracer. The radiotracer may be any compound which is prepared by nucleophilic substitution using  18 F. Examples of radiotracers include [ 18 F]-FDG, 3′-deoxy-3′-(18)F-fluorothymidine (FLT), FMISO and sodium fluoride. 
         [0083]    In embodiments wherein the desired radiotracer is [ 18 F]-FDG, the precursor is mannose triflate. Suitable precursors for the preparation of other common radiolabelled substances will be known by those skilled in the art. 
         [0084]    In some embodiments, the method comprises transferring the organic-based solution comprising the released radioisotope to a reactor where the radioisotope is reacted with the precursor. The reactor may form a part of the microfluidic cell or a separate device. 
         [0085]    In some embodiments, the method further comprises transferring the organic-based solution comprising the released radioisotope to a mixer where the radioisotope is mixed with the precursor. The mixture may then be transferred to a reactor where the reaction between the radioisotope and the precursor is effected, for example by heating. 
         [0086]    In some embodiments, a microfluidic system comprises a RIM and a RPM. The RIM comprises the chamber. The RPM comprises a microreactor and/or a micromixer. Where the organic-based solution comprising the released radioisotope is for transfer from the RIM to the RPM, heating of the chamber may be achieved by heating part or all of the RPM directly. The chamber is not heated directly but is heated indirectly through heat transfer from the RPM. The RPM may comprise a reactor. The reactor may be heated directly. The RPM may comprise a mixer. The mixer may be heated directly. 
         [0087]    Alternatively, the released radioactive isotope may be reacted with the precursor in the chamber. In some embodiments, the organic-based solution may additionally comprise the precursor such that the radioisotope reacts with the precursor in the chamber upon release of the radioisotope from the first electrode. Aptly, the cell may comprise one or more additional inlets for providing the precursor. 
         [0088]    The method may further comprise additional chemical processing steps which may be necessary for obtaining the final radiopharmaceutical. The additional chemical processing steps may include hydrolysis, deprotection and/or purification. For example, in embodiments wherein the radioisotope is [ 18 F]-fluoride and the precursor is mannose triflate, the nucleophilic substitution reaction generates acetylated [ 18 F]-FDG which must then be hydrolysed to remove the protecting groups in order to produce the final radiotracer [ 18 F]-FDG. The appropriate chemical processing steps required for the synthesis of a desired radiopharmaceutical will be apparent to those skilled in the art. 
         [0089]    The overall yield of radiolabelled substances prepared in accordance with the methods of certain embodiments of the invention may be at least 90% or at least 95%. 
         [0090]    In some embodiments, a method for separating and recovering a radioisotope from an aqueous solution comprising the radioisotope comprises:
       providing a flow of the aqueous solution to a chamber of a microfluidic cell, the chamber comprising a first electrode and a second electrode;   generating a first electric field between the first and second electrodes, thereby trapping the radioisotope on the first electrode;   providing a flow of an organic-based solution through the chamber comprising the first and the second electrodes;   generating a second electric field between the first and the second electrodes, wherein the second electric field has an opposing polarity to the first electric field, thereby releasing the radioactive isotope from the first electrode into the organic based solution,   wherein the first electrode is formed from a carbon rod,   wherein the chamber has a volume of no greater than approximately 40 μl,   wherein the first electric field is generated by applying a voltage of no greater than 30 V across the first and second electrodes,   wherein the second electric field is generated by applying a voltage of no greater than 5 V across the first and second electrodes,   optionally, wherein the organic-based solution comprises from 1 to 10% water;   optionally, wherein the first electrode has a polished surface layer;   optionally, wherein the flow rate of the aqueous solution is from approximately 0.1 mL/min to approximately 0.3 mL/min;   optionally, wherein the flow rate of the organic-based solution is from approximately 0.1 mL/min to approximately 0.2 mL/min;   optionally, wherein the surface area of the first electrode which comes into contact with the flow of aqueous solution is at least 20 mm 2 ; and   optionally, wherein the method further comprises heating the chamber and/or the organic-based solution to a temperature of from 40 to 100° C., e.g. 50 to 100° C., e.g. 40 to 80° C. prior to applying the second electric field.       
 
         [0105]    In some embodiments, the apparatus is a microfluidic cell. 
         [0106]    In some embodiments, the apparatus comprises a pair of opposing plates. The plates may support the first and second electrodes in a face-to-face arrangement. The plates may be formed from any suitable material such as glass, quartz, silicon or ceramics, or a polymer such as polymethylmethacrylate (PMMA), polycarbonate, polydimethylsiloxane (PDMS), polydicyclopentadiene (DCPD) and polyetheretherketone (PEEK). In some embodiments, the plates are made of glass. The plates may be machined or moulded to the required size and shape. 
         [0107]    In some embodiments, the plates are fixed together. The plates may be fixed together so as to provide a liquid-tight seal around the chamber. Any suitable means may be used to fix the plates together, including screws, adhesives, double-sided tape, ultrasonic bonding or thermal bonding. It will be appreciated that the type of fixing that is appropriate will depend on the material from which the plates are made. For plastic plates, double-sided tape, adhesives or ultrasonic bonding may be used. For glass plates, thermal bonding may be most suitable. The plates may be in direct contact in the assembled cell, or they may be spaced apart. 
         [0108]    In some embodiments, the apparatus further comprises a spacer or gasket between the opposing plates. The spacer may be formed from any suitable material. In some embodiments, the spacer is formed from silicon. 
         [0109]    A chamber may be defined, at least in part, by a cavity, channel or cut-out in one or both plates and/or in the spacer. In some embodiments, the chamber is defined by a channel in the spacer. The channel may be straight, or it may be serpentine. 
         [0110]    In some embodiments, the apparatus is configured to receive fluid at a flow rate of at least 1 mL/min, at least 0.05 mL/min, at least 0.1 mL/min, at least 0.2 mL/min or at least 0.3 mL/min. 
         [0111]    In some embodiments, the chamber has one or more side walls, a floor and a ceiling. The side wall(s) of the chamber may be formed by the plate(s) and/or the spacer. A part or the whole of the floor of the chamber may be formed by one of the first and second electrodes. A part or the whole of the ceiling of the chamber may be formed by the other of the first and second electrodes. 
         [0112]    The chamber may be any suitable size or shape. In some embodiments, the chamber has a rectangular cross-section. In other embodiments, the chamber is coin-shaped. 
         [0113]    The apparatus may further comprise leads which connect the electrodes to a circuit. One or more holes may be provided in one or both plates to enable electrical connection with the electrodes. 
         [0114]    In some embodiments, the inlet is provided by a hole or a channel in one of the pair of opposing plates. The outlet may be provided by a hole or a channel in the same plate in which the inlet is provided, or in the other of the pair of opposing plates. Aptly, the cell may comprise one or more further inlets. 
         [0115]    In some embodiments, the apparatus further comprises a heater. The heater may be configured for heating the chamber, or it may be configured for heating fluid before the fluid enters the chamber. In some embodiments, the heater is configured for indirect heating of the chamber. In some embodiments, the heater comprises a hot plate, a heating element or a heat mat. In some embodiments, the chamber, or the entire apparatus or microfluidic cell, is positioned on the heater. The heater may be manually or automatically operable. 
         [0116]    In some embodiments, the invention provides a microfluidic system comprising:
       a) a RIM as described herein, for example the RIM comprising an apparatus for separating and recovering a radioisotope from an aqueous solution comprising the radioisotope, the apparatus comprising:
           an inlet;   an outlet; and   a chamber in fluid communication with the inlet and the outlet to form a fluid pathway, wherein the chamber comprises a first electrode and a second electrode; and   
           b) a RPM, the RPM comprising a micromixer and/or a microreactor.       
 
         [0122]    Aptly the RIM constitutes or comprises a microfluidic cell of the invention. 
         [0123]    The microfluidic system may further comprise a heater configured to heat all or part of the RPM. Aptly the heater may be configured to heat the micromixer and/or the microreactor. Aptly heating all or part of the RPM results in heat transfer to the chamber of the RIM. In some embodiments, the heater comprises a hot plate, a heating element or a heat mat. The heater may be manually or automatically operable. 
         [0124]    Temperature plays an important role in the reaction rate and efficiency of chemical and electrochemical processes. Where one or more of these processes occurs within an integrated system, for example within a microfluidic system (e.g. in an integrated microfluidic chip), it may be advantageous to control and/or differentiate the temperature at which each process occurs. For example, it may be advantageous if the chamber is maintained at a temperature which is lower than that of all or part of the RPM. 
         [0125]    Aptly, the microfluidic system may further comprise a heat reduction means. The heat reduction means operates to reduce heat transfer from the RPM to the chamber when the RPM is heated directly. In some embodiments, the heat reduction means is a slot formed in the microfluidic system (e.g. in a microfluidic chip comprising the RIM and the RPM) between the chamber and all or part of the RPM. 
         [0126]    In some embodiments, there is a temperature difference of greater than 50° C. between the directly heated area of the RPM and the chamber, particularly when the RPM is heated to temperatures in excess of 90° C., for example to approximately 100° C. 
         [0127]    In some embodiments, an apparatus for separating and recovering a radioisotope from an aqueous solution comprising the radioisotope comprises a microfluidic cell comprising:
       an inlet;   an outlet; and   a chamber in fluid communication with the inlet and the outlet to form a fluid pathway, wherein the chamber comprises a first electrode and a second electrode,   wherein the first electrode is formed from a carbon rod,   wherein the chamber has a volume of no greater than approximately 40 μL,   optionally, wherein the first electrode has a polished surface layer,   optionally, wherein the distance between the first electrode and the second electrode is no greater than 0.5 mm,   optionally, wherein the surface area of the first electrode which comes into contact with the flow of aqueous solution is at least 20 mm 2 ;   optionally wherein the apparatus is a microfluidic device e.g. a microfluidic chip or cell       
 
         [0137]    In certain embodiments, the system further comprises a holder device for holding a microfluidic chip comprising a microfluidic cell. The holder device may comprise a holder body having a cavity configured to hold and secure the chip. Aptly, the holder body comprises an upper layer and a lower layer. Aptly, the upper layer comprises an upper surface which comprises one or more through holes configured to accommodate one or more connecting elements. Aptly, at least one of the one or more through holes is configured to accommodate an optical fibre and/or a wire element. Aptly, each through hole is configured to enable the connecting element to connect the chip to further downstream elements for example to a detector and/or a source to a detection zone of the chip. 
         [0138]    The apparatus of the present invention is simple and cheap to fabricate, and enables electro-trapping and release of radioisotopes with high efficiency. The compact design conveniently enables fast and safe dose-on-demand production of radiopharmaceuticals. The use of a microfluidic cell circumvents the need for azeotropic evaporation, thereby significantly shortening production time. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0139]    Certain embodiments of the present invention relate to a microfluidic cell, a microfluidic chip and system comprising such a cell and/or chip. A fluid flow can be described as “microfluidic” (i.e. “microfluidic fluid flow”) if a fluid passes through a channel having at least one dimension of less than 1 mm, in particular a channel having a dimension of less than 1 mm, e.g. less than 500 μm, less than 250 μm, less than 200 μm, or less than 150 μm. This creates laminar flow characteristics (generally having a Reynolds number of less than 100) where diffusion is the dominant cross stream chemical interaction. Consequently, microfluidic fluid flow occurs during the manipulation of small volumes, for example from 1 μm to 100 μl, within microstructured devices that features dimensions of the order of 10&#39;s to 100&#39;s micrometers. 
         [0140]    In certain embodiments, a “microfluidic flow system” comprises a system having at least one channel for fluid flow, the channel having at least one dimension of less than one 1 mm, for example less than 500 μm, e.g. less than 300 μm, e.g. less than 200 μm, e.g. less than 150 μm, e.g. less than 100 μm, e.g. less than 50 μm. The microfluidic flow system comprises a microfluidic device but may also comprise other components that are in fluid communication with the microfluidic device. 
         [0141]    In one embodiment, the system comprises one or more channels having a width of, for example, between about 100 μm to about 200 μm e.g. about 150 μm and a depth of for example about 50 μm deep. 
         [0142]    A “microfluidic chip” or a “microfluidic cell” can be identified by the fact that it has one or more channels with at least one dimension less than 1 mm, for example, less than 500 μm, e.g. less than 300 μm, e.g. less than 200 μm, e.g. less than 150 μm, e.g. less than 100 μm, e.g. less than 50 μm, in particular a channel having a dimension of less than 1 mm, e.g. for example, between about 100 μm to about 200 μm e.g. a width of about 150 μm and a depth of between about 40 to 60 μm, for example about 50 μm. The microfluidic chip may be part of a microfluidic flow system. The one or more channels may form a fluid flow path in the chip or cell. 
         [0143]    As used herein, a “radiopharmaceutical” is an isotopically labelled analogue of a pharmaceutical molecule. Aptly, a radiopharmaceutical can be used for diagnostic or therapeutic purposes. 
         [0144]    As used herein, a “radiotracer” is a radiopharmaceutical having a largely unaltered metabolic pathway compared to the unlabelled analogue. It is therefore possible to follow and quantify processes on a particular metabolic pathway by detecting the radioactive decay of the labelling radioisotope. Radiotracers are used for diagnostic purposes. Examples of radiotracers include, but are not limited to,  18 F-FLT ([ 18 F]fluoro thymidine),  18 F-FDDNP (2-(1-{6-[(2-[ 18 F]fluoroethyl)(methyl)amino]2-naphthyl}ethylidene)malonitrile),  18 F-FHBG (9-[4-[ 18 F]fluoro-3-(hydroxymethyl)butyl]guanine or [ 18 F]-penciclovir),  18 F-FESP ([ 18 F]-fluoroethylspiperone),  18 F-p-MPPF (4-(2-methoxyphenyl)-1-[2-(N-2-pyridinyl)-p-[ 18 F]fluorobenzamido]ethylpiperazine),  18 F-FDG (2-[ 18 F]fluoro-2-deoxy-D-glucose),  18 F-FMISO ( 18 F-fluoromisonidazole) and  18 F-sodium fluoride. 
     
    
     
         [0145]    Embodiments of the invention will now be described by way of example only and with reference to the accompanying Figures in which: 
           [0146]      FIG. 1  is an exploded view of a microfluidic cell in accordance with an embodiment of the present invention; 
           [0147]      FIG. 2  is a perspective view of the microfluidic cell of  FIG. 1 ; 
           [0148]      FIG. 3  is an exploded view of a microfluidic cell in accordance with another embodiment of the present invention; 
           [0149]      FIG. 4 a    is a schematic side view of a microfluidic cell in accordance with a further embodiment of the present invention; 
           [0150]      FIG. 4 b    is an exploded view of the microfluidic cell of  FIG. 4   a;    
           [0151]      FIG. 5  is a schematic diagram of a microfluidic system comprising the microfluidic cell of  FIG. 4 ; and 
           [0152]      FIG. 6 a    depicts a top view of a microfluidic system comprising a RIM and RPM as described herein which comprises a heat reduction means; and 
           [0153]      FIG. 6 b    illustrates the temperature difference observed between the directly heated area of the RPM and the chamber of the RIM as measured with an infrared thermometer; 
           [0154]      FIG. 7A-C  illustrate the microfluidic system of  FIG. 6  secured in a holder. 
       
    
    
       [0155]    With reference to  FIG. 1 , a microfluidic cell  10  comprises upper  12  and lower  14  rectangular glass plates, which have been machined to the desired size and shape. It will be appreciated that in alternative embodiments, the plates may be formed from other materials such as quartz or polymer. In the centre of the lower plate  14  there is a circular cut-out  16  which extends through the entire depth of the plate  14 , and which receives a carbon disk (10 mm diameter) that forms the working electrode  18 . The working electrode  18  is cut to size and polished. A first lead  20  connects the working electrode  18  to an electrical circuit (not shown). The working electrode  18  is secured in place by a first cylindrical glass liner  22  which is sized to fit into the cut-out  16  of the lower plate  14 . The glass liner  22  may be a stock component, or it may be machined to the required shape and size. 
         [0156]    The upper plate  12  has a circular cut-out  24  in a position corresponding to that of the cut-out  16  in the lower plate  14 . The upper plate further comprises two holes  26 , one either side of the cut-out  24 , each of which is fluidly connected to the cut-out  26  via a channel  28 . One of the holes  26   a  provides an inlet to allow fluids into the cell  10 , while the other  26   b  provides an outlet. A circular platinum counter electrode  30  (10 mm diameter) is received in the cut-out  24  in the upper plate  12 . The counter electrode  30  is connected to the circuit by a second lead  32 . The counter electrode  30  is secured in place by a second cylindrical glass liner  34  which is sized to fit into the cut-out  24  in the upper plate  12 . The glass liner  34  may be a stock component, or it may be machined to the required shape and size. 
         [0157]      FIG. 2  shows the cell  10  of  FIG. 1  with the components assembled. To assemble the cell, the upper and lower plates  12 ,  14  are aligned before being thermally bonded together. Once the plates  12 ,  14  are bonded the counter electrode  30  is placed within the cut-out  24  in the upper plate  12 . The electrode lead  32  is inserted and a sealant is applied to the interior surface of the cut-out  24  before inserting the second glass liner  34 . The sealant is allowed to set before the process is repeated for securing the working electrode  18  in the cut-out  16  in the lower plate  14 . 
         [0158]    Once assembled, the upper and lower plates  12 ,  14  together with the first and second glass liners  22 ,  34  define a chamber which houses the working electrode  18  and the counter electrode  30  in a face-to-face arrangement. The gap between the electrodes is 250 μm, and the volume of the chamber is 19.6 μL. Fluids can be flowed through the chamber via the inlet and outlet  26   a ,  26   b.    
         [0159]      FIG. 3  shows an alternative embodiment of a microfluidic cell  110  comprising upper  112  and lower  114  glass plates which are substantially square. The lower plate  114  comprises a cavity  116  which provides a chamber for electro-trapping and release. The cavity  116  can be any depth or shape. In the embodiment shown, the cavity  116  is key-shaped, i.e. it comprises a circular portion  116   a  and an elongate portion  116   b  extending from the circular portion  116   a . A sputter coated metallic layer formed on a surface of the cavity  116  provides the counter electrode  130 . Two holes  126  extend through the lower plate  114  into the cavity  116 , providing an inlet  126   a  and an outlet  126   b  for fluids. 
         [0160]    The microfluidic cell  110  further comprises a disk-shaped working electrode  118  formed from a carbon rod. The working electrode  118  is greater in diameter than the circular portion  116   a  of the cavity  116  in the lower plate  114  which forms the chamber. The working electrode  118  is housed within a circular recess  124  in the upper plate  112 . The upper plate  112  further comprises a first hole  132  in the approximate centre of the plate which extends into the recess  124 , enabling electrical connection of the working electrode  118 . A second hole  134  in the upper plate  112  is provided in a position corresponding to the elongate portion  116   b  of the cavity  116  in the lower plate  114 , thereby enabling electrical connection with the counter electrode  130 . 
         [0161]    To assemble the cell  110 , the upper plate  112 , lower plate  114  and working electrode  118  are loosely assembled and the plates  112 ,  114  aligned before the plates  112 ,  114  are thermally bonded together. The electrical contacts (not shown) are then secured in the holes  132 ,  134  in the upper plate  112  using a sealant 
         [0162]    With reference to  FIGS. 1-3 , an embodiment of a method of radioisotope recovery (solvent exchange) by electro-trapping and release using a microfluidic cell in accordance with the present invention will now be described. The method first comprises flowing  18 O-enriched water containing  18 F into the chamber of the microfluidic cell  10 ,  110  via the inlet  26   a ,  126   a . An electric field is generated between the electrodes  18 ,  118 ,  30 ,  130  by applying a voltage across the electrodes such that the negatively charged  18 F ions are attracted to the working electrode  18 ,  118 , causing them to be trapped on the working electrode  18 ,  118 . Waste H 2   18 O is removed from the cell  10 ,  110  via the outlet  26   b ,  126   b . The chamber is then washed by flowing acetonitrile through the chamber. An organic-based solution comprising K222 and K 2 CO 3  in acetonitrile (and optionally 4% H 2 O) is introduced into the chamber via the inlet  26   a ,  126   a . The polarity of the electric field is then reversed, causing the  18 F ions to be released from the working electrode  18 ,  118  into the organic-based solution. The organic-based solution containing the released  18 F ions can then be removed from the chamber via the outlet  26   b ,  126   b , and transferred to a separate reactor where the  18 F ions are reacted with a precursor to generate a radiotracer. Alternatively, the precursor may be provided in the organic-based solution so that the nucleophilic substitution reaction between the precursor and the  18 F ions occurs within the chamber. 
         [0163]    Turning to  FIG. 4 a   , in a further embodiment a microfluidic cell  210  comprises a rectangular upper plate  212 , an opposing lower plate  214  of the same size and shape as the upper plate  212 , and a silicon spacer  216  disposed between the upper and lower plates  212 ,  214 . The plates  212 ,  214  are machined to the required size and shape. The upper and lower plates  212 ,  214  and the silicon spacer  216  are held together within a clamp  217 . A cut-out is formed in the centre of silicon spacer  216 , thus defining a channel which constitutes a reaction chamber  215 . A rectangular working electrode  218  formed from a section of carbon rod is positioned against the upper plate  212  and forms a ceiling of the reaction chamber  215 . The working electrode  218  is polished prior to assembly of the cell  210 . A platinum foil counter electrode  230  is positioned on the lower plate  214 , facing the working electrode  218 , forming a floor of the reaction chamber  215 . The electrodes are held in place by the force of the clamp  217  and are sealed using sealant. Leads connect the working and counter electrodes  218 ,  230  to a circuit. Two holes  226  are formed in the upper plate which provide an inlet  226   a  and an outlet  226   b  to allow fluids to flow through the chamber  215  via capillary tubing.  FIG. 4 b    shows an exploded view of the microfluidic cell  210 . 
         [0164]      FIG. 5  shows a microfluidic system  240  for the preparation of a radiotracer. In the embodiment shown, the system is set up for the synthesis of [ 18 F]-FDG, but it will be appreciated that the system could be used to prepare any desired radiopharmaceutical. The system eliminates evaporation steps for solvent exchange, and enables dose-on-demand production of radiotracers in an integrated system. 
         [0165]    The system  240  comprises the microfluidic cell  210  for solvent exchange of  18 F by electro-trapping and release. The microfluidic cell  210  is supplied with fluids from a set of four syringes, a first syringe  242  containing a source of  18 F, a second syringe  244  containing acetonitrile for washing, a third syringe  246  containing an organic-based solvent and a fourth syringe  248  containing water. A line  250  connects the syringes  242 ,  244 ,  246 ,  248  to the inlet  226   a  of the microfluidic cell  210 . The flow of fluids into the cell  210  is controlled by a first valve  252  positioned at a point at which the line  250  diverges between a first branch  254  leading to the first syringe  242  and a second branch  256  leading to the second  244 , third  246  and fourth  248  syringes. 
         [0166]    The system further comprises a microfluidic reactor  258  and a fifth syringe  260 , which contains mannose triflate. A flow path  262  connects the fifth syringe  260  to the microfluidic reactor  258 . A line  264  feeds fluids from the outlet  226   b  of the microfluidic cell  210  into the flow path  262 . A second valve  266  is positioned at the junction of the line  264  with the flow path  262 , and controls the flow of fluids into the microfluidic reactor  258 . 
         [0167]    The system additionally comprises a C 18  deprotection column  268 , a sixth syringe  270  containing water for elution and a seventh syringe  272  containing sodium hydroxide for hydrolysis. A flow path  274  supplies fluids from the sixth and seventh syringes  270 ,  272  to the deprotection column  268 . A line  276  feeds fluids from the microfluidic reactor  258  into the flow path  274 . A third valve  278  is positioned at the junction of the line  276  with the flow path  274 , and thus controls the flow of fluids into the deprotection column  268 . 
         [0168]    Recovery of  18 F from an aqueous solution into an organic-based solution by electro trapping and release is carried out using the microfluidic cell  210 , as described above. The organic-based solution containing the released  18 F ions leaves the microfluidic cell  210  via the outlet  226   b , and is transferred to the microfluidic reactor  258  via the line  264 . Mannose triflate is supplied to the microfluidic reactor  258  from the fifth syringe  260  via the flow path  262 . Within the microfluidic reactor  258  the mannose triflate undergoes a nucleophilic substitution reaction with the  18 F ions, producing acetylated [ 18 F]-FDG. The acetylated [ 18 F]-FDG is then transferred to the deprotection column  268  via line  276  and flow path  274 , where it undergoes hydrolysis in the presence of a sodium hydroxide solution to produce [ 18 F]-FDG. 
         [0169]      FIG. 6 a    shows a top view of a microfluidic system  300  for the preparation of a radiotracer. In certain embodiments, the system  300  is a combined RIM and RPM as described herein. In the embodiment shown, the system comprises a microfluidic cell  310  comprising a chamber  311  for solvent exchange of  18 F by electro-trapping and release. Adjacent to the microfluidic cell is valve  330 . The system further comprises a microfluidic reactor  320 . Area  321  of the microfluidic reactor is heated directly by means of a heater (not shown) positioned directly below area  321 . Heat reduction means is provided in the form of a slot  340 . The slot reduces heat transfer from the directly heated area ( 321 ) to the chamber of the microfluidic cell.  FIG. 7A-C  illustrates the microfluidic system of  FIG. 6  which secured in a holder. The system may further comprise one or more valves which can control flow of solutions within and/or between each module. 
         [0170]      FIG. 6 b    is a graphical representation of the temperature difference between area  321  and microfluidic cell  310  as measured by an infrared thermometer following direct heating of area  321 . 
       Examples 
       [0171]    Microfluidic cells similar to that shown in  FIG. 4  were fabricated by clamping together two glass plates, a pair of electrodes (including a platinum counter electrode) and a silicon spacer comprising a channel. Details of the cell s are provided in Table 1. 
         [0172]    The cells were tested with a low volume of  18 O-water comprising  18 F from an ABT cyclotron. Trapping and release efficiencies were probed at different voltages with variation of the flow rate. The results are shown in Tables 2 and 3. Efficiencies of up to 99% in trapping (Table 2) and up to 98% in release (Table 3) were observed. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Cell 
                   
                   
               
               
                 design 
                 Working electrode 
                 Cell volume (μL) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 Graphite (15 × 4 mm) 
                 30 
               
               
                 2 
                 Carbon disk (10 mm 
                 32 
               
               
                   
                 diameter) 
               
               
                 3 
                 Carbon disk (10 mm 
                 16 
               
               
                   
                 diameter) 
               
               
                 4 
                 Carbon 8 × 3 × 0.25 mm 
                 6 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Trapping conditions 
                   
               
             
          
           
               
                   
                 Volume 
                   
                   
                   
               
               
                 Cell 
                   18 O-water 
                 Flow rate 
                   
                 Trapping 
               
               
                 design 
                 (mL) 
                 (mL/min) 
                 Voltage (V) 
                 efficiency % 
               
               
                   
               
               
                 1 
                 0.2 
                 0.2 
                 20 
                 99 
               
               
                 2 
                 0.3 
                 0.2 
                 20 
                 99 
               
               
                 3 
                 0.3 
                 0.2 
                 15-20 
                 98 
               
               
                 4 
                 0.3 
                 0.2 
                 14-20 
                 96-99 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Release conditions 
                 Release 
               
             
          
           
               
                 Cell 
                   
                 Volume 
                 Flow rate 
                 Voltage 
                   
                 efficiency 
               
               
                 design 
                 Solvent 
                 (mL) 
                 (mL/min) 
                 (V) 
                 Temperature 
                 % 
               
               
                   
               
               
                 1 
                 Water 
                 0.2 
                 0.1 
                 1.6-3.8 
                 r.t. 
                 44-69 
               
               
                 2 
                 Water-K222 
                 0.3 
                 0.1 
                 1.6-3.8 
                 r.t. 
                 94 
               
               
                 2 
                 K222-K 2 CO 3   
                 0.3 
                 0.1 
                 1.6-3.8 
                 r.t. 
                 85 
               
               
                   
                 MeCN-4% H 2 O 
               
               
                 2 
                 K222-K 2 CO 3   
                 0.3 
                 0.1 
                 1.6-4.1 
                 r.t. 
                 73 
               
               
                   
                 MeCN 
               
               
                 3 
                 K222-K 2 CO 3   
                 0.1 
                 0.1 
                 1.6-4.1 
                 r.t. 
                 76 
               
               
                   
                 MeCN-4% H 2 O 
               
               
                 3 
                 K222-K 2 CO 3   
                 0.1 
                 0.1 
                 1.6-4.1 
                 80° C. 
                 98 
               
               
                   
                 MeCN-4% H 2 O 
               
               
                 4 
                 K222-K 2 CO 3   
                 0.1 
                 0.1 
                 1.6-4.1 
                 r.t. 
                 82 
               
               
                   
                 MeCN-4% H 2 O 
               
               
                 4 
                 K222-K 2 CO 3   
                 0.1 
                 0.1 
                 1.6-4.1 
                 80° C. 
                 96-98 
               
               
                   
                 MeCN-4% H 2 O 
               
               
                   
               
             
          
         
       
     
         [0173]    In cell design 1 graphite was used as the anode (the working electrode). A trapping efficiency of 99% was achieved at 20 V within 1 minute but only 44-69% release efficiency could be obtained within 3 minutes. The low release efficiency was thought to be due to deformation of the graphite electrode. The cell was modified by replacing the graphite electrode with an electrode produced by cutting a small piece from a carbon rod (cell design 2, 3 and 3) and a significant improvement in the release efficiency was observed (Table 3). No significant difference in performance of cells 3 and 4 was observed over more than 40 runs, demonstrating excellent stability. 
         [0174]    A system similar to that shown in  FIG. 5  was fabricated and used to synthesise [ 18 F]-FDG in accordance with the following method. 0.2-0.3 mL of  18 O-enriched H 2 O containing  18 F was passed through the cell at a flow rate of 0.2 mL/min under a constant electric potential (14-20 V) applied between the carbon and Pt electrodes. The cell was then washed by flushing with anhydrous MeCN (0.5 mL/min, 1 minute) while the voltage was disconnected. Next, 0.1 mL of organic-based solution containing K222 and KHCO 3  in MeCN—H 2 O (1-10%) was passed through the cell at flow rate of 0.1 mL/min under a reversed potential (2-4 V). The cell was heated to a pre-set temperature of 80° C. and the released solution was stored in a sample loop. The released solution containing  18 F, K222 and KHCO 3  was pushed by MeCN at flow rate of 0.02 mL/min to mix with 0.1 mL of mannose triflate solution (0.02 mL/min) inside a Y-micromixer. The mixture was then transferred to a microreactor (volume 0.05 mL) heated to 100° C. The reaction solution was mixed with H 2 O at a flow rate of 0.04 mL/min and then passed through a C18-monolith column for trapping the labelled precursor. The monolith was washed with water and dried with N 2 . 0.4 mL of 2 N NaOH solution was loaded into the monolith and hydrolysis was maintained at room temperature for 2 minutes. The product [ 18 F]-FDG was eluted out with 1-5 mL of water. 
         [0175]    Using this system,  18 F fluoride (initial activity up to 30 mCi) can be trapped with an efficiency of 94-99% and subsequently released with a release efficiency of 96-98%. Using the organic solution containing released  18 F fluoride for fluorination of mannose triflate, up to 100% acetylated [ 18 F]-FDG. (ACY-[ 18 F]-FDG) can be obtained. 100% ACY-[ 18 F]-FDG can be obtained within 1.2 min at 100° C. After basic hydrolysis on the deprotection column (aptly a monolith) 98.3% [ 18 F]-FDG was obtained. Optionally, the [ 18 F]-FDG can be further purified by passing through cation-, anion-, silica- and C18-monoliths. 
         [0176]    The use of microfluidic cells for processing of low volume of heavy water feed from the cyclotron offers an excellent method for solvent exchange to carry out nucleophilic substitution reactions with standard precursors. In addition to the excellent trap and release performance, the benefits of the invention include the use of a low voltage, re-use of electrodes and simple operation. The present invention thus offers significant potential in efficient dose-on-demand radiotracer production. 
         [0177]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to” and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0178]    Features, integers, characteristics or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of the features and/or steps are mutually exclusive. The invention is not restricted to any details of any foregoing embodiments. The invention extends to any novel one, or novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. 
         [0179]    The reader&#39;s attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.