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Electrochemical drying of [F-18]fluoride

Overview

Photograph and schematic of electrochemical trapping

Figure 1: (Left) Photograph of Pt-transition metal electrode cell assembled with fluidic and electrical connections. (Top center) Cross section of the flow cell. A patterned adhesive layer defining the fluid path (bottom center) is sandwiched between an adsorption electrode and a reference electrode. Fluid flows a serpentine path to maximize time spent within the electric field between the two electrodes. During trapping (top right), the negatively charged [F-18]fluoride ions are attracted to the positively charged adsorption electrode and adsorb to the surface while the [O-18]water continues to the outlet. (Middle right) [F-18]fluoride is dried using an anhydrous solvent. (Bottom right) [F-18]fluoride is released into a solution suitable for downstream radiochemical synthesis while applying a (generally smaller) reverse potential.

In the vast majority of F-18 radiochemistry, [F-18]fluoride ion is incorporated into the tracer via a nucleophilic substitution reaction. Prior to such reaction, it is important to thoroughly remove water, which otherwise strongly interacts with [F-18]fluoride and reduces its reactivity.
In conventional radiochemistry systems, water is typically removed from [F-18]fluoride via a combination of solvent-exchange on a quaternary methyl ammonium (QMA) cartridge followed by several steps of azeotropic distillation (with acetonitrile). An alternative drying method based on solvent-exchange within an electrochemical cell has been reported, and has recently been implemented in a microdevice. The [F-18]fluoride in [O-18]water is flowed through the cell and the [F-18]fluoride ion is attracted to the positive electrode. Once all water has been eliminated, the [F-18]fluoride is released into an anhydrous solution.  Aside from the convenience of integration, electrochemical separation of [F-18]fluoride from water offers additional advantages such as faster drying, complete dehydration of the adsorbed fluoride ions without the need for repeated azeotropic drying and potential use of the platform for monitoring residual water concentration in the cell (via electrical impedance measurement).

To date, there have been reports that have investigated the effects of various carbon electrode geometries on the fluoride extraction and release. All existing electrochemical solvent exchange approaches use glassy carbon or graphite as the anode electrode in combination with a metal cathode (usually platinum). However, glassy carbon surfaces degrade with use, resulting in low efficiencies and large deviations in trap and release after multiple experiments. In addition, the erosion of the glassy carbon can be associated with the release of particulates and clogging of downstream microfluidic components, adversely affecting the performance and reliability of the synthesis.

Results

We have developed an electrochemical cell with two metal electrodes that eliminate these shortcomings. We investigated a number of different metals, observing marked differences in performance of trapping from water and release into an organic solvent dependent on the nature of the metal.  X-ray photoelectron spectroscopy (XPS) analysis of the surfaces of several metals used in the electrochemical cell provided some insight into the unique fluoride adsorption and desorption properties. The performance of the device showed a relation to the surface oxides, depth of intercalation, and the nature of chemical bonds formed on the surface during the electrochemical process. By avoiding the use of glassy carbon, the rapid electrode erosion is eliminated, making the cell reusable with consistent performance. Furthermore, operation at higher voltages (which improves trapping speed but would have accelerated the erosion of glassy carbon) is possible, allowing efficient trapping at much faster speeds compared with that of glassy carbon. The same cell was used in more than 100 experiments over a period of several months with only a minor decrease in performance and no need for replacement or repair. 

Related Publications

  • S. Sadeghi, V. Liang, S. Cheung, S. Woo, C. Wu, J. Ly, Y. Deng, M. Eddings, and R. M. van Dam, “Reusable electrochemical cell for rapid separation of [18F]fluoride from [18O]water for flow-through synthesis of 18F-labeled tracers,” Applied Radiation and Isotopes, vol. 75, pp. 85–94, May 2013. (PMC Link)

Team Members

Former:

  • Saman Sadeghi (postdoctoral scholar)
  • Vincent Liang (undergraduate student)
  • Shilin Cheung (postdoctoral scholar)
  • Suh Woo (undergraduate student)
  • Curtis Wu (undergraduate student)
  • Jimmy Ly (graduate student)
  • Yuliang Deng (exchange student)
  • Mark Eddings (postdoctoral scholar)