[11C]Methyl iodide is the principle reagent for introduction of C-11 into radiopharmaceuticals. Although there have been many variations on the method used for its production, all methods involve the reduction of [11C]CO2 with LiAIH4 to provide the [11C]methyl alkoxy aluminate followed by conversion with HI or triphenylphosohine diiodide to [11C]CH3I. There are two problems with the existing methodology. First, even under the most carefully controlled conditions, the specific activity of [11C]CH3l can be significantly lowered by adsorption of atmospheric CO2 and carbonaceous material in the LiAlH4 reagent solvent. Second, the methyl alkoxy aluminate and excess LiAlH4 must be decomposed to liberate [11C]methanol prior to conversion to (II C]CH3I. This requires that any residual activity must be allowed to decay and the apparatus thoroughly cleaned, dried and set-up before additional [11C]CH3l preparations. We are currently examining the feasibility of the on-line halogenation of [11C]methane over various supported Lewis acid and platinum catalyst (1). [11C]CO2 is quantitatively converted to [11 C]CH4 by passage over Ni catalyst at 400 C. Methane has been reported to be converted to a mixture of methylene chloride and methyl chloride by reaction of chlorine over 10% ZrOF2/AL2O3 (2). Conditions necessary for improving the selectivity for monohalogenated species are currently under investigation. A new C-11 target manifold has been designed and fabricated which allows for the on-line analysis of target gases and products obtained from subsequent chemistry. The GC utilizes a state of the art helium discharge ionization detector (DID) capable of detecting products in the 50 ppb range, below that necessary for high specific activity determinations.
