Use of 11C as a tracer for studying the synthesis of [ 11C]urea from [ 11C]cyanide

The use of reversed-phase liquid chromotography and radiochemical detection with carbon-11 (t 1 2 = 20.4 min) as a tracer allowed the study of the preparation of [ 11C]urea from [ 11C]cyanide at no carrier-added concentrations. [ 11C]cyanate was readily prepared by permanganate oxidation of [ 11C]cyanide at 75°C. The conversion of NH 4O 11CN (≈0.03 mM) to [ 11C]urea in the presence of excess ammonium ions (0.28 M) was found to best fit pseudo first order reaction kinetics with a rate constant of 0.065 ± 0.008 min −1 at 75°C. Heating at higher temperatures (180–200°C) revealed that the conversion of NH 4O 11CN to [ 11C]urea occured in high yield in less than 3 min. The hydrolysis of [ 11C]cyanate to [ 11C]carbonate, a possible side reaction, was found to proceed at a rate of 0.010 ± 0.001 min −1 at 113°C.