Labelling of the solvent DMSO as side reaction of methylations with n.c.a. [11C]CH3I

Competing labelling of solvent dimethyl sulfoxide (DMSO) can occur during the 11C-methylation of amine precursors. A kinetic analysis of the methylation reaction of DMSO with n.c.a. [11C]CH3I was performed at 120°C resulting in rate constants. The rate constant for the formation of the intermediate, methylated DMSO ([11C]DMSO-M), is compared to the reaction of [11C]CH3I with two tertiary amines, namely Dexetimide and Desmethyloxotremorine-M. The specific activity of the labelled product is reduced due to partial 12C-methylation of the precursor amines by [11C]DMSO-M in cases of significant DMSO labelling as side reaction.