Comparative study of specific activity of [¹¹C]methyl iodide: A search for the source of carrier carbon

Any no-carrier-added ¹¹C-compounds prepared from [¹¹C]CH₃I seem to coexist with at least about 0.2 μmol of its non-radioactive form (carrier). In order to elucidate the source of the carrier, ¹¹C]CH₃I prepared under various conditions was converted into several ¹¹C-compounds and their specific activities were measured by high performance liquid chromatography. Carbon contamination of LiAlH₄ was also examined by gas chromatography. It was shown that only a negligible amount of CO₂ was brought with the irradiated target N₂ gas and that the carrier amount decreased remarkably with the reduction of the amount and concentration of the LiAlH₄THF solution but not to less than 0.2 μmol. The use of Et₂O or Pr₂O instead of THF was found to give EtOH or PrOH, respectively, together with MeOH, all in small amounts. Hence, the 0.2 μmol carrier can be regarded as being originated in MeOH formed from a trace of THF remaining in the LiALH₄ after its drying step.