Parameters of carbon labelling using ionized gases

We have recently described a rapid non-synthetic method for producing¹⁴C-and¹¹C-labelled biomolecules. The apparatus consists of a high vacuum system in which small amounts of¹⁴CO gas are ionized by electron impact. The resulting ionized, excited and dissociated species drift toward a thin layer of organic molecules where they interact to produce¹⁴C-labelled compounds. In this paper, details are given on the mechanisms of interaction of the electron beam and on the parameters influencing the labelling yields. Using cholesterol as a model compound labelling yields were measured while the electron energy, thickness of the organic film, gas pressure and time of exposure were varied over a wide range. The results suggest that¹⁴C⁺ and/or¹⁴CO⁺ are the principal species involved in the labelling reactions. Supported by the Medical Research Council of Canada Grant MA-6137.