Abstract
A novel technique is described for measuring the site selectivity of methods for labelling the major CFC‐alternative, 1,1,1,2‐tetrafluoroethane (HFA 134a), with fluorine‐18 (t~1/2~ = 109.7 min). The carbon–carbon bond in radiofluorinated HFA 134a is broken in the ion source of an isotope separator. Radioactivity associated with the ion beam of the [CF~2~ ^18^F]^+.^ fragment (m/z = 68) is collected, measured and divided by the integrated mass of the simultaneously collected ion beam for the [CF~3~]^+.^ fragment (m/z = 69) to give the ‘specific radioactivity’ (in nCi nmol^–1^) of the radiolabel in the 1‐position. Similarly, the ‘specific radioactivity’ of the radiolabel in the 2‐position is calculated from the measured radioactivity of the ion beam from the [CH~2~ ^18^F]^+.^ fragment (m/z = 32) and the integrated mass of the simultaneously collected ion beam from the [CH~2~F]^+.^ fragment (m/z = 33). The selectivity of the labelling procedure for a particular position is then given by the decay‐corrected ratio of specific radioactivity at that position to the sum of specific radioactivities. The labelling of HFA 134a by the reaction of [^18^F] fluoride with trifluoroethylene was found to have 97% selectivity for the CF~3~ group, whereas labelling by the reaction of [^18^F] fluoride with 2,2,2‐trifluoroethyl p‐toluenesulphonate was found to have 91% selectivity for the CH~2~F group. This information is of value for tracer studies of the fate of HFA 134a in man following its inhalation as a drug propellant. The described technique is of potentially wider value for determining the position of fluorine‐18 in labelled polyfluorinated molecules.