Novel use of an isotope separator to determine the position of fluorine-18 in labelled 1,1,1,2-tetrafluoroethanes

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 (t1/2 = 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 [CF2 18F]+. fragment (m/z = 68) is collected, measured and divided by the integrated mass of the simultaneously collected ion beam for the [CF3]+. 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 [CH2 18F]+. fragment (m/z = 32) and the integrated mass of the simultaneously collected ion beam from the [CH2F]+. 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 [18F] fluoride with trifluoroethylene was found to have 97% selectivity for the CF3 group, whereas labelling by the reaction of [18F] fluoride with 2,2,2‐trifluoroethyl p‐toluenesulphonate was found to have 91% selectivity for the CH2F 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.