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 [CF218F]+. 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 [CH218F]+. 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.