NOVEL USE OF AN ISOTOPE SEPARATOR TO DETERMINE THE POSITION OF F-18 IN LABELED 1,1,1,2-TETRAFLUOROETHANES

A novel technique is described for measuring the site selectivity of m ethods for labelling the major CFC-alternative, 1,1,1,2-tetrafluoroeth ane (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 i sotope separator. Radioactivity associated with the ion beam of the [( CF2F)-F-18]+. fragment (m/z = 68) is collected, measured and divided b y 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 'speci fic radioactivity' of the radiolabel in the 2-position is calculated f rom the measured radioactivity of the ion beam from the [(CH2F)-F-18+. fragment (m/z = 32) and the integrated mass of the simultaneously col lected 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 t he reaction of [F-18] fluoride with trifluoroethylene was found to hav e 97% selectivity for the CF, group, whereas labelling by the reaction of [F-18] 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 i ts inhalation as a drug propellant. The described technique is of pote ntially wider value for determining the position of fluorine-18 in lab elled polyfluorinated molecules.