or is a precursor group selected from halogen or trialkylstannyl that in the transformation depicted scheme 2 becomes an occurrence of r 1 in formula i which is a radioisotope of either hydrogen or a halogen the intermediates of formula xii and the processes for transforming compounds of formula xii to compounds of formula i are yet further aspects of the invention particular embodiments of this aspect of the invention are described below one or more occurrences of r 2 in formula xii is halogen preferably bromine or iodine and is transformed to a compound of formula i wherein the corresponding occurrence of r 1 is tritium by a process comprising treatment of the compound of formula xii with tritium gas in the presence of a transition metal catalyst suitable transition metal catalysts include palladium platinum rhodium which may be in the form of the element including as metal blacks oxides hydroxides and on various supports in a particular embodiment of this aspect of the invention the compound of formula xii is one or more occurrences of r 2 in formula xii is a trialkylstannyl group for example a trimethylstannyl group or a tributylstannyl group and is transformed to a compound of formula i wherein the corresponding occurrence of r 1 is halogen by a process comprising treatment of the compound of formula xii with an electrophilic form of a halogen radioisotope suitable electrophilic forms of the halogen include the elemental halogen the n halosuccinimide.