The masses of the neutron-deficient nuclei 7 0 , 7 1 Se have been measured using a direct time-of-flight technique following the fragmentation of a 7 8 Kr beam (73 MeV/nucleon). Mass excesses of -62.31 ± 0.35(syst.) ± 0.30(stat.) and -63.49 ± 0.25(syst.) ± 0.20(stat.) MeV, respectively, were deduced. In addition, a novel technique based on stripping was developed for the purification of heavy fragmentation beams. This technique provides a good selectivity - typically isotopes of a single Z are transmitted - without altering the beam quality.