Packed capillary liquid chromatography with electrospray mass spectrometry was used for direct determination of the specific radioactivity by calculation of isotope ratios between the Br-76- and Br-76-labeled analogues of N-((3 aminomethyl)benzyl) 4-bromobenzamide. Using 20 muL injections on packed capillary columns, sufficient mass sensitivity was attained for the determination on an injected amount of radioactivity corresponding to approximately 2 MBq (0.3 pmol of the Br-76 isotopic analogue). NUCL MED BIOL 27;6:851-853, 2000. (C) 2000 Elsevier Science Inc. All rights reserved.