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The technique for tagging isobars in a mixed beam by measuring energy loss by time-of-flight has been tested. With this method, isobar separation should improve by allowing more energy logs (thicker absorber), but only if one can control the absorber homogeneity. Measurements of beam energy loss and energy spread obtained under such conditions were shown to be close to predicted values using both collisional and charge exchange contributions to energy straggling. The calculation of energy straggling allows us to study the efficacy of this method for isobar separation when applied to different mass ranges and beam energies. Partial separation in a most difficult case, an analyzed beam of A = 132 isobars at energies near 3 MeV/A, has been demonstrated. The time-of-flight information can be added online as an additional tag to the data stream for events of interest. Such event-by-event tagging enables one to study the effect of differences in isobaric mixture in the beam on the reaction outcome even when isobar separation is not complete.