We have designed and tested a transition radiation detector able to select different mass particles in a high-energy unseparated beam and to provide a prompt identification signal that can be used as a first-level trigger for the physical event acquisition. A measurement on electron/hadron test beam at CERN is presented: on the basis of the results obtained we show that we can promptly separate electrons from pions starting from 1 GeV/c momentum and we discuss about the possibility to distinguish as well pions from protons in the hundred GeV/c range of momentum at the level of a few percent of particle mis-identification. (C) 2001 Elsevier Science B.V. All rights reserved.