title: Isotope amount ratio measurement challenge.
authors: Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116, Braunschweig, Germany. olaf.rienitz@ptb.de.
authors: Olaf
authors: O
authors: Rienitz
authors: Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116, Braunschweig, Germany.
authors: Axel
authors: A
authors: Pramann