Before entering a mass spectrometer, a beam of singly charged ions are filtered by a velocity selector with crossed electric and magnetic fields of E=374 V/m and B=4 mT, respectively, such that v, E and B are mutually perpendicular to one another. The ions entering the slit are forced to move in a semi-circular path due to the presence of a uniform magnetic field Bo=0.14 T perpendicular to the plane of the ion trajectory, and produce a spot a distance D=22 cm from the entrance slit, on a sensitive plate. Calculate the mass (in au, atomic unit) of an ion in the beam. Electronic charge e=1.610-19 C, 1 au=1.6710-27 kg.