Abstract
A high sensitivity mass spectrometer with very low background was constructed for quantitative determinations of extremely small amounts of gas isotopes. The improvement of the ion source was carried out in order to reduce the background ions originating in the ion source itself. The ions desorbed from the electrode surface of the ion source by electron bombardment were found to give a large fraction of the background ions at pressures below 10-9 Torr. The peak heights of these surface ions, e. g., H+, F+ and Cl+ changed with the geometrical structure and the operating condition of the ion source in a remarkably different way compared with the residual gas ions. The surface ions could be distinguished through their distinct feature of the dependence of their peak heights on the repeller voltage, and their reduction was accomplished by the improvement of the structure of the ionization chamber.
