Kinetic-energy-sensitive mass spectrometry for separation of different ions with the same m/z value

A double-focusing mass spectrometer (MS) equipped with a superconducting-tunnel-junction (STJ) detector has been applied to measure relative ionization cross-sections for the production of ions that are accompanied by different ion species with the same mass-to-charge (m/z) value. The STJ detector fabricated for this study enables kinetic energy (E) measurement of incoming individual ions at a counting rate of up to ~100 k ions/s and an energy resolution (ΔE/E) of 15%. Both high counting rate and high-energy resolution are necessary to independently determine both m and z and not the m/z value only in ion-counting MS experiments. Ions such as ¹⁴N₂ ²⁺ and ¹⁴N⁺ with the same m/z value can be clearly discriminated using a kinetic-energy-sensitive MS. This fine discrimination capability allows direct determination of relative ionization cross-sections of the homonuclear diatomic ions ¹⁴N₂ ²⁺/¹⁴N₂ ⁺ and ¹⁶O₂ ²⁺/¹⁶O₂ ⁺, which are difficult to measure due to the strong interference by the signals of their dissociated atomic ions with noticeably large ionization cross-sections. The new instrument requires no low-abundance heteronuclear diatomic molecules of the forms ¹⁴N¹⁵N or ¹⁶O¹⁷O to carry out ionization studies and thus, is expected to be useful in fields such as atmospheric science, interstellar science, or plasma physics. Copyright © 2008 John Wiley & Sons, Ltd.