Enhancement of negative ion currents by argon/CO 2 mixtures in an electron impact ion source

Argon and mixtures of argon with carbon dioxide in the range of 1–50% have been used as moderator gases for negative ion production in an electron impact ion source of a VG ZAB‐2F mass spectrometer. The enhancement of quasi‐molecular anion ([M – H] − ) currents of oxygen‐containing samples has been studied. It is a function of the composition of moderator gas mixture, moderator gas pressure, and sample pressure in the ion source. Most intensive quasi‐molecular anion currents have been observed at a pressure in the ion source of c. 2.5 × 10 −5 Torr (measured by the ion gauge remote from the ion source of the ZAB‐2F) using a moderator gas mixture of 10% CO 2 in argon. Fragmentation of the samples may be increased by increasing the concentration of CO 2 in the moderator gas.