Minimisation of pressure dependent mass discrimination in the ion source of the Helix MC Plus noble gas mass spectrometer

When tuned for maximum sensitivity, the electron bombardment ion source on the ANU (the Australian National University) Helix MC Plus noble gas mass spectrometer produces an Ar ion beam that is mass discriminated as a function of Ar partial pressure. This effect can be reduced substantially or eliminated by adjusting the repeller and trap voltages, although with a sensitivity loss of up to 50%. A recommended procedure for tuning an ion source to reduce pressure dependent mass discrimination is described. Similar procedures can be applied to other noble gases and other ion sources to ensure that pressure dependent mass discrimination is avoided.