Positive ion SIMS-SSAMS for trace analysis of materials

Abstract

By using a single-stage accelerator mass spectrometer (SSAMS) as a detector for a secondary ion mass spectrometry (SIMS) instrument, spatially resolved trace analysis of elements and isotopes in solid materials is significantly improved. This is especially true for masses greater than ∼90 u, where a mass resolving power in excess of 10,000 is generally needed to separate abundant molecular isobars from atomic ions of interest, particularly when the atomic species are present in trace quantities. Prior techniques using SIMS in high-resolution mode can suffer from residual molecular interferences that effect detection limits and isotope analysis precision. The 300 kV SSAMS at the U.S. Naval Research Laboratory typically measures +1 ions up to mass 300 u, as they are more abundant for most elements. Additionally, there can be no molecule fragments in a lower charge state at the same m/q and E/q. Interferences from higher charge states either injected into the SSAMS or generated by the stripper tube are limited to mimicking masses below ∼120 u. Molecule fragments can also be chosen for analysis, when a molecular ion is more abundant than an atomic one. Electrostatic peak switching through the SIMS and SSAMS magnets enables rapid cycling up to ±6.5% about a central mass during a depth profile measurement. For typical operating conditions using Ar gas in the stripper tube, molecular ions are reduced by 4–6 orders of magnitude while better than 20% of desired ions are transmitted. These numbers depend on gas thickness in the stripper, mass of the ion, and molecule involved. Cross sections for molecule destruction and ion transmission, and charge state +1 ion yields are described for a number of different stripper gases and ions. Example trace analysis measurements demonstrate the instrument’s capabilities.