Trace element AMS at NRL: Initial use of a modified SIMS ion source

There has been recent interest to attach a high-energy ion accelerator to the output of a secondary ion mass spectrometer (SIMS). NRL has undertaken such an effort, to improve the rejection of molecular ions and thus lower the limit of detection for masses above 90 Da. In this mass range molecules containing H, C, N, O and other light species that are present can create molecular interferences with M/ΔM values greater than 10,000. These create a background difficult to remove in conventional SIMS analysis. Using a tandem accelerator and a gas stripper cell, molecules can be broken apart, leaving only atomic ions to measure. By measuring the energy of collected ions, residual interferences caused by molecular fragments in altered charge states can be rejected. The NRL AMS facility provides a further enhancement, in that this system is designed to transmit ions in parallel over a broad mass range. This enables higher precision measurements, for example of isotope ratios, but also allows efficient and sensitive data collection over a relatively broad mass spectrum. This is important, for example, for Genesis Return Mission samples that have trapped solar wind atoms in the surface of collector plates of various materials. This paper presents initial results obtained using the new configuration of the NRL AMS system.