High mass resolution SIMS

We are developing a method to conduct SIMS analysis at high mass resolution (m/Dm > 50,000), to facilitate the examination and study of complex organic and biomolecules on surfaces. The approach uses a primary-ion beam probe (rastered 25 keV Gaþ ion source), providing high (100 nm) spatial resolution, and an ion cyclotron resonance (ICR) cell, capable of mass analysis at a resolution in excess of 105 and mass accuracy of less than 1.0 ppm. The apparatus includes a time-of-flight (ToF) mass analyzer, offering rapid chemical mapping at low (m/Dm < 10,000) mass resolution to identify points of interest for subsequent high mass resolution analysis. To combine both ICR and ToF mass analysis in one instrument requires the sample to be near ground potential, to inject secondary ions into the grounded ICR cell at low enough kinetic energies to permit high trapping efficiency. Consequently, the optics and detector for ToF analysis must be electrically floating at the desired acceleration potential for the secondary ions. For high-resolution mass analysis, secondary ions are transmitted to the ICR cell, which is in line-of-sight to the sample and immersed in a homogeneous magnetic field (7 T). Initial results from the instrument show that it is possible to trap simple ions effectively for high-resolution analysis, but high-mass ions (m/z > 300), although, readily detected by the ToF analyzer, are inefficiently transmitted to the ICR cell. This has stimulated the design of a new ion optics coupling arrangement, which provides a higher mass resolution over a wider mass range. # 2004 Elsevier B.V. All rights reserved.