SIMS Isotopic Measurements at High Mass Resolution

One of the most promising capabilities of the ion microprobe mass analyzer which has not been fully realized so far is the measurement of isotopic ratios in small samples. In situ measurement of mineral assemblages in the ion probe avoids the time consuming desaggreagation of the sample into its mineral components [1]. However, the ubiquitous presence of interfering species (oxides, hydrides and other polyatomic ions as well as multiply charged ions) in the secondary ion spectrum has in the past restricted isotopic measurements by SIMS to elements like Li, Be, B [2] or Mg [3–8] where interferences are either virtually absent or their contributions can be corrected for. In all these studies interferences were small and/or the measured isotopic effects large. In the case of Pb in zircons [9] the number of interfering species is large and their contributions considerable. As a consequence the validity of correction procedures is questionable. Energy filtering offers the possibility of eliminating molecular interferences [10] but the method suffers from a large loss in the secondary ion signal and cannot be applied in the case of interfering hydrides and multiply charged ions. Another approach is to separate off interfering ions by using a secondary ion mass spectrometer capable of high mass resolution. The recently commercially available CAMECA IMS 3F achieves a mass resolving power which allows the elimination of most of the main molecular interferences in the atomic mass range 1–60. This opens the possibility to extend isotopic measurements to other elements and to cases where contributions from interfering ions are dominant [11].