text: An alternative approach used to measure the relative abundance of radiogenic isotopes when working with a solid surface is secondary-ion mass spectrometry (SIMS).  This type of ion-microprobe analysis normally works by focusing a primary (oxygen) ion beam on a sample in order to generate a series of secondary positive ions that can be focused and measured based on their mass/charge ratios.
  SIMS is a common method used in U-Pb analysis, as the primary ion beam is used to bombard the surface of a single zircon grain in order to yield a secondary beam of Pb ions.  The Pb ions are analyzed using a double focusing mass spectrometer that comprises both an electrostatic and magnetic analyzer.  This assembly allows the secondary ions to be focused based on their kinetic energy and mass-charge ratio in order to be accurately collected using a series of Faraday cups.
  A major issue that arises in SIMS analysis is the generation of isobaric interference between sputtered molecular ions and the ions of interest.  This issue occurs with U–Pb dating as Pb ions have essentially the same mass as HfO.  In order to overcome this problem, a sensitive high-resolution ion microprobe (SHRIMP) can be used.  A SHRIMP is a double-focusing mass spectrometer that allows for a large spatial separation between different ion masses based on its relatively large size.  For U-Pb analysis, the SHRIMP allows for the separation of Pb from other interfering molecular ions, such as HfO.