Secondary Ion Mass Spectrometry (SIMS) Microscopy

The aim of SIMS microscopy is the elemental and isotopic analysis of a solid surface by an ion beam coupled with a mass spectrometer. This technique is based upon the sputtering of a few atomic layers from the surface of a sample, induced by the bombardment of focused primary ions (O2+, O−, Cs+, or Ar+) with sufficiently high energy [several kiloelectron volts (KeV)]. These primary ions penetrate the solid surface and transfer some of their kinetic energy to the target particles, creating collision cascades that induce the emission of surface particles (atoms or molecules) in a charged or uncharged state. The secondary ions are representative of the elemental and isotopic composition of the bombarded analyzed area. They are accelerated and analyzed with a mass spectrometer (electrostatic sector and magnetic sector) on the basis of the mass-to-charge ratio. The sputtered ions stemming from each point of the bombarded surface are focused into an image by an immersion objective lens. The SIMS analysis was performed using a CAMECA IMS 4F E7 instrument. For this study, O2+ beam bombardment was used to enhance the ionization field of electropositive species such as uranium [Energy (E)=12 KeV; Intensity (I)=500 picoampere (pA)]. In this scanning microscope, the primary beam was focused into a small shaft (around 0.5μm), which scans the sample surface. The collected secondary ions were measured with an electron multiplier and also sequentially converted into an image. Mass resolution could reach 10,000 (M/ΔM), and the lateral resolution of the imaging was 0.5μm. For each area analyzed, mass spectra at around the mass of U238 and ion images were obtained. Na+23 images gave the histological structure of the sample, and U+238 images showed uranium fixation within the structures. Analyses were performed as follows: two controls, two animals of series 1, and five animals of series 2 were analyzed. For each animal, an average of 10 areas (200×200μm) were fully mapped for Na+23 and U+238.