Tracking donor cells in a popliteal lymph node of a BALB/c mouse that has been injected in the footpad with 2 × 107 spleen cells from a C57Bl/6 mouse fed for 2 weeks on a 15N diet. (a,b) Parallel quantitative mass imaging at 12C14N- and 12C15N-. (a) A mosaic of 12C14N- images, showing the topology of a lymph-node section. (b) A mosaic of the 12C15N-/12C14N- ratio images, showing 15N-labeled donor cells. Tile field: 100 μm × 100 μm (256 × 256 pixels); acquisition time 2 min per tile (16-tile mosaic). (c-e) Higher-resolution parallel mass imaging at 12C-,13C-, 12C14N- and 12C15N- of the field indicated by the arrow in (a,b). (c) 12C14N- image, (d) 12C15N-/12C14N- ratio image and (e) 13C-/12C- ratio image of the same field. Field: 30 × 30 μm (256 × 256 pixels); acquisition time 40 min.