Fig. 2

Chemical imaging of Na-containing biological particles. a SEM image b elemental Na map and c EDX spectra of the biological particle. The Cu and Si in the EDX spectra are background peaks originating from the substrate and various instrument parts inside the SEM chamber. STXM images of the same biological particle: d pre edge (1070 eV), e Na peak (1079 eV) and f Na optical density map. Color bars indicate optical density. g–i Representative NanoSIMS images of ²³Na⁺ of selected biological particles showing various distributions of sodium within particles contours. Scale bars are 2 µm