Principle of SXRF imaging. (a) X-ray fluorescence is based on the photoelectric effect: an incident X-ray from the synchrotron source is absorbed by an atom of the target, transferring its energy to an inner shell electron and causing this electron to be ejected, creating a vacancy. This vacancy is filled by an electron from the outer shells, resulting in the emission of an X-ray with an energy characteristic of the excited atom and equal to the difference between the two electron binding energies. (b) Schematic of a synchrotron facility, the linear accelerator (linac) that first accelerates the electrons and provides the initial energy, the booster ring where the electrons are accelerated to high energies (up to several GeV), the storage ring where the electrons are stored and their trajectory is bent using bending magnets, and insertion devices to produce the synchrotron radiation that is directed to the experimental stations called beamlines. (c) Typical XRF spectrum of a biological sample (neuron) showing the characteristic X-rays emitted by the chemical elements that constitute the sample. (d) Nano-SXRF elemental maps (P, S, Cl, K, Ca, Br, Cu, and Zn) of the dendrites of a single primary rat hippocampal neuron. Right: optical imaging of the same primary rat hippocampal neuron in culture expressing the GFP fluorescent protein postsynaptic density PSD-95. The rectangle indicates the area scanned during analysis. Min-max range bar units are arbitrary. Scale bar: 10 µm. Adapted from Perrin et al. [30].