The corrected fluorescence counts for each metal atom are shown in histogram format (i.e., sample well counts minus counts from a blank well). The corrected counts for nickel are very far above background. The inset shows electron orbitals in schematic form with incident X-ray, the transition from higher to lower energy orbitals, and emission of X-ray fluorescence illustrated. The incident X-ray (E_o) knocks out a 1s electron, the unstable core hole is filled by the subsequent transition (seen in color), and a fluorescent X-ray of energy characteristic for the metal atom is emitted (E_f).