Quantitative chemical analysis of ocular melanosomes in stained and non-stained tissues

► Combined quantitative EELS and EDX yielded the chemical composition of melanosomes. ► Melanosomes of rat RPE and choroid and of sepia ink sac were analyzed. ► Eumelanin and pheomelanin can be distinguished using the S/N mole fraction ratio. ► Trace elements, particularly Zn, can be analyzed quantitatively if staining was omitted. ► Melanosomes have especially high N and O mole fractions and show clear Ca-O correlation.