A process is provided for synthesizing metal salts of perfluorinated polyethers containing at least one carboxylic acid group. The method comprises the step of reacting 2-hydroxy-6-aminocarbonyl naphthalene and a isocyanate in an organic solvent at a temperature of 90\u2013200\xb0 C. The target may also comprise at least two different foils layered sequentially one of the other, wherein X-rays are produced when an electron beam impinges the foil closest to the source fo the electron beam; wherein the energy of the electron beam is selectively changed to produce X-rays of a least one preselected energy characteristic of at least one of the foils.