1. W.G. Pfann, “Why Ultra-Pure Metals?,” in Ultra-High Purity Metals, American Society for Metals, Metals Park Ohio, Reinhold Publishing Corp., 1962, p. 1–11.
2. G. Horz, and E. Fromm, “Niob,” in Gase und Kohlenstoff in Metallen, ed. by E. Fromm and E. Gebhardt; Springer-Verlag, Berlin, 1976, p. 460–494.
3. E. Fromm, “Interaction of Oxygen and Nitrogen with Clean Transition Metal Surfaces,” Surface Science 79(1978) p. 259–275.
4. J. Bigot, “Le Niobium,” in Monographies sur les metaux de haute purete, tome 2, ed. by G. Chaudron; Masson, Paris, 1977, p. 472–496.
5. G. Tölg, “Bulk and Distribution Analysis-Linked Strategy in Analytic Chemistry of High-Purity Materials,” Proc. 5th Int. Symp.: High Purity Materials in Science and Technology, Vol. II., Dresden, GDR, 1980, p. 53–73.