**Acknowledgement**

This work was funded by the Research fund of the University of Applied Sciences Niederrhein in Krefeld / Germany. The authors are grateful to the staff of Shimadzu Europe, Duisburg, Germany, specially to Uwe Oppermann who promoted the work with technical support concerning atomic absorption spectroscopy. Thanks to the library of St.Josef, Krefeld, Germany (C. Zettner) and the Brigitten Bibliothek, Kaldenkirchen, Germany (K.J. Dors †) for the possibility to measure books out of their book inventory.
