1. Art Institute of Chicago, Department of Conservation and Science, 111 S. Michigan Ave, Chicago, IL 60603, USA
2. Center for Scientific Studies in the Arts, Northwestern University, Tech Building, 2145 Sheridan Road, Evanston, IL 60208, USA
3. Radboud University, Institute for Molecules and Materials, (Analytical Chemistry & Chemometrics), P. O. Box 9010, Nijmegen, 6500 GL, The Netherlands