The poisoned samples from the Herbarium Centrale Italicum: A study of the mercury contamination by ion beam analysis techniques

Museum herbaria are collections of preserved, dried plants, bearing an exceptional value as archives of botanical history and biodiversity, an invaluable resource for modern research in several fields. The Museum of Natural History of the University of Florence (Italy) houses the Florentine Botanical Collections (FI), which include some of the most ancient and largest herbaria in the world, such as the Herbarium Centrale Italicum (FI-HCI). Like many herbaria worldwide, these collections are affected by Hg contamination resulting from the historical use of corrosive sublimate (HgCl2) as a pesticide for specimen preservation. Although this practice was discontinued over a century ago, elemental Hg degassing from the samples and residual Hg compounds on them prevent the full accessibility and valorisation of these collections.This work aims at characterizing the concentrations and in-depth distributions of Hg and other possibly related elements (Cl, and S) within contaminated samples from the Florentine collections using external beam ion beam analysis (IBA) techniques. Nine representative specimens – eight from the 19th century and a recent, untreated one (blank) – were selected for PIXE and EBS measurements, with a 3 MeV proton beam, at the INFN LABEC 3 MV Tandem accelerator.The study revealed that contamination variably affects both the plant matrix and the support paper: Hg concentrations measured on dark stains on paper, which were identified as superficial accumulations of Hg, were significantly higher than those measured on clear paper and on the plant matrix, where Hg penetrates deeply instead. It was also possible to make some hypotheses about the speciation of Hg in the samples.