Developing a simple screening method for the identification of historic biocide residues on herbarium material in museum collections

The National Museum of Wales houses c.250,000 higher plant specimens, with material dating back to the 17th century. Herbaria have been a major source of botanical research and reference for centuries and the collections have increased over time from donations and through collecting. Due to its organic content, botanical material is susceptible to insect and fungal attack. Even aged, dried material is a source of sugar and protein. Institutions and collectors have prevented such attacks through the application of pesticides. Treatments containing compounds of arsenic, lead and mercury were commonplace, and have remained stable over time. Consequently, present-day handling of these collections presents a potential health risk to staff and visitors through inhalation and skin absorption, particularly since the quantity and nature of the pesticide applied is unknown. Occasionally the residues are visible, but research has shown that herbarium sheets, which appear untouched, have been previously treated, and contain high concentrations of toxic metals. The use of a UV hand-held lamp has helped to identify sheets that have been treated, even though treatment is not visible to the naked eye. The UV causes areas to fluoresce on the herbarium mount sheet. These areas were analysed by particle induced X-ray emission (PIXE), and have been found to correlate with pesticide applications containing mercury and arsenic. This research has shown the feasibility of using a hand-held UV lamp as a screening method for identifying contaminated samples within museum collections and provided a means to prioritise which collections require immediate re-mounting