‘Dash on the Poison': Analysis of an 1837 Bound Herbarium Laced with Colorless Arsenic Trioxide

Abstract

Herbaria, collections of dried plants, have been essential resources in the study of botany for hundreds of years. Like other natural historians, botanists have always struggled to protect their specimens from insects, and heavy metal pesticides were considered indispensable until relatively recently. Prominent nineteenth-century botanists usually recommended the liberal ‘poisoning’ of dried plant specimens with ‘corrosive sublimate,’ mercury (II) chloride in alcohol, among other pesticides. This study describes the instrumental analysis of a book of dried plant specimens assembled by an anonymous botanist in 1837. It was suspected that the plants in this bound herbarium had been poisoned with corrosive sublimate but, instead, arsenic was identified throughout with X-ray fluorescence (XRF) spectroscopy. Further investigations were made with Fourier transform infrared spectroscopy (FTIR), micro-Raman spectroscopy, and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDX), to identify the source of the arsenic in the volume. XRF and SEM-EDX confirmed that a colorless arsenic compound, likely arsenic trioxide, had been dissolved into the gum Arabic adhesive that affixes the plant specimens to each page. The authors are not aware of other instances in which a poisoned adhesive was used to mount herbarium specimens. This botanist’s unusual technique appears to have been largely successful at both securing the plant specimens to the book’s pages and protecting them from insect activity. The ubiquity of arsenic in nineteenth-century life is discussed, in addition to historical pest control measures, and the USA's gradual regulation of arsenic, mercury, and other poisonous substances.