Scientists have embarked on a groundbreaking journey, firing lasers at Charles Darwin's invaluable specimens, and here's why. These specimens, collected by Darwin and fellow naturalists aboard the HMS Beagle during their voyage to the Galapagos from 1831-1836, have been carefully preserved in rows of sealed jars in London's Natural History Museum for over two centuries. Darwin's observations during this voyage laid the foundation for his renowned theory of natural selection and evolution. The specimens, including mammals, reptiles, fish, and shrimps, have provided scientists with invaluable insights. However, the question of the liquids in which these priceless specimens are suspended has remained unanswered until now.
The preservation fluids used throughout history have varied, from alcohols like ethanol and methanol to the late 19th-century favorite, formaldehyde. Dutch anatomist Frederik Ruysch's method involved steeping aromatic spices in an ethanol-water base, while French histologist Pol Bouin favored a recipe of formaldehyde, picric acid, and acetic acid. German pathologist Carl Kaiserling's technique involved sequential dipping in formaldehyde, potassium nitrate, and glycerin. These variations have led to heterogeneity across collections, making it challenging to determine the precise preservation fluids used.
To probe the insides of the jars without endangering them, a team of scientists, including Wren Montgomery and Sara Mosca, employed a portable laser spectroscopy technique called spatially offset Raman spectroscopy (SORS). SORS measures the 'excitement' in a material's molecular structure after a laser hit, revealing its chemical makeup. However, traditional Raman spectroscopy wouldn't work for these jars, as the laser light is scattered within the first few hundred micrometers, dominated by the container's surface.
SORS overcomes this by taking multiple Raman measurements at different offsets, allowing the researchers to accurately identify the preservation fluids in nearly 80% of the jars. The study revealed that mammals and reptiles were often 'fixed' in formalin and suspended in ethanol, while invertebrates, especially jellyfish and shrimps, were stored in formaldehyde or buffered formaldehyde with glycerol or phenoxetol for tissue integrity.
This technique is not only crucial for Darwin's collection but also for the over 100 million fluid-preserved specimens in museums worldwide, many of which are too fragile to open. By monitoring and caring for these invaluable specimens without compromising their integrity, scientists can ensure their preservation for future research. The research was published in ACS Omega, marking a significant advancement in the care and preservation of historical specimens.
