Amber, a relatively simple resin secreted by trees, is among nature’s sturdiest and perhaps most elegant compounds. It is tough enough to withstand millions of years’ worth of environmental abuse, yet it remains capable of capturing light in a gem-like fashion suitable for jewelry.
“They will put our modern, synthetic polymers to shame every single time,” says Jennifer Poulin, a senior conservation scientist with the Canadian Conservation Institute in Ottawa. “These wonderful natural polymers have survived ice ages, prolonged UV radiation and huge geographic upheavals. Yet they remain strong and resilient,” Poulin says.
Whether polished or raw, the ancient compound amber holds up under scientific scrutiny.Photo credit: Canadian Conservation Institute
Despite her fascination with the stuff, however, amber has been frustrating Poulin for much of her career with the institute. She and colleague Kate Helwig, as well as other researchers around the world, have been trying to unravel the intricate polymeric links that provide these remarkable properties. Poulin’s interest in characterizing amber stemmed from requests by archaeologists, who wanted to know if First Nations objects containing amber components found at sites in Canada might actually have originated elsewhere across the country or even Europe, thereby demonstrating an ancient trade route. “You can’t extract the individual polymeric units from amber with solvents,” Poulin says, referring to a desire to determine the geographic provenance of any given piece of amber by assessing its mix of components. “They’re strongly connected together and we only see the compounds that are freely contained within the macromolecular structure. So it’s very difficult to classify something that you can’t fully characterize.”
During her search for distinct chemical features to make the appropriate comparison, Poulin subjected samples to pyrolysis-gas chromatography-mass spectrometry, a flash heating method of sample introduction that breaks apart the amber matrix into individual units, which then are separated and detected. This method is used throughout the world to identify the polymeric structure and to classify amber.
Nevertheless, it was still not enough to satisfy Poulin. She adapted a thermal separation probe to perform the pyrolysis step of the analysis. This allowed the amber samples to be heated more slowly and to a lower final temperature. When she did so, the amber polymers were broken into not only the individual units, but also into larger molecular fragments that showed important linkages within the structure. The results were particularly exciting for Baltic amber and amber from the Canadian High Arctic, which showed internal crosslinking of the polymer matrix by succinic acid. “Although these succinic acid cross-linkages were first hypothesized in 1972, no one has been able to show definitive proof,” she says. “Here we finally have molecular evidence of this cross-linking.”
The Canadian Conservation Institute has been working with a collection of amber taken from 11 sites across the country. This discovery not only introduces a new means of classifying these samples but opens up the possibility of tackling other natural polymers in the same way. “We now have the ability to more fully understand some of the mechanisms that help to make this remarkable substance so resilient and tough,” Poulin adds.