Slower Py–GC–MS Unravels Cross-link Relationship in Amber
Photo Credit: Richard Stanley/Getty Images
Amber is formed from plant resin that has polymerized and fossilized to form a resilient natural polymer. Jennifer Poulin and Kate Helwig, conservation scientists from the Canadian Conservation Institute in Ottawa, Canada, have developed a novel pyrolysis gas chromatography–mass spectrometry (py-GC–MS) method to characterize the structure of Class I amber that could be applied to other natural polymeric structures.1 Published in the journal Analytical Chemistry, the method demonstrates how a slower py-GC–MS technique can be performed to analyze natural polymers that contain cross-linked chemical moieties.
Amber can be split into different categories, the most common being Class I amber that has four subcategories — Ia, Ib, Ic, and Id. Class I amber is characterized by its extremely resilient structure formed by a polylabdanoid matrix, making simple sample extraction near impossible and ensuring that its structure has remained mysterious. Jennifer Poulin told The Column: “Our study into the chemical structure of these resilient natural polymers was born through the specific needs of our clients. Historically, amber was highly traded in Canada and many First Nation objects contain amber components. We received requests from archaeologists who had recovered amber objects at sites across Canada and they wanted to know if we could determine the origin of the amber. It was thought that amber discovered at one site might have originated from another geographical area, thus, possibly establishing trade routes.”
In a previous study, Poulin and Helwig had collected amber samples recovered from 11 different sites in Canada for analysis using py-GC–MS to characterize amber structure based on polyabdanoid compounds.2 This led to the establishment of Class 1d amber, similar in structure to Baltic amber, but can only be found in Canada. It also contained high levels of succinic acid.
At this point, the research was concluded, but the introduction of a new GC–MS system with a thermal separation probe (TSP) led the authors to analyze the samples with a longer pyrolysis time at a slightly lower temperature. Poulin told The Column: “Although designed more specifically for thermal desorption experiments, at that time I was deeply interested in the pyrolysis of amber and I tested it using the new Class Id amber at the TSP’s maximum temperature (450 ºC) and maximum heating rate (900 ºC/min).” She added: “I was surprised to see peaks eluting quite late in the chromatogram, which I had not observed using traditional flash pyrolysis methodology.”
Succinylated ozol pyrolysate compounds were detected, showing that the succinic acid acts as a cross-linker within the Class 1d amber, which was again seen when applying the method to Baltic amber. “Naturally, I wanted to try this pyrolysis methodology on Baltic amber, as researchers have hypothesized for decades that the macromolecular structure is esterified through succinic acid linkages to communol moieties in the macromolecule structure. I was terrifically excited to find similar larger fragments of the polymer eluting for Baltic amber samples as well.” According to Poulin, this points to succinic acid as a key contributor to the strength, resilience, and stability of Class 1d and Baltic amber.
Poulin said: “Now utilizing pyrolysis with a slower heating rate, lower final temperature and longer isothermal dwell time, larger fragments of the polymer remain intact and show us a better picture of how the macromolecules are constructed. One obvious future application of the slower pyrolysis method is the continuation of our research to determine if any form of cross-linking is occurring in the other Class I ambers that do not contain succinic acid. Certainly, there are some Class Ib ambers that have shown to be more resilient than others.”
Although the method was developed for amber analysis, Poulin told The Column that it could also have interesting applications for the characterization of other natural polymeric systems. — B.D.
1. J. Poulin and K. Helwig, Analytical Chemistry DOI: 10.1021/ac501073k (2014).