Researchers Use Ultra-High Resolution Mass Spectrometry to Analyze Ancient Bitumen

Components of the oldest bitumen sample were detected in an ancient vase by Russian scientists, who also accurately determined the age of this sample. The Journal of Mass Spectrometry features an article where the researchers recommend a new and efficient method to carry out organic compound analysis and introduce customized software. These researchers are from the Moscow Institute of Physics and Technology (MIPT), the Skolkovo Institute of Science and Technology (Skoltech), the Institute for the History of Material Culture, the Talrose Institute for Energy Problems of Chemical Physics, and the Emanuel Institute of Biochemical Physics of the Russian Academy of Sciences (IBCP RAS).

This image shows bitumen amphora. CREDIT: MIPT

A type of petroleum commonly found in natural deposits is bitumen. Bitumen has been used since the Stone Age. For instance, the word ‘mummy’ is derived from the Persian ‘mūm’, or ‘bitumen’, as this substance was utilized in embalming. Bitumen was used in medicine, construction, and warfare by the Greeks. There is a possibility that the renowned ‘Greek fire’ is based on bitumen. The earliest amphora containing bitumen (5ᵗʰ century BCE) was found by Russian archeologists on the Taman Peninsula, which is a highly volcanically active region with several petroleum seeps and a potential source of bitumen imported by the Greeks.

More oxygen as time passes by

Ancient bitumen sample analysis can disclose their age and origin. A mummy has been used by US scientists to prove that early Egyptian bitumen did not appear entirely from the Dead Sea. If the bitumen sample age from the amphora is around 2,500 years, this leads to an extended period of biodegradation because of the bacterial activity. This results in oxidation of the organic molecules in bitumen, which is the steady introduction of supplementary oxygen into the sample. This proves that older samples possess more oxygen atoms.

Elemental analysis was carried out on the Taman sample and it specified an oxygen (O) content of 11%, differing from the 1% or less in fresh petroleum samples, while the other elements such as carbon (C), nitrogen (N), hydrogen (H), and sulfur (S) were present in the usual amounts.

This shows that the sample had undergone degradation for a very long time within the amphora; for around 2,500 years. However, elemental analysis cannot spot the types of molecules present in the sample. The scientists used ultrahigh-resolution mass spectrometry to identify the molecules.

Mass spectrometry is an analytical method that arranges charged particles in a magnetic and/or electric field on the basis of their mass-to-charge ratio (m/z). The molecules with an initial z value of 0 are said to be charged (ionised) molecules. A magnetic/electric field assists in the spatial separation of ions. The detector finds a particle’s mass-to-charge ratio based on its ‘landing’ point. A mass spectrum is a graph of detector signal intensity (on the vertical axis) vs mass-to-charge ratio (on the horizontal axis). A mass spectrum is a pattern of peaks, each of them equivalent to an ion with a particular mass-to-charge ratio. It helps identify the components in the sample.

A closer look

Differentiating the various peaks in the bitumen mass spectra from each other is difficult. To carry out this process, the scientists used the highly developed ultrahigh-resolution mass spectrometry techniques developed at the lab, which enabled them to differentiate molecules with different masses, by only a part of that of an electron. None of the sole components in the bitumen sample among the thousands have escaped their attention. The researchers also found the elemental composition of the components.

The analysis showed that amongst substances in the Taman bitumen sample that have oxygen, most of them have four to nine oxygen (O) atoms. However, ordinary petroleum samples have several compounds with two oxygen atoms and only a few of them have three or four oxygen atoms. On exposure to ozone (O3), petroleum is oxidized and provides substances with oxygen content same as that of the bitumen from amphora which is in accordance to the theory that the Taman sample stands the consequence of prolonged oxidation.

Oxygen can be detected in various ‘segments’ of a molecule. Such ‘segments’ that influence the chemical properties of the material are known as functional groups. The researchers utilized the hydrogen/deuterium exchange reaction to discover the functional groups of the substances within the material. The basic theory underlying this method is that deuterium (D), referred to as heavy hydrogen, is capable of only substituting those hydrogen (H) atoms that belong to a functional group. Replacing a deuterium atom for a hydrogen atom within a molecule results in a peak on the mass spectrum chart to be transferred. There was an added hydrogen/deuterium exchange in the case of the early bitumen, in comparison to the fresh sample. Thus, the authors concluded that the sample from the amphora consisted of molecules of not just with one −OH group but with two −OH groups that are the degradation process products.

Professor Evgeny Nikolaev is the research supervisor of the study. He is the scientific head at MIPT’s Laboratory of Ion and Molecular Physics, the same name lab at the Institute of Energy Problems of Chemical Physics (of the Russian Academy of Sciences) and the Laboratory of Mass Spectrometry at Skoltech Space Centre. He and Yuri Kostyukevich, postdoctoral researcher at Skoltech, who conducted major experiments in this study, shared his views on the finding;

Ultrahigh-resolution mass spectrometry is an immensely powerful technique in analytical chemistry. Applied to petrochemistry, archaeology, and medicine it provides a valuable insight into the molecular composition of a substance. The analysis of ancient bitumen has already revealed much about the transformations that petroleum undergoes over the course of millennia. Thanks to mass spectrometry, we might be able to obtain new information about the goods traffic and trade routes in the ancient world.

Professor Evgeny Nikolaev, Scientific Head, MIPT

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