A day at the lab with Rachèl: Taking samples for isotope analyses.
Over the next months, we – Veronica Jackson and Rachèl Spros – will show you what it’s like to be a researcher for the Make-up of the City project, showing you what we do during a typical day in the lab. While our osteoarchaeologist Veronica investigates all the pathologies visible with the naked eye, I (Rachèl) look at the bones and teeth of the individuals of the Ieper collection at elemental level and today, I will explain the world of the tiniest atoms within the big world of archaeology.
Fig. 1: Atom of lithium (6Li) (Image source: CK-12 Foundation, Credit: Jodi So, License: CC BY-NC 3.0)
Everything around us, from the waters in the ocean, to the air we breathe, is made of elements. Our bodies themselves are no exception to this: they contain many elements like carbon, nitrogen, oxygen, and many others. These elements are the building blocks for our bones, our teeth, and every cell in our body. And if we go even smaller, we find that these tiny elements themselves are made up of a cloud of electrons and within this the centre (or ‘nucleus’) made up of protons and neutrons, and all together they are called ‘atom’ (Fig. 1). Within an atom, it is actually the number of protons that determines which element it is. For example, a cloud with only one proton is the element hydrogen, but a cloud with six protons is the element carbon. The number of neutrons, on the other hand, determines the ‘’version’’ of the element. These different versions of the same element are called isotopes. The element carbon, for example, can have 6, 7, or 8 neutrons within its little cloud, changing the mass of the atom and creating the isotopes carbon 12 (12C), carbon 13 (13C), and carbon 14 (14C), where the 12, 13, and 14 refer to the total number of both protons and neutrons in the nucleus. Carbon atoms with 6 neutrons (12C) are most common in our body, but sometimes we find a heavier carbon atom with 7 neutrons (13C). The same is true for the isotopes of the nitrogen atoms in our body, and for most of the other elements too. Our bodies will contain more or fewer heavy isotopes instead of lighter isotopes of an element when we eat a specific type of food or drink a specific type of drink from specific areas in the world. During digestion, our bodies will indeed take up nutrients, and those nutrients are of course made of chemical elements as well. The ratio of heavier to lighter isotopes from an element in our food can depend on many things: trophic level, temperature, humidity, the type of soil, photosynthesis, and many other things. So the isotopes in our bodies will always reflect our diet and the environment our food grew in: We are what and where we eat!
So, what I will do is analyse the amount of heavy and light isotopes of different elements in the archaeological bones and teeth from the late medieval Ieper population, to uncover their diets and where their food and drinks came from. Yet, before the exact isotopic composition of the material reveals itself, many steps must be made and all atoms that I don’t want to count need to be chemically removed first until nothing else is left but the elements I need. But let’s start at the beginning: Taking samples from a skeleton.
Our bodies contain about 7 octillion atoms (that’s 27 zeros!). Luckily, I don’t need to count that many atoms to know the isotopic composition of an individual. Instead, I only need to cut out small sections from a thighbone, a rib, and two teeth to get an idea of the individual’s entire life history from birth to death. Our bodies need time to form teeth and to make bone tissue, this doesn’t just happen overnight so each part of our bodies will show an homogenised amount of isotopes covering many years, and each body part shows a different age. Teeth, for example, show our childhood, while bones show our adult lives. The size of the bone fragments required depends on the preservation of the skeleton, because over time, skeletons will degrade and many of those 7 octillion atoms will disappear. Fortunately for me, many of the skeletons found at Ieper were very well preserved. Some of the bones even had traces of cartilage and the spongy shaped inside of the bones were still intact! So only a few grams of bone and a few milligrams of tooth enamel are needed per individual (Fig. 2). Though the better preserved, the longer it takes to sample, because when I try to cut out small parts using a small circular shaped drill, the blade struggles to cut through the dense material and the friction between the blade and the material will create heat. The atoms in our body don’t like it when they become too hot and as a result they will be altered if they’re exposed to high temperatures for too long. This problem means that I’m only able to take samples of about five individuals a day. Sampling all individuals from Ieper will therefore take a very long time but it’s all worth it once I get the results from all the analyses at the end. The chemical treatment to obtain the atoms I want will be the next step in this long process and I will tell you all about it next time.
Fig. 2: From bone to collagen: the sampled area indicated with a red circle (left), and the collagen end product after chemical pre-treatment (right).