ELI5/ How can isotope testing determine the geographical region someone is from?

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I see in a lot of Jane/John Doe cases where isotope testing tells the detectives where the person is from or grew up. How does this work? Do you have to live somewhere for a minimum amount of time to have the isotopes appear? If you move from one coast to the other do your early age isotopes disappear? Are there geographical areas with overlapping isotope data that muddies the return? Thanks in advance.

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>I see in a lot of Jane/John Doe cases where isotope testing tells the detectives where the person is from or grew up. How does this work?

It doesn’t – fictional crime dramas are just that, fictional. Some have a more firm connection to reality than others, but series like Bones for example are entirely fueled by imaginary future magic.

Stables isotopes like O18, C13 H2 accumulate in keratin cells (hair, nails). Since the isotopes are stable and keratin is also inert after they form you can analyze the ratio of these isotopes and determine where someone is from because the distribution of these isotopes vary geographically.

Yes you have to live for sometime in the area for the isotope to accumulate. I don’t know if there are overlaps in data but it’s not impossible.

but I’ve heard that it’s used in analysis of migration of animals. I didn’t know it was also used in forensics because humans don’t consume the food that only comes from the region they live in which would make isotope analysis a lot harder.

A lot of the food you are eating comes from farms aroud your local area. Especially milk is full of minerals and hard to transport long distances. But in general food is fairly local, although it is becoming more and more global. So the minerals in your body is mostly made up of the local minerals in the ground around where you live. And the isotope composition varies around the world. So by analyzing the isotope composition of your bones it is possible to get a rough idea of where you have lived the last few years.

This used to be more accurate. Firstly food used to be a lot more local. But we have huge global trade routes giving access to any type of food all year around irregardless of season. And things like the iron curtain also prevented food trading in large parts of the world. Another important factor was that the Chernobyl disaster spread different isotopes of minerals to different parts of the world which were tracable.

Atoms consist of protons, neutrons and electrons, and isotopes are elements with the same amount of protons (so oxygen always has 8 protons), but different amount of neutrons.

Only some of those isotopes are stable, but for example carbon has two stable isotopes in Carbon 12 (6 protons, 6 neutrons) and Carbon 13 (6 protons, 7 neutrons).

Now, the ratios of those stable isotopes are different everywhere on earth, as we eat food and drink water humans absorb those isotopes in different parts of the body. For our childhood that would be our tooth enamel, where we recently lived that would be our hair and bone will store the last 10 years or so. Teeth enamel and hair have the advantage that once they’re created they don’t absorb new materials, but bone will tend to reflect the average ratios of where you’ve lived (since it’s living tissue). However, by comparing different bones (with different replacement ratios) you can still get a pretty good idea of where someone has lived longterm. Hair grows relatively quickly, so it will store a isotopes fairly narrowly (down to a few weeks of residency). On the other hand it grows fairly quickly, so on a human male with short hair you will have a month to 4 months of data.

The main contributor is water, as water contains Oxygen 16 and Oxygen 18 in different ratios (Oxygen 16 vs 18 varies mostly based on how far inland you are) as well as Strontium 86 and 87 (Strontium 87 is technically radioactive, but it has a halflife of that’s 4 times the age of the universe, so the ratio won’t change noticably in a few decades. Strontium ratios tend to be relatively unique to a watershed). In some places you get additional clues from various lead isotopes or unusual amounts of heavy metals (isotope analysis always only a part of more comprehensive spectroscopic analysis)

Yes. You can get a false reading since teeth enamel and bones stores the average intake over a period of time, but along with other clues you can get a relatively accurate idea of which region. There are two main limiters. 1. Reference material. Obviously to pinpoint a source of water someone must have taken and logged a sample at some point. On top of that bone formation is complex enough that you need a fair extensive set of references (someone must have taken reference samples specificly with isotope analysis in mind). 2. Since drinking water is the main source of isotope reference material you can’t pinpoint it anymore specific than the supply of drinking water. Sometimes that can be pretty exact (even down to a specific lake), sometimes it’s no more specific than “this part of a continent”.