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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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”.
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