“YOU ARE WHAT YOU EAT”
This rule also applies at the atomic level: your body’s atoms come from your food and drink. Atoms of almost all the chemical elements (carbon, oxygen, hydrogen, etc.) have more than one possible atomic weight. These different atoms of the same element are called isotopes. For example oxygen (O) has three naturally occurring isotopes: O-16, O-17, and O-18 (16O, etc.) These isotopes never disintegrate; we call them stable isotopes to distinguish them from the better known radioactive isotopes used in medical treatment.

The relative abundances of the stable isotopes of an element are almost the same in all samples containing that element. However, we can detect tiny variations which are the result of natural processes. For example, the 18O/16O ratio in rain and snow varies by up to 3% depending on the location where it fell. We can determine 18O/16O with a precision of better than one part in 10,000, using a stable isotope ratio mass spectrometer [SIRMS] (Figure 1). These analyses give the degree to which a sample is enriched or depleted in 18O compared to the world standard (seawater); the analyses are called delta 18O ( 18O) values. In rain they always tend to be negative numbers, because rain is always depletedin 18O compared to sea-water.

Most of the O atoms in our body come from the water we drink, and is usually isotopically like the precipitation where we live. Therefore we can often learn where a person lived from the isotopic composition of their teeth and bones. Fortunately we now have maps showing the distribution of 18O/16O ratios in precipitation falling over North America and Europe (Figure 2) which we can use to help us to trace the place of origin of a murder victim. Even burned remains can be analyzed this way.