Forensics 101: Strontium—You Are What You Eat

In past Forensics 101 posts, we’ve discussed skeletal identification through adult or pre-pubescent aging, sex and race. But what if only partial remains are found, perhaps only a few bones? Is there a way to start the identification process when the skull or pelvis is missing and all you have to work with is a single femur? The bones themselves can still share information with investigators, and one of crucial piece of evidence is the analysis of strontium content in the bone.

Elemental strontium is found in the soil, water supplies and bedrock of our planet. Due to an extremely long half-life, strontium isotope levels remain constant in the environment for extended periods of time. Plants that grow in strontium-rich soils naturally incorporate the element into their cellular structure. Herbivores in turn consume the plants, absorbing the strontium. Similar to calcium, strontium becomes part of the mineral structure of bone.

The key to strontium analysis is its four stable isotopes—strontium-84, -86, -87 and -88. Geographic distribution differences exist for all four isotopes, and, as a result, different geographic areas of the world have characteristic ratios between isotopes and will transfer those identical ratios to local plants. Small sections of bone are analyzed through mass spectrometry to reveal their strontium profile. Match the strontium ratio of recovered remains to a geographic location, and you’ve learned an important detail about your victim.

But strontium analysis can be even more precise. As children grow and bones lengthen, the strontium they consume becomes a part of their skeletal structure. But, like calcium, the strontium content in bone turns over approximately every six years throughout life. As new strontium is integrated into bone, it leaves a geographic fingerprint that lasts for the next six years of life. Conversely, strontium is incorporated permanently into tooth enamel during dental development in children, leaving a lifelong indictor in adult teeth as to where the individual spent his or her childhood years.

While not leading directly to a definitive identification, information about where a victim grew up or lived during the past six years could be crucial in providing investigators with a starting point for missing persons’ searches. DNA would be the next logical step in victim identification, and I’ll discuss that in the next Forensics 101 post.

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