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Investigating forensics

Investigating forensics

Isotopes

 Resources / Isotopes

Stable Isotopes

Isotopes are different forms of an element that share the same chemical properties, but that differ in mass and the number of neutrons they contain. Common elements that possess isotopes include carbon, oxygen, hydrogen, and nitrogen. Each element has a specific identifier, like 'C' for carbon, while a number placed before it identifies the isotope (e.g. 13C and 12C). Some elements have many isotopes, but there are two basic types: stable and unstable or radioactive. Stable isotopes do not change over time while radioactive isotopes decrease or decay over predictable periods. To distinguish different isotopes from each other, scientists use special instruments called mass spectrometers.

Isotopes are everywhere in the environment. They are incorporated into the tissues of plants through soil and water and into animals through their eating, drinking and breathing. An organism takes in isotopes throughout its lifetime, replacing them as the tissues (e.g., skin, hair, bone) are replaced.

Different tissues are replaced at different rates, but at death, the tissues stop integrating isotopes because they stop growing altogether. At this point, the unstable isotopes begin to decay while the stable isotopes remain at the level they were when the individual died. Using the relative amount of one isotope compared to another in an organism's tissues, scientists can determine important features about that organism's life. For example, archaeologists can use isotopes to calculate how long ago an organism lived, study the dietary habits of an individual or group, or determine where a person grew up or where they lived in the last 20-25 years of their life. Forensic investigators can also use this information to learn more about an individual's life history, or to narrow down a list of missing persons as a potential match for unidentified remains.

It may not identify the person per se, but it may lead the investigation and thus the human and financial resources in that direction. This is important when resources are scares and there is no money to explore every single lead the investigation opens.

Elements used for isotope analyses:

The most common elements used for isotopic analyses are carbon, nitrogen, hydrogen, oxygen, and strontium. Carbon (C) is present in the atmosphere, water and soil and is taken up by plants during photosynthesis, the conversion of sunlight into useable energy. Different plants take up carbon at different rates depending on the kind of plant and the climate it lives in. For example, tropical plants have different isotopic ratios in their cells than plants in temperate climates like BC. For this reason, carbon can be used to determine the kind of environment an individual came from and their likely diet.

Nitrogen (N) is also present in the atmosphere, but is incorporated into the tissues of plants through nitrogen fixation, a process that converts nitrogen into ammonia, an essential part of DNA synthesis. As animals eat plants and other animals eat those animals, the ratio of nitrogen isotopes changes. As a result, nitrogen ratios can be used to determine what level of the food chain an organism was on and whether they were herbivore, a carnivore or an omnivore.

Hydrogen (H) and oxygen (O) are the basic components of water. Consequently, they are present throughout the environment in multiple forms. For both elements, the ratio of their isotopes varies with temperature and climate and comparisons can be used to determine if an individual moved to different  geographic areas during their lifetime or if two or more people are from the same area (and drank the same water). Although hydrogen and oxygen have the potential to be very useful for isotopic studies, the techniques are still being developed and the results must be interpreted carefully.

Strontium (Sr) is a pure metal with many stable and unstable isotopes. It is abundant in nature and is found primarily in volcanic rock and soils. As soils erode and filter into water and food sources, strontium is taken into the body where it is incorporated into bone tissues in the same way as calcium. The ratio of strontium isotopes also varies from one geographical location to another. As a result, analyzing the isotopic ratios in the bone of an individual can help determine where a person came from or distinguish individuals  whose remains are comingled but who come from different places.

Tissues used for isotopic analyses:

Bones and teeth are the most frequently analyzed tissues because they are hard and long lasting and so commonly preserved in archaeological and forensic contexts, and are hard and long lasting. Bone consists of two components: an organic matrix composed mostly of protein collagen and an inorganic mineral component made largely of calcium phosphate.

Bone is a living tissue, and remodels over time, it is replaced as we growth and get older. However, this process is relatively slow and dense cortical bone reflects approximately the last 10-15 years of an individual's life.

Teeth also consist of organic and inorganic materials, but unlike bone, once formed, dental enamel does not remodel. Consequently, teeth are very useful for determining the environment and conditions of an individual's early life. (i.e., when their teeth formed). In addition, comparisons of teeth and bone tissues in a single individual whether a person moved geographic locations since childhood: their teeth will show where they lived where they were young and their bones will show where they lived in the years before their death.

Hair and fingernails may also be useful for isotopic analyses. Although they are more fragile than bone, they grow at predictable rates, and reflect an individual's very recent past. This may be useful for determining if an individual recently changed locations or for studying seasonal dietary changes.

Finally, blood and soft tissues can also be used for some isotopic studies. However, these materials decay quickly and may be more prone to contamination.

An abundance of the heavier stable isotopes of carbon and nitrogen in an individual, 13C and 15N, respectively, reflect what he eats and therefore stable isotope analysis can show if a person is a vegan or eats a lot of meat. In the same way, the abundance of the heavier stable isotopes of hydrogen and oxygen in the tissues, 2H and 18O, respectively, reflects the water we consume (by way of drinking it, in meals and beverages, and in fruit and vegetables).

Diets of ancient times (paleodiets) may be easier to analyse, as foods and water were sourced locally rather than globally or bottled, respectively as it is today.

Further Applications of isotopic analyses:

As discussed briefly in other sections, there are many applications for isotopic analyses that are useful for archaeologists and forensic investigators.

On an individual level, carbon and nitrogen isotopes can be used to determine the dietary signatures of a person or the geographic origin of specific products like illegal drugs or wildlife parts.

Strontium isotopes can be used to determine the place of origin for an unidentified individual or separate individuals whose remains may have become mixed as the result of accidental or deliberate disturbance, as in a plane crash.

Hydrogen and oxygen isotopes are increasingly being tested for their ability to determine the geographic location or climatic region where an individual lived.  These isotopes are also being developed for their ability to demonstrate the origin and trace the movement of illegally obtained wildlife or drugs.

Isotopic studies can also answer broader, population level questions. For example, isotope studies have been used to address questions related to the introduction of agriculture in various parts of the world and to consider the social and economic implications of this shift. Isotopes can also be used to determine the extent to which a population relied on different food sources, like fish or game, or when a group changed their subsistence strategy. For example, isotopes have been used to determine that Neanderthals were top-level predators and accomplished hunters, not the lumbering "sub-humans" they have sometimes been portrayed as. Finally, nitrogen and carbon isotopes have also been used to help archaeologists determine when people in the past weaned their children and what foods they might have used to make the transition from breastfeeding to solid foods.

Limitations of isotopic studies:

Although isotopic studies are enormously useful, they are not without challenges.

The most important of these is diagenesis - the chemical and structural changes that occur in materials as they are exposed to the environment and begin to decay. The original state of the material (if it was broken or burned material for example) and exposure to different conditions can affect which isotopes are present and in what ratios. Different isotopes and different tissues are affected in different ways and analysts must be very conscious of how such changes can influence the resulting isotopic signatures.

Contamination is also a concern for isotopic studies. Because isotopes are present throughout the environment, there is always the possibility that the signature picked up in a tissue came from another source than the body - either by leaching in with water, through exposure to local soils or at the lab. Although there are techniques available to minimize its effects and researchers are constantly improving their methods and protocols, contamination remains a challenging problem for isotopic studies.

Isotopic studies are further challenged by the fact that ratios may not always provide clear results. For example, carbon and nitrogen values for various dietary patterns overlap to some extent. As a result it may not be possible to say with confidence that an individual or group ate a specific diet, especially if it was a mixed one. Similarly it may not be possible to distinguish two populations if they lived in similar climates or ate the same kinds of foods. Even though they may be culturally distinct, the isotopic signatures will look very similar. In contrast, a dietary signature indicating foreign foods may merely indicate that a group was importing foods from elsewhere, rather than having migrated themselves. Despite these challenges, isotopic research offers an additional line of evidence to understand past and present populations or individuals, as well as tremendous opportunities for new research.

Report writing

A report is a formal description of an event or investigation. A forensic report explains what an investigator did, how they did it and what they think the evidence shows.  A forensic investigator's report is especially important because it must be able to explain the results of the investigation to a judge and possibly a jury who would not be able to attend a crime scene and observe an investigation first-hand. There are no agreed-upon protocols or standards for writing forensic reports in Canada, but most forensic scientists use a scientific format that includes the following:

  • Report summary
  • Background (how the author became involved in the case)
  • Qualifications of the author (what makes the author an authority on the subject)
  • Materials, methods and limitations (what work was done, how and why it was conducted, and any barriers to further investigation/analysis)
  • Results (what the evidence found)
  • Interpretation of results (what the evidence means, within the area of expertise)
  • Conclusions (another short summary of the case, the findings and their importance)
  • Bibliography (what sources of information - professional literature, interviews etc - were used).