When investigating for a forensic case, the first step is to collect physical evidence. In some cases, however, the evidence required can only be uncovered by means of analytical techniques. Anything found at a crime or forensic scene could be the link investigators need to determine the past events that led to the investigation itself.
Isotope fingerprints, the unique chemical signature contained in any sample or material, are used more and more to uncover evidence for forensic cases. In general, the study of isotope fingerprints can be applied to forensics investigations in three ways.
Isotope ratios could be used to determine if someone or something is associated with (or excluded from) a crime.
When talking about food forensics, isotope fingerprints can reveal if food and beverages were adulterated or mislabeled, and can provide more evidence about their origin (click here to read more about how isotopes are used for food and beverage analysis).
The analysis of stable isotopes can indicate where something is from or where someone has been. For instance, isotope ratios in human and animal hair and nails can suggest a region where someone lived, while isotope fingerprints measured in a drug of abuse, such as heroin, can indicate its geographic source (read more here).
Being quantitative empirical evidence that is reproducible and easy to validate, isotope measurements have been used in courts as evidence for forensics cases. Examples which then were reported to the media are many. As reported by Leslie Chesson in this executive summary in 2016, The Economist reported the work performed by the U.S. Drug Enforcement Administration (DEA) to test cocaine brought into the country illegally. To determine the original source of the drug, the DEA measured a combination of trace chemical profiles and isotope ratios to form a telltale ”fingerprint” of the cocaine.
In the case of the crash of Flight 1549 in 2009 (case known as the "Miracle on the Hudson”, as it landed in the Hudson River after having struck a flock of geese) experts from the Smithsonian’s Museum Conservation Institute labs in Suitland, MD, examined hydrogen isotope fingerprints from feathers to confirm whether the geese were from resident or migratory populations.
The hydrogen isotope fingerprints measured in feathers can be employed as geographic markers, as they reflect the types of vegetation in the bird’s diet at the time it grew new feathers after molting. Using isotope ratio mass spectrometers, the scientists compared the bird-strike feather samples with samples from migratory Canadian geese and from resident geese close to LaGuardia Airport. Their analysis revealed that the isotope values of the geese involved in the crash of Flight 1549 were most similar to migratory Canadian geese from the Labrador region and significantly different from feathers collected from Canada geese living in the New York City region.
Isotope fingerprints may not identify the person per se, but they may lead the investigation in the right direction. This is important when resources are scarce and there are limited funds to explore every single lead the investigation opens. For example, in 2000, IRMS was used to assist in the investigation of an unidentified set of human remains that were found near the Great Salt Lake in Utah. The differences in the oxygen isotope ratios in hair segments helped predict where the individual had traveled, based on the changes in the isotopes fingerprints of water drunk by the person. These predictions worked to restrict the potential location origin. The analysis eventually led to the discovery of a missing person’s report that fit many features of the remains, and a DNA test between living relatives confirmed the identity of the individual.
On April 20, 2010, the explosion on the Deepwater Horizon oil rig in the Gulf of Mexico triggered a gusher of oil from the Macondo well at the seafloor that continued for 87 days and released about 160 million barrels of oil into the environment. Christopher M. Reddy, a senior scientist in marine chemistry and geochemistry at the Woods Hole Oceanographic Institution in Massachusetts, studied oil from this spill. As he and his colleagues wrote in the Proceedings of the National Academy of Sciences, “Compositional data for released oil is … necessary for forensic analyses when distinguishing Macondo well oil from hydrocarbons released from other sources in the Gulf of Mexico.
For more examples and insights on the analytical methods, “Stable Isotope Forensics” by Wolfram Meier-Augenstein, a German-born scientist working in the United Kingdom, is considered to be the reference for this application. The book reports the use of Isotope ratio mass spectrometry to help identify murder victims, to trace illicit international goods, and even more dramatic cases in which Meier-Augenstein’s himself was involved.
But how can isotope fingerprints support these investigations? Isotopes can be found everywhere in the environment. For example, soil and water are incorporated into the tissues of plants and into animals through their diet. Via the dieting habits, any organism incorporates specific isotopes which trace the organisms’ real history.
Read more about the answers that Isotope fingerprints can give about criminal forensic cases in this infographic. And the video below gives an overview of how isotope fingerprints are used for forensics applications by following the investigation of the Isotope Hunter, a man on a mission to reveal real samples’ history by means of the isotope fingerprints.
In conclusion, from drugs of abuse to oil spills, isotope fingerprints deliver valuable forensic information and are widely used in courts, in conjunction with other techniques, as quantitative empirical evidence that is reproducible and easy to validate.
To learn more about how isotope fingerprints are used for forensics cases click on the resources below:
Follow the adventure of the Isotope Hunter as he investigates the origin and authenticity of samples with isotope fingerprints. Using isotope ratio mass spectrometry (IRMS), the Isotope Hunter gains access to information on geographic region, botanical processes, soil and fertilization processes, and fraudulent practices. Are you an Isotope Hunter? Investigate here.