Isotopic analysis
Isotopic analysis is a scientific technique used to identify variations of the same chemical element that differ in neutron number, affecting their atomic weight. This method has significant applications in forensic science, particularly in analyzing physical evidence from crime scenes. By measuring the ratios of stable isotopes—such as carbon, nitrogen, oxygen, strontium, and lead—scientists can derive insights about the composition and origin of materials. For instance, isotopic ratios can help trace soil samples found on a suspect’s shoes back to a specific crime scene.
Stable isotopes remain unchanged over time and reflect natural variations, while radioisotopes, which decay over time, have limited forensic applications. Isotopic analysis is valuable for establishing the geographic origins of biological materials, including human remains, and has been increasingly used to identify missing persons in conflict situations. The precision of this analysis is achieved using mass spectrometry, which measures isotopic composition against established standards regulated by organizations such as the International Atomic Energy Agency. As the field of isotopic analysis continues to grow, its relevance in forensic investigations and quality control in food industries is becoming increasingly recognized.
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Isotopic analysis
DEFINITION: Identification of forms of the same chemical element that have the same number of protons but a different number of neutrons in the nucleus.
SIGNIFICANCE: Organic and inorganic materials vary in chemical properties that can be detected in evidence from crime scenes. Forensic applications of isotopic analysis usually focus on identifying the chemical composition of physical evidence samples.
Isotopes of the same element have different atomic weights and travel at slightly different rates in chemical reactions. Isotopes exist in two forms: stable isotopes and radioisotopes. Stable isotopes do not change over time and reflect isotopic variation that exists in nature. Stable isotope values are reported as the ratio of the rarer, heavy isotope to the lighter, more abundant isotope. For example, carbon has two stable isotopes, 13C and 12C, found in the atmosphere in abundances of 99 percent and 1 percent, respectively. Isotope ratios of both organic and inorganic materials can be precisely measured using a mass spectrometer and compared against standards of known isotopic composition. The International Atomic Energy Agency regulates these standards to maintain quality control between laboratories.
Unlike stable isotopes, radioisotopes are unstable forms that radioactively decay over time. These are rarely used in forensic science, although recent applications in have been successful in determining whether a person was born before or after testing of the atomic bomb through the study of radioactive decay of carbon in human skeletal remains. This can be used to determine the antiquity of human remains and whether the remains are forensically significant.
The reason that stable isotopes vary in nature is based on the principle of isotopic fractionation—the differential incorporation of the heavy and light isotope during chemical reactions. Isotope ratios are usually measured against a known standard and are converted into a “permil” (parts per thousand) value. Several important stable isotopes are used in the forensic sciences, including carbon, nitrogen, oxygen, strontium, and lead.
Stable isotope analysis is typically used to trace the composition and origin of physical evidence. A forensic scientist can analyze an unknown substance using a mass spectrometer to determine its isotopic composition for different elements. If a sample has a unique chemical composition, isotope values can be used to determine its probable geological or biological origin. For example, soil evidence recovered from a suspect’s shoes or vehicle may be traced to soil samples found at a crime scene. Food companies may consult with forensic scientists to evaluate the isotopic composition of imported foods as a means of quality control. Forensic anthropologists may use stable isotope analysis of human bones and teeth to trace the probable geographic birthplace of an unidentified individual. Isotopic analysis is a growing field of study in the forensic sciences. In the twenty-first century, isotopic analysis was used to examine remains and identify missing people killed in conflicts ranging from the former Yugoslavia, Colombia, and Iraq.
Bibliography
Bell, Suzanne. Forensic Chemistry. Upper Saddle River, N.J.: Pearson Prentice Hall, 2006.
Chesson, Lesley A., and Gregory E. Berg. "The Use of Stable Isotopes In Postconflict Forensic Identification." Wiley, 21 July 2021, www.anthropologyapps.com/PDFs/Chesson%20and%20Berg%202021.pdf. Accessed 16 Aug. 2024.
Fry, Brian. Stable Isotope Ecology. New York: Springer, 2006.
Yinon, Jehuda, ed. Forensic Applications of Mass Spectrometry. Boca Raton, Fla: CRC Press, 1995.