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 participate at slightly different rates in chemical reactions and physical processes, leading to their separation (which can be measured). 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 about 1 percent and 99 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. Techniques such as isotope-ratio mass spectrometry (IRMS) are commonly used for stable isotope analysis, and accelerator mass spectrometry (AMS) enables highly sensitive detection of rare radioactive isotopes. The International Atomic Energy Agency supports the development, dissemination, and interlaboratory comparison of isotopic reference materials, while isotopic standards used in mass spectrometry are established through international scientific consensus and multiple standard-setting bodies rather than by a single regulatory authority. These reference materials include Vienna Pee Dee Belemnite (VPDB) for carbon isotope measurements and Vienna Standard Mean Ocean Water (VSMOW) for hydrogen and oxygen isotopes, which serve as baseline standards for reporting isotope ratios.

Unlike stable isotopes, radioisotopes are unstable forms that radioactively decay over time. These are rarely used in forensic science; nevertheless, radiocarbon analysis in human skeletal remains—particularly bomb-pulse dating— has been successful in determining whether a person was born before or after testing of the atomic bomb. 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 isotopes 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. Isotopic analysis is also used in wildlife forensics to determine illegal poaching, captive animals, and illegal trade of animals.

Bibliography

Bell, Suzanne. Forensic Chemistry. 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. Springer, 2006.

Hopkins, John B., 3rd, et al. “Forensic Application of Stable Isotopes to Distinguish Between Wild and Captive Turtles.” Biology, vol. 11, no. 12, 29 Nov. 2022, p. 1728, doi:10.3390/biology11121728. Accessed 22 Jan. 2026.

Yinon, Jehuda, editor. Forensic Applications of Mass Spectrometry. CRC Press, 1995.