Stable Isotope Analysis

What Are Stable Isotopes?
Stable isotopes are naturally occurring, non-radioactive forms of elements that have the same number of protons but a different number of neutrons, causing differences in the atomic masses of the isotopes: for example, carbon-12, carbon-13.
Much of the hydrosphere and biosphere is made up of several key elemental building blocks, and these include the elements, and the stable isotopes of hydrogen (H), oxygen (O), nitrogen (N), carbon (C), and sulfur (S). While there are many isotopes of elements found in the periodic table, these five “light isotopes” are the most abundant and useful.

What is Stable Isotope Analysis?
Isotopic signatures allow for tracing of the sources and fate of components moving through the environment. The differences in atomic mass

[between the isotopes] cause a quantitative difference in reaction rate, leading to isotopic fractionation effects between the sources and products involved in a chemical transformation. Fractionation refers to the separation of one isotope relative to another, leading to differences in the isotopic ratios of reactant and product. Isotopic composition is often expressed in standard delta notation (δ), being used to compare a sample to an international standard, where:

where, δ = [(R_sample/R_standard) – 1] x 1000.
R_sample and R_standard are the measured isotopic ratios of sample and standard material.
The δ signatures are measured using mass spectrometry, which determines the isotopic composition based on the element’s mass-to-charge ratio.

How Do Scientists Measure Stable Isotopes?
Scientists measure stable isotopes using an instrument called an isotope ratio mass spectrometer (IRMS). Samples are generally converted by high temperature combustion to a simple gas. For example, all ¹³C: ¹²C determinations are made on 100% CO₂ gas, and so any carbon bearing sample is burned and converted to pure CO₂ before mass spectrometric analysis. There are many different instruments, and applications for IRMS.

What are the Uses of Stable Isotope Analysis?
Stable isotopes are often used to trace the geochemical cycling of nutrients and contaminants moving through a natural environment. Applications extend to both terrestrial and aquatic environments, including uses in food web studies, contaminant source apportionment, migratory animal tracking, nutrient cycling evaluations, food adulteration, environmental forensics, and numerous other interdisciplinary uses.