Stable isotope analysis, primarily conducted using isotope-ratio mass spectrometry (IRMS), is a technique for determining the relative abundance of stable (i.e. non-radioactive) isotopes a substance contains. Isotopic composition reflects a material’s history, incorporating information about factors such as geographical origin, environmental conditions, and metabolic processes.
In addition to helping researchers investigate the origin of ecological, geological, and archeological samples, isotope analysis has commercial applications in the food and textile industries for supply chain verification.
Most commonly studied stable isotopes and sample materials
Stable isotope analysis typically focuses on carbon, nitrogen, oxygen, sulfur, and hydrogen isotopes. These elements are ideal for analysis, as they are present in a range of naturally occurring materials in relatively large concentrations and tend to contain a sufficiently high concentration of the rarer isotope to enable precise determination with IRMS.
Stable isotope analyses are commonly denoted using delta (δ) and the rarer isotope:
δ13C analysis of carbon, focusing on the 13C to 12C ratio
δ15N analysis of nitrogen, focusing on the 15N to 14N ratio
δ34S analysis of sulfur, focusing on the 34S to 36S ratio
δ18O analysis of oxygen, focusing on the 18O to 16O ratio
δ2H analysis of hydrogen, focusing on the 2H to 1H (deuterium to protium) ratio
Several isotopes can be covered in one study, yielding a more comprehensive picture of the sample material’s origin. For example, the δ18O and δ2H values of water are closely interconnected and are regularly studied together. It is also often possible to determine carbon and nitrogen isotope ratios simultaneously, given that the sample’s carbon-to-nitrogen ratio is sufficiently low.
In addition to water, typical sample matrices for isotope ratio analysis include plant and animal tissues, teeth, bone, hair, soil, sediment, food, fabrics, oils, rock, and minerals.
How are the results expressed?
The results of stable isotope analysis are expressed in per mille (‰) deviations from an internationally recognized reference standard, which itself is given a value of 0‰. The primary standards for the most commonly analyzed isotope ratios are the Vienna Standard Mean Ocean Water (VSMOW) for oxygen and hydrogen ratios, the Vienna Peedee Belemnite (VPDB) for carbon ratios, the Vienna Canyon Diablo Trolite (VCDT) for sulfur ratios, and atmospheric air (AIR) for nitrogen ratios.
Secondary standards are used for calibration and as matrix-specific reference materials, against which specific types of samples can be compared. For example, the δ18O value of silicates can be evaluated in comparison to the NBS-28 reference, which has a value of +9.6 on the VSMOW scale.
Applications in ecological and geological research
Isotope ratios provide insights into ecological and geological processes, such as the movement of water and nutrients within an ecosystem and the environmental conditions under which rocks and minerals originated. This gives stable isotope analysis numerous applications within these fields, including the following:
Food web studies, where δ13C and δ15N composition of animal tissues can be analyzed to determine where organisms fall within the food chain.
Analysis of rock formation, where sulfur, oxygen, and hydrogen isotopes can provide insight into the conditions under which the rock formed, including its source (igneous, metamorphic, or sedimentary).
Pollution tracking, where nitrogen ratios can be studied to identify the sources of pollution in soil, water, and air.
Archeological research
In archaeology, stable isotope analysis helps researchers understand past human behaviors and living conditions. Studies often focus on reconstructing past diets through the analysis of carbon isotopes in human and animal tissues, bones, teeth, and hair. δ13C variations across such samples provide information about relative plant and animal consumption and the proportion of C3 and C4 plants in the diet. This information can, in turn, shed light on a range of phenomena from social hierarchies and cultural dietary choices to migration patterns and the introduction of new agricultural practices.
Food authenticity testing
Different plants have distinct isotopic compositions due to variations in environmental conditions, which is also reflected in the isotope composition of animals that feed on plants and the honey bees produce from them. These unique “isotopic fingerprints” make it possible to use stable isotope analysis in food authenticity studies to determine whether measured isotope ratios match the product’s claimed composition and origin.
Commercial stable isotope analyses for food authentication often focus on the following products:
Honey: δ13C ratios can be used to detect honey adulteration with C4 plant-based sugars (e.g. corn or sugarcane), as bees generally use C3 plants to produce honey. Carbon, nitrogen, sulfur, and oxygen isotopes can be analyzed for further insight into botanical and geographical origin.
Wine: The δ2H and δ18O fingerprints of wine reflect local rainfall patterns, and can therefore be used to determine geographical origin or detect adulteration through the addition of water.
Organic produce: The nitrogen fingerprints of organic and synthetic fertilizers are different, which makes it possible to use δ15N values to determine whether synthetic fertilizers have been used to grow vegetables that are marketed as organic.
Other commonly analyzed products include coffee, olive oil, spirits, and bottled water, all of which are susceptible to falsification of origin, alteration with cheaper ingredients, or both.
Supply chain verification in the textile industry
The carbon, hydrogen, and oxygen isotope ratios of cotton reflect the conditions it was cultivated in, including soil composition, climate, water sources, and agricultural practices. This makes it possible to use stable isotope analysis for cotton origin confirmation. Often, the goal is to ensure that cotton does not originate from areas where unethical work practices are widespread, such as Xinjiang in China.
All stable isotope analyses in one place
Measurlabs provides high-quality stable isotope analysis services to researchers and businesses in multiple academic fields and industries. In addition to the applications and samples outlined above, we can offer analyses for a range of other sample matrices. Do not hesitate to get in touch with our experts for more information.