STABLE ISOTOPE ANALYSIS AND CSIA

Stable Isotope Analysis

Most of the elements of a chemical compound are usually present as a mixture of different isotopes. The ratio between isotopes varies basing on origin or manufacturing process but also on physical, chemical and biological processes affecting the substances over time. That’s why the isotope ratio is an intrinsic fingerprinting, which measurements allow to obtained very useful informations for a broad range of applications, in the environment, the energy, until nowadays forensic chemistry sectors.

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:

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.

What is CSIA?

Compound specific isotope analysis (CSIA) involves the determination of isotopic values at the molecular level for individual compounds of interest, as opposed to traditional bulk analysis. This method is used for the analysis of naturally occurring stable isotopes in environmental samples, and has been used for a variety of elements and sample types depending on the application and the industry of interest. It can be used to gain information on a compound’s origin, sources, degradation pathways, and its movements through the natural environment.