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The science of stable isotope ratio analysis

How does origin authentication using stable isotopes work?

Stable isotope ratio analysis (SIRA) boils down to “you are what you eat.”

From trees to water to meat, all natural products are made up of elements consumed, or eaten. Stable isotopes are location-specific parts of these elements, which means that they are linked to the origin of the product.

What are isotopes? This short video, made by the International Atomic Energy Agency (IAEA) explains!

Using water as an example

Water is made up of hydrogen and oxygen. Both elements, hydrogen and oxygen, have heavier ‘versions’ that are known as stable isotopes that have not decayed since they were formed billions of years ago. There are different amounts of heavy hydrogen and heavy oxygen in water, depending where you are in the world.

Heavy hydrogen, also known as deuterium or ‘D’, varies from place to place.

For example, in the middle of the ocean, roughly 1 hydrogen atom in every 6240 atoms is heavy hydrogen. In Scotland, the ratio would be very different, as there is less heavy hydrogen in Scottish water.

With water in the South of England, the ratio changes again, and we see more heavy hydrogen than Scotland.

By exploiting this natural variation as a tool, we can verify the origin of organic materials, such as food, timber and clothing, and ensure traceability in a supply chain.

What does a stable isotope signature look like?

Stable isotope analysis SIRA test signature

Agroisolab is able to analyse up to 8 signatures (measurements of the stable isotopes) per sample. The more signatures that correlate with the geography/provenance, the better the resolution of the results.

Different products require a different number of signatures needed for origin analysis. Some products, such as bottled water, do not require as many signatures for origin authentication, whereas we typically test all 8 against meat for optimal results.

How are stable isotopes measured?

SIRA stable isotopes IRMS mass spectrometer

SIRA results are taken from Isotope Ratio Mass Spectrometry (IRMS). This is where food or products are chemically broken down and injected into a mass spectrometer (see image). The relative abundance of its constituent stable isotopes (how enriched/depleted an isotope is) is measured by their deflection through a magnetic field.

The abundance of the product’s stable isotopes is compared to a standard. For water, this is Vienna Standard Mean Ocean Water (VSMOW). Results are expressed as a parts-per-thousand ratio (units = ‰)

How do you find the origin from the results?

Once the stable isotope signatures have been detected, they are then compared against our reference database for that kind of product.

Reference databases are built by taking samples from verified locations, measuring their isotope ratios, and then adding the isotope information to the database. The result is an entire library of reference samples from many different locations. The more reference samples in a database, the closer the product can be matched to a specific region. 

For example – a tomato sample declared to originate from Italy would be measured against the reference database of Italian tomatoes to see if the stable isotope ratios match, or are likely to match.

Agroisolab delivers reports to the customer which contain a likelihood of whether or not the product is likely to come from its declared origin. Generally, the results are scored by a traffic light code – green, amber and red based off the level of certainty, with clarifying comments in the report.

How does SIRA help verify organic products?

When verifying organic products, SIRA focuses on the amount of nitrogen present, since organic and conventional fertilisers affect the levels of nitrogen in products in different ways.

For more information about testing organic products, click here.

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