Carbon 14 dedicated to biopolymer analysis

Since 2005, CIRAM laboratories have been analysing, dating and authenticating works of art and archaeological artefacts. Specialists in carbon-14 dating, we also use this method to quantify the fraction of biogenic and fossil carbon present in industrial products.

Both private, independent and innovative, CIRAM laboratories are located near Bordeaux, on the Montesquieu technology park in Martillac.

POLYMER ANALYSIS, A MAJOR CHALLENGE FOR INDUSTRY 

In the current context, the analysis of biobased products is becoming a necessity to limit the environmental impact of polymers. Although biobased products are increasingly present in industrial production (biofuels, detergents, paints, construction materials, etc.), there are still no strict regulations concerning the minimum contents to be used.

The only exception is the European legislation on plastic bags, which must contain at least 50% biobased carbon from 2020 and 60% from 2025.

A reliable and objective analysis for biopolymers 

While there are no standards in place, many sectors use voluntary reporting (mass balance) and/or life cycle assessment. It is therefore important to use objective, reliable and rapid methods to qualify and quantify the biobased part of industrial materials. The implementation of regulations would make it possible to avoid fraud and to verify the quality of the raw materials used both during R&D and for a finished product.

Polymer chemistry remains at the forefront of technological advances in the biobased product sectors. Between the certification bodies (TUV AUSTRIA and DIN

Geprüft) and international standards (ISO 16620-2:2019 and ASTM D6866-22), the use of carbon-14 is essential to qualify, quantify, classify and certify biopolymers.

Dissociation between modern and ancient carbon

Biopolymers contain mainly carbon, hydrogen and oxygen. While it is possible to quantify these elements using a variety of analytical methods, it is impossible to distinguish biobased materials from petroleum-based materials on a molecular level. Radiocarbon analysis is a reliable quantification method for this problem. The 14C isotope has a half-life of 5730 years, after which time its concentration will be halved. After 10 periods, this isotope disappears completely. Oil comes from the decomposition of organic matter over millions of years, well beyond the life expectancy of carbon-14, which is about 60,000 years. Oil therefore no longer contains 14C, so it can be said to contain only old carbon. Conversely, biomass (i.e. living organisms) has a full reservoir of carbon-14, which is known as "modern" carbon.

Using radiocarbon analysis, our laboratory scientists distinguish between the proportion of biobased (modern) and fossil carbon (ancient) in your biopolymers. We measure the ratio of 14C, 13C and 12C isotopes in a sample. Although the proportion of 14C is very small (1 atom of 14C for every 1012 atoms of 12C), it is still present in all molecules containing 'modern' carbon.

RADIOCARBON ANALYSIS OF BIOPOLYMERS 

Radiocarbon analysis involves several steps. First, the total carbon content of the sample must be measured. This is done using a dedicated elemental carbon and nitrogen analyser. The content of biobased carbon in relation to the total carbon content is expressed as a percentage of modern carbon (pMC).

The reference value, an important variable 

In order to provide a reliable and meaningful measurement, we use a reference value REF which corresponds to 100% biomass. The REF value varies depending on the date of analysis and the nature of the biomass. The reference value is based on CO2 measurements in the air in a rural area in the Netherlands (Lutjewad, Groningen) carried out by the CIO (Centre for Isotope Research, University of Groningen).

AMS analysis of carbon isotopes and interpretation of results

The mass spectrometer (MAS) allows the measurement of 12C, 13C and 14C carbon in a carbon sample, which we analyse through the prism of a reference material.

All modern percentage carbon (pMC) values must be corrected using stable isotope data (13C/12C ratios) obtained on CO2 from the combustion of the sample. Isotope fractionation is the fluctuation in the isotope ratio caused by biochemical processes. These variations are independent of time and natural radioactive decay. The accuracy of AMS measurements is between 0.3 and 0.5% absolute pMC and the detection limit is approximately 0.3% pMC.

The measurement of carbon 14 is carried out in relation to the total carbon in the sample. Example: if 50g of a 100% biobased product is mixed with 50g of a 0% biobased product, the pMC measurement (% biobased carbon) is unlikely to be 50%, as the number of carbon atoms (% mass of C) is not equivalent from one molecule to the other. It is important to remember that the radiocarbon method is simple, but no shortcuts should be taken. Similarly, the evaporation of some volatile compounds during the synthesis or manufacture of biopolymers can distort the results between the expected values and the biobased carbon content measured on the finished product.

Although the radiocarbon method works "blind", i.e. without the need to know the chemical composition of the sample, it seems interesting and constructive to have a preliminary exchange, in order to fully understand the problematic of the industrialist and to clearly expose the benefits and constraints of the analytical techniques.

Characterisation of biopolymers by radiocarbon 

To summarise, the use of radiocarbon for the characterisation of biopolymers is a reliable and accurate method that works on all types of matrices (liquid, solid and gel) without the need to know the chemical composition. It is also relevant to use radiocarbon at all stages of the industrial process (from Research and Development to the finished product, but also sourcing, validation of the different treatments and even for obtaining a label).