For those seeking carbon credits for selling or using biofuels, it’s important to know that the fuel actually came from plants that were recently alive, and not from fossil fuels, which are made of plant matter that’s been dead for hundreds of millions of years. Researchers now have an easier way of determining the source of those hydrocarbon molecules. A team from the National Institute of Standards & Technology has designed a benchtop spectrometer that can do radiocarbon analysis more easily and less expensively than standard methods (J. Phys. Chem. Lett. 2017, DOI: 10.1021/acs.jpclett.7b02105).
The most reliable way to tell fossil fuels apart from biofuels lies in their carbon isotope composition: In fossil fuels, all of the carbon-14, which has a 5,730-year half-life, has undergone radioactive decay long ago, while biofuels retain a 14C signal. The trouble is that even in biofuels, the 14C concentration is miniscule, so distinguishing these two carbon sources requires isotope detection at limits that challenge most benchtop instruments.
The gold standard for 14C analysis—best known as a way to date fossils and artifacts—is accelerator mass spectrometry, which can determine the age of a specimen to within 0.2-0.4%. The technique works by accelerating ions from a sample to high speeds in order to separate the isotopes by mass and count them. But it’s expensive: accelerator mass spectrometers cost about $6 million, so most scientists must ship their samples to a dedicated facility, where sample preparation and analysis can come to about $500 per sample.
So researchers, including the NIST team, have been developing potentially cheaper, more accessible radiocarbon analysis using optical techniques. In cavity ring-down absorption spectrometry, for example, near-infrared light from a continuous-wave laser tuned to the wavelength absorbed by 14C is bounced between two mirrors inside an optical cavity, until the signal eventually leaks out or “rings down” and is detected by the instrument. When a sample of 14C-containing gas is introduced into the cavity, it absorbs some of the light, changing the ring-down interval, which can be used to calculate the amount of 14C present.
Another group previously used a variation on this ring-down technique to measure 14C with 0.4% precision, but their method required fairly complex modeling to determine the 14C concentration (Optica 2016, DOI:10.1364/OPTICA.3.000385). To get around this limitation, Adam J. Fleisher, David A. Long, and their colleagues at NIST decided instead to use linear cavity ring-down spectrometry, which requires less signal analysis, yielding more direct measurements. To detect 14C at or below the parts per trillion levels in biofuel samples, the team designed a unit with a stable temperature of -55 ˚C; a very precise, fixed distance between mirrors; and improved laser technology and optics. They achieved a precision of 11% for analyzing samples of CO2 produced by the combustion of fossil fuels and bioethanol.
Jocelyn C. Turnbull, a radiocarbon scientist at GNS Science, a geoscience consultancy in New Zealand, calls the result impressive. “The payoff will be huge when the instrument becomes available for widespread use at presumably much lower cost than traditional radiocarbon measurement methods.” She says its current level of precision would be most useful for verifying bioproducts, but it’s a long way from being useful for dating archaeological or geological artifacts, or for tracing the sources and fate of fossil fuel emissions in the atmosphere, which would require a precision of 0.2%.
The NIST team is now designing a next-generation instrument to target this threshold. The company Planetary Emissions Management, which focuses on applications for monitoring greenhouse gas emissions, has signed an agreement with NIST to commercialize the technology.