Carbon that has been out of contact with the air for more than about 40,000 years has no carbon-14 left that a lab can detect. Carbon in last season's corn crop carries about as much as the air it grew in. CO₂ recovered at an ammonia plant or drawn from a natural well sits at the first end of that scale. CO₂ stripped out of biogas sits at the other, and an accelerator mass spectrometer can tell them apart.
That measurement is what stands behind the word biogenic. Under the GHG Protocol's Scope 3 Standard, biogenic CO₂ in a company's value chain is reported separately from the scope totals, while fossil CO₂ counts in full. A 12 ounce can carbonated to 3.8 volumes holds about 2.6 grams of CO₂, so every million cans puts about 2.6 metric tons in one column or the other depending on where the carbon came from. A supply contract can state the origin. A radiocarbon result is how someone checks it.
Why Fossil Carbon Reads Zero
Carbon-14 forms in the upper atmosphere and enters plants through photosynthesis, then moves into animals and microbes through the food chain. Once an organism stops taking in carbon, the carbon-14 it holds decays with a half-life of about 5,700 years and is never replaced.
Coal, oil and natural gas formed millions of years ago, so their carbon-14 is gone. Burning them, or making ammonia from natural gas, produces CO₂ with none at all. Biogas comes from microbes breaking down recent plant and animal matter such as manure, food waste, paper and yard waste. Raw biogas runs 35 to 45 percent CO₂, and the carbon in that CO₂ carries the signature of the material it came from.
How the Result Is Reported
The method is ASTM D6866, Standard Test Methods for Determining the Biobased Content of Solid, Liquid, and Gaseous Samples Using Radiocarbon Analysis. The current edition is D6866-24a. Method B uses accelerator mass spectrometry, and Beta Analytic, one of the main labs running the test, describes it as the more accurate and precise of the two methods in the 2016 edition. Labs convert hydrocarbon gases such as methane to CO₂ for measurement. A sample of CO₂ product arrives already in that form.
The raw result is percent modern carbon, or pMC. It compares the sample's carbon-14 with a NIST reference material, 4990C, that approximates the air in 1950. The lab divides pMC by an atmospheric adjustment factor called REF to arrive at percent biobased carbon. ASTM D6866 quotes an uncertainty of plus or minus 3 percentage points, absolute, on that final figure.
The Bomb Pulse Sets the Reference
The reference year is 1950 because of what came after it. Atmospheric weapons testing between 1952 and 1963 raised the carbon-14 in the air by about 90 percent. Beta notes in its explanation of results that a plant living in 1963 would measure about 190 pMC. The excess has been washing out since, to about 140 pMC by 1975, 120 by 1985 and 102 by 2015.
REF exists to correct for that drift. Biomass growing today still reads a little above the 1950 reference, so dividing by REF scales this year's growth to 100 percent. The 2016 edition of the standard had REF falling about half a point a year.
Landfill Gas Can Read Above 100
Landfill carbon bends that arithmetic. Paper, wood and yard waste in a landfill grew years or decades before the gas comes off it, often when the air held more carbon-14 than it does now. Isotope work by Coleman, Hackley and colleagues, published in 1995 and 1996, found elevated carbon-14 in landfill methane and leachate, a trace of bomb-era carbon in the waste.
So a landfill CO₂ sample can come back above the current reference, and a straight division would put it over 100 percent biobased. The high reading is the fingerprint of decades-old paper and wood, and the standard has a provision for it. Beta's explanation of results says ASTM D6866-16 directs labs to use averaged past atmospheric values for REF when a sample measures above 100 pMC.
Dairy and food waste digesters sit closer to the present. Cattle on current feed are eating carbon from recent crops, so CO₂ from a dairy digester would be expected to read near present-day air.
Federal Reporting Already Uses It
EPA's Greenhouse Gas Reporting Program relies on the same test for stack CO₂. A unit that burns municipal solid waste as its primary fuel splits its CO₂ into biogenic and fossil shares with ASTM D7459 sampling and ASTM D6866 analysis. Under 40 CFR 98.34(d) it samples at least once every calendar quarter, each sample collected over at least 24 hours of normal operation, and the year's biogenic share is the average of those results. Units that co-fire biomass with fossil fuel may use the same procedure.
That rule was written for waste combustors, where the fuel is a mix and the biogenic fraction shifts week to week. The sampling logic carries over to any CO₂ product. One test describes one sample. A repeating schedule describes a supply.
What a Buyer Should Ask to See
For a beverage company reporting Scope 3, a single percentage on a certificate is a thin basis. The details that let an auditor rely on the result fit in four lines.
- The ASTM D6866 edition and method used, with Method B the stronger choice
- The pMC result and the REF value used to convert it
- Where the sample was drawn: the source gas, the product tank or the trailer
- The sample date and how often the test repeats
The sample point carries the most weight. A test on raw biogas confirms the feedstock. A test on product liquid confirms what was sold, including any CO₂ from another source that shared the tank. When a plant runs on one biogas source and fills its own storage, those two results describe the same carbon.
Purity and Origin Are Separate Tests
Beverage grade and biogenic are two claims checked by two different tests. Purity is settled by a certificate of analysis against the International Society of Beverage Technologists guidelines, 99.9 percent CO₂ minimum, with individual impurity limits as low as 20 parts per billion for benzene. Origin is settled by carbon-14. A load can pass one and have no basis for the other.
CleanCycleCarbon captures the CO₂ separated during renewable natural gas upgrading, where the carbon comes from biogas, and purifies it with patent pending cryogenic technology to beverage grade, the spec we certify to. We entered continuous commercial beverage grade production in December 2024.
Every biogenic claim on a load of CO₂ reduces to a count of carbon-14 atoms that an accredited lab can repeat. A buyer reporting Scope 3 should ask to see that count, along with where and when the sample was drawn.
