ASTM D6866-24

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D6866 − 24
Standard Test Methods for
Determining the Biobased Content of Solid, Liquid, and
Gaseous Samples Using Radiocarbon Analysis
This standard is issued under the fixed designation D6866; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* may randomly contaminate an unknown sample producing
inaccurately high biobased results. Despite vigorous attempts
1.1 This standard is a test method that teaches how to
to clean up contaminating artificial C from a laboratory,
experimentally measure biobased carbon content of solids,
isolation has proven to be the only successful method of
liquids, and gaseous samples using radiocarbon analysis. These
avoidance. Completely separate chemical laboratories and
test methods do not address environmental impact, product
extreme measures for detection validation are required from
performance and functionality, determination of geographical
laboratories exposed to artificial C. Accepted requirements
origin, or assignment of required amounts of biobased carbon
are:
necessary for compliance with federal laws.
(1) disclosure to clients that the laboratory(s) working with
1.2 These test methods are applicable to any product con- 14
their products and materials also works with artificial C
taining carbon-based components that can be combusted in the
(2) chemical laboratories in separate buildings for the
presence of oxygen to produce carbon dioxide (CO ) gas. The 14
handling of artificial C and biobased samples
overall analytical method is also applicable to gaseous
(3) separate personnel who do not enter the buildings of the
samples, including flue gases from electrical utility boilers and
other
waste incinerators.
(4) no sharing of common areas such as lunch rooms and
1.3 These test methods make no attempt to teach the basic
offices
principles of the instrumentation used although minimum
(5) no sharing of supplies or chemicals between the two
requirements for instrument selection are referenced in the
(6) quasi-simultaneous quality assurance measurements
References section. However, the preparation of samples for
within the detector validating the absence of contamination
the above test methods is described. No details of instrument
within the detector itself. (1, 2, and 3)
operation are included here. These are best obtained from the
1.5 This standard does not purport to address all of the
manufacturer of the specific instrument in use.
safety concerns, if any, associated with its use. It is the
1.4 Limitation—This standard is applicable to laboratories
responsibility of the user of this standard to establish appro-
working without exposure to artificial carbon-14 ( C). Artifi-
priate safety, health, and environmental practices and deter-
cial C is routinely used in biomedical studies by both liquid
mine the applicability of regulatory limitations prior to use.
scintillation counter (LSC) and accelerator mass spectrometry
NOTE 1—ISO 16620-2 is equivalent to this standard.
(AMS) laboratories and can exist within the laboratory at levels
1,000 times or more than 100 % biobased materials and
1.6 This international standard was developed in accor-
100,000 times more than 1% biobased materials. Once in the dance with internationally recognized principles on standard-
laboratory, artificial C can become undetectably ubiquitous
ization established in the Decision on Principles for the
on door knobs, pens, desk tops, and other surfaces but which
Development of International Standards, Guides and Recom-
mendations issued by the World Trade Organization Technical
1 Barriers to Trade (TBT) Committee.
These test methods are under the jurisdiction of ASTM Committee D20 on
Plastics and are the direct responsibility of Subcommittee D20.96 on Environmen-
tally Degradable Plastics and Biobased Products.
Current edition approved Feb. 1, 2024. Published February 2024. Originally
approved in 2004. Last previous edition approved in 2022 as D6866 - 22. DOI: The boldface numbers in parentheses refer to a list of references at the end of
10.1520/D6866-24. this standard.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6866 − 24
2. Referenced Documents marine, or forestry materials living in a natural environment in
3 equilibrium with the atmosphere.
2.1 ASTM Standards:
D883 Terminology Relating to Plastics 3.3.6 biogenic—containing carbon (organic and inorganic)
of renewable origin like agricultural, plant, animal, fungi,
2.2 Other Standards:
microorganisms, macroorganisms, marine, or forestry materi-
CEN/TS 16640:2014 Biobased Products—Determination of
als.
the biobased carbon content of products using the radio-
carbon method
3.3.7 biobased carbon content—the amount of biobased
CEN/TS 16137:2011 Plastics—Determination of biobased
carbon in the material or product as a percent of the total
carbon content
organic carbon (TOC) in the product.
ISO 16620-2:2015 Plastics—Biobased content—Part 2: De-
3.3.8 biogenic carbon content—the amount of biogenic
termination of biobased carbon content
carbon in the material or product as a percent of the total
EN 15440:2011 Solid recovered fuels—Methods for the de-
carbon (TC) in the product.
termination of biomass content
3.3.9 biobased carbon content on mass basis—amount of
ISO 13833:2013 Stationary source emissions—
biobased carbon in the material or product as a percent of the
Determination of the ratio of biomass (biogenic) and
total mass of product.
fossil-derived carbon dioxide—Radiocarbon sampling
and determination
3.3.10 biogenic carbon content on mass basis—amount of
biogenic carbon in the material or product as a percent of the
3. Terminology
total mass of product.
3.1 The definitions of terms used in these test methods are
3.3.11 break seal tube—the sample tube within which the
referenced in order that the practitioner may require further
sample, copper oxide, and silver wire is placed.
information regarding the practice of the art of isotope analysis
3.3.12 coincidence circuit—a portion of the electronic
and to facilitate performance of these test methods.
analysis system of an LSC which acts to reject pulses which are
3.2 Terminology D883 should be referenced for terminol-
not received from the two Photomultiplier Tubes (that count
ogy relating to plastics. Although an attempt to list terms in a
the photons) within a given period of time and are necessary to
logical manner (alphabetically) will be made as some terms
rule out background interference and required for any LSC
require definition of other terms to make sense.
used in these test methods (9, 6, 12).
3.3 Definitions:
3.3.13 coincidence threshold—the minimum decay energy
3.3.1 AMS facility—a facility performing Accelerator Mass
required for an LSC to detect a radioactive event. The ability to
Spectrometry.
set that threshold is a requirement of any LSC used in these test
methods (6, 12).
3.3.2 accelerator mass spectrometry (AMS)—an ultra-
sensitive technique that can be used for measuring naturally
3.3.14 contemporary carbon—a direct indication of the
occurring radio nuclides, in which sample atoms are ionized,
relative contributions of fossil carbon and “living” biospheric
accelerated to high energies, separated on basis of momentum,
carbon can be expressed as the fraction (or percentage) of
charge, and mass, and individually counted in Faraday collec-
contemporary carbon, symbol f . This is derived from “frac-
C
tors. This high energy separation is extremely effective in
tion of modern” (f ) through the use of the observed input
M
filtering out isobaric interferences, such that AMS may be used
function for atmospheric C over recent decades, representing
14 12
to measure accurately the C ⁄ C abundance to a level of 1 in
the combined effects of fossil dilution of C (minor) and
10 . At these levels, uncertainties are based on counting
nuclear testing enhancement (major). The relation between f
C
statistics through the Poisson distribution (4,5).
and f is necessarily a function of time. By 1985, when the
M
3.3.3 automated effıciency control (AEC)—a method used particulate sampling discussed in the cited reference was
performed, the f ratio had decreased to approximately 1.2 (4,
by scintillation counters to compensate for the effect of
M
quenching on the sample spectrum (6). 5).
3.3.4 background radiation—the radiation in the natural 3.3.15 chemical quenching—a reduction in the scintillation
environment; including cosmic radiation and radionuclides
intensity (a significant interference with these test methods)
present in the local environment, for example, materials of seen by the Photomultiplier Tubes (PMT, pmt) due to the
construction, metals, glass, concrete (7,8,9,4,6-14).
materials present in the scintillation solution that interfere with
the processes leading to the production of light. The result is
3.3.5 biobased—containing organic carbon of renewable
fewer photons counted and a lower efficiency (8, 9, 12).
origin like agricultural, plant, animal, fungi, microorganisms,
3.3.16 chi-square test—a statistical tool used in radioactive
counting in order to compare the observed variations in repeat
counts of a radioactive sample with the variation predicted by
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
statistical theory. This determines whether two different distri-
Standards volume information, refer to the standard’s Document Summary page on
butions of photon measurements originate from the same
the ASTM website.
photonic events. LSC instruments used in this measurement
Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org. should include this capability (6, 12, 15).
D6866 − 24
3.3.17 cocktail—the solution in which samples are placed 3.3.31 modern carbon—explicitly, 0.95 times the specific
for measurement in an LSC. Solvents and Scintillators— activity of SRM 4990B (the original oxalic acid radiocarbon
chemicals that absorb decay energy transferred from the standard), normalized to δ C = −19 % (Currie, et al., 1989).
solvent and emits light (photons) proportional in intensity to Functionally, the fraction of modern carbon equals 0.95 times
the deposited energy (8, 9, 6, 12). the concentration of C contemporaneous with 1950 wood
(that is, pre-atmospheric nuclear testing). To correct for the
3.3.18 decay (radioactive)—the spontaneous transformation
post 1950 bomb C injection into the atmosphere (5), the
of one nuclide into a different nuclide or into a different energy
fraction of modern carbon is multiplied by a correction factor
state of the same nuclide. The process results in a decrease,
representative of the excess C in the atmosphere at the time
with time, of the number of original radioactive atoms in a
of measurements.
sample, according to the half-life of the radionuclide (4, 6, 12).
3.3.32 noise pulse—a spurious signal arising from the elec-
3.3.19 discriminator—an electronic circuit which distin-
tronics and electrical supply of the instrument (6, 12, 23, 24).
guishes signal pulses according to their pulse height or energy;
3.3.33 phase contact—the degree of contact between two
used to exclude extraneous radiation, background radiation,
phases of heterogeneous samples. In liquid scintillation
and extraneous noise from the desired signal (6, 12, 13, 16).
counting, better phase contact usually means higher counting
3.3.20 dpm—disintegrations per minute. This is the quantity
efficiency (6, 12).
of radioactivity. The measure dpm is derived from cpm or
3.3.34 photomultiplier tube (PMT, pmt)—the device in the
counts per minute (dpm = cpm − bkgd / counting efficiency).
LSC that counts the photons of light simultaneously at two
There are 2.2 × 10 dpm / μCi (6, 12).
separate detectors (24, 16).
3.3.21 dps—disintegrations per second (rather than minute
3.3.35 pulse—the electrical signal resulting when photons
as above) (6, 12).
are detected by the PMTs (6, 12, 13, 16).
3.3.22 effıciency—the ratio of measured observations or
3.3.36 pulse height analyzer (PHA)—an electronic circuit
counts compared to the number of decay events which oc-
which sorts and records pulses according to height or voltage
curred during the measurement time; expressed as a percentage
(6, 12, 13, 16).
(6, 12).
3.3.37 pulse index—the number of after-pulses following a
3.3.23 external standard—a radioactive source placed adja-
detected coincidence pulse (used in three dimensional or pulse
cent to the liquid sample to produce scintillations in the sample
height discrimination) to compensate for the background of an
for the purpose of monitoring the sample’s level of quenching
LSC performing (6, 13, 24, 16).
(6, 12).
3.3.38 quenching—any material that interferes with the
3.3.24 figure of merit—a term applied to a numerical value
accurate conversion of decay energy to photons captured by the
used to characterize the performance of a system. In liquid
PMT of the LSC (7, 8, 9, 6, 10, 12, 17).
scintillation counting, specific formulas have been derived for
quantitatively comparing certain aspects of instrument and
3.3.39 region—regions of interest, also called window
cocktail performance and the term is frequently used to
and/or channel in regard to LSC. Refers to an energy level or
compare efficiency and background measures (6, 12, 17).
subset specific to a particular isotope (8, 6, 13, 23, 24).
3.3.25 flexible tube cracker—the apparatus in which the
3.3.40 renewable—being readily replaced and of non-fossil
sample tube (Break Seal Tube) is placed (18, 19, 20, 21).
origin; specifically not of petroleum origin.
3.3.26 fluorescence—the emission of light resulting from
3.3.41 scintillation—the sum of all photons produced by a
the absorption of incident radiation and persisting only as long
radioactive decay event. Counters used to measure this as
as the stimulation radiation is continued (6, 12, 22).
described in these test methods are Liquid Scintillation Coun-
ters (LSC) (6, 12).
3.3.27 fossil carbon—carbon that contains essentially no
radiocarbon because its age is very much greater than the 5,730
3.3.42 scintillation reagent—chemicals that absorbs decay
year half-life of C (4, 5). energy transferred from the solvent and emits light (photons)
proportional in intensity to the decay energy (8, 6, 24).
3.3.28 half-life—the time in which one half the atoms of a
particular radioactive substance disintegrate to another nuclear 3.3.43 solvent-in scintillation reagent—chemical(s) which
form. The half-life of C is 5,730 years (4, 6, 22).
act as both a vehicle for dissolving the sample and scintillator
and the location of the initial kinetic energy transfer from the
3.3.29 intensity—the amount of energy, the number of
decay products to the scintillator; that is, into excitation energy
photons, or the numbers of particles of any radiation incident
that can be converted by the scintillator into photons (8, 6, 12,
upon a unit area per unit time (6, 12).
24).
3.3.30 internal standard—a known amount of radioactivity
3.3.44 specific activity (SA)—refers to the quantity of radio-
which is added to a sample in order to determine the counting
activity per mass unit of product, that is, dpm per gram (6, 12).
efficiency of that sample. The radionuclide used must be the
same as that in the sample to be measured, the cocktail should 3.3.45 standard count conditions (STDCT)—LSC condi-
be the same as the sample, and the Internal Standard must be tions under which reference standards and samples are
of certified activity (6, 12). counted.
D6866 − 24
3.3.46 three dimensional spectrum analysis—the analysis of 4.6 Acceptable SI unit deviations (tolerance) for the practice
the pulse energy distribution in function of energy, counts per of these test methods is 65 % from the stated instructions
unless otherwise noted.
energy, and pulse index. It allows for auto-optimization of a
liquid scintillation analyzer allowing maximum performance.
Although different manufacturers of LSC instruments call 5. Safety
Three Dimensional Analysis by different names, the actual
5.1 The specific safety and regulatory requirements associ-
function is a necessary part of these test methods (6, 12, 13).
ated with radioactivity, sample preparation, and instrument
operation are not addressed in these test methods. It is the
3.3.47 true beta event—an actual count which represents
responsibility of the user of these test methods to establish
atomic decay rather than spurious interference (20, 21).
appropriate safety and health practices. It is also incumbent on
the user to conform to all the federal and state regulatory
4. Significance and Use
requirements, especially those that relate to the use of open
4.1 This testing method provides accurate biobased/
radioactive source, in the performance of these test methods.
biogenic carbon content results to materials whose carbon
Although C is one of the safest isotopes to work with, State
source was directly in equilibrium with CO in the atmosphere
2 and Federal regulations must be followed in the performance of
at the time of cessation of respiration or metabolism, such as
these test methods.
the harvesting of a crop or grass living its natural life in a field.
5.2 The use of glass and metal, in particular with closed
Special considerations are needed to apply the testing method
systems containing oxygen that are subjected to 700°C tem-
to materials originating from within artificial environments.
peratures pose their own safety concerns and care should be
Application of these testing methods to materials derived from
taken to protect the operators from implosion/explosion of the
CO uptake within artificial environments is beyond the
glass tube. Safety Data Sheets should always be followed with
present scope of this standard.
special concern for eye, respiratory, and skin protection.
4.2 Method B utilizes AMS along with Isotope Ratio Mass Radioactive C compounds should be handled and disposed of
Spectrometry (IRMS) techniques to quantify the biobased in accordance with State and Federal regulations.
content of a given product. Instrumental error can be within
NOTE 2—Prior to D6866 - 11, this standard contained a Method A,
0.1-0.5 % (1 relative standard deviation (RSD)), but controlled
which utilized LSC and CO absorption into a cocktail vial. Error was
studies identify an inter-laboratory total uncertainty up to
cited as 615 % absolute due to technical challenges and low radiocarbon
counts. Empirical evidence now indicates error may be 620 % or higher
63 % (absolute). This error is exclusive of indeterminate
in routine use. This method was removed in this revision due to the
sources of error in the origin of the biobased content (see
inapplicability of this low precision method to biobased analysis.
Section 22 on precision and bias).
NOTE 3—Prior to D6866-16, this standard contained a CARBONATE
OPTION A (CARBONATE SUBTRACTION) procedure, to exclude
4.3 Method C uses LSC techniques to quantify the biobased
inorganic carbonate from the biobased result. Empirical evidence now
content of a product using sample carbon that has been
indicates error may be unreasonably high in routine use, especially in
converted to benzene. This test method determines the
products with very low in organic carbon and very high in inorganic
carbonate. This method was removed in this revision due to potential low
biobased content of a sample with a maximum total error of
precision results which are not observed in CARBONATE OPTION B
63 % (absolute), as does Method B.
(ACID RESIDUE COMBUSTION).
4.4 The test methods described here directly discriminate
5.3 In Method C, benzene is generated from the sample
between product carbon resulting from contemporary carbon
carbon. Benzene is highly toxic and is an EPA-listed carcino-
input and that derived from fossil-based input. A measurement
gen. It must be handled accordingly, using all appropriate eye,
14 12 14 13
of a product’s C/ C or C/ C content is determined relative
skin, and respiratory protection. Samples must be handled and
to a carbon based modern reference material accepted by the
disposed of in accordance with State and Federal regulations.
radiocarbon dating community such as NIST Standard Refer-
Other hazardous chemicals are also used, and must be handled
ence Material (SRM) 4990C, (referred to as OXII or HOxII). It
appropriately (see Safety Data Sheets for proper handling
is compositionally related directly to the original oxalic acid
procedures).
radiocarbon standard SRM 4990B (referred to as OXI or
METHOD B: AMS
HOxI), and is denoted in terms of f , that is, the sample’s
M
fraction of modern carbon. (See Terminology, Section 3.)
6. Apparatus and Reagents
4.5 Reference standards, available to all laboratories prac-
6.1 AMS and IRMS Apparatus:
ticing these test methods, must be used properly in order that
6.1.1 A vacuum manifold system with capabilities for air
traceability to the primary carbon isotope standards are
and non-condensable gas evacuation, sample introduction,
established, and that stated uncertainties are valid. The primary
water distillation, cryogenic gas transfer, and temperature and
standards are SRM 4990C (oxalic acid) for C and RM 8544
pressure monitoring. The following equipment is required:
(NBS 19 calcite) for C. These materials are available for
distribution in North America from the National Institute of 6.1.2 Manifold tubing that is composed of clean stainless
steel and/or glass.
Standards and Technology (NIST), and outside North America
from the International Atomic Energy Agency (IAEA), Vienna,
6.1.3 Vacuum pump(s) capable of achieving a vacuum of
Austria. 101 Pa or less within the vacuum region.
D6866 − 24
6.1.4 Calibrated pressure transducers with coupled or inte- 8.7 With the manifold closed to the vacuum pump, the
grated signal response controllers. quartz tubing is cracked, the sample CO is liberated and
6.1.5 A calibrated sample collection volume with associated immediately cryogenically (with liquid nitrogen) transferred to
temperature readout. a sample collection bulb attached to a separate port on the
manifold.
6.1.6 Clean quartz tubing for sample combustion and sub-
sequent gas transfer, quantification and storage.
8.8 The contents of the sample collection bulb shall be
6.1.7 A hydrogen/oxygen torch or other heating device
distilled to remove residual water using a dry ice/alcohol slurry
and/or gas for sealing quartz tubing.
maintained at approximately −76°C. Simultaneously the
sample CO gas is released and immediately condensed in a
7. AMS and IRMS Reagents
calibrated volume.
7.1 A stoichiometric excess of oxygen for sample combus-
8.9 The calibrated volume is then closed and the CO shall
tion; introduced into sample tube as either a pure gas or as solid
equilibrate to room temperature.
copper (II) oxide.
8.10 Recovery shall be determined using the ideal gas law
7.2 A stoichiometric excess of silver, nominally 30 mg,
relationship.
introduced into sample tube for the removal of halogenated
8.11 The sample shall be transferred to a borosilicate break
species.
seal tube for storage and delivery to an AMS facility for
14 12 13 12
7.3 A −76°C slurry mixture of dry ice (frozen CO ) and analysis of C/ C and C/ C isotopic ratios.
alcohol distillation and removal of sample water.
9. Analysis, Interpretation, and Reporting
7.4 Liquid nitrogen.
14 12 13 12
9.1 C/ C and C/ C isotopic ratios are measured using
14 12 13 12
AMS. The isotopic ratios of C/ C or C/ C are determined
8. Sample Preparation
relative to a standard traceable to the NIST SRM 4990C (oxalic
8.1 Method B is a commonly used procedure to quantita- acid) modern reference standard. The calculated “fraction of
tively combust the carbon fraction within product matrices of
modern” (f ) represents the amount of C in the product or
M
varying degrees of complexity. The procedure described here material relative to the modern standard. This is most com-
for Method B is recommended based on its affordability and
monly referred to as percent modern carbon (pMC), the percent
extensive worldwide use. Nevertheless, laboratories with alter- equivalent to f (for example, f 1 = 100 pMC).
M M
native instrumentation such as continuous flow interfaces and
9.2 All pMC values obtained from the radiocarbon analyses
associated CO trapping capabilities are equally suitable pro-
must be corrected for isotopic fractionation using stable isotope
vided that the recovery of CO is quantitative, 100 6 5 %. 13 12
data (25). Correction shall be made using C/ C values
determined directly within the AMS where possible. In the
8.2 Based on the stoichiometry of the product material,
absence of this capability (and citable absence of fractionation
sufficient sample mass shall be weighed such that 1-10 mg of
within the AMS) correction shall be made using the delta 13C
carbon is quantitatively recovered as CO . Weighed sample
(δ C) measured by IRMS, CRDS (cavity ring down spectros-
material shall be contained within a pre-cleaned quartz sample
copy) or other equivalent technology that can provide precision
container, furnace-baked at 900°C for ≥2 h, and torch sealed at
one end. Typically 2 mm OD/1 mm ID quartz tubing is to 60.3 per mil. Reference standard must be traceable to
Vienna Pee Dee Belemite (VPDB) using NIST SRM 8539,
sufficient, however any tubing configuration needed to accom-
modate large sample volumes is acceptable. 8540, 8541, 8542 or equivalent.
9.3 Zero pMC represents the entire lack of measurable C
8.3 The weighed sample shall then be transferred into an
atoms in a material above background signals thus indicating a
appropriately sized quartz tube, typically 6 mm OD/4 mm ID.
fossil (for example, petroleum based) carbon source. One
8.4 The sample, thus configured shall then be adapted to a
hundred pMC indicates an entirely modern carbon source. A
vacuum manifold for evacuation of ambient air to a pressure
pMC value between 0 and 100 indicates a proportion of carbon
101 Pa or less.
derived from fossil vs. modern source.
8.5 If the material is known to be volatile or contains 14
9.4 The correction factor is based on the C activity in the
volatile components, the sample material within the tube shall
atmosphere at the time of testing. The first version of this
be frozen with liquid nitrogen to –196°C prior to evacuation.
standard (ASTM D6866-04) in 2004 referenced a value of
The evacuated tube shall be torch sealed then combusted in a
107.5 pMC based on measurements of CO2 in the air in a rural
temperature controlled furnace at 900°C for 2 to 4 h.
area of the Netherlands (Lutjewad, Groningen) and the ASTM
8.6 After combustion, the quartz sample tube shall be scored D6866-10 version (2010) cited 105 pMC. These data points
to facilitate a clean break within a flexible hose portion of a equated to a decline of 0.5 pMC per year. From the period of
“tube cracker” assembly adapted to the manifold. One example 2004 through 2019 the annual decrease in the REF was
configuration of a tube cracker is shown in Fig. X1.2. The maintained at the projected 0.5 pMC per year, based on then
materials are composed of stainless steel. Compression fittings available data through 2010. From the existing historical data
with appropriate welds are used to assemble the individual and recently published long term C atmospheric data com-
parts. This and alternative assemblies are given in the Refer- piled at sites from around the world “Atmospheric Radiocar-
ences section (18, 19, 20, 21). bon for The Period 1950—2019 Quan Hua et al.,” it has been
D6866 − 24
shown that the projected decrease of 0.5 pMC per year slowed 10.6 Optimized counting regions to provide very low back-
to approximately 0.3 pMC per year on average beginning in ground counts while maintaining counting efficiency greater
about 2014. Therefore, on January 2 of each year, the values in than 60 % of samples 0.7 to 1.5 g in clean, 3-mL, 7-mL or
Table 1 are used as REF for the years 2024 through 2026 20-mL low potassium glass counting vials. Alternatively, clean
reflecting a 0.3 pMC decrease per year. The REF (pMC) values PTFE or quartz counting vials may be used in this method.
for 2019 through 2024 were determined to be 100.0, based on
10.7 No single LSC is specified for this method. However,
continued measurements at The Netherlands (Lutjewad, Gron-
minimum counting efficiency and control of background inter-
ingen) through 2022 and recently published long term C
ference is specified. Like all analytical instruments, LSCs
atmospheric data compiled at sites from around the world
require study as to their specific components and counting
“Atmospheric Radiocarbon for The Period 1950—2019 Quan
optimization.
Hua et al.,” which is reflective of a slowing in the dilution of
14 10.8 Standardization of sample preparation is required.
excess C in the atmosphere. References for reporting carbon
14 13
10.9 Standardization and optimization of clean sample
isotopic ratio data are given in Refs. (15, 26) for C and C,
vials, which must be made of either PTFE, quartz, or low-
respectively.
potassium glass with PTFE tops. Sample vials may be either
TABLE 1 Percent Modern Carbon (pMC) Reference
3-mL, 7-mL or 20-mL in volume. Plastic vials must not be used
Year REF (pMC)
2015 102.0 for this method.
2016 101.5
10.10 Counting efficiency and background optimization
2017 101.0
2018 100.5
should be performed using a suitable reference standard (for
2019 100.0
example, NIST SRM-4990B or SRM-4990C oxalic acid) using
2020 100.0
the same reagents and counting parameters as the samples.
2021 100.0
2022 100.0
10.11 Counting efficiency (E) shall be determined by divid-
2023 100.0
ing the measured cpm by the known dpm, and multiplying this
2024 99.7
2025 99.4
by 100 to obtain the counting efficiency as a percentage. For
2026 99.1
example, for the Oxalic Acid I standard, E = (cpm/g Oxalic
9.5 Calculation of % biobased carbon content is made by Acid/ 14.27 dpm/g) × 100, where E = counting efficiency in %,
dividing pMC by REF and multiplying the result by 100. (for
cpm/g Oxalic Acid is the net activity per gram measured for the
example, [102 (pMC) / 102 (REF)] × 100 = 100 % biobased oxalic acid after subtracting background, and 14.27 dpm/g is
carbon. Results are reported as % biobased carbon content or
the absolute value of the NIST “OxI” reference standard. (SRM
% biogenic carbon content rounded to the nearest 1 unit with 4990B). The NIST “OxII” standard (SRM 4990C) has a
an applied error of 3 % absolute (see 4.2).
slightly different C activity level. ANU sucrose (NIST SRM
8542) can be used as a suitable standard in place of oxalic acid.
9.6 See 22.7 for calculating and reporting results for mate-
10.12 Counting interference concerns that must be ad-
rials which calculate to greater than 100 % biobased carbon
dressed as part of specific instrument calibration and normal-
content.
ization include luminance, chemical or color quench, static
9.7 As stated in 4.1, this testing standard is applicable to
electricity, random noise, temperature, and humidity variability
materials whose carbon source was directly in equilibrium with
(27).
CO in the atmosphere at the time of cessation of respiration or
10.13 Alternate regions of interest parameters may be used
metabolism. See 22.11 for calculating and reporting results for
based upon testing of 20, or more, 6-h counts of the same
materials from marine and aquatic environments.
reference (STDCT) standard that record the raw data and
spectrum for keV regions of interest 4 through 96. Optimal
METHOD C: Liquid Scintillation Counting
counting conditions should be established by maximizing the
Figure of Merit (E /bkg) values to obtain the highest count
10. Detailed Requirements
efficiency and the lowest background and other interference.
NOTE 4—Acceptable tolerance levels of 65 % are standard to this
Counting efficiency of less than 60 % is unacceptable and can
method unless otherwise stated.
be improved by LSC instrument optimization and sample/
10.1 Low level LSCs with active shielding that can produce reagent compatibility or shielding improvements.
consistent background counts of less than 5 dpm.
10.14 Samples will be equilibrated with reference standards
under identical conditions of time and temperature.
10.2 Anti-coincidence systems such as two and three PMTs
(multidetector systems).
10.15 Samples will be counted for a minimum of 10 h with
region of interest (ROI) channels including ROI energy levels
10.3 Coincidence circuits.
of 0-155 keV such that E /B is 1,000 or higher in 20 to 120-min
10.4 Software and hardware that include thresholds and
subsets with raw data saved to disk for later statistical analysis
statistics, pulse rise and shape discrimination, and three-
and documentation of stable counting conditions.
dimensional spectrum analysis.
10.16 Before commercial testing, laboratories that intend to
10.5 Use of external and internal standards must be used in implement this method must participate in an inter-laboratory
LSC operation. comparison study to assess between laboratory reproducibility.
D6866 − 24
11. Apparatus and Reagents 12.6 As an alternative combustion approach for volatile
materials, the samples can be combusted in a bomb that is
11.1 Benzene Synthesis Apparatus:
pressurized with oxygen to 300-400 psi. The CO generated in
11.1.1 A benzene synthesis unit will be required to convert
the bomb is subsequently released to a dry ice trap for moisture
sample carbon to benzene. These units are commercially
removal, followed by a liquid nitrogen cold trap for CO
available, but can also be homemade if desired. Examples of
collection.
benzene synthesis units are discussed in (28) and (29).
12.7 The collected CO is reacted with a stoichiometric
11.2 LSC Apparatus:
excess (3:1 lithium:carbon ratio) of molten lithium which has
11.2.1 LSC as described in Section 10.
been preheated to 700°C. Li C is produced by slowly bleeding
2 2
11.2.2 Clean low potassium scintillation vials with a volume
the CO onto the molten lithium in a stainless steel vessel (or
of 3-mL, 7-mL, or 20-mL.
equivalent) while under a vacuum of ≤135 mPa.
11.3 LSC and Benzene Synthesis Reagents:
12.8 The Li C is heated to about 900°C and placed under
2 2
11.3.1 High purity oxygen used for converting sample
vacuum for 15-30 minutes to remove any unreacted gases and
carbon to CO . Alternatively, technical grade oxygen can be
to complete the Li C synthesis reactions (15).
2 2
used if scrubbed with a suitable material such as Ascarite.
12.9 The Li C is cooled to room temperature and gently
11.3.2 High purity nitrogen used to combine with the 2 2
hydrolyzed with distilled or de-ionized water to generate
oxygen when combusting highly volatile samples.
acetylene gas (C H ) by applying the water in a drop-wise
Alternatively, technical grade nitrogen can be used if scrubbed 2 2
fashion to the carbide. The evolved acetylene is dried by
with a suitable material such as Ascarite.
passing it through dry ice traps, and the dried acetylene is
11.3.3 Cupric oxide wire for conversion of CO to CO when
subsequently collected in liquid nitrogen traps.
combusting highly volatile samples with oxygen/nitrogen
blends.
12.10 The acetylene gas is purified by passing it through a
11.3.4 Reagent grade powdered lithium or lithium rod (each
phosphoric acid or potassium chromate (in sulfuric acid) trap to
packed in argon) for converting CO to lithium carbide
remove trace impurities, and by using dry ice traps to remove
(Li C ).
water.
2 2
11.3.5 Reagent grade potassium chromate (in sulfuric acid)
12.11 The C H gas is catalyzed to benzene (C H ) by
2 2 6 6
or phosphoric acid for purifying acetylene gas.
bleeding the acetylene onto a chromium catalyst which has
11.3.6 Suitable catalyst material such as a Si O /Al O
2 3 2 3
been preheated to ≥90°C, or onto a vanadium catalyst (the later
substrate activated with either chromium (as Cr O ) or vana-
2 3
activates at ambient temperature). In the former case, the
dium (as V O ) for converting acetylene gas to benzene (30).
2 5
reaction is cooled with a water jacket to avoid decomposition
11.3.7 Scintillation cocktail.
from excessive heat generated during the exothermic reaction.
11.3.8 De-ionized or distilled water for hydrolysis of Li C
2 2
12.12 The benzene is thermally evolved from the catalyst at
to acetylene gas.
70-110°C and then collected under vacuum at roughly –78°C.
The benzene is then frozen until it is counted. Radon can be
12. Sample Preparation and Analysis
removed by pumping on the benzene while it is at dry ice
12.1 Tolerance of 65 % is to be assumed unless otherwise
temperatures.
stated.
12.13 The C content shall be determined in an LSC with
12.2 Standard procedures are to be employed for the con-
optimization of the instrument as described in Section 10.
version of original sample material to benzene using the liquid
Either single vial counting or “chain” counting is acceptable.
scintillation dating technique (28).
12.14 Radiocarbon activity in the sample is to be deter-
12.3 Based on the stoichiometry of the product material,
mined by “benzene cocktail” analysis, consisting of a scintil-
sufficient sample mass shall be weighed such that quantitative
lator plus sample benzene, in constant volume and proportion.
recovery of the carbon would theoretically yield 1.00-4.00 g of
A recommended scintillator is butyl-PBD or PPO/POPOP
carbon for conversion to benzene.
dissolved in toluene or equivalent (27) and (11). Alternatively,
some scintillators (including butyl-PBD) can be added to the
12.4 The carbon within each sample shall first be combusted
benzene as a solid.
to CO by placing the sample in a closed system which is
purged or evacuated of air.
12.15 Standard methods consist of counting a cocktail
containing sample benzene plus a scintillation solution. For
12.5 The system is then purged several times with pure
example, a cocktail might contain 4-mL sample benzene plus
nitrogen. After verifying the integrity of the closed system, the
0.5-mL scintillation solution. In this example, if 4-mL of
sample is bathed in 100 % oxygen (non-volatile samples) or a
sample benzene is not available, reagent grade (99.999 % pure)
mixture of nitrogen and oxygen (volatile samples) and ignited.
thiophene-free benzene can be added to bring the sample
Samples ignited using a nitrogen/oxygen mix must pass
volume to 4-mL. Larger or smaller volumes may be utilized
through a cupric oxide furnace at 850°C to avoid carbon loss to
depending upon the configuration of the specific laboratory’s
CO. The generated sample CO is collected using liquid
counting protocols.
nitrogen cold traps. If desired, the CO can be passed through
a series of chemical traps to remove various contaminants prior 12.16 LSCs are to be monitored for background and stabil-
to cryogenic collection of the CO (28). ity with traceable documentation.
D6866 − 24
12.17 Should anomalies appear during sample counting, the must be corrected for isotopic fractionation (25) after perform-
benzene is to be re-measured in another counter to verify the ing stable carbon isotope analyses. Correction shall be made
activity, or the sample must be completely re-analyzed. using the δ C measured by IRMS, CRDS or other equivalent
technology that can provide precision to 63 per mil. IRMS
12.18 Traceable quench detection should be performed on
Reference standard must be traceable to Vienna Pee Dee
each sample to ensure benzene purity. In the event the sample
Belemite (VPDB) using NIST SRM 8539, 8540, 8541, 8542,
is substantially quenched, the data should be discarded and the
or equivalent. The δ C must be measured on combustion CO
sample should be re-analyzed.
or benzene.
12.19 Measurements are to be made on an interval basis
13.3 The pMC in biobased products can be greater than
(usually 50 or 100 minutes) to allow statistical analysis of the
100 % because of the 1950s nuclear testing programs, which
measurement.
resulted in a considerable enrichment of C in the atmosphere
12.20 Prior to removing the sample from the counter,
(see 22.5). The first version of this standard (ASTM D6866-04)
stability is to be verified and the data scrutinized for anomalies.
in 2004 cited an atmospheric correction factor (REF) of 107.5
If the distribution does not closely follow Gaussian statistics,
pMC based on measurements of CO in air from a rural area in
the sample should be transferred and counted in another
the Netherlands (Lutjewad, Groningen). An annual decrease
counter for verification, or the sample should be completely
in atmospheric C from the bomb testing programs was
re-analyzed.
determined to be 0.5 pMC over the period of 2004 through
2018, based upon on-going measurements of CO in air from
12.21 Counting should be performed as needed to obtain an 2
the Netherlands. The C activity in the atmosphere reached the
accuracy of 2 % or better.
1950 level of 13.56 dpm per gram carbon, which is defined as
12.22 Calculation of the data should be performed only after
100 pMC by 2019, and remained at that level through 2024.
cross-checking all transcribed numbers, synthesis records,
Because all sample C activities are referenced to a “pre-
cocktail preparation, counting data, and counting analysis.
bomb” standard, and because nearly all new biobased products
12.23 Any unused sample material shall be maintained at
are produced in a post-bomb environment, all pMC values
the laboratory for potential re-analysis for a minimum of 180
(after correction for isotopic fractionation) must be adjusted by
days. The sample will then be disposed of in accordance with
an atmospheric correction factor (REF) to obtain the true
state and federal regulations.
biobased content of the sample. The atmospheric correction
factor is based on the C activity in the atmosphere at the time
12.24 Because of problems in storing benzene over ex-
of testing. Long term C atmospheric data compiled at sites
tended periods of time, it may be necessary to re-distill (to
from around the world “Atmospheric Radiocarbon for The
remove scintillant) and re-weigh the benzene if re-analysis is
Period 1950—2019; Quan Hua et al.,” and on-going Nether-
desired at a later date. Alternatively, a fresh portion of the
lands data through 2022, reflected a significant slowing in the
biobased product can be processed to obtain a fresh benzene
rate of the annual dilution of excess C in the atmosphere to
sample.
0.3 pMC. Therefore, on January 2 of each year, the values in
NOTE 5—The benzene derived from the sample carbon is toxic and is a
known carcinogen. Special handling and disposal procedures will be Table 2 are used as the REF for the period of 2024 through the
required.
end of 2026. References for reporting carbon isotopic ratio data
14 13
are given in Refs. (15, 26) for C and C, respectively.
13. Interpretation and Reporting
TABLE 2 Percent Modern Carbon (pMC) Reference
Year REF (pMC)
13.1 The counts shall be compared, directly or through
14 2015 102.0
secondary standards, to the primary NIST C oxalic acid SRM
2016 101.5
4990C (or other suitable standard traceable to SRM 4990C),
2017 101.0
2018 100.5
with stated uncertainties. Significantly lower C counts than
2019 100.0
the standard indicate the presence of C-depleted carbon
2020 100.0
source. The lack of any measurable C counts above the
2021 100.0
2022 100.0
background signal in a material indicates a fossil (for example,
2023 100.0
petroleum based) carbon source. A sample that has the
2024 99.7
same C activity level (after correction for the post-1950
2025 99.4
2026 99.1
bomb injection of C into the atmosphere) as the oxalic acid
standard is 100 % biobased and signifies an entir
...