EN ISO 21644:2021

SLOVENSKI STANDARD
01-marec-2021
Nadomešča:
SIST EN 15440:2011
SIST EN 15440:2011/AC:2011
Trdna alternativna goriva - Metode za določevanje biomase (ISO 21644:2021)
Solid recovered fuels - Methods for the determination of biomass content (ISO
21644:2021)
Feste Sekundärbrennstoffe - Verfahren zur Bestimmung des Gehaltes an Biomasse (ISO
21644:2021)
Combustibles solides de récupération - Méthode de détermination de la teneur en
biomasse (ISO 21644:2021)
Ta slovenski standard je istoveten z: EN ISO 21644:2021
ICS:
27.190 Biološki viri in drugi Biological sources and
alternativni viri energije alternative sources of energy
75.160.10 Trda goriva Solid fuels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 21644
EUROPEAN STANDARD
NORME EUROPÉENNE
January 2021
EUROPÄISCHE NORM
ICS 75.160.10 Supersedes EN 15440:2011
English Version
Solid recovered fuels - Methods for the determination of
biomass content (ISO 21644:2021)
Combustibles solides de récupération - Méthode de Feste Sekundärbrennstoffe - Verfahren zur
détermination de la teneur en biomasse (ISO Bestimmung des Gehaltes an Biomasse (ISO
21644:2021) 21644:2021)
This European Standard was approved by CEN on 22 November 2020.

CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this
European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by
translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 21644:2021 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 21644:2021) has been prepared by Technical Committee ISO/TC 300 "Solid
recovered materials, including solid recovered fuels" in collaboration with Technical Committee
CEN/TC 343 “Solid Recovered Fuels” the secretariat of which is held by SFS.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by July 2021, and conflicting national standards shall be
withdrawn at the latest by July 2021.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN 15440:2011.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 21644:2021 has been approved by CEN as EN ISO 21644:2021 without any modification.

INTERNATIONAL ISO
STANDARD 21644
First edition
2021-01
Solid recovered fuels — Methods for
the determination of biomass content
Combustibles solides de récupération – Méthode de détermination de
la teneur en biomasse
Reference number
ISO 21644:2021(E)
©
ISO 2021
ISO 21644:2021(E)
© ISO 2021
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
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CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved

ISO 21644:2021(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviations . 3
5 Principle . 4
6 Determination of biomass content . 4
6.1 Sampling . 4
6.2 Sample preparation . 4
6.3 Applicable methods . 4
7 Expression of results . 5
8 Performance characteristics . 5
9 Test report . 6
Annex A (normative) Determination of the biomass content based on the C method .7
Annex B (normative) Determination of biomass content using the selective dissolution
method (SDM) .25
Annex C (normative) Determination of biomass content using the manual sorting method
(Msort) .34
Annex D (informative) Limitations of the determination methods .39
Annex E (informative) Performance data .42
Bibliography .45
ISO 21644:2021(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www .iso .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 300, Solid recovered fuels.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2021 – All rights reserved

ISO 21644:2021(E)
Introduction
The biomass content of solid recovered fuels is relevant for the evaluation of the impact of energy
production on greenhouse gas emission. Instrumental methods, wet chemical and manual procedures
are available for the calculation of the renewable energy fraction. Instrumental methods are based on
the determination of C content while manual procedures are based on separation of different fractions
by visual inspection. The wet chemical procedure differentiate biomass from non-biomass materials as
function of the acid dissolution behaviour.
The fraction of biomass is expressed:
— by mass;
— by energy content (gross or net calorific value);
— by carbon content.
This document is primarily intended for laboratories, producers, suppliers and purchasers of solid
recovered fuels, but is also useful for the authorities and inspection organizations.
INTERNATIONAL STANDARD ISO 21644:2021(E)
Solid recovered fuels — Methods for the determination of
biomass content
1 Scope
This document specifies three methods for the determination of the biomass content in solid recovered
fuels: the C content method, the selective dissolution and the manual sorting methods.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 21637:2020, Solid recovered fuels — Terminology, definitions and descriptions
1)
ISO 21645 , Solid recovered fuels — Methods for sampling
2)
ISO 21646 , Combustibles solides de récupération — Préparation des échantillons
3)
ISO 21654 , Solid recovered fuels — Determination of calorific value
4)
ISO 21656 , Solid recovered fuels — Determination of ash content
ISO 21663, Solid recovered fuels — Methods for the determination of total carbon (C), hydrogen (H),
nitrogen (N) and sulphur (S) by the instrumental method
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 21637:2020 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
ash content on dry basis
mass of inorganic residue remaining after ignition of a fuel under specified conditions, expressed as
mass fraction in percent of the dry matter in the fuel, also includes removed ash contributors
Note 1 to entry: This is typically expressed as a percentage of the mass of dry matter in the fuel source.
Note 2 to entry: Depending on the combustion efficiency the ash may contain combustibles.
Note 3 to entry: If a complete combustion is realized, ash contains only inorganic, non-combustible components.
[SOURCE: ISO 21637:2020, 3.3]
1) Under preparation. Stage at the time of publication ISO/FDIS 21645.
2) Under preparation. Stage at the time of publication ISO/DIS 21646.
3) Under preparation. Stage at the time of publication ISO/FDIS 21654.
4) Under preparation. Stage at the time of publication ISO/FDIS 21656.
ISO 21644:2021(E)
3.2
biogenic
produced in natural processes by living organisms but not fossilized or derived from fossil resources
3.3
biomass
material of biological origin excluding material embedded in geological formations and/or fossilized
[SOURCE: ISO 16559:2014, 4.32, modified — Notes 1 and 2 to entry have been removed.]
3.4
calorific value
quantity of heat produced by the complete combustion, at a constant pressure equal to 1 013,25 mbar,
of a unit volume or mass of gas, the constituents of the combustible mixture being taken at reference
conditions and the products of combustion being brought back to the same conditions
[SOURCE: EN 437: 2018, modified — Second paragraph (the list) has been removed.]
3.5
gross calorific value
calorific value where the water produced by combustion is assumed to be condensed
[SOURCE: ISO 21637:2020, 3.34]
3.6
isotope abundance
fraction of atoms of a particular isotope of an element
3.7
laboratory sample
part of the sample (3.13) sent to or received by the laboratory
Note 1 to entry: When the laboratory sample is further prepared (reduced) by subdividing, mixing, grinding, or
by combinations of these operations, the result is the test sample. When no preparation of the laboratory sample
is required, the laboratory sample is the test sample. A test portion is removed from the test sample for the
performance of the test or for analysis.
Note 2 to entry: The laboratory sample is the final sample from the point of view of sample collection, but it is the
initial sample from the point of view of the laboratory.
Note 3 to entry: Several laboratory samples may be prepared and sent to different laboratories or to the same
laboratory for different purposes. When sent to the same laboratory, the set is generally considered as a single
laboratory sample and is documented as a single sample.
3.8
moisture
water removable under specific conditions
[SOURCE: ISO 21637:2020, 3.46]
3.9
net calorific value at constant volume
calorific value where the water produced by combustion is assumed to be in the vapour state
[SOURCE: ISO 21637:2020, 3.47]
3.10
nominal minimum particle size
aperture size of the sieve used for determining the particle size distribution of solid recovered fuels
through which no more than 5 % by mass of the material passes
2 © ISO 2021 – All rights reserved

ISO 21644:2021(E)
3.11
nominal top size
smallest aperture size of the sieve used for determining the particle size distribution of solid recovered
fuels through which at least 95 % by mass of the total material passes through the sieve
[SOURCE: ISO 21637:2020, 3.48]
3.12
percentage modern Carbon
pmC
carbon mass fraction from biogenic origin
Note 1 to entry: The internationally accepted radiocarbon dating reference value is 95 percent of the activity, in
AD 1950, of this NBS oxalic acid SRM4990B.
Note 2 to entry: In 2015, the value of 100 % biogenic carbon was set at 102 pmC.
Note 3 to entry: The biogenic origin is expressed in percentage.
3.13
sample
quantity of material, from a larger amount for which the quality is to be determined
[SOURCE: ISO 21637:2020, 3.63, modified — Notes 1–3 to entry have been removed.]
3.14
sample preparation
actions taken to obtain representative laboratory samples (3.7) or test portions from the original sample
(3.13) as received
[SOURCE: ISO 21637:2020, 3.66]
4 Symbols and abbreviations
For the purposes of this document, the following symbols and abbreviations apply.
C symbol for element carbon
D diameter (mm)
C carbon isotope with an atomic mass of 14 u
LSC Liquid Scintillation Counter or Liquid Scintillation Counting
Msort manual sorting method
RSD relative standard deviation
SDM selective dissolution method
SRF solid recovered fuel
TC total carbon content
u atomic mass unit
w mass fraction expressed as a percentage by mass
w content expressed as a percentage of the energy content
cal
w content expressed as a percentage of the total carbon content
TC
ISO 21644:2021(E)
The different references used in this document are indicated by the following indices:
— for air dried (dried at room temperature 20–25 °C for 24 hours)
(ad)
— for as received
(ar)
— for dry
(d)
— for dry and ash free, where appropriate.
(daf)
EXAMPLE w means the fraction of energy content in the non-biomass fraction by calorific value, on
cal,NB()d
dry basis.
5 Principle
The determination of the biomass content is based on selective dissolution, manual sorting or C
measurement of biomass in solid recovered fuel. The choice for the method to be used is described
in Clause 6. The biomass content gives an estimation of the content of the biogenic fraction in solid
recovered fuel.
6 Determination of biomass content
6.1 Sampling
Sampling, transport, storage of the solid recovered fuel and sample preparation in the field shall be
conducted according to ISO 21645 and ISO 21646.
6.2 Sample preparation
Preparation of the test sample for the C or SDM shall be conducted according to ISO 21646. For the
Msort, no sample preparation is performed.
Since SRF is considered as a heterogeneous material, the minimum sample amount to be used for each
test shall be:
— C method: a quantity between 0,4 and 2 g of the material with a nominal top size of 1 mm or
less, depending on the device used for combustion (bomb, combustion tube furnace or elemental
analyser) or the quantity indicated by the constructor in the case of the use of a laboratory scale
combustion apparatus;
— selective dissolution method (SDM): at least 5 g of the material with a nominal top size of 1 mm or less;
— manual sorting method (Msort): at least as big as the minimum sample size according to ISO 21645
(as received), as calculated in ISO 21646.
6.3 Applicable methods
For the determination of biomass content three methods are available:
1) the instrumental C method shall be according to Annex A. This method is based on the
14 14
determination of the ratio of C to the total carbon content; the C is proportional to the biomass
content of the SRF. This method is suitable for samples of all types of fuel and shall be according to
Annex A. A value of 10 % biogenic carbon can be considered as the lower range of application of C
method by liquid scintillation counter (LSC);
2) the selective dissolution method (SDM) shall be according to Annex B. The determination of the
biomass content by the SDM is based on the property of biomass that it can be dissolved in a
sulphuric acid / hydrogen peroxide mixture. This method has limitations that makes it less suitable
4 © ISO 2021 – All rights reserved

ISO 21644:2021(E)
if the content of natural and/or synthetic rubber in the SRF is more than 10 %, or if the sum of the
content of hard coal, coke, brown coal, lignite, degradable plastics of fossil origin, non-degradable
plastic of biogenic origin, oil or fat present as a constituent of biomass, wool, viscose, nylon,
polyurethane or other polymers containing molecular amino groups and silicon rubber exceeds
5 %. Additional information about these limitations is found in Annex D. The selective dissolution
method (SDM) is applicable for the biomass percentage content between 10 % and 90 %;
3) the manual sorting method (Msort) shall be according to Annex C. The determination of the
biomass content by the manual sorting method is based on the visual examination of fractions and
their separation on the basis of their nature and origin. The method is suitable for samples with a
particle size >10 mm.
For the limitations of the three methods see Annex D.
7 Expression of results
Depending on the use of the results, three different dimensions are used to express the biomass content:
a) biomass in percent by mass w ;
B
b) biomass in percent by calorific value w ;
B,cal
c) biomass in percent by carbon content w .
B,TC
The expression of results by C method shall be according to Annex A.
The expression of results by SDS method shall be according to Annex B.
The expression of results by Msort method shall be according to Annex C.
8 Performance characteristics
External data for the calculation of the expanded uncertainty of measurements are presented in
Annex E where results of round robin and validation studies are summarized. These values should be
used in combination with individual laboratory performance characteristics and a desired coverage
factor to get the overall uncertainty.
Practical examples of use of the data from Annex E:
EXAMPLE 1
A laboratory wants to determine the expanded uncertainty of measurement of SDM method (% by mass).
The intra-laboratory reproducibility for the laboratory calculated from internal validations studies and control
charts was determined to be 2,5 % (RSD).
[3]
The round robin results from the QUOVADIS study (Table E.2) give a RSD value of 3,43 % (at 67,79 % level).
2 2
u c,rel = √(2,5 +3,43 ) = 4,24 %
U rel = 2 × u c,rel = 8,48 %
where u c,rel is the combined uncertainty of measurement and U rel is the expanded uncertainty of measurement
using a coverage factor of 2 (~95 % confidence interval).
ISO 21644:2021(E)
EXAMPLE 2
A laboratory measures the biomass content by C method – LSC B (% by TC).
The intra-laboratory reproducibility for the laboratory calculated from internal validations studies and control
charts was determined to be 2,4 % (RSD).
[3]
The round robin results from the QUOVADIS study (Table E.6) give a RSD value of 2,5 % (at 55,5 % level).
2 2
u c,rel = √(2,4 +2,5 ) = 3,5 %
U rel = 2 × u c,rel = 7,0 %
where u c,rel is the combined uncertainty of measurement and U rel is the expanded uncertainty of measurement
using a coverage factor of 2 (~ 95 % confidence interval).
9 Test report
The test report shall contain at least the following information:
a) identification of the laboratory performing the test;
b) date of the test;
c) identification of product (sample) tested;
d) sample preparation (e.g. method of size reduction, drying, subdivision);
e) storage conditions;
f) date of receipt of laboratory sample and dates of the test (beginning and end);
g) a reference to this document (ISO 21644:2020) and the method used;
h) in case of C-method, the results of the test including the basis on which they are expressed and
application of the isotope correction;
i) the biomass content expressed as a percentage by mass, calorific value and/or carbon content,
rounded to the nearest 0,1 %;
j) any operation not included in this document, or regarded as optional;
k) any unusual features noted during the test procedure.
6 © ISO 2021 – All rights reserved

ISO 21644:2021(E)
Annex A
(normative)
Determination of the biomass content based on the C method
A.1 General
The two proposed methods for C measurement, Proportional Scintillation Method (PSM) or
Accelerated Mass Spectrometry (AMS), require specialised personnel and instrumentation. However,
the preparation step for instrumental analysis can be completed as normal routine laboratory activity.
For the collection from the sample of the C fraction, generally accepted methods for the conversion of
the carbon present in the sample to CO are described.
A.2 Principle
The methods for the determination of the biomass content specified in this annex are based on the
determination of the C content. The amount of biomass carbon in solid recovered fuel is proportional
to this C content.
The carbon present in the sample is converted to CO by combustion. The combustion is carried out
in a way to comply with the requirements of the subsequent measurement of the C content. This
measurement is carried out according to one of the two following methods, Proportional Scintillation
Method (PSM) or Accelerated Mass Spectrometry (AMS). These methods are considered equivalent,
giving the same results within the scope of this document. The results are expressed as the percentage
biomass carbon of the total carbon content. The fraction of biomass content by mass and the fraction of
biomass by energy content are calculated from the carbon content of biomass, using the carbon content
of biomass and the energy content of the biomass fraction that is present in the sample.
A.3 Limitations
For the limitation of this method see Annex D.
A.4 Symbols
For the purposes of this annex, the following symbols apply.
C symbol for element carbon
C carbon isotope with an atomic mass of 14
AMS Accelerator Mass Spectrometry
β beta particle, electron emitted during radioactive decay
Bq Bequerel, disintegrations per second
d on dry base
DPM disintegrations per minute
CPM counts per minute
ISO 21644:2021(E)
CV coefficient of variation
GM Geiger Müller
LCV Low calorific value
LLD Lower Limit of Detection
m mass expressed as a percentage by mass
M moisture expressed as a percentage by mass
MOP 3-Methoxy 1-propyl amine
NCV Net Calorific Value
LSC Liquid Scintillation Counter or Liquid Scintillation Counting
REF reference value of 100 % biogenic carbon
pmC percentage modern Carbon
PSM Proportional Scintillation-counter Method
X fraction expressed as a percentage by mass
RSD Relative Standard Deviation
SRF Solid recovered fuel
TC Total carbon content
A.5 Reagents and materials
CO absorber for LSC (methoxypropylamine or equivalent).
Universal LSC cocktail for aqueous and non-aqueous sample.
−1
2 to 4 mol l KOH or NaOH absorption liquid (standard glass bottles with plastic screw caps that are
resistant to alkaline solutions shall be used).
For the preparation of a carbonate free adsorption liquid, preparation using freshly opened KOH or
NaOH pellet containers is sufficient. Dissolve the KOH (NaOH) pellets in a small amount of water (the
heat produced during the dissolution process will enhance the dissolution process). When NaOH is
used, small amounts of precipitation are an indication of the presence of Na CO . By decanting the clear
2 3
phase, the almost carbonate free solution shall be diluted to the desired volume. As the dissolution
of KOH or NaOH is an exothermic process, extra care should be taken as boiling of the concentrated
solution during dilution can occur.
For high precision measurements the following procedure shall be used to produce a 0,7 l carbonate
−1
free KOH (NaOH) 4 mol l solution.
— 670 ml demineralised water (water from a system producing ultrapure water for laboratory use);
— 156,8 g KOH pellets (112 g NaOH);
— 30 ml saturated Ba(OH) solution. [2,4 ÷ 2,6 g Ba(OH) in 30 ml demineralised water];
2 2
— dissolve the KOH (NaOH) pellets in the demineralised water (use magnetic stirrer);
8 © ISO 2021 – All rights reserved

ISO 21644:2021(E)
— heat the solution and the saturated Ba(OH) solution to 80 °C, and mix the two solutions. Cool down
the solution to −8 °C, stop the stirring and leave the solution overnight at −8 °C. After filtration the
solution is ready for use. Keep stored in a well-sealed container.
A.6 Procedure for the conversion of the carbon present in the sample to CO for
C determination by PSM
A.6.1 General
Three procedures are allowed for the conversion of the sample to a form that can be used for the
determination of the C content:
1) combustion in a calorimetric bomb,
2) combustion in a tube furnace,
3) combustion in a laboratory scale combustion apparatus.
NOTE The method mentioned under 3) is not validated.
Other apparatus may be used which provide a complete combustion in the reported experimental
conditions. As an example, the combustion may be performed by using elemental analyser. The CO
formed is then absorbed in a suitable solution, which depends on the combustion method and the
selected method for the subsequent C measurement. Two absorption solutions are available: in
case substantial chemical or optical quenching is foreseen (high NO values, formation of coloured
x
substances) collection of the CO shall be done in the NaOH solution. The use of pure oxygen or a mix of
oxygen and argon during combustion will reduce the formation of nitrous oxides to an acceptable level.
A.6.2 Combustion of the sample in a calorimetric bomb
A.6.2.1 Procedure
For the combustion according to the determination of the calorific value of the sample, ISO 21654 shall
be used. The test sample is a general analysis sample passing through a sieve with 1 mm aperture and
prepared according to ISO 21646. The test sample mass of less than 1 g is pressed in the form of a
pellet by using a suitable pressing device (manual or pneumatic). For SRF materials with high content
of plastic or rubber showing higher LCV values, the test sample mass should be reduced to a mass in
the range from 0,4 to 0,8 g to be suitable for safe bomb operation. For materials difficult to combust
(e.g. material with high ash content >30 % on dry basis) it is recommended to use a combustion aid.
The appropriate mass of the test sample to be combusted depends on the total carbon content in order
to have similar amount of absorbed CO in the scintillation cocktail for subsequent C measurement to
reduce the measurement bias due to different quenching conditions: for this purpose, the total carbon
content of the sample shall be determined before the combustion step.
After combustion, the combustion gases are collected in a suitable mixture. Alternatively, the gases
are collected in a gas bag as described in A.6.2.2. For the determination of the C content the CO
shall be collected in cooled (<10 °C) absorbing solution or a cooled mixture of absorbing solution and
scintillation liquid.
As the bomb volume is released to atmospheric pressure, there will be a residual amount leftover in the
bomb that is directly related to the pressure in the bomb after the combustion (with a residual pressure
of 2,5 MPa 4 % of the combustion gas will be left after release to atmospheric pressure).
To overcome this issue:
a) perform the calibration and the analysis taking account of this residual amount by using the
pressure correction factor,
b) use the vacuum pump to remove the residue gases;
ISO 21644:2021(E)
c) flush the bomb with Argon or (CO -free) N and collect the CO in the rinsing gases as well.
2 2 2
A.6.2.2 Absorption of the gas sample
If a gas sample bag is used, it shall be connected to a small pump with a connection line into a 20 ml
glass vial, filled with 10 ml of absorbing solution or a mixture of 10 ml of the absorbing liquid and 10 ml
of the scintillation cocktail, placed in an ice bath or cooling device at <10 °C, to remove the heat of the
−1
exothermic carbamate formation reaction. The pumping speed shall be about 50 ml·min . The transfer
of the gas from the bag takes about 2 h to 3 h. After the sample is collected, it is ready to be counted on
a LSC. Blank samples should also be counted at the same time to allow that small day-to-day variations
in the background can be accounted for.
In case of direct absorption of combustion gases, the outlet of the combustion device (e.g. oxygen
combustion bomb) is connected via transfer line, equipped with a valve, to an empty cooled impinger
for water removal and a second cooled impinger with 10 ml absorbing solution or scintillation cocktail.
Using the proper safe device, the bomb valve is opened and the gases are collected at a low flux rate
−1
(optimal value 50–60 ml·min ). The transfer takes about 3 h. Blank samples should also be counted at
the same time to allow that small day-to-day variations in the background can be accounted for.
Measurements shall be started after leaving the vial to cool down to the measurement temperature and to
reduce any chemiluminescence due to manipulation. Absorbing solution are stable at least for 1 week after
−1
sampling: alternatively, the CO shall be collected in a 4 mol l KOH (NaOH) solutions for longer period.
NOTE 1 There are strong indications that the NO formed during the combustion reacts with the absorption
x
mixture resulting in yet unexplained errors after a few days of storage.
NOTE 2 The initial and final mass of the absorbing solution is an estimation of the recovery of CO and
indicates the possible sources of error in the procedure, such as a leakage in the absorption system.
A.6.3 Combustion of the sample in a tube furnace
A suitable amount of sample (seaved at 1 mm nominal size) is weighed in the boat to be inserted in
the combustion tube. The test mass depends on the carbon content of the SRF and the capacity of the
absorption solution; generally, up to 2 g are processed in macro instruments and typically 200 mg in
microtube furnace. The gas bubbles set is filled with the absorption solution. The temperature of the
furnace is raised at the operation value (e.g. 1 100 or 1 350 °C): when temperature reaches the set
point the oxygen supply is connected and the flow rate is adjusted at the desired value. The sample
boat is inserted and combustion is started. At the end of the combustion the impingers are removed
and the absorption solutions collected for subsequent analysis. The oxygen flow rate should not exceed
the maximum flow rate allowed for the gases to be quantitatively collected in the absorption solution.
In case of the use of microtube the representativity of the test sample should be verify according to the
grain size.
For the determination of the C content the CO shall be collected using an impinger filled with a
−1
cooled absorbing solution or a mixture of absorbing and scintillation liquid; alternatively, a 4 ml l KOH
(NaOH) solution may be used (see A.6.2.2, NOTE 1 and 2).
As an alternative, the CO may be trapped by means of a cryogenic trap. In that case the cryogenic trap
shall consist of a water trap (dry ice in ethanol or acetone) followed by a cryogenic trap. Care shall be
taken to avoid formation of liquid oxygen, which shall be achieved by heating the trap slightly above the
boiling point of oxygen, using liquid argon or by performing the separation at diminished pressure.
A.6.4 Combustion of the sample in a laboratory scale combustion apparatus
The combustion condition and test sample amount depend on the apparatus used and manufacturer
instructions.
The CO may be trapped using different methods, depending on the laboratory. Some examples are
reported below.
10 © ISO 2021 – All rights reserved

ISO 21644:2021(E)
The lab-scale combustion apparatus shall be able to combust a suitable SRF-sample at a constant rate,
with a complete conversion of the carbon present to CO . For the determination of the C content
the CO shall be collected using a suitable impinger filled with a cooled mixture of absorbing solution
−1
and a suitable scintillation liquid or a 4 ml l KOH (NaOH) solution (see A.6.2.2, NOTE 1 and 2). As a
result of the absorption of the CO a large volume reduction of the gas volume will be observed after
trapping. Therefore, the gas pump is to be positioned in front of the impinger, and the gas pump used
shall be gas tight.
As an alternative, the CO may be trapped by means of a cryogenic trap, which consist of a water trap
(dry ice in ethanol or acetone) followed by a cryogenic trap. Care shall be taken to avoid formation of
liquid oxygen, which shall be achieved by heating the trap slightly above the boiling point of oxygen,
using liquid argon or performing the separation at diminished pressure. As an alternative, when AMS
is being used, CO may be collected by mixing homogenized SRF with cupric oxide (CuO) in a sealed,
evacuated quartz or high silica, high temperature glass tube. Water vapour (up to 3 Pa) can be added
to the tube prior to introduction of the CO to help remove sulphur compounds. The tube is heated to
900 °C for 3 h to 5 h. The CO is collected by breaking the tube using a tube-cracker connected to an
evacuated glass collection line.
A.6.5 Measurements
If collected samples are sent to specialized laboratories, the samples shall be stored in a way that no
CO from air can enter the absorption solution. A check on the leak of CO from air shall be performed
2 2
by preparing laboratory blanks during the sampling stage.
For the determination of the 0 % biomass content the combustion of a coal reference material
(e.g. BCR 182) shall be used.
For the 100 % biomass content the Oxalic acid primary standard (SRM 4990c) is available: however, this
material is difficult to combust due to its low calorific value and the difficulties in the preparation of a
suitable pellet. Other reference materials such as Lichen BCR 482 may be used because their biomass
content is 100 % by total carbon by definition (the reported organic C content is 42,1 % and total carbon
is 44,7 %). A control by using independent 100 material (laboratory internal reference materials such
as collected vegetable prepared for other type o chemical analysis are suitable) is recommended.
NOTE Oxalic acid can be difficult to completely combust under the conditions used in this document for the
bomb combustion.
Among the materials which may be used the following are included:
— vegetable, with known TC content, assuming 100 % biomass;
— L-ascorbic acid from natural sources, with known purity and TC content from chemical formula;
— material with recent carbon, with reference biomass content from AMS method.
This aspect regarding uncertainty evaluation should be considered:
— 0 and 100 % biomass reference values should be used without uncertainty since they derive from
theoretical consideration and not from accepted averages values from round robin test;
— C content should be based on reference method determination such as AMS; or on theoretical
consideration from actual CO average level.
ISO 21644:2021(E)
A.7 Procedure for the C determination by Proportional Scintillation-counter
Method (PSM)
A.7.1 General
This procedure describes the determination of the C by Proportional Scintillation-counter Method
(PSM) in absorbing solutions obtained from the combustion of SRF samples in a calorimetric bomb, a
tube furnace or a laboratory scale combustion device as described in A.6.
A.7.2 Principle
PSM (also called Liquid Scintillation Counter method, LSC) determines the isotope abundance of C
indirectly, through its emission of β particles due to the radioactive decay of the C isotope. The β
particles are observed through their interaction with scintillation molecules. The CO formed by the
combustion of SRF is trapped in an absorbing solution. This solution is mixed with the organic solution
containing the scintillation molecules and the C activity of this mixture is measured in a Proportional
(Liquid) Scintillation Counter. In case substantial chemical or optical quenching is foreseen (high
NO values, formation of coloured substances) collection of the CO shall be done in the NaOH (KOH)
x 2
solution. As an alternative the use of pure oxygen or a mix of oxygen and argon during combustion will
reduce the formation of nitrous oxides to an acceptable level. In this case either absorption solutions
may be used.
A.7.3 Reagents and materials
— scintillation liquid;
— absorbing solution (e.g. 100 % ethanolamine or 5M ethanolamine in 2-methoxyethanol or
3-methoxypropylamine);
— commercial C labelled substance in solution with known dpm/ml valid to check instrument
performances;
— oxalic acid primary standard (SRM 4990c) or other suitable reference material;
— a C-free reference material.
A.7.4 Apparatus
−12
The extremely low natural levels of radiocarbon in the earth's atmosphere (about 1,10 %) requires
extra precautions for accurate measuremen
...