SIST EN ISO 21362:2026

SLOVENSKI STANDARD
01-april-2026
Nanotehnologije - Analiza nanoobjektov s frakcioniranjem asimetričnega in
centrifugalnega pretoka skozi polje (ISO 21362:2026)
Nanotechnologies - Analysis of nano-objects using asymmetrical flow and centrifugal
field-flow fractionation (ISO 21362:2026)
Nanotechnologien - Analyse von Nanoobjekten mit Hilfe von Asymmetrischer-Fluss-
Feldflussfraktionierung und zentrifugaler Feldflussfraktionierung (ISO 21362:2026)
Nanotechnologies - Analyse des nano-objets par fractionnement flux asymétrique et flux
force centrifuge (ISO 21362:2026)
Ta slovenski standard je istoveten z: EN ISO 21362:2026
ICS:
07.120 Nanotehnologije Nanotechnologies
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 21362
EUROPEAN STANDARD
NORME EUROPÉENNE
February 2026
EUROPÄISCHE NORM
ICS 07.120 Supersedes CEN ISO/TS 21362:2021
English Version
Nanotechnologies - Analysis of nano-objects using
asymmetrical flow and centrifugal field-flow fractionation
(ISO 21362:2026)
Nanotechnologies - Analyse des nano-objets par Nanotechnologien - Analyse von Nanoobjekten mit
fractionnement flux asymétrique et flux force Hilfe von Asymmetrischer-Fluss-
centrifuge (ISO 21362:2026) Feldflussfraktionierung und zentrifugaler
Feldflussfraktionierung (ISO 21362:2026)
This European Standard was approved by CEN on 11 January 2026.

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, Türkiye 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
© 2026 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 21362:2026 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 21362:2026) has been prepared by Technical Committee ISO/TC 229
"Nanotechnologies" in collaboration with Technical Committee CEN/TC 352 “Nanotechnologies” the
secretariat of which is held by AFNOR.
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 August 2026, and conflicting national standards shall
be withdrawn at the latest by August 2026.
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 CEN ISO/TS 21362:2021.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
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, Türkiye and the
United Kingdom.
Endorsement notice
The text of ISO 21362:2026 has been approved by CEN as EN ISO 21362:2026 without any modification.

International
Standard
ISO 21362
First edition
Nanotechnologies — Analysis of
2026-02
nano-objects using asymmetrical
flow and centrifugal field-flow
fractionation
Nanotechnologies — Analyse des nano-objets par fractionnement
flux asymétrique et flux force centrifuge
Reference number
ISO 21362:2026(en) © ISO 2026
ISO 21362:2026(en)
© ISO 2026
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
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Email: copyright@iso.org
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Published in Switzerland
ii
ISO 21362:2026(en)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms. 8
5 Principles of operation .10
5.1 Field-flow fractionation (FFF) — General .10
5.2 Specific applications by applied field.11
5.2.1 Flow field .11
5.2.2 Centrifugal field . 13
6 Method development for asymmetrical flow field-flow fractionation (AF4) .15
6.1 General . 15
6.2 Sample specifications . 15
6.3 Mobile phase specifications .16
6.4 Fractionation .17
6.4.1 Channel and membrane selection .17
6.4.2 Injection and relaxation .19
6.4.3 Optimizing flow conditions . 20
6.4.4 Elution programme . 20
6.4.5 Using field-flow fractionation (FFF) theory to select initial flow settings .21
7 Method development for centrifugal field-flow fractionation (CF3) .21
7.1 General .21
7.2 Choice of mobile phase .21
7.3 Field strength selection . 22
7.4 Field decay programme . 22
7.5 Channel flow rate selection . 22
7.6 Calculation of the relaxation time . 23
7.7 Calculation of sample injection delay . 23
7.8 Using field-flow fractionation (FFF) theory to select initial settings . 23
8 Analysis of nano-objects .23
8.1 General . 23
8.2 Online size analysis . 23
8.3 Online concentration analysis . 25
8.3.1 General . 25
8.3.2 Mass-based methods . 25
8.3.3 Number-based methods . . 26
8.4 Online material identification or composition .27
8.5 Off-line analysis (fraction collection) .27
8.6 Alternative and emerging methods . 28
9 Qualification, performance criteria and measurement uncertainty .29
9.1 System qualification and quality control . 29
9.1.1 Basic system qualification . 29
9.1.2 Focusing performance . 30
9.1.3 Flow rate of the carrier liquid. 30
9.1.4 Separation field . 30
9.2 Method performance criteria .31
9.2.1 Recovery .31
9.2.2 Selectivity .32
9.2.3 Retention ratio .32
9.2.4 Resolution .32

iii
ISO 21362:2026(en)
9.3 Method precision and measurement uncertainty .32
10 General procedures for measurement of samples .33
10.1 Introduction . 33
10.2 Calibration of retention time for online size analysis . 33
10.2.1 Calibration of the asymmetrical flow field-flow fractionation (AF4) channel. 33
10.2.2 Calibration of asymmetrical flow field-flow fractionation (AF4) retention time
for online size measurements . . 34
10.3 Asymmetrical flow field-flow fractionation (AF4) general measurement procedure . 34
10.4 Centrifugal field-flow fractionation (CF3) general measurement procedure . 35
11 Test report .36
11.1 General . 36
11.2 Apparatus and measurement parameters . 36
11.2.1 Asymmetrical flow field-flow fractionation (AF4) recording and reporting
specifications . . 36
11.2.2 Centrifugal field-flow fractionation (CF3) recording and reporting specifications .37
11.3 Test report . 38
Annex A (informative) Summary of interlaboratory comparison .39
Bibliography .54

iv
ISO 21362:2026(en)
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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
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 jointly by Technical Committee ISO/TC 229, Nanotechnologies and Technical
Committee IEC/TC 113, Nanotechnology for electrotechnical products and systems, in collaboration with the
European Committee for Standardization (CEN) Technical Committee CEN/TC 352, Nanotechnologies, in
accordance with the Agreement on technical cooperation between ISO and CEN (Vienna Agreement). The
draft was circulated for voting to the national bodies of both ISO and IEC.
This first edition cancels and replaces ISO/TS 21362:2018, which has been technically revised.
The main changes are as follows:
— addition of subclause 8.6 addressing alternative and emerging methods;
— revision of technical content to reflect the current state of the art;
— addition of Annex A summarizing an interlaboratory comparison conducted through VAMAS.
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.

v
ISO 21362:2026(en)
Introduction
The capacity to isolate and analyse diverse populations of nano-objects and their agglomerates or aggregates,
often suspended in, or extracted from, complex matrices, is critical for applications ranging from materials
discovery and nanomanufacturing to regulatory oversight and environmental risk assessment. Furthermore,
the ability to characterize these analytes with minimal perturbation of their natural or native state is
highly desirable. The list of available techniques capable of achieving such objectives is relatively short,
and while all techniques have advantages and disadvantages, and no single technique is solely adequate
or appropriate for all possible applications and materials, a group of related separation techniques known
collectively as field-flow fractionation (FFF), conceptually proposed in Reference [1] offers many advantages
for nanotechnology applications. In FFF, the analyte, suspended in a liquid medium, is fractionated by
the application of a field (e.g. flow, centrifugal, electric, thermal-gradient, magnetic) perpendicular to the
direction of flow of the analyte and mobile phase eluting through a thin defined channel. Separation occurs
when the analyte responds to the applied field, such that populations with different response sensitivities
reach equilibrium positions (i.e. in equilibrium with diffusional forces) higher or lower in the laminar flow
streamlines perpendicular to channel flow, thus eluting differentially.
Among the FFF variants, asymmetrical flow FFF (variously abbreviated in the literature as AF4, A4F, AFFFF,
AfFFF or AsFlFFF) and centrifugal FFF (abbreviated as CF3, also called sedimentation FFF associated with the
abbreviation SdFFF), are available commercially and have been most widely adopted in the nanotechnology
field (for convenience and simplicity, the abbreviations AF4 and CF3 are used throughout this document).
AF4 is arguably the most versatile technique with respect to the wide range of applications, materials and
particle sizes to which it has been applied. Symmetrical flow FFF (fFFF), the original “flow-based” technique
[2] [3]
as first described in 1976, has been supplanted commercially by AF4, introduced in 1987, due to several
advantages, including a simpler channel design, the ability to visualize the sample through a transparent
top channel wall, and reduced analyte band width. The theory and application of CF3 as it is presently
[4]
applied was described in 1974, although a centrifugal field-based FFF system was first developed and
[5]
tested independently in 1967. Other FFF field variants, such as thermal, electrical and magnetic, provide
unique capabilities, but are limited in the scope of their applications vis-à-vis nanotechnology or commercial
availability.
Where FFF was once predominantly the domain of specialists, these instruments are now commonly and
increasingly utilized in government, industry and academic laboratories as part of the nano-characterization
toolbox. Two factors are driving this increase in nanotechnology utilization: maturation of commercial
instrumentation and versatility with respect to coupling a wide range of detectors to FFF systems. In the
latter case, recent developments have led to the use of highly sensitive elemental detectors (e.g. inductively
coupled plasma mass spectrometer or ICP-MS), which offer enhanced characterization and quantification for
many materials. Additionally, traditional concentration or sizing detectors, such as ultraviolet-visible (UV-
Vis) absorbance, fluorescence, multi-angle light scattering (MALS) and dynamic light scattering (DLS), yield
online data for eluting populations, and theoretically provide more accurate information than obtainable
using off-line measurements of unfractionated polydisperse systems. The measured retention time of an
eluting peak can also be used to estimate the hydrodynamic size by AF4 based on theoretical relationships
or calibration with a known size standard. CF3 has the unique capacity to rapidly separate species of the
same size but differing in density.
Although FFF based techniques have the capacity to separate and characterize analytes over an extremely
broad size range, from about 1 nm up to tens of micrometres, this document focuses primarily on materials
in the nanoscale regime and their associative structures. However, the basic underlying principles,
experimental approach, and hardware described here can be more broadly applied.
For general references and further reading for FFF theory and practise, as well as AF4 and CF3 applications
to nanotechnology, see References [6] to [18]. Annex A summarizes a Versailles Project on Advanced
Materials and Standards (VAMAS) interlaboratory comparison conducted to evaluate the capacity of AF4
and CF3 techniques to separate and characterize components of a complex multimodal mixture of analytes
reproducibly and with acceptable recovery and resolution across laboratories using different commercial
instrument platforms and instrument configurations.

vi
International Standard ISO 21362:2026(en)
Nanotechnologies — Analysis of nano-objects using
asymmetrical flow and centrifugal field-flow fractionation
1 Scope
This document describes the general principles of field-flow fractionation and specifies parameters,
conditions and minimal reporting requirements, as part of an integrated measurement system, required
to develop and validate methods for the application of asymmetrical flow and centrifugal field-flow
fractionation in the analysis of nano-objects and their aggregates and agglomerates in aqueous media.
General guidelines and procedures are provided to aid the user.
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 80004-1:2023, Nanotechnologies – Vocabulary — Part 1: Core vocabulary
ISO/TS 80004-6:2021, Nanotechnologies — Vocabulary — Part 6: Nano-object characterization
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 80004-1 and ISO/TS 80004-6 and
the following, apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at https:// www .electropedia .org/
— ISO Online browsing platform: available at https:// www .iso .org/ obp
3.1
nanoscale
length range approximately from 1 nm to 100 nm
[SOURCE: ISO 80004-1:2023, 3.1.1]
3.2
nano-object
discrete piece of material with one, two, or three external dimensions in the nanoscale (3.1)
[SOURCE: ISO 80004-1:2023, 3.1.5]
3.3
nanoparticle
nano-object (3.2) with all external dimensions in the nanoscale (3.1)
Note 1 to entry: If the dimensions differ significantly (typically by more than three times), terms such as nanofibre or
nanoplate are preferred to the term nanoparticle.
[SOURCE: ISO 80004-1:2023, 3.3.4]

ISO 21362:2026(en)
3.4
field-flow fractionation
FFF
separation technique where a field is applied to a liquid suspension passing along a narrow channel (3.7) in
order to induce separation of the particles present in the liquid, dependent on their differing mobility under
the force exerted by the field
Note 1 to entry: The field can be, for example, gravitational, centrifugal, liquid flow, electrical or magnetic.
Note 2 to entry: Using a suitable detector after or during separation allows determination of the mean size and size
distribution of nano-object (3.2) populations.
3.5
asymmetrical flow field-flow fractionation
AF4
separation technique that uses a cross flow (3.19) field applied perpendicular to the channel flow (3.21) to
achieve separation based on analyte diffusion coefficient or size
Note 1 to entry: Cross flow occurs by means of a semipermeable (accumulation) wall in the channel, while cross flow is
zero at an opposing nonpermeable (depletion) wall.
Note 2 to entry: By comparison, in symmetrical flow, the cross flow enters through a permeable wall (frit) and exits
through an opposing semipermeable wall and is generated separately from the channel flow.
Note 3 to entry: Nano-objects (3.2) generally fractionate by the “normal” mode, where diffusion dominates and the
smallest species elute first. In the micrometre size range, the “steric-lift hyperlayer” mode of fractionation is generally
dominant, with the largest species eluting first. The transition from normal to steric-lift hyperlayer mode (3.33) can be
affected by material properties or measurement parameters, and therefore is not definitively identified; however, the
transition can be defined explicitly for a given experimental set of conditions; typically, the transition occurs over a
particle size range from about 0,5 µm to 2 µm.
Note 4 to entry: Including both normal and steric-lift hyperlayer modes, the technique has the capacity to separate
particles ranging in size from approximately 1 nm to about 50 µm.
3.6
centrifugal field-flow fractionation
CF3
separation technique that uses a centrifugal field applied perpendicular to a circular channel (3.7) that spins
around its axis to achieve size separation of particles from roughly 10 nm to roughly 50 µm.
Note 1 to entry: Separation is governed by a combination of size and effective particle density.
Note 2 to entry: Applicable size range is dependent on and limited by the effective particle density.
3.7
channel
thin ribbon-like chamber with a parabolic flow profile required for separation
under the influence of a field applied perpendicular to the channel flow (3.21)
Note 1 to entry: Channel thickness (3.9) can vary and is nominally determined by a spacer (3.8) insert, while fixed-
height channels have a predefined thickness and do not use inserts.
Note 2 to entry: In asymmetrical flow field-flow fractionation (3.5), a trapezoidal channel is commonly used, typically
with a maximum breadth of ca. 20 mm to 25 mm and length of ca. 100 mm to 300 mm.
Note 3 to entry: In asymmetrical flow, one channel surface [depletion wall (3.12)] is solid (impermeable) and the
opposing surface [accumulation wall (3.11)] consists of a semipermeable membrane on a porous frit.
Note 4 to entry: In centrifugal field-flow fractionation (3.6), both the inner and outer walls of the channel are solid
(non-porous) and the channel is curved. A trapezoidal channel is commonly used, typically with a breadth of 10 mm to
20 mm and length of 300 mm to 550 mm.

ISO 21362:2026(en)
3.8
spacer
thin plastic film with a cut-out that defines the thickness and lateral dimensions
of the channel (3.7)
Note 1 to entry: Trapezoidal or rectangular cut-outs are most commonly used in asymmetrical flowfield-flow
fractionation (3.5).
Note 2 to entry: Typical spacer thickness used for separation of nano-objects (3.2) ranges from 190 µm to 500 µm.
Note 3 to entry: Fixed-height channels do not use a spacer; in this case the channel shape and thickness are predefined.
3.9
channel thickness
w
nominal thickness as defined by the spacer (3.8) or predefined in a fixed-height
channel (3.7)
3.10
effective channel thickness
w
eff
varying from the nominal value due to compressibility or swelling of the
semipermeable membrane at the accumulation wall (3.11)
Note 1 to entry: The value of the effective thickness can differ from the nominal value for a given spacer (3.8) and may
be determined using a well-defined analyte of known diffusivity under the test conditions.
Note 2 to entry: The measured effective channel thickness can depend on other factors, such as interactions between
the analyte and the membrane and variability in spacer manufacturing.
3.11
accumulation wall
surface of a field-flow fractionation (3.4) channel toward which sample components are forced by the applied
field acting perpendicular to the channel flow (3.21)
Note 1 to entry: In asymmetrical flow field-flow fractionation (3.5), the accumulation wall is flat and consists of a
semipermeable membrane on a porous frit substrate.
Note 2 to entry: In centrifugal field-flow fractionation, the accumulation wall is impermeable and curved, and is
located farther from the axis of rotation relative to the depletion wall (3.12). In the rare case that the particles have a
lower density than the aqueous medium, the depletion and accumulation walls are reversed.
3.12
depletion wall
surface of a field-flow fractionation (3.4) channel opposite the accumulation wall (3.11), which is depleted in
analyte due to the movement of analyte toward the accumulation wall in the applied field
Note 1 to entry: In asymmetrical flow field-flow fractionation, the depletion wall is flat and impermeable.
Note 2 to entry: In centrifugal field-flow fractionation (3.6), the depletion wall is impermeable and curved, and located
closer to the axis of rotation relative to the accumulation wall. When the effective particle density is lower than the
density of the medium, the depletion and accumulation walls are reversed.
3.13
mobile phase
carrier liquid
eluent
liquid phase used to achieve separation and transport of analytes
Note 1 to entry: The eluent or mobile phase can contain one or more salts, surfactants, and other chemical constituents
that are required for optimized separation and recovery (3.35) of an analyte.

ISO 21362:2026(en)
Note 2 to entry: In this document, only aqueous phases are relevant, but organic solvents can also be used if equipment
and channel are compatible.
3.14
elution
process by which analytes in the mobile phase (3.13), or eluent, are transported
through, and exit from, the fractionation channel (3.7)
Note 1 to entry: Elution begins after injection, focusing (3.16) and other pre-elution steps have completed.
Note 2 to entry: Elution can occur with or without an applied field.
3.15
elution time
elapsed time after initiation of elution (3.14) and excluding preliminary steps such
as injection, focusing (3.16) or other transitions
Note 1 to entry: Elution and retention share the same timeline and can be used interchangeably.
Note 2 to entry: The horizontal (time) axis of a fractogram (3.34) is generally expressed as elution time.
3.16
focusing
application of counter-balanced flow from opposite ends
(inlet and outlet) of the channel (3.7) to focus sample components into a thin band near the inlet port at the
accumulation wall (3.11)
Note 1 to entry: This step is required to minimize band broadening (3.30) and to allow components to achieve an
equilibrium localization [relaxation (3.17)] within the channel.
Note 2 to entry: Focusing does not occur during frit-inlet injection.
3.17
relaxation
process by which the sample components assume their equilibrium state with
respect to the opposing forces of diffusion and the applied field before elution (3.14) is initiated
Note 1 to entry: In flow field-flow fractionation (3.4) there are two means to achieve relaxation: normal focusing (3.16),
relaxation and frit inlet or hydrodynamic relaxation.
Note 2 to entry: In centrifugal field-flow fractionation (3.6), stop-flow is used to achieve relaxation.
3.18
injection flow
flow that drives the sample out of the injection loop and into the fractionation
channel (3.7)
Note 1 to entry: Depending on instrument design, injection can occur via a separate injection port or through the
channel inlet port.
3.19
cross flow
flow field applied perpendicular to the channel flow (3.21) to achieve
separation of analytes
Note 1 to entry: In asymmetrical flow field-flow fractionation (3.5), cross flow is created by the pressure differential
across a permeable membrane at the accumulation wall (3.11), which results in a force directed toward the
accumulation wall that decreases with increasing distance from the accumulation wall.
Note 2 to entry: Cross flow is generated by using a flow controller combined with a single pump or by use of a second
dedicated pump.
ISO 21362:2026(en)
3.20
inlet flow
mobile phase (3.13), or eluent, that enters the channel (3.7) at the front end
(upstream)
Note 1 to entry: In asymmetrical flow field-flow fractionation (3.5), inlet flow is split between cross flow (3.19) and
channel flow during elution (3.14).
3.21
channel flow
parabolic laminar flow through the channel (3.7) and parallel to the accumulation
wall (3.11)
Note 1 to entry: Channel flow is generally equivalent to the flow exiting the channel and entering the detectors under
typical experimental conditions but can differ if flow exiting the channel is split.
Note 2 to entry: In asymmetrical flow field-flow fractionation (3.5), fluid loss through the permeable accumulation wall
leads to a linearly decreasing channel-flow velocity. This gradient can be compensated using a trapezoidal channel
design with decreasing channel breadth toward the outlet.
3.22
void volume
V
fluid volume defined by the channel (3.7) dimensions plus the volume between the
channel exit and the first detector
3.23
void peak
peak appearing in the fractogram (3.34) that corresponds to unretained material
not in equilibrium with the separation field
Note 1 to entry: The void peak travels at the average carrier fluid velocity and elutes before retained components.
Note 2 to entry: In this context, unretained means components that are not separated by the field and elute with the
void peak. Unretained has a different meaning in traditional enthalpic-based chromatographic separations.
Note 3 to entry: A void peak is generated by the mechanical disruption or change in flow conditions when elution
(3.14) is initiated; in this context the void peak should ideally contain only mobile phase (3.13) and small molecular
species unaffected by the applied field.
3.24
void time
t
time between initiation of elution (3.14) and detection of the void peak (3.23) defined at its maximum signal
intensity
3.25
retention time
t
R
time between initiation of elution (3.14) and detection of an analyte peak defined at its maximum signal
intensity
Note 1 to entry: For a Gaussian peak, the maximum and peak centre are equivalent.
Note 2 to entry: Retention time and elution time (3.15) represent equivalent timelines. The latter is generic, while the
former is typically used in the context of an analyte peak.
Note 3 to entry: The net retention time can be obtained by subtracting the void time (3.24) from the measured peak
retention time. This equates to normalizing retention time to the elution of unretained material traveling at the mean
velocity of the channel parabolic flow.

ISO 21362:2026(en)
3.26
retention parameter
λ
dimensionless parameter equal to the ratio of the analyte zone centre-of-mass
distance from the accumulation wall (3.11) to the channel (3.7) thickness
Note 1 to entry: A measure of the strength of interaction between the applied field and the analyte.
3.27
retention ratio
R
ratio of the mean velocity of the analyte zone to the mean velocity of the mobile
phase (3.13) in the channel (3.7) during elution (3.14)
Note 1 to entry: This can be calculated theoretically or determined empirically from the ratio of the elution times
(3.15) associated with the void and analyte peaks and is directly related to the retention parameter (3.26).
3.28
selectivity
measure of the ability of a method to separate analytes of different diffusion
coefficient or size
Note 1 to entry: Empirically, selectivity is calculated from the slope of a double logarithmic plot of diffusion coefficient
versus retention ratio (3.27) or retention time (3.25) versus analyte diameter.
Note 2 to entry: A high selectivity reflects a large change in retention time with a small variation in analyte size.
Note 3 to entry: In centrifugal field-flow fractionation (3.6), selectivity is also dependent on effective mass, but the
empirical relationship is defined in the same manner as asymmetrical flow field-flow fractionation (3.5).
3.29
resolution factor
fractionation power
R
s
ratio of the difference in retention time (3.25) to the average of the peak widths measured as the full width at
half maximum for two adjacent eluting analytes
Note 1 to entry: Measure of the degree of separation between neighbouring or overlapping peaks.
3.30
band broadening
overall dispersion or widening of an analyte band as the analyte passes through a separation system
3.31
zone broadening
broadening of the width of the sample zone during separation in the channel (3.7)
3.32
normal mode
Brownian mode
mode of elution (3.14) in which diffusion is the dominant opposing force to the
applied orthogonal force (e.g., cross flow (3.19) or centrifugal), resulting in an elution (3.14) sequence where
smaller particles elute before larger particles
Note 1 to entry: All
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