ISO 5667-27:2025

Water quality - Sampling - Part 27: Guidance on sampling for microplastics in water

This document specifies the basic methods for sampling suspended microplastics in water (domestic water, freshwater, seawater, treated wastewater and untreated wastewater), for their subsequent characterization. Suspended particles can also include synthetic or semi-synthetic polymeric materials (such as rubber). This document does not cover chemical analysis, biological (ecotoxicological) methods or physical methods, nor the pre-treatment or digestion methods intrinsic to such analyses. This document covers general methodologies: - for grab sampling, sampling using a set of successive filters of different pore sizes (cascade filtration), for water samples with low, medium and high content of suspended solids, and - for net sampling using, for example, manta, plankton or neuston nets.

Qualité de l'eau — Échantillonnage — Partie 27: Recommandations pour l'échantillonnage des microplastiques dans l'eau

General Information

• Status: Published
• Publication Date: 02-Mar-2025
• ICS: 13.060.45 - Examination of water in general
• Technical Committee: ISO/TC 147/SC 6 - Sampling (general methods)
• Drafting Committee: ISO/TC 147/SC 6 - Sampling (general methods)
• Current Stage: 6060 - International Standard published
• Start Date: 03-Mar-2025
• Due Date: 25-Aug-2024
• Completion Date: 03-Mar-2025

Overview

ISO 5667-27:2025 - "Water quality - Sampling - Part 27: Guidance on sampling for microplastics in water" provides standardized guidance for collecting suspended microplastics from a range of water matrices (domestic water, freshwater, seawater, treated and untreated wastewater). It defines microplastics (particles 1 µm to 5 mm) and gives practical procedures for grab sampling, volume‑reduced sampling (cascade filtration) and net sampling (for example, manta, plankton or neuston nets). The document focuses on sampling methodology, sample handling and quality control; it does not cover chemical, biological/ecotoxicological or physical analysis methods nor pre‑treatment/digestion protocols.

Key Topics and Technical Requirements

• Sampling methods
  • Grab sampling (spot samples) - collect discrete volumes for laboratory separation.
  • Cascade filtration (volume‑reduced sampling) - successive filters of different pore sizes to capture particle size fractions; methods adapted for low, medium and high suspended solid content.
  • Net sampling - surface or near‑surface collection using nets such as manta, plankton or neuston nets.
• Suspended solids categories (for method selection)
  • Low: 1–100 mg/L
  • Medium: 100–500 mg/L
  • High: >500 mg/L
• Quality control
  • Use of field blanks and other blanks to detect contamination during sampling.
  • Recommendations on materials and handling to minimize plastic contamination (e.g., inert filters for blanks).
  • Guidance on sampling volume, frequency, timing and sampling points to reduce variability.
• Apparatus and sample handling
  • Lists of suitable equipment for grab sampling, cascade filtration and net sampling, and instructions for transport and storage to preserve sample integrity.
• Reporting
  • Required sampling report elements to ensure data traceability and comparability.

Practical Applications and Users

ISO 5667-27 is intended for:

• Environmental laboratories and analytical teams setting up microplastics sampling programs
• Water utilities and wastewater operators monitoring plastic contamination
• Governmental and regulatory bodies establishing surveillance or compliance sampling
• Researchers conducting field studies on microplastics distribution in rivers, coastal waters and drinking water supplies
• NGOs and consultants performing baseline surveys and impact assessments

Benefits include improved comparability of datasets, reduced sampling bias, and clearer documentation for monitoring programs.

Related Standards (if applicable)

• ISO 5667 series (general water sampling standards)
• ISO 5667-3 (Preservation and handling of water samples)
• ISO 5667-17 (related sampling guidance)
• ISO/TR 21960 and ISO/TR 22930-1 (terminology and detection/limit definitions referenced)

Keywords: ISO 5667-27, microplastics sampling, water quality, cascade filtration, grab sampling, manta net, field blanks, suspended microplastics, sampling protocol.

Standards Content (Sample)

International
Standard
ISO 5667-27
First edition
Water quality — Sampling —
2025-03
Part 27:
Guidance on sampling for
microplastics in water
Qualité de l'eau — Échantillonnage —
Partie 27: Recommandations pour l'échantillonnage des
microplastiques dans l'eau
Reference number
© ISO 2025
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
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles and general considerations . . 3
4.1 Methodologies .3
4.1.1 General .3
4.1.2 Grab sampling .3
4.1.3 Volume-reduced sampling .3
4.2 Selecting the most appropriate sampling method .4
4.3 Sampling volume .6
4.4 Quality control .6
4.4.1 General .6
4.4.2 Sources of sampling errors .7
4.4.3 Blanks .7
4.5 Sampling plan .9
4.5.1 General .9
4.5.2 Sampling points .9
4.5.3 Sampling frequency, duration and timing .9
5 Reagents . 9
6 Apparatus . 10
6.1 Grab sampling method .10
6.2 Cascade filtration methods .11
6.3 Net sampling . 12
7 Sample handling .16
7.1 General .16
7.2 Grab sampling method .16
7.3 Cascade filtration methods .16
7.4 Net sampling .17
8 Procedures . 17
8.1 Grab sampling method .17
8.2 Cascade filtration method 1: Sampling of domestic water, treated wastewater and
water samples with low suspended solid content .18
8.3 Cascade filtration method 2: Sampling of untreated wastewater or water samples with
medium or high suspended solid content .19
8.4 Net sampling . 20
9 Sampling report .21
Bibliography .26

iii
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 document 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 by Technical Committee ISO/TC 147 Water quality, Subcommittee SC 6,
Sampling (general methods).
A list of all parts in the ISO 5667 series can be found on the ISO website.
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
Introduction
Microplastic occurrence in the environment is a prominent concern both to the public and to the scientific
community. Determining the amount and distribution of microplastics in water bodies and domestic water
[1]-[6]
is therefore a critical task. However, the methodology for sampling microplastics in water samples is
still lacking in precision. Consistent methodology is only starting to emerge, but still no universal protocol
exists for the sampling of these contaminants in water.
[7]
The presence of small plastic fragments in the ocean was first reported in 1972, but it was in 2004 that
the term “microplastics” was proposed for the first time to describe plastic particles of a few micrometres in
[8]
diameter. Since then, a wealth of information became available on the abundance and type of microplastics
in the marine environment, freshwater and estuarine systems. However, the different studies have used
diverse techniques to sample, extract, treat and detect microplastic present in water.
There are many reasons why different studies investigating microplastic occurrence in water and
wastewater show different results. The disparity between some of the findings (for microplastic type
and abundance) can be partially explained by the fact that differing sampling techniques have been used.
Variables pertaining to both time of year and time of day, flow rate and volume of water sampled, grab
sampling or sieving the water over an extended period, the use of plastic containers or tubing, selection of a
few parts of the sample for analysis, or dissimilar devices to capture the microplastic fragments, can be the
causes of variation in study results.
While several standards for water sampling and water quality already exist (e.g. ISO 5667 series and, in
particular, ISO 5667-17), microplastics as particular determinands pose a specific challenge which requires
a more specific approach. For example, microplastics sampling requires the use of very specific materials
for collecting, handling and storing to avoid cross-contamination. Also, microplastic buoyancy can vary
depending on their composition, size, shape or colonization by microorganisms, and microplastics are not
homogeneously distributed in the water column. Therefore, a more targeted and detailed set of sampling
protocols is required to account for these differences. To better understand the fate and impact of
microplastics in the environment, a more specific standardized sampling approach should be adopted and
applied.
v
International Standard ISO 5667-27:2025(en)
Water quality — Sampling —
Part 27:
Guidance on sampling for microplastics in water
WARNING — Persons using this document should be familiar with normal laboratory practice. This
document does not purport to address all of the safety problems, if any, associated with its use. It
is the responsibility of the user to establish appropriate safety and health practices and to ensure
compliance with any national regulatory conditions.
IMPORTANT — It is essential that tests conducted according to this document be carried out by
suitably trained staff.
1 Scope
This document specifies the basic methods for sampling suspended microplastics in water (domestic water,
freshwater, seawater, treated wastewater and untreated wastewater), for their subsequent characterization.
Suspended particles can also include synthetic or semi-synthetic polymeric materials (such as rubber). This
document does not cover chemical analysis, biological (ecotoxicological) methods or physical methods, nor
the pre-treatment or digestion methods intrinsic to such analyses.
This document covers general methodologies:
— for grab sampling, sampling using a set of successive filters of different pore sizes (cascade filtration), for
water samples with low, medium and high content of suspended solids, and
— for net sampling using, for example, manta, plankton or neuston nets.
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 5667-3, Water quality — Sampling — Part 3: Preservation and handling of water samples
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
microplastic
solid plastic or synthetic polymer particle insoluble in water with the largest dimension between 1 μm and 5 mm
Note 1 to entry: Microplastics can show various shapes.
Note 2 to entry: This definition encompasses the ISO/TR 21960 definitions of large microplastics and microplastics.

3.2
plastic
material which contains as an essential ingredient a high polymer and which, at some stage in its processing
into finished products, can be shaped by flow
[SOURCE: ISO 472:2013, 2.702, modified — Notes 1 and 2 to entry have been deleted.]
3.3
rubber
family of polymeric materials which are flexible and elastic
[SOURCE: ISO 1382:2020, 3.420, modified — the domain and Notes 1 and 2 to entry have been deleted.]
3.4
suspended solid
solid remaining in suspension in water, which can be removed by sedimentation, filtration or centrifugation
[SOURCE: ISO 6107:2021, 3.554]
3.4.1
high suspended solid content
concentration of solids suspended in water above 500 mg/l
3.4.2
medium suspended solid content
concentration of solids suspended in water ranges between 100 mg/l to 500 mg/l
3.4.3
low suspended solid content
concentration of solids suspended in water ranges between 1 mg/l to 100 mg/l
3.5
grab sample
single discrete sample directly collected from a water body at a specific time and location (and depth when
relevant)
3.6
blank
sample identical to the sample of interest, but in the absence of the determinand
3.7
field blank
blank (3.6) to verify possible contamination during sampling
Note 1 to entry: A field blank is prepared in the laboratory using water (filtered before use through an inert filter with
a pore size smaller than 1 μm) and sent with the sampling personnel for exposure to the sampling environment.
3.8
domestic water
water either in its original state or after treatment
Note 1 to entry: Domestic water is intended for human use, such as cooking, food preparation, washing, drinking or
other domestic purposes.
3.9
limit of detection
smallest true value of the measurand which ensures a specified probability of being detectable by the
measurement procedure
Note 1 to entry: For microplastics (3.1), the limit of detection is defined as the methods’ capability of reliably detecting
at least one particle or a defined mass.

[SOURCE: ISO/TR 22930-1:2020, 3.6, modified — the term has been changed from "detection limit" to "limit
of detection", Note 1 to entry has been replaced and Note 2 to entry has been deleted.]
4 Principles and general considerations
4.1 Methodologies
4.1.1 General
The microplastic sampling approaches described in this document can be grouped mainly as grab sampling
and volume-reduced sampling.
Grab sampling (also known as spot sampling) consists of collecting a given volume of water (without
reducing it during this procedure). Subsequent separation, pre-treatment (when required) and analysis
of the microplastics are done in a laboratory. In volume-reduced sampling, the microplastics are collected
while reducing the volume of water during the sampling process, by passing the water through nets or
sieves. The collected microplastics are preserved for further processing in a laboratory.
4.1.2 Grab sampling
The methodology for the sampling of microplastics via grab sampling entails filling non-plastic containers
(or made of materials which are less present in the environment or not part of the measurement concept)
with domestic water, freshwater, treated or untreated wastewater, or surface, near surface or deep-water
samples, for the subsequent pre-treatment and analysis in a laboratory. The container material must be
suitable for the sample. The sampling date, time, location, depth (when relevant) and collected volume are
recorded.
The main advantage of grab sampling is that all the microplastics present in the sampled water are collected
without size limitation, in contrast to volume-reduced sampling, where the selected mesh size determines
the smallest size of sampled particles. In grab sampling, risks of contamination are reduced when compared
to volume-reducing sampling in an open system (e.g. using manta trawls, plankton or neuston nets), because
handling of the sample and time of exposure to the surrounding environment are shorter. However, the main
[9]
disadvantage of grab sampling is the limited volume of sample that can be collected, stored and processed.
4.1.3 Volume-reduced sampling
In volume-reduced sampling, the microplastics are extracted and aggregated from the medium before
analysis. This can be done via cascade filtration, where a volume of water passes through a series of filters
with decreasing mesh sizes; or by using manta trawls, plankton or neuston nets. While volume-reducing
methods allow for a greater area and/or volume of water to be sampled, their main disadvantages are
the limits on the minimum microplastic size that can be collected. This is determined by the sizes of the
[10][11]
filters, sieves used and mesh selectivity, alongside a higher risk of contamination because the time of
exposure to the surrounding environment is long.
The methodology of cascade filtration allows for the sampling of domestic, treated and untreated
wastewater, surface and subsurface waters with low, medium or high suspended solid content, and consists
of passing the water to be sampled through a series of filters of different pore sizes, for the retention of the
microplastics in the different filters. The sampling date, time, location, mesh pore size and the flow rate of
the water passing through the filters over the sampling time interval are recorded, in order to express the
1)
results in terms of collected particles per unit volume, or mass per unit volume.
1) Cascade filtration systems can be limited by the minimum pore size of the sieves used or volume of water sampled
(see Table 1). The use of sedimentation boxes or continuous flow centrifuges have recently been proposed as potential
alternatives of volume-reduced sampling of microplastics from large volumes of water, with initial studies showing the
capability to capture the smallest microplastics (<30 µm) from river waters. While these techniques are currently outside
the scope of this document, References [12] to [16] can be consulted for further information of these recent proposals.
General information on sedimentation boxes and continuous flow centrifuges can be found in ISO 5667-17 (not related to
microplastic sampling).
For sampling surface waters (up to about 100 cm below surface), the use of manta trawls, plankton and neuston
nets in dynamic and stationary methods are described. Other types of nets are also used for subsurface water
sampling. The sampling date, time, location, mesh pore size and water passing through the nets over the
sampling time should be recorded. Additionally, for dynamic sampling, recording of log speeds of boats or
[6]
vessels, tow duration, area covered, wind velocities and significant wave heights is recommended.
4.2 Selecting the most appropriate sampling method
Sampling strategies can differ depending on the targeted environmental compartment that needs to be
monitored and depending on the target question (see Table 1).
Grab sampling can be used not only to collect microplastics from the water surface but also from the water
column, by using a container or a submersible water pump. Grab sampling is applicable to all categories of
water samples (domestic water, freshwater, seawater, treated wastewater and untreated wastewater) and
is preferred over net tows for smaller microplastics, which typically cannot be collected by the larger mesh
sizes. However, given the more limited volume that can practically be collected and stored, the detection
limit for grab samples can be higher, particularly when sampling water bodies such as rivers, lakes or sea
water. Replicates or combined spot samples can compensate for this deficiency.
Column waters can also be sampled by pumping, followed by a set of sieves to isolate microplastics of
different size ranges (cascade filtration). This method is applicable to domestic water, freshwater, treated
wastewater and untreated wastewater. Cascade filtration systems are also limited by the minimum pore size
of the filters or sieves used. While mesh sizes can be down to 1 μm, samples such as untreated wastewater
require larger pore sizes to avoid clogging. Cascade filtration systems allow more significant capture of
smaller microplastics when compared to nets.
For sampling the water surface and water column in rivers, lakes and sea, the most common method is the net
[2][6][17]-[19]
tow, using neuston or plankton nets or manta trawls. Manta trawls and neuston nets are mainly
used for microplastics sampling in the ocean, while plankton nets are used for sampling in rivers. Net mesh
[2][6][18][20]
sizes vary between 50 μm and 500 μm, with 330 μm the most commonly used. Plankton nets have
the smallest mesh pore sizes (between 50 μm to 100 μm) and they usually need to be towed at lower speeds
[9]
in order to reduce clogging. Microplastics smaller than the minimum mesh size are not collected, resulting
in a potential underestimation of their total abundance in the sampled water. It is important to note that
the frequency of occurrence of dispersed microplastics is likely to vary inversely with size (i.e. there will
[2][21]
be fewer larger items) and needs to be taken into account when planning the sampling method and
sampling strategy.
In net tow, the collection of microplastics can be performed by a dynamic or stationary sampling method.
In dynamic sampling, trawls are towed by a boat with a rope, keeping the nets outside the waves from the
vessel to prevent disturbance or dispersion of the particles to be collected. It is recommended to position the
[6]-[9]
net on the side of the boat, by a suitable pole installed.
For streams, creeks and smaller rivers with variable water regime not entirely navigable, stationary
sampling is recommended. For this, the floating nets need to be fixed on the banks. The nets filter water
using the stream current with their mouths skimming the surface, with a weight used to maintain a
continuous and consistent submersion depth. The nets are collocated in the opposite direction of the water
[9]
flow. Plankton nets are more commonly used for stationary sampling in rivers.

Table 1 — Comparison of different methodologies for sampling microplastics in water
Method Overview and application Advantages Limitations
Discrete sample collection with non-plastic containers (or made of Relatively quick and straightforward method. Sampling smaller volumes of water.
materials which are less present in the environment or not part of
Reduced risk of contamination. It requires transportation of large or multiple containers to the lab.
the measurement concept).
No limitation of microplastic size. Less representative.
Grab sampling
Applicable to domestic water, freshwater, treated wastewater and
It can sample surface and water column. If sampling in deeper waters such as sea, middle of a lake or river,
untreated wastewater.
a boat or ship can be needed.
Applicable for surface waters and water column.
Volume-reduced sampling. Applicable to domestic water, freshwater, Mesh sizes can vary between 1 µm to 5 mm, allowing The minimum mesh pore size and mesh selectivity determine
seawater, treated wastewater and untreated wastewater. for a more efficient and wider range of particle sizes the minimum particle size to be captured.
when compared to manta or neuston nets.
Applicable for surface waters and water column. It requires electric energy (unless sampling from a pressurized
Cascade filtering using finer filters can collect system or if a manual or hand-operated pump is used).
Cascade filters using non-plastic sieves (or made of materials which
smaller-sized particles more efficiently than the
are less present in the environment or not part of the measurement It involves more equipment than grab sampling.
use of manta, plankton or neuston nets.
concept).
Cascade filtration
Filters can be clogged, particularly when using smaller mesh sizes
It allows sampling large volumes of water.
or when sampling water with high-suspended solid contents.
It permits size fractionation directly in the field.
Higher risk of sample contamination from apparatus and due
It can sample surface and water column. to manipulation.
If sampling in deeper waters such as sea, middle of a lake or river,
a boat or ship can be needed.
Volume-reduced sampling. Widely used for sampling water in nav- It allows sampling large volumes of water and Most available nets have mesh sizes between 500 and 50 µm, with
igable rivers, lakes and sea. covers large areas. 330 µm being the most common one. This limits the minimum
microplastic size that can be captured.
Applicable for surface waters and water column.
Clogging problems.
Meshes are towed by a boat with a rope. The position of the net
should be away from the wake from the vessel. It requires a boat or ship.
Ideally, mesh materials should be made of compounds which are less Risk of sample loss or contamination due to manipulation and
Net sampling:
present in environment or not part of the measurement concept. transfer from the nets.
Dynamic sampling
The mesh pore size determines the minimum particle size to be
captured. The mesh opening must be reported.
For multiple depth collection at the water subsurface, nets can be
tethered at predetermined increments. Multiple opening or closing
nets can also be used for water column sampling (e.g. vertical multi-
ple-opening plankton sampler, VMPS or multiple opening and closing
net with environmental sensing system, MOCNESS).
Volume-reduced sampling. Widely used for sampling surface waters. It allows sampling large volumes of water and Most available nets have mesh sizes between 500 and 50 µm, with
sampling over longer periods of time. 330 µm being the most common one. This can limit the minimum
Recommended for streams, creeks and smaller rivers with variable
microplastic size that can be captured, with potential underesti-
water regime not entirely navigable.
mation of the real quantity of microplastics in the sampled water.
The nets are collocated in the opposite direction of the water flow,
Net sampling:
Clogging problems.
with a weight used to maintain a continuous and consistent sub-
Stationary
mersion depth, and fixed on the banks. Anchoring the nets to the riverbed can be difficult.
sampling
Ideally, mesh materials should be made of compounds which are less Mostly for surface sampling (up to about 100 cm below surface).
present in environment or not part of the measurement concept, Not usually applicable for deep water column.
The mesh pore size determines the minimum particle size to be
Risk of sample loss or contamination due to manipulation and
captured. The mesh opening must be reported.
transfer from the nets.
4.3 Sampling volume
Sampling volume must be adjusted according to the analysis purpose, to the microplastics content and sizes
[22][ 23]
expected, and to the water quality (e.g. a high quantity of suspended solids can cause filter clogging).
Different factors should be considered when determining the optimum water volume to be sampled. For
microplastics, the limit of detection can be defined as the methods’ capability of reliably detecting at least
[4][21][23]-[26]
one particle or a defined mass with statistical rigour.
Limits of detection also depend on the microplastic size range or concentrations to be studied. Sampling
small volumes reduces the chance of detecting microplastics and increases the margin of error. Therefore,
[26]
the detection limit benefits from large sample volumes. In addition, larger sampling volumes are
more representative, as microplastics are not distributed homogeneously in the sampled matrix. Smaller
microplastics are more abundant, therefore smaller volumes can be required when targeting exclusively
small microplastics (e.g. <50 µm). However, if the objective of the study includes the detection of larger
[4][21][24][26]
microplastics, larger volumes are needed. This is particularly important if the subsequent
analysis will report total mass of microplastics per volume, or per sample. Larger particles will dominate the
total mass, with just one or two particles containing more mass than several smaller particles. Therefore,
the results of mass per volume or sample can become biased if an insufficient volume is collected.
For domestic waters (e.g. drinking water or water from a household tap), the minimum volume for grab
[22][26]
sampling is one litre, as reported in several scientific publications. This volume is representative for
daily human consumption (WHO guidelines) and corresponds to the recommendations of ISO standards
for water contaminants analysis. For sampling domestic waters in cascade filtration, the volume can be
[27][28]
extended. Previous studies have indicated that the presence of large microplastic particles is very low
(usually one particle per metre cube). Therefore, sample volumes of 1 000 litres or more are recommended,
[26][ 29]
whenever possible, for a sufficiently representative sample which includes large microplastics.
For treated wastewater, previous studies have reported a wide range of microplastic particles (values

varying between 10 particles per metre cube to 30 000 particles per metre cube). For untreated wastewater
influents, the number of particles are expected to be even higher (but the number of large microplastics is
[29]-[34]
expected to be lower). The mass loads for wastewater treatment plant effluents have been reported
−3 −3 [35]
between 200 µg m and 3 800 µg m . Therefore, volumes of 10 l to 100 l of untreated wastewater
are recommended for a sufficiently representative sample. Larger volumes are recommended for treated
wastewater or when sizes up to 5 000 μm need to be quantified. Lesser volumes are used when targeting
exclusively smaller microplastics or when sampling untreated wastewater influent with cascade filtration
(because of high amounts of suspended solids which can clog the filters).
For collecting microplastics in rivers, lakes and sea, the most common method is by using nets. Manta
trawls and neuston nets are mainly used for microplastics sampling in the ocean, while plankton nets are
mainly used for sampling in rivers. Due to the low concentrations of large microplastics usually reported in
[21][26] 3 3
previous studies, sample volumes of at least 200 m to 500 m have been suggested in the literature,
2 [6][9][26]
which is equivalent of surveying an area of approximately 1 000 m . However, for remote locations
with expected very low particle numbers, greater volumes can be needed.
4.4 Quality control
4.4.1 General
Particular importance should be given to careful sampling performed on-site and to correct recording of
the water sampling and handling. Reference should be made to ISO 5667-14 regarding quality assurance
of water sampling and handling. This guidance is independent of the quality assurance required for the
laboratory analysis of samples.
Sampling quality programmes include documented evidence that:
— the individuals who collect samples are competent and well trained;
— appropriate sample collection and sample handling methods are employed;

— equipment is maintained and calibrated;
— correct practices are followed;
— records are both complete and secure.
Sampling programmes should include field blank samples handled as if genuine samples, to assess
contamination of samples and use of appropriate replicate samples to assess precision and repeatability.
Addition-recovery tests with standard plastic particles can also be employed to check how accurately the
standard plastic particles are sorted and collected in the process of sampling.
Weather, tidal currents and river inflows can also affect the content and composition of microplastics in the
water. In addition, the content and composition of microplastics can be affected in areas near land following
[6]
rainfall. Therefore, in order to ensure that the samples are as representative as possible and to account for
seasonal variations or peak releases, it is recommended to repeat the sample collection at different times of
either the day or the year, or both. Time-proportional composite sampling is also recommended, particularly
when sampling untreated wastewater (i.e. combining discrete volume aliquots collected at different time
intervals).
4.4.2 Sources of sampling errors
Sources of sampling errors include the following:
a) contamination by cross-contamination between samples;
b) incorrect storage; the choice of sampling vessels, containers and the options for preservation can affect
the integrity of the microplastics, particularly with samples such as untreated wastewater. Sampling
vessels and containers can be a source of contamination. The choice of preservation agents largely
depends on the research question being considered; care should be taken when selecting the preserving
agent (and the concentration used) as they can damage some polymers;
c) incorrect sampling; deviation from the sampling procedure can be a source of error;
d) losses of materials due to incorrectly applied sampling or filling technology, (e.g. incorrect use of suction
pumps, incorrect transfer method, multiple transfer or turbulent filling of sample).
e) use of unsuitable or insufficiently purified facilities (e.g. flasks, filtration equipment, tubing, nets); this
can cause sampling errors and contamination;
f) contamination of samples from air or vessel owing to not covering equipment and samples during
sampling, processing and storage;
g) unsuitable clothing (i.e. synthetic fibres) during sampling;
h) confusion of sampling collection location by, for example, inadequate documentation. Correct
documentation should contain the position coordinates and photos (taken in different seasons if
necessary). GPS navigation handsets have proved useful;
i) confusion of samples due to inadequate labelling or incomplete or incorrectly completed sampling
reports;
j) sampling in non-representative, non-homogeneous or other inappropriate sampling points.
4.4.3 Blanks
This technique can be used to identify any errors relating to contamination of sampling containers and the
sampling process.
For grab sampling, field blank samples are laboratory blank samples which are taken into the field, treated
as samples and analysed as a check on sampling procedures (see Figure 1).

At the laboratory, divide a blank sample of pure water (filtered through inert filter with a pore size smaller
than 1 μm) into two parts: part A and part B. Part A is retained in the laboratory. Part B is the “field blank”
transported into the field.
Divide part B into two portions b1 and b2; portion b1 should be processed, as far as is practical, with the
same technique as real samples, using the same sampling equipment and containers as the real samples.
For grab sampling, portion b1 should be transferred with the same technique as real samples, as far as is
practical, to the same type of sampling containers as real samples.
For sampling using filtration equipment, portion b1 should be processed using equivalent sampling
containers and filtration equipment, as far as is practical, with the same technique as the real samples.
Portion b2 should be retained and returned to the laboratory without any further processing in the field.
The comparison of results of part A and portion b1 identifies errors due to handling, processing and
transportation.
The comparison of results of part A and portion b2 identifies errors due to container and sample
transportation only.
The comparison of results of portion b1 and portion b2 identifies errors due to contamination of sampling
containers or sampling processes.
Refer to ISO 5667-14:2014, 11.3 for additional guidance.
Figure 1 — Flow chart of the the field blank sampling technique for grab sampling
For the cascade filtration, initial validation of the system should be performed. For this, the entire sampling
system shall be characterized prior to the sampling phase, in order to determine systematic microplastics
contamination which can be coming from this system. This can be done by repeating the sampling protocol
at least three times (using different volumes of pre-filtered ultrapure water) and analysing the resulting
samples. The laboratory analyses should demonstrate the effectiveness of the device cleaning protocol in
preventing cross contamination of the samples.
When using net sampling (such as manta, plankton or neuston nets), it is impractical to bring additional
laboratory water or plastic-free seawater to pass through the nets in the field. When using these methods, a
“blank” sampling can be conducted by washing unused nets (prepared, stored and transported in the same
way as nets used for sampling) in the same manner as the nets used for sampling. The sample and the blank

sample shall be stored and transported back to the laboratory in identical containers and shall be processed
in the same way.
4.5 Sampling plan
4.5.1 General
Sampling strategy includes identification of the area under investigation, choice of sampling procedure,
choice of location and number of sampling sites, seasonality and input patterns.
Aspects such as the sample identification method, the number of sampling points, locations and the number
of samples to be taken at each site shall be defined in the sampling plan. Necessary adjustments can then
be performed in the field, in which case the sampling record should be updated to include a reasonable
justification for the changes.
The types of local substrates, hydrographic features in the area to study, information on nearby sources of
discharge and (where available) knowledge from past measurements should be considered during sampling,
in order to ensure that results are as accurate as possible.
For further guidance on how to devise sampling programmes, see ISO 5667-1, ISO 5667-4, ISO 5667-6 and
ISO 5667-9.
4.5.2 Sampling points
Sampling points should be chosen so that the samples are representative of the wider environment or
area under consideration. When identifying sources of microplastic discharge, the sampling points should
be appropriately placed in relation to the source of emissions. There are often practical considerations
to account for, such as access to the sampling points, suitable locations for the sampling equipment or
protecting the sampling equipment from vandalism or theft.
The sampling locations should be documented using its latitude and longitude coordinates as well as the
precise location of each measuring point. The sampling points can also be recorded with 1:5 000 and
1:25 000 scale maps and images, as well as a description of the access route. Please refer to ISO 5667-17:2008,
4.4 for additional guidance.
4.5.3 Sampling frequency, duration and timing
The sampling frequency, duration and timing depend on the main purpose of the investigation. A single
sampling exercise can be sufficient depending on the problem being investigated. However monthly or
weekly
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