ISO 15767:2003
(Main)Workplace atmospheres — Controlling and characterizing errors in weighing collected aerosols
Workplace atmospheres — Controlling and characterizing errors in weighing collected aerosols
ISO 15767:2003 provides recommendations for controlling the analytical uncertainty associated with aerosol collection medium instability, where collection medium or substrate includes any article used to collect particles (e.g. filter or foam material) as well as those supporting elements which must be analysed by weighing. ISO 15767:2003 is applicable to results compiled both from the literature and, if necessary and feasible, through laboratory experiment. Expected errors associated with given aerosol capture methods are quantified where possible. Recommendations as to materials to be used are given. Means of controlling or correcting errors arising from instability are provided. Recommendations for the weighing procedure are given. A procedure for estimating weighing errors is described. Finally, recommendations are given for the reporting of measured masses.
Atmosphères des lieux de travail — Contrôle et caractérisation des erreurs de pesée des aérosols collectés
L'ISO 15767:2002 fournit des recommandations pour le contrôle des incertitudes analytiques liées à l'instabilité des supports de collecte d'aérosols, où le support ou substrat de collecte inclut tout objet utilisé pour recueillir des particules (par exemple un filtre ou un matériau en mousse), ainsi que les éléments connexes devant être soumis à l'analyse par pesée. L'ISO 15767:2002 est applicable aux résultats compilés à la fois à partir de sources bibliographiques et, lorsque cela est nécessaire et faisable, à partir d'expériences en laboratoire. Les erreurs prévisibles liées aux méthodes de prélèvement d'aérosols sont quantifiées chaque fois que possible. Des recommandations sur les matériaux à utiliser sont indiquées. Des moyens de contrôle ou de correction des erreurs dues à l'instabilité sont fournis. Des recommandations sur le mode opératoire de pesée sont données. Une procédure d'estimation des erreurs de pesée est décrite. Enfin, des recommandations sont données pour établir des rapports des masses mesurées.
Zrak na delovnem mestu – Pregled in opis napak pri tehtanju zbranih aerosolov
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INTERNATIONAL ISO
STANDARD 15767
First edition
2003-02-15
Workplace atmospheres — Controlling
and characterizing errors in weighing
collected aerosols
Atmosphères des lieux de travail — Contrôle et caractérisation des
erreurs de pesée des aérosols collectés
Reference number
ISO 15767:2003(E)
©
ISO 2003
---------------------- Page: 1 ----------------------
ISO 15767:2003(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2003
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2003 — All rights reserved
---------------------- Page: 2 ----------------------
ISO 15767:2003(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 Weight instability — Causes and minimization . 2
4.1 General. 2
4.2 Moisture sorption. 2
4.3 Electrostatic effects. 3
4.4 Effects of volatile compounds (other than water) . 3
4.5 Handling damage . 4
4.6 Buoyancy changes . 4
5 Correcting for weight instability by use of blanks . 4
5.1 General. 4
5.2 Minimum number of blanks . 4
5.3 Weighing times and sequence . 4
5.4 Conditioning times. 5
5.5 Storage stability . 5
6 Transport of samples to laboratory . 5
6.1 General. 5
6.2 Recommended packaging. 5
7 Weighing equipment and procedure. 6
7.1 The balance . 6
7.2 Recommended environmental controls . 6
7.3 Other equipment requirements. 6
7.4 Procedure. 7
8 Recommendations for the reporting of measured mass relative to LOD and LOQ. 7
Annex A (normative) Estimation of measurement errors . 8
Annex B (informative) Interpretation of LOD and LOQ. 12
Annex C (informative) Method evaluation example . 14
Annex D (normative) Test of transportation integrity. 15
Bibliography . 17
© ISO 2003 — All rights reserved iii
---------------------- Page: 3 ----------------------
ISO 15767:2003(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 15767 was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 2, Workplace
atmospheres.
iv © ISO 2003 — All rights reserved
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ISO 15767:2003(E)
Introduction
Assessment of airborne aerosol hazards in the occupational setting entails sampling onto a collection medium,
followed by analysis of the collected material. The result is generally an estimated concentration of a
hazardous material in the air. The accuracy of such estimates depends on several factors, one of which
relates to the specific type of analysis employed.
This International Standard deals with a specific type of analysis which finds the most general application in
the sampling of aerosols, namely the weighing of sampled material. Gravimetric analysis, though apparently
simple, is subject to errors from instability in the mass of the sampling medium and other elements which must
be weighed. An example is provided by aerosol samplers designed to collect particles so as to agree with the
inhalable aerosol sampling convention. For some sampler types, filter and cassette are weighed together to
make estimates. Therefore, if the cassette, for example, absorbs or loses water between the weighings
required for a concentration estimation, then errors may arise. This International Standard describes such
potential errors and provides solutions for their minimization.
© ISO 2003 — All rights reserved v
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INTERNATIONAL STANDARD ISO 15767:2003(E)
Workplace atmospheres — Controlling and characterizing
errors in weighing collected aerosols
1 Scope
This International Standard provides recommendations for controlling the analytical uncertainty associated
with aerosol collection medium instability, where collection medium or substrate includes any article used to
collect particles (e.g. filter or foam material) as well as those supporting elements which must be analysed by
weighing.
This International Standard is applicable to results compiled both from the literature and, if necessary and
feasible, through laboratory experiment. Expected errors associated with given aerosol capture methods are
quantified where possible. Recommendations as to materials to be used are given. Means of controlling or
correcting errors arising from instability are provided. Recommendations for the weighing procedure are given.
A procedure for estimating weighing errors is described. Finally, recommendations are given for the reporting
of measured masses.
2 Normative references
The following referenced documents are indispensable for the application 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 7708, Air quality — Particle size fraction definitions for health-related sampling
EN 482, Workplace atmospheres — General requirements for performance of procedures for the
measurement of chemical agents
EN 13205:2001, Workplace atmospheres — Assessment of performance of instruments for measurement of
airborne particle concentrations
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
substrate
aerosol sampling filter, foam, etc., together with whatever mounting is weighed as a single item
NOTE As an example of the converse, the 25-mm or 37-mm plastic filter cassette often used for “total dust” sampling
in either its closed-face or open-face version is not part of the substrate in the definition above, since it is not weighed.
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ISO 15767:2003(E)
3.2
equilibration time
time constant characterizing an approximately exponentially damped approach of the mass of an aerosol
collection medium to a constant value
NOTE 1 The constant can be defined as the mean difference of the mass from equilibrium per mean mass loss or gain
rate as measured over a finite time interval.
NOTE 2 There may be important instances in which several time constants are required to describe the approach to
equilibrium.
NOTE 3 Equilibration time is expressed in seconds.
3.3
field blank
blank substrate that undergoes the same handling as the sample substrate, generally including conditioning
and, often, loading into the samplers or transport containers, as well as transportation between lab and
sampling site, but without being exposed to sampling
3.4
lab blank
blank substrate that undergoes the same handling as the sample substrate in the laboratory, including
conditioning and loading into the samplers or transport containers if this is done in the laboratory
3.5
blank substrate
collection medium or substrate taken from the same batch as the sampling medium, but unexposed to sampling
3.6
limit of detection
LOD
three times the estimated standard deviation of the mass of the sample, accounting for the double weighing
(exposed vs. unexposed) and for the uncertainty associated with any correction blanks used
NOTE The value LOD can be used as a threshhold value to assert the presence of a substance with confidence in
the method. Annex B describes how to estimate, on the basis of the method evaluation, the false positive rate in such
assertions.
3.7
limit of quantitation
LOQ
ten times the estimated standard deviation of the mass of the sample
NOTE The value LOQ can be used as a threshhold value to assure measurement of a substance accurately. For
details, see Annex B.
4 Weight instability — Causes and minimization
4.1 General
[1] to [11]
Weight instability of sampling substrates can be attributed to several causes . The following subclauses
address the more important of these.
4.2 Moisture sorption
4.2.1 Moisture sorption is the most common cause of weight instability. Water can be directly collected by
the filter or foam or other substrate material that is weighed. Water sorption by any part of the sampling
system which is weighed must be suspected as well. For example, the sampling cassette itself, if weighed,
[1]
can be the cause of significant error .
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ISO 15767:2003(E)
4.2.2 The effects of water sorption can be reduced by using non-sorptive materials. However, there may
exist specific sampling needs for which a hydrophobic material is not feasible. Table 1 presents a list of
common aerosol sampling substrates with different water adsorption features.
Table 1 — Water sorption characteristics of some aerosol sampling media
Water sorption
Substrate or cassette type
Very low Low High Very high
Cellulose fibre filter *
Glass fibre filter *
Quartz fibre filter *
Cellulose ester membrane filter *
Polytetrafluoroethylene filter *
PVC membrane filter * *
Polycarbonate filter *
Silver membrane filter *
Polyurethane foam *
Greased Mylar impaction substrate * *
Greased aluminium foil impaction substrate *
Carbon-filled resin *
Aluminium cassette * *
Stainless steel cassette *
NOTE 1 References [2] to [4] provide further details. Also, reference [5] reports that filters of evidently the same
material, but originating from different manufacturers, can have widely differing variabilities.
NOTE 2 There is generally a trade-off between hydrophobicity and conductivity in many materials [9]. Therefore, one
must be aware of the possibility of creating sampling problems when reducing hygroscopicity.
NOTE 3 Pre-treatments of substrates, such as greasing, can also affect water sorption.
4.3 Electrostatic effects
Electrostatic effects are a common source of weighing problems. These effects can usually be minimized by
discharging the substrate through the use of a plasma ion source or a radioactive source. Using conductive
materials may reduce such problems. See also reference [7].
4.4 Effects of volatile compounds (other than water)
[3]
4.4.1 Volatile compounds can be present in unused collection media , or can be adsorbed onto media
during sampling.
4.4.2 Desorption of volatiles from unused media can be controlled, for example, by heating or oxygen
plasma treatment prior to conditioning and weighing. Alternatively, losses may be compensated by the use of
blanks (see Clause 5).
4.4.3 When volatile materials collected during sampling form part of the intended sample, standardized
written procedures are required to ensure that any losses are minimized or at least controlled, for example by
conditioning under tightly specified conditions.
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ISO 15767:2003(E)
4.4.4 When volatile materials collected during sampling are not part of the intended sample, it may be
difficult to eliminate them if weighing is the only form of analysis. Non-sorptive media should preferably be
used.
4.5 Handling damage
4.5.1 If friable substrates are used, procedures are needed to avoid mechanical damage.
4.5.2 The air-sampling equipment should be designed so that the substrate is not damaged during
assembly and disassembly.
4.5.3 Flat tipped forceps are recommended for handling filters. Non-oxidizing metal tins may be used to
weigh delicate substrates without direct handling.
4.5.4 Parts to be weighed shall not be touched with the hands, unless gloved.
4.5.5 Gloves, if used, shall leave no residue on what is weighed.
4.5.6 Handling shall take place in a clean environment, to avoid contamination.
4.6 Buoyancy changes
[8]
Corrections for air buoyancy , equal to the density of air multiplied by the air volume displaced, are not
necessary for small objects, such as a 37-mm diameter membrane filter. However, there may exist
circumstances (e.g. if an entire sampling cassette were weighed without the use of correcting blanks) in which
the object to be weighed is so large that buoyancy must be corrected. For example, if the volume weighed
3
exceeds 0,1 cm , then correction would be required in order to weigh down to 0,1 mg if pressure changes of
the order of 10 % between weighings are expected (e.g. at different altitudes). If such a correction is
necessary, the atmospheric pressure and temperature at the time of weighing should be recorded.
5 Correcting for weight instability by use of blanks
5.1 General
[12] to [20]
Many approaches to controlling weight instability exist . The use of blanks is the most important
practical tool for reducing errors due to weight instability. Correction for weight instability depends on the
specific application and should follow a written procedure. The general principles are as follows. Blank
sampling media are exposed, as closely as possible, to the same conditions as the active sampling media,
without actually drawing air through. Correction is effected by subtracting the average blank mass change
from the mass change of the active samples. Of course, if the atmosphere to be sampled contains water (or
other volatile) droplets, then the use of blanks alone cannot correct. Blanks shall be matched to samples, i.e. if
the sample consists of a filter within a cassette which is weighed, the blank shall be the same type of filter
within the same type of cassette.
NOTE The effect of filter variations due to their manufacture is generally eliminated through the use of blanks.
5.2 Minimum number of blanks
Generally, at least one blank is recommended for each 10 samples. Measurement schemes in current use
require between one and four blanks per batch. See Annex A for advantages of multiple blanks.
5.3 Weighing times and sequence
Blanks shall be interspersed with samples, before and after use, so as to detect systematic variations in mass
(e.g. due to sorption or evaporation of a contaminant during weighing).
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ISO 15767:2003(E)
5.4 Conditioning times
Conditioning times for reaching equilibrium with the weighing environment may vary from a few hours to
several weeks, depending on the specific sampling media. Typically, for workplace sampling applications,
overnight conditioning is satisfactory. For sampling media with longer conditioning times, error correction
through the use of blank substrates is particularly important.
5.5 Storage stability
Unused substrates shall be stored prior to weighing and conditioning in a clean laboratory, whose
environmental conditions do not differ too greatly from the environment of the balance. Pre-weighed
substrates shall be stored together with weighed blanks and used in any case within the assigned shelf-life.
The assigned shelf-life and storage requirements shall be documented as part of a written weighing procedure.
NOTE Shelf-life depends on substrate material, storage conditions, cassette material and required LOQ or LOD.
Archived samples shall be stored together with weighed blanks in a clean laboratory whose environmental
conditions do not differ too greatly from the environment of the balance. Note that transfers of mass between
filters and cassettes could occur where these media are stored together.
6 Transport of samples to laboratory
6.1 General
The transportation of samples shall form part of a written procedure. The transport procedure shall be
validated to ensure that significant losses do not occur. Follow the test method given in Annex D.
The main problems occurring during handling and transport of sampling media are described below.
With substrates designed to be separated from sampling cassettes, dust can migrate from substrate to
the transport container, and hence be lost.
On the other hand, contamination of the sampling cassette and cover lid (if supplied) can be a significant
source of error if the cassette (including cover lid) is part of the substrate.
If a cover lid is not supplied, dust can be lost from the cassette to the transport container.
Dust can migrate from sampling cassette to substrate.
NOTE Transportation losses are discussed in references [12] and [13].
6.2 Recommended packaging
6.2.1 Each substrate that is not mounted in a sampling cassette shall be transported in a Petri dish, tin or a
similar closed container.
6.2.2 Sampling cassettes (i.e. with mounted filters) should preferrably have cover lids during transport. If the
sample consists of all dust deposited inside the sampling cassette (with filter), then dust which migrates during
transport from cassette to cover lid shall also be weighed.
6.2.3 The sealed substrates shall be transported in a suitable container or package. The floor, ceiling and
walls of the container should be lined with a spongy material (preferably electrically conducting) which may
absorb some mechanical shock and thus protect the samples during transport.
6.2.4 The samples shall be protected from excessive heating or cooling during transport.
NOTE 1 Special procedures are generally used for the transport of unstable particles or biological materials.
© ISO 2003 — All rights reserved 5
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ISO 15767:2003(E)
NOTE 2 If there is a possibility for dust to be lost from the substrate, the losses can be recovered by transporting the
substrate within a container that can itself be weighed.
7 Weighing equipment and procedure
7.1 The balance
The balance should be matched to the task. The choice of balance depends on the desired limits of
quantitation for the application (see Clause 8) and on the maximum tare masses of the samples to be weighed.
Workplace-air sampling typically requires a balance capable of weighing to an accuracy of either five or six
figures. The balance shall be regularly calibrated using reference masses traceable to International Standards.
NOTE The performance of different balances was compared and reported in reference [5]. In one experiment, repeat
weighings of 25-mm filters were made with filters stored between weighings in ventilated tins with conditions not strictly
controlled. A balance weighing to 1 µg (six figures) was compared to a balance weighing to 10 µg (five figures). It was
concluded that using a 1-µg balance vs. a 10-µg balance approximately halves the standard deviation of repeat weighing.
Intra-day standard deviation was smaller than the inter-day deviation and is expected to be of greater importance when
blanks are used to correct inter-day variation in the balance room. (See also reference [11].)
7.2 Recommended environmental controls
7.2.1 Equilibration and weighing shall be carried out under the same conditions, i.e. in the same room or
chamber. Environmental control can be achieved in different ways:
by means of a balance room containing balance, samples and the weighing personnel;
by means of an environmentally-controlled chamber containing balance and samples, sited within a clean
laboratory.
NOTE It may be possible to achieve an adequate level of environmental control without the need for active air
conditioning. However the quality of gravimetric analysis depends strongly on the quality of the environmental control.
7.2.2 For sensitive (i.e. hygroscopic) samples, temperature and humidity control in the weighing chamber or
balance room are important. In these cases, temperature should be maintained constant within ± 2 °C of the
set point, and humidity should be constant to within ± 5 % relative humidity (RH) at the target temperature.
The target temperature and humidity should be in the range of operating conditions recommended by the
manufacturer of the balance [e.g. (20 ± 2) °C and (50 ± 5) % RH]. Very dry atmospheres (e.g. < 20 % RH) are
to be avoided, as electrostatic buildup on the samples is more likely in such conditions. The environmental
controls shall be capable of compensating for heat and humidity sources, such as people working in the room
[3]
or electrically-powered instruments in the room .
NOTE Air conditioning in a weighing chamber is not necessary, e.g. filtered laboratory air can be fed into a positive-
pressure chamber.
7.2.3 The particulate content of the balance room or chamber air should be minimized by filtration [e.g. by
High Efficiency Particulate Air (HEPA) filtration].
7.2.4 Fresh air should be supplied consistent with the health and comfort requirements of personnel
working in the balance room or laboratory. Turbulent air movement generated by ventilation or humidity
control in the balance room or chamber should be minimal, so as not to affect the balance reading.
7.3 Other equipment requirements
The balance should be located in an area which is free from excessive vibration [e.g. due to lifts (elevators) or
rotating machinery]. A massive weighing table (e.g. one made of 200 kg of marble) is one means to dampen
ambient vibration. The area should be away from doors, windows, air ducts, and sources of radiant energy
6 © ISO 2003 — All rights reserved
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ISO 15767:2003(E)
such as direct sunlight or ovens. The electrical supply to the balance should be stable, and the balance should
not be located near sources of strong electromagnetic radiation.
7.4 Procedure
7.4.1 The weighing procedure shall be documented.
7.4.2 Equilibration of the sample to the temperature and humidity of the balance room or chamber shall take
place for a period appropriate to the sample. The samples shall be kept in clean containers but open to the
atmosphere so that equilibration can occur.
NOTE Desiccation prior to equilibration is sometimes used to remove excess water from the samples taken in a
humid environment. This only applies to samples with excess water. (See also references [21] to [23].)
7.4.3 Elimination (if possible) of static electricity from the sample is extremely important, and should be
done immediately prior to placement of the sample on the balance pan. Alternatively, a static eliminator can
be placed inside the balance chamber.
7.4.4 The balance reading shall not be recorded until after it has stabilized.
7.4.5 Re-zero the balance as determined necessary.
NOTE A defective substrate can sometimes be identified by an abnormal initial mass.
8 Recommendations for the reporting of measured mass relative to LOD and LOQ
8.1 If the measured mass is above the limit of quantitation (LOQ, Annexes A to C), then it is reported.
8.2 If the result falls between the limit of detection (LOD, Annexes A to C) and LOQ, then it is reported that
the measured mass is between the values of LOD and LOQ, and the measured mass is reported as well.
8.3 If the measured mass falls below the value of the LOD, it is reported that the estimate is below the value
of the LOD.
NOTE 1 If the value of the LOD is exceeded, then the false-positive error rate in asserting detection is < 1 % if the
method is evaluated with as many degrees of freedom (25) as specified in Annexes A to C.
NOTE 2 In some applications a series of measured masses, each below the the value of the LOD, may be of help, e.g.
in asserting the presence of mass over the entire series, even if the individual measurements are too small to assert
detection with confidence. Applications such as these employ actual measured values (even if negative).
8.4 LOD and LOQ values shall be determined and shall be given in the report. Annexes A to C contain
suitable procedures for these determinations.
© ISO 2003 — All rights reserved 7
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ISO 15767:2003(E)
Annex A
(normative)
Estimation of measurement errors
A.1 Symbols (and abbreviated terms)
The following symbols and abbreviated terms are used in th
...
SLOVENSKI STANDARD
SIST ISO 15767:2004
01-junij-2004
Zrak na delovnem mestu – Pregled in opis napak pri tehtanju zbranih aerosolov
Workplace atmospheres -- Controlling and characterizing errors in weighing collected
aerosols
Atmosphères des lieux de travail -- Contrôle et caractérisation des erreurs de pesée des
aérosols collectés
Ta slovenski standard je istoveten z: ISO 15767:2003
ICS:
13.040.30 Kakovost zraka na delovnem Workplace atmospheres
mestu
SIST ISO 15767:2004 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
---------------------- Page: 1 ----------------------
SIST ISO 15767:2004
---------------------- Page: 2 ----------------------
SIST ISO 15767:2004
INTERNATIONAL ISO
STANDARD 15767
First edition
2003-02-15
Workplace atmospheres — Controlling
and characterizing errors in weighing
collected aerosols
Atmosphères des lieux de travail — Contrôle et caractérisation des
erreurs de pesée des aérosols collectés
Reference number
ISO 15767:2003(E)
©
ISO 2003
---------------------- Page: 3 ----------------------
SIST ISO 15767:2004
ISO 15767:2003(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but
shall not be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In
downloading this file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat
accepts no liability in this area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation
parameters were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In
the unlikely event that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2003
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means,
electronic or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or
ISO's member body in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2003 — All rights reserved
---------------------- Page: 4 ----------------------
SIST ISO 15767:2004
ISO 15767:2003(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative references . 1
3 Terms and definitions. 1
4 Weight instability — Causes and minimization . 2
4.1 General. 2
4.2 Moisture sorption. 2
4.3 Electrostatic effects. 3
4.4 Effects of volatile compounds (other than water) . 3
4.5 Handling damage . 4
4.6 Buoyancy changes . 4
5 Correcting for weight instability by use of blanks . 4
5.1 General. 4
5.2 Minimum number of blanks . 4
5.3 Weighing times and sequence . 4
5.4 Conditioning times. 5
5.5 Storage stability . 5
6 Transport of samples to laboratory . 5
6.1 General. 5
6.2 Recommended packaging. 5
7 Weighing equipment and procedure. 6
7.1 The balance . 6
7.2 Recommended environmental controls . 6
7.3 Other equipment requirements. 6
7.4 Procedure. 7
8 Recommendations for the reporting of measured mass relative to LOD and LOQ. 7
Annex A (normative) Estimation of measurement errors . 8
Annex B (informative) Interpretation of LOD and LOQ. 12
Annex C (informative) Method evaluation example . 14
Annex D (normative) Test of transportation integrity. 15
Bibliography . 17
© ISO 2003 — All rights reserved iii
---------------------- Page: 5 ----------------------
SIST ISO 15767:2004
ISO 15767:2003(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 15767 was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 2, Workplace
atmospheres.
iv © ISO 2003 — All rights reserved
---------------------- Page: 6 ----------------------
SIST ISO 15767:2004
ISO 15767:2003(E)
Introduction
Assessment of airborne aerosol hazards in the occupational setting entails sampling onto a collection medium,
followed by analysis of the collected material. The result is generally an estimated concentration of a
hazardous material in the air. The accuracy of such estimates depends on several factors, one of which
relates to the specific type of analysis employed.
This International Standard deals with a specific type of analysis which finds the most general application in
the sampling of aerosols, namely the weighing of sampled material. Gravimetric analysis, though apparently
simple, is subject to errors from instability in the mass of the sampling medium and other elements which must
be weighed. An example is provided by aerosol samplers designed to collect particles so as to agree with the
inhalable aerosol sampling convention. For some sampler types, filter and cassette are weighed together to
make estimates. Therefore, if the cassette, for example, absorbs or loses water between the weighings
required for a concentration estimation, then errors may arise. This International Standard describes such
potential errors and provides solutions for their minimization.
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INTERNATIONAL STANDARD ISO 15767:2003(E)
Workplace atmospheres — Controlling and characterizing
errors in weighing collected aerosols
1 Scope
This International Standard provides recommendations for controlling the analytical uncertainty associated
with aerosol collection medium instability, where collection medium or substrate includes any article used to
collect particles (e.g. filter or foam material) as well as those supporting elements which must be analysed by
weighing.
This International Standard is applicable to results compiled both from the literature and, if necessary and
feasible, through laboratory experiment. Expected errors associated with given aerosol capture methods are
quantified where possible. Recommendations as to materials to be used are given. Means of controlling or
correcting errors arising from instability are provided. Recommendations for the weighing procedure are given.
A procedure for estimating weighing errors is described. Finally, recommendations are given for the reporting
of measured masses.
2 Normative references
The following referenced documents are indispensable for the application 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 7708, Air quality — Particle size fraction definitions for health-related sampling
EN 482, Workplace atmospheres — General requirements for performance of procedures for the
measurement of chemical agents
EN 13205:2001, Workplace atmospheres — Assessment of performance of instruments for measurement of
airborne particle concentrations
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
substrate
aerosol sampling filter, foam, etc., together with whatever mounting is weighed as a single item
NOTE As an example of the converse, the 25-mm or 37-mm plastic filter cassette often used for “total dust” sampling
in either its closed-face or open-face version is not part of the substrate in the definition above, since it is not weighed.
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3.2
equilibration time
time constant characterizing an approximately exponentially damped approach of the mass of an aerosol
collection medium to a constant value
NOTE 1 The constant can be defined as the mean difference of the mass from equilibrium per mean mass loss or gain
rate as measured over a finite time interval.
NOTE 2 There may be important instances in which several time constants are required to describe the approach to
equilibrium.
NOTE 3 Equilibration time is expressed in seconds.
3.3
field blank
blank substrate that undergoes the same handling as the sample substrate, generally including conditioning
and, often, loading into the samplers or transport containers, as well as transportation between lab and
sampling site, but without being exposed to sampling
3.4
lab blank
blank substrate that undergoes the same handling as the sample substrate in the laboratory, including
conditioning and loading into the samplers or transport containers if this is done in the laboratory
3.5
blank substrate
collection medium or substrate taken from the same batch as the sampling medium, but unexposed to sampling
3.6
limit of detection
LOD
three times the estimated standard deviation of the mass of the sample, accounting for the double weighing
(exposed vs. unexposed) and for the uncertainty associated with any correction blanks used
NOTE The value LOD can be used as a threshhold value to assert the presence of a substance with confidence in
the method. Annex B describes how to estimate, on the basis of the method evaluation, the false positive rate in such
assertions.
3.7
limit of quantitation
LOQ
ten times the estimated standard deviation of the mass of the sample
NOTE The value LOQ can be used as a threshhold value to assure measurement of a substance accurately. For
details, see Annex B.
4 Weight instability — Causes and minimization
4.1 General
[1] to [11]
Weight instability of sampling substrates can be attributed to several causes . The following subclauses
address the more important of these.
4.2 Moisture sorption
4.2.1 Moisture sorption is the most common cause of weight instability. Water can be directly collected by
the filter or foam or other substrate material that is weighed. Water sorption by any part of the sampling
system which is weighed must be suspected as well. For example, the sampling cassette itself, if weighed,
[1]
can be the cause of significant error .
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4.2.2 The effects of water sorption can be reduced by using non-sorptive materials. However, there may
exist specific sampling needs for which a hydrophobic material is not feasible. Table 1 presents a list of
common aerosol sampling substrates with different water adsorption features.
Table 1 — Water sorption characteristics of some aerosol sampling media
Water sorption
Substrate or cassette type
Very low Low High Very high
Cellulose fibre filter *
Glass fibre filter *
Quartz fibre filter *
Cellulose ester membrane filter *
Polytetrafluoroethylene filter *
PVC membrane filter * *
Polycarbonate filter *
Silver membrane filter *
Polyurethane foam *
Greased Mylar impaction substrate * *
Greased aluminium foil impaction substrate *
Carbon-filled resin *
Aluminium cassette * *
Stainless steel cassette *
NOTE 1 References [2] to [4] provide further details. Also, reference [5] reports that filters of evidently the same
material, but originating from different manufacturers, can have widely differing variabilities.
NOTE 2 There is generally a trade-off between hydrophobicity and conductivity in many materials [9]. Therefore, one
must be aware of the possibility of creating sampling problems when reducing hygroscopicity.
NOTE 3 Pre-treatments of substrates, such as greasing, can also affect water sorption.
4.3 Electrostatic effects
Electrostatic effects are a common source of weighing problems. These effects can usually be minimized by
discharging the substrate through the use of a plasma ion source or a radioactive source. Using conductive
materials may reduce such problems. See also reference [7].
4.4 Effects of volatile compounds (other than water)
[3]
4.4.1 Volatile compounds can be present in unused collection media , or can be adsorbed onto media
during sampling.
4.4.2 Desorption of volatiles from unused media can be controlled, for example, by heating or oxygen
plasma treatment prior to conditioning and weighing. Alternatively, losses may be compensated by the use of
blanks (see Clause 5).
4.4.3 When volatile materials collected during sampling form part of the intended sample, standardized
written procedures are required to ensure that any losses are minimized or at least controlled, for example by
conditioning under tightly specified conditions.
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4.4.4 When volatile materials collected during sampling are not part of the intended sample, it may be
difficult to eliminate them if weighing is the only form of analysis. Non-sorptive media should preferably be
used.
4.5 Handling damage
4.5.1 If friable substrates are used, procedures are needed to avoid mechanical damage.
4.5.2 The air-sampling equipment should be designed so that the substrate is not damaged during
assembly and disassembly.
4.5.3 Flat tipped forceps are recommended for handling filters. Non-oxidizing metal tins may be used to
weigh delicate substrates without direct handling.
4.5.4 Parts to be weighed shall not be touched with the hands, unless gloved.
4.5.5 Gloves, if used, shall leave no residue on what is weighed.
4.5.6 Handling shall take place in a clean environment, to avoid contamination.
4.6 Buoyancy changes
[8]
Corrections for air buoyancy , equal to the density of air multiplied by the air volume displaced, are not
necessary for small objects, such as a 37-mm diameter membrane filter. However, there may exist
circumstances (e.g. if an entire sampling cassette were weighed without the use of correcting blanks) in which
the object to be weighed is so large that buoyancy must be corrected. For example, if the volume weighed
3
exceeds 0,1 cm , then correction would be required in order to weigh down to 0,1 mg if pressure changes of
the order of 10 % between weighings are expected (e.g. at different altitudes). If such a correction is
necessary, the atmospheric pressure and temperature at the time of weighing should be recorded.
5 Correcting for weight instability by use of blanks
5.1 General
[12] to [20]
Many approaches to controlling weight instability exist . The use of blanks is the most important
practical tool for reducing errors due to weight instability. Correction for weight instability depends on the
specific application and should follow a written procedure. The general principles are as follows. Blank
sampling media are exposed, as closely as possible, to the same conditions as the active sampling media,
without actually drawing air through. Correction is effected by subtracting the average blank mass change
from the mass change of the active samples. Of course, if the atmosphere to be sampled contains water (or
other volatile) droplets, then the use of blanks alone cannot correct. Blanks shall be matched to samples, i.e. if
the sample consists of a filter within a cassette which is weighed, the blank shall be the same type of filter
within the same type of cassette.
NOTE The effect of filter variations due to their manufacture is generally eliminated through the use of blanks.
5.2 Minimum number of blanks
Generally, at least one blank is recommended for each 10 samples. Measurement schemes in current use
require between one and four blanks per batch. See Annex A for advantages of multiple blanks.
5.3 Weighing times and sequence
Blanks shall be interspersed with samples, before and after use, so as to detect systematic variations in mass
(e.g. due to sorption or evaporation of a contaminant during weighing).
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5.4 Conditioning times
Conditioning times for reaching equilibrium with the weighing environment may vary from a few hours to
several weeks, depending on the specific sampling media. Typically, for workplace sampling applications,
overnight conditioning is satisfactory. For sampling media with longer conditioning times, error correction
through the use of blank substrates is particularly important.
5.5 Storage stability
Unused substrates shall be stored prior to weighing and conditioning in a clean laboratory, whose
environmental conditions do not differ too greatly from the environment of the balance. Pre-weighed
substrates shall be stored together with weighed blanks and used in any case within the assigned shelf-life.
The assigned shelf-life and storage requirements shall be documented as part of a written weighing procedure.
NOTE Shelf-life depends on substrate material, storage conditions, cassette material and required LOQ or LOD.
Archived samples shall be stored together with weighed blanks in a clean laboratory whose environmental
conditions do not differ too greatly from the environment of the balance. Note that transfers of mass between
filters and cassettes could occur where these media are stored together.
6 Transport of samples to laboratory
6.1 General
The transportation of samples shall form part of a written procedure. The transport procedure shall be
validated to ensure that significant losses do not occur. Follow the test method given in Annex D.
The main problems occurring during handling and transport of sampling media are described below.
With substrates designed to be separated from sampling cassettes, dust can migrate from substrate to
the transport container, and hence be lost.
On the other hand, contamination of the sampling cassette and cover lid (if supplied) can be a significant
source of error if the cassette (including cover lid) is part of the substrate.
If a cover lid is not supplied, dust can be lost from the cassette to the transport container.
Dust can migrate from sampling cassette to substrate.
NOTE Transportation losses are discussed in references [12] and [13].
6.2 Recommended packaging
6.2.1 Each substrate that is not mounted in a sampling cassette shall be transported in a Petri dish, tin or a
similar closed container.
6.2.2 Sampling cassettes (i.e. with mounted filters) should preferrably have cover lids during transport. If the
sample consists of all dust deposited inside the sampling cassette (with filter), then dust which migrates during
transport from cassette to cover lid shall also be weighed.
6.2.3 The sealed substrates shall be transported in a suitable container or package. The floor, ceiling and
walls of the container should be lined with a spongy material (preferably electrically conducting) which may
absorb some mechanical shock and thus protect the samples during transport.
6.2.4 The samples shall be protected from excessive heating or cooling during transport.
NOTE 1 Special procedures are generally used for the transport of unstable particles or biological materials.
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NOTE 2 If there is a possibility for dust to be lost from the substrate, the losses can be recovered by transporting the
substrate within a container that can itself be weighed.
7 Weighing equipment and procedure
7.1 The balance
The balance should be matched to the task. The choice of balance depends on the desired limits of
quantitation for the application (see Clause 8) and on the maximum tare masses of the samples to be weighed.
Workplace-air sampling typically requires a balance capable of weighing to an accuracy of either five or six
figures. The balance shall be regularly calibrated using reference masses traceable to International Standards.
NOTE The performance of different balances was compared and reported in reference [5]. In one experiment, repeat
weighings of 25-mm filters were made with filters stored between weighings in ventilated tins with conditions not strictly
controlled. A balance weighing to 1 µg (six figures) was compared to a balance weighing to 10 µg (five figures). It was
concluded that using a 1-µg balance vs. a 10-µg balance approximately halves the standard deviation of repeat weighing.
Intra-day standard deviation was smaller than the inter-day deviation and is expected to be of greater importance when
blanks are used to correct inter-day variation in the balance room. (See also reference [11].)
7.2 Recommended environmental controls
7.2.1 Equilibration and weighing shall be carried out under the same conditions, i.e. in the same room or
chamber. Environmental control can be achieved in different ways:
by means of a balance room containing balance, samples and the weighing personnel;
by means of an environmentally-controlled chamber containing balance and samples, sited within a clean
laboratory.
NOTE It may be possible to achieve an adequate level of environmental control without the need for active air
conditioning. However the quality of gravimetric analysis depends strongly on the quality of the environmental control.
7.2.2 For sensitive (i.e. hygroscopic) samples, temperature and humidity control in the weighing chamber or
balance room are important. In these cases, temperature should be maintained constant within ± 2 °C of the
set point, and humidity should be constant to within ± 5 % relative humidity (RH) at the target temperature.
The target temperature and humidity should be in the range of operating conditions recommended by the
manufacturer of the balance [e.g. (20 ± 2) °C and (50 ± 5) % RH]. Very dry atmospheres (e.g. < 20 % RH) are
to be avoided, as electrostatic buildup on the samples is more likely in such conditions. The environmental
controls shall be capable of compensating for heat and humidity sources, such as people working in the room
[3]
or electrically-powered instruments in the room .
NOTE Air conditioning in a weighing chamber is not necessary, e.g. filtered laboratory air can be fed into a positive-
pressure chamber.
7.2.3 The particulate content of the balance room or chamber air should be minimized by filtration [e.g. by
High Efficiency Particulate Air (HEPA) filtration].
7.2.4 Fresh air should be supplied consistent with the health and comfort requirements of personnel
working in the balance room or laboratory. Turbulent air movement generated by ventilation or humidity
control in the balance room or chamber should be minimal, so as not to affect the balance reading.
7.3 Other equipment requirements
The balance should be located in an area which is free from excessive vibration [e.g. due to lifts (elevators) or
rotating machinery]. A massive weighing table (e.g. one made of 200 kg of marble) is one means to dampen
ambient vibration. The area should be away from doors, windows, air ducts, and sources of radiant energy
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such as direct sunlight or ovens. The electrical supply to the balance should be stable, and the balance should
not be located near sources of strong electromagnetic radiation.
7.4 Procedure
7.4.1 The weighing procedure shall be documented.
7.4.2 Equilibration of the sample to the temperature and humidity of the balance room or chamber shall take
place for a period appropriate to the sample. The samples shall be kept in clean containers but open to the
atmosphere so that equilibration can occur.
NOTE Desiccation prior to equilibration is sometimes used to remove excess water from the samples taken in a
humid environment. This only applies to samples with excess water. (See also references [21] to [23].)
7.4.3 Elimination (if possible) of static electricity from the sample is extremely important, and should be
done immediately prior to placement of the sample on the balance pan. Alternatively, a static eliminator can
be placed inside the balance chamber.
7.4.4 The balance reading shall not be recorded until after it has stabilized.
7.4.5 Re-zero the balance as determined necessary.
NOTE A defective substrate can sometimes be identified by an abnormal initial mass.
8 Recommendations for the reporting of measured mass relative to LOD and LOQ
8.1 If the measured mass is above the limit of quantitation (LOQ, Annexes A to C), then it is reported.
8.2 If the result falls between the limit of detection (LOD, Annexes A to C) and LOQ, then it is reported that
the measured mass is between the values of LOD and LOQ, and the measured mass is reported as well.
8.3 If the m
...
NORME ISO
INTERNATIONALE 15767
Première édition
2003-02-15
Atmosphères des lieux de travail —
Contrôle et caractérisation des erreurs de
pesée des aérosols collectés
Workplace atmospheres — Controlling and characterizing errors in
weighing collected aerosols
Numéro de référence
ISO 15767:2003(F)
©
ISO 2003
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ISO 15767:2003(F)
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Publié en Suisse
ii © ISO 2003 — Tous droits réservés
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ISO 15767:2003(F)
Sommaire Page
Avant-propos. iv
Introduction . v
1 Domaine d'application. 1
2 Références normatives. 1
3 Termes et définitions . 1
4 Instabilité de pesée — Causes et minimisation. 2
4.1 Généralités. 2
4.2 Adsorption d'humidité. 3
4.3 Effets électrostatiques. 3
4.4 Effets dus aux composants volatils (autres que l'eau). 3
4.5 Détériorations dues aux manipulations. 4
4.6 Variations dues à la poussée d'Archimède.4
5 Corrections liées à l'instabilité de pesée. 4
5.1 Généralités. 4
5.2 Nombre minimal de blancs . 5
5.3 Durée et séquence des pesées. 5
5.4 Temps de conditionnement . 5
5.5 Stabilité au cours de l'entreposage. 5
6 Transport des échantillons jusqu'au laboratoire. 5
6.1 Généralités. 5
6.2 Recommandations concernant l'emballage. 6
7 Équipement et mode opératoire de pesée.6
7.1 Balance. 6
7.2 Contrôles environnementaux recommandés. 6
7.3 Autres exigences liées aux équipements. 7
7.4 Mode opératoire . 7
8 Recommandations sur la consignation de la masse mesurée par rapport aux valeurs LOD
et LOQ . 7
Annexe A (normative) Estimation des erreurs de mesurage. 9
Annexe B (informative) Interprétation de LOD et LOQ . 13
Annexe C (informative) Exemple d'évaluation de méthode . 15
Annexe D (normative) Essai d'intégrité pendant le transport. 16
Bibliographie . 18
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ISO 15767:2003(F)
Avant-propos
L'ISO (Organisation internationale de normalisation) est une fédération mondiale d'organismes nationaux de
normalisation (comités membres de l'ISO). L'élaboration des Normes internationales est en général confiée
aux comités techniques de l'ISO. Chaque comité membre intéressé par une étude a le droit de faire partie du
comité technique créé à cet effet. Les organisations internationales, gouvernementales et non
gouvernementales, en liaison avec l'ISO participent également aux travaux. L'ISO collabore étroitement avec
la Commission électrotechnique internationale (CEI) en ce qui concerne la normalisation électrotechnique.
Les Normes internationales sont rédigées conformément aux règles données dans les Directives ISO/CEI,
Partie 2.
La tâche principale des comités techniques est d'élaborer les Normes internationales. Les projets de Normes
internationales adoptés par les comités techniques sont soumis aux comités membres pour vote. Leur
publication comme Normes internationales requiert l'approbation de 75 % au moins des comités membres
votants.
L'attention est appelée sur le fait que certains des éléments du présent document peuvent faire l'objet de
droits de propriété intellectuelle ou de droits analogues. L'ISO ne saurait être tenue pour responsable de ne
pas avoir identifié de tels droits de propriété et averti de leur existence.
L'ISO 15767 a été élaborée par le comité technique ISO/TC 146, Qualité de l'air, sous-comité SC 2,
Atmosphères des lieux de travail.
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ISO 15767:2003(F)
Introduction
L'évaluation des risques liés aux aérosols en suspension dans l'air en milieu professionnel nécessite
l'échantillonnage sur un support de collecte, suivi d'une analyse des matières recueillies. Le résultat obtenu
permet généralement d'évaluer la concentration des matières nocives dans l'air. La précision de telles
estimations repose sur plusieurs facteurs, dont l'un est lié au type spécifique d'analyse employé.
La présente Norme internationale décrit un type d'analyse particulier, la pesée du matériau prélevé, qui est
appliqué de façon très générale dans le prélèvement des aérosols. L'analyse gravimétrique, bien que simple
en apparence, est propice aux erreurs en raison de l'instabilité de la masse du support de prélèvement
d'échantillonnage et des autres éléments à peser. Le cas se présente par exemple avec les échantillonneurs
d'aérosols conçus pour recueillir des particules conformément à la convention d'échantillonnage des aérosols
inhalables. Dans certains cas, les échantillonneurs comprennent un filtre et une cassette pesés ensemble
pour permettre les estimations. Ainsi, par exemple, l'absorption ou la déperdition d'eau dans la cassette entre
les différentes pesées requises pour estimer une concentration peuvent donner lieu à des erreurs. La
présente Norme internationale décrit les erreurs potentielles de ce type et fournit des solutions visant à les
minimiser.
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NORME INTERNATIONALE ISO 15767:2003(F)
Atmosphères des lieux de travail — Contrôle et caractérisation
des erreurs de pesée des aérosols collectés
1 Domaine d'application
La présente Norme internationale fournit des recommandations pour le contrôle des incertitudes analytiques
liées à l'instabilité des supports de collecte d'aérosols, où le support ou substrat de collecte inclut tout objet
utilisé pour recueillir des particules (par exemple un filtre ou un matériau en mousse), ainsi que les éléments
connexes devant être soumis à l'analyse par pesée.
La présente Norme internationale est applicable aux résultats compilés à la fois à partir de sources
bibliographiques et, lorsque cela est nécessaire et faisable, à partir d'expériences en laboratoire. Les erreurs
prévisibles liées aux méthodes de prélèvement d'aérosols sont quantifiées chaque fois que possible. Des
recommandations sur les matériaux à utiliser sont indiquées. Des moyens de contrôle ou de correction des
erreurs dues à l'instabilité sont fournis. Des recommandations sur le mode opératoire de pesée sont données.
Une procédure d'estimation des erreurs de pesée est décrite. Enfin, des recommandations sont données pour
établir des rapports des masses mesurées.
2 Références normatives
Les documents de référence suivants sont indispensables pour l'application du présent document. Pour les
références datées, seule l'édition citée s'applique. Pour les références non datées, la dernière édition du
document de référence s'applique (y compris les éventuels amendements).
ISO 7708, Qualité de l'air — Définitions des fractions de taille des particules pour l'échantillonnage lié aux
problèmes de santé
EN 482, Atmosphères des lieux de travail — Exigences générales concernant les performances des
procédures de mesurage des agents chimiques
EN 13205:2001, Atmosphères de lieux de travail — Évaluation des performances des instruments de
mesurage des concentrations d'aérosols
3 Termes et définitions
Pour les besoins du présent document, les termes et définitions suivants s'appliquent.
3.1
substrat
filtre de prélèvement, mousse, etc. et tout accessoire de montage associé, pesés en tant qu'élément unique
NOTE À titre d'exemple contraire, la cassette en plastique servant de support aux filtres de 25 mm ou 37 mm
fréquemment utilisée pour échantillonner les «poussières totales», disponible en version à face fermée ou face ouverte,
ne fait pas partie du substrat défini ci-dessus, puisqu'elle n'est pas prise en compte dans la pesée.
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ISO 15767:2003(F)
3.2
temps d'équilibrage
constante temporelle qui caractérise une approche de la masse d'un support de collecte d'aérosols,
s'amortissant de façon approximativement exponentielle jusqu'à une valeur constante
NOTE 1 La constante peut être définie comme le rapport de la différence moyenne entre la masse obtenue à l'équilibre
et le rapport moyen de perte ou de gain de masse, sur une période déterminée.
NOTE 2 Dans certaines situations importantes, plusieurs constantes temporelles peuvent être requises pour décrire
une approche de l'équilibre.
NOTE 3 Le temps d'équilibrage est exprimé en secondes.
3.3
blanc de terrain
substrat à blanc soumis aux mêmes manipulations que le substrat de prélèvement, incluant généralement une
phase de conditionnement et nécessitant souvent une mise en place dans les échantillonneurs ou récipients
de transport, ainsi que l'acheminement entre le laboratoire et le site de prélèvement, mais sans être soumis
au prélèvement
3.4
blanc de laboratoire
substrat à blanc soumis aux mêmes manipulations de laboratoire que le substrat de prélèvement, incluant une
phase de conditionnement et une mise en place dans les échantillonneurs ou récipients de transport, si ces
opérations sont effectuées en laboratoire
3.5
substrat à blanc
support ou substrat de collecte provenant du même lot que le support de prélèvement, mais sans exposition
au prélèvement
3.6
limite de détection
LOD
trois fois l'écart type estimé de la masse de l'échantillon, avec la prise en compte de la double pesée
(exposée et non exposée) et de l'incertitude associée à la correction de blanc
NOTE La LOD peut être utilisée comme seuil de mise en évidence d'une substance présente, avec le niveau de
confiance qui caractérise la méthode. L'Annexe B décrit le mode d'estimation, sur la base de l'évaluation de la méthode,
du taux de faux positifs de ces mises en évidence.
3.7
limite de quantification
LOQ
dix fois l'écart type estimé de la masse de l'échantillon
NOTE La LOQ peut être utilisée comme seuil de garantie pour la précision du mesurage d'une substance. Voir les
détails à l'Annexe B.
4 Instabilité de pesée — Causes et minimisation
4.1 Généralités
[1] à [11]
L'instabilité de pesée des substrats de prélèvement peut être attribuée à plusieurs causes . Les plus
importantes sont décrites dans les paragraphes qui suivent.
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4.2 Adsorption d'humidité
4.2.1 L'adsorption d'humidité est la cause la plus fréquente d'instabilité de pesée. L'eau peut être prélevée
directement sur le filtre, la mousse ou tout autre matériau de substrat pesé. L'adsorption d'eau dans une
partie quelconque du système de prélèvement pesé peut également être soupçonnée. Par exemple, la
[1]
cassette elle-même peut, lors de sa pesée, être à l'origine d'erreurs significatives .
4.2.2 Les effets de l'adsorption d'eau peuvent être réduits par l'emploi de matériaux non adsorbants.
Toutefois, les besoins liés à certains prélèvements ne permettent pas l'emploi de matériaux hydrophobes. Le
Tableau 1 présente une liste de substrats de prélèvement d'aérosols courants, ayant des propriétés
adsorbantes différentes vis-à-vis de l'eau.
Tableau 1 — Propriétés adsorbantes vis-à-vis de l'eau de quelques supports
de prélèvement d'aérosols
Type de substrat ou de cassette Très faible Faible Elevée Très élevée
Filtre en fibre de cellulose *
Filtre en fibre de verre *
Filtre en fibre de quartz *
Filtre à membrane en ester de cellulose *
Filtre en polytétrafluoroéthylène *
Filtre à membrane PVC * *
Filtre en polycarbonate *
Filtre à membrane argent *
Mousse de polyuréthanne *
Substrat d'impaction en mylar graissé * *
Substrat d'impaction à feuille d'aluminium graissée *
Résine au carbone *
Cassette en aluminium * *
Cassette en acier inoxydable *
NOTE 1 Les références [2] à [4] comportent des détails supplémentaires. Par ailleurs, la référence [5] établit que les
filtres réputés constitués du même matériau, mais provenant de fabricants divers, peuvent présenter des caractéristiques
très inégales.
NOTE 2 Pour de nombreux matériaux, il existe en général un compromis entre l'hydrophobicité et la conductivité [9].
Néanmoins, il doit être tenu compte du fait que la réduction de l'hygroscopicité peut engendrer des problèmes de
prélèvement.
NOTE 3 Le prétraitement des substrats, tel qu'un graissage, peut également influer sur l'adsorption d'eau.
4.3 Effets électrostatiques
Les effets électrostatiques sont une source fréquente de problèmes lors des pesées. Ces effets peuvent
généralement être minimisés par l'élimination des charges électriques du substrat, par l'emploi d'une source
ionique à plasma ou d'une source radioactive. Ces problèmes peuvent être réduits par l'usage de matériaux
conducteurs. Voir également la référence [7].
4.4 Effets dus aux composants volatils (autres que l'eau)
[3]
4.4.1 Des substances volatiles peuvent être présentes sur certains supports de collecte inutilisés , ou
introduites par adsorption sur le support pendant le prélèvement.
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4.4.2 La désorption des substances volatiles du support inutilisé peut être contrôlée, par exemple, par un
traitement thermique ou un traitement par plasma à oxygène avant le conditionnement et la pesée. À l'opposé,
les pertes peuvent être compensées par l'usage de blancs (voir l'Article 5).
4.4.3 Lorsque les matières volatiles collectées lors du prélèvement font partie de l'échantillon, des
procédures normalisées écrites doivent être rédigées pour garantir que les pertes éventuelles sont
minimisées, ou au minimum contrôlées, par exemple grâce à un conditionnement rigoureusement défini.
4.4.4 Lorsque les matières volatiles recueillies lors du prélèvement ne font pas partie de l'échantillon, leur
élimination peut être difficile si la pesée constitue l'unique forme d'analyse. Il convient d'utiliser de préférence
des supports non adsorbants.
4.5 Détériorations dues aux manipulations
4.5.1 Si les substrats utilisés sont friables, des procédures sont requises pour éviter les détériorations
d'origine mécanique.
4.5.2 Il convient que l'équipement de prélèvement de l'air soit conçu pour que le substrat ne subisse
aucune détérioration lors de l'assemblage et du désassemblage.
4.5.3 L'utilisation de pinces à embouts plats est recommandée pour manipuler les filtres. Les substrats
délicats peuvent être pesés sans manipulation directe, dans des récipients en métal inoxydable.
4.5.4 Les éléments à peser ne doivent pas être manipulés avec les doigts, sauf si les mains sont gantées.
4.5.5 Si des gants sont utilisés, ils ne doivent laisser aucun résidu sur les éléments pesés.
4.5.6 La manipulation doit avoir lieu dans un environnement propre, pour éviter les contaminations.
4.6 Variations dues à la poussée d'Archimède
[8]
Les corrections liées à la poussée d'Archimède , égale à la masse volumique de l'air multipliée par le volume
d'air déplacé, ne sont pas nécessaires pour les objets de petite taille tels que les filtres à membrane de 37 mm.
Toutefois, dans certaines circonstances (par exemple pour peser une cassette de prélèvement entière sans
recourir à des blancs correcteurs), le volume de l'objet pesé est tel qu'une compensation de la poussée
3
atmosphérique est nécessaire. Si, par exemple, le volume pesé est supérieur à 0,1 cm , alors une correction
est requise pour permettre des pesées allant jusqu'à 0,1 mg, en cas de variation de pression prévue de l'ordre
de 10 % entre chaque pesée (par exemple à des altitudes différentes). Lorsqu'une telle correction est
nécessaire, il convient de consigner la pression atmosphérique et la température au moment de la pesée.
5 Corrections liées à l'instabilité de pesée
5.1 Généralités
[12] à [20]
Plusieurs approches pour contrôler l'instabilité de pesée existent . L'utilisation de blancs est la
principale solution pratique pour réduire les erreurs dues à l'instabilité de pesée. La correction de l'instabilité de
pesée dépendant de chaque application particulière, il convient qu'elle soit conforme à une procédure écrite. Le
principe général est le suivant. Des supports de prélèvement à blanc sont exposés à des conditions aussi
proches que possible de celles du support de prélèvement actif, sans toutefois procéder à un pompage d'air.
La correction est appliquée en soustrayant le changement de masse moyenne des échantillons à blanc du
changement de masse relevée sur les échantillons actifs. Bien entendu, dans le cas d'échantillons contenant
des gouttelettes d'eau (ou d'autres substances volatiles), l'usage de blancs seuls ne permet aucune correction.
Les blancs doivent être appariés aux échantillons: si l'échantillon comporte un filtre contenu dans la cassette
pesée, le blanc doit être constitué d'un filtre de même type contenu dans le même type de cassette.
NOTE Les effets des variations de l'état du filtre, qui résultent de leur fabrication, sont généralement éliminés par
l'usage de blancs.
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5.2 Nombre minimal de blancs
Il est généralement conseillé d'utiliser au moins un blanc pour dix échantillons. Les procédures de mesurage
courantes nécessitent l'emploi d'un à quatre blancs par lot. Pour connaître les avantages liés à l'emploi de
plusieurs blancs, se reporter à l'Annexe A.
5.3 Durée et séquence des pesées
Des blancs doivent être intercalés entre les supports utilisés pour le prélèvement, avant et après utilisation,
afin de détecter les variations systématiques en masse (dues par exemple à l'adsorption ou à l'évaporation
d'un contaminant durant la pesée).
5.4 Temps de conditionnement
Le temps de conditionnement nécessaire à l'obtention d'un équilibre dans l'environnement de pesée peut varier
de quelques heures à plusieurs semaines, suivant le support de prélèvement spécifique. En règle générale, un
temps de conditionnement égal à une nuit est satisfaisant pour les applications liées au prélèvement sur les
lieux de travail. Lorsque le support de prélèvement nécessite un temps de conditionnement plus long, l'usage
de substrats à blanc pour corriger les erreurs est particulièrement important.
5.5 Stabilité au cours de l'entreposage
Les substrats non encore utilisés doivent être entreposés, avant leur pesée et leur conditionnement, dans un
laboratoire propre, dont les conditions environnementales ne sont pas trop éloignées de celles de
l'environnement de la balance. Les substrats soumis à une pesée préalable doivent être conservés au même
endroit que les blancs pesés et doivent être utilisés, dans tous les cas, au cours de leur durée de
conservation. Les exigences concernant la durée de conservation et l'entreposage doivent faire l'objet d'une
documentation incluse dans le mode opératoire de pesée.
NOTE La durée de conservation dépend du matériau constitutif du substrat, des conditions d'entreposage, du
matériau de la cassette et des valeurs de LOQ ou LOD requises.
Les échantillons archivés doivent être conservés avec les blancs pesés, dans un laboratoire propre dont les
conditions environnementales ne sont pas trop éloignées de celles de l'environnement de la balance. Il est à
noter que des transferts de masse peuvent se produire entre les filtres et les cassettes, lorsque ces supports
sont entreposés au même endroit.
6 Transport des échantillons jusqu'au laboratoire
6.1 Généralités
Le transport des échantillons doit faire l'objet d'une documentation écrite. La procédure de transport doit être
validée, afin de garantir qu'aucune perte significative ne peut avoir lieu. La méthode d'essai indiquée à
l'Annexe D doit être suivie.
Les principales difficultés liées à la manipulation et au transport de supports de prélèvement sont les
suivantes.
Dans le cas de substrats conçus pour être séparés des cassettes de prélèvement, des poussières
peuvent migrer du substrat vers le récipient de transport, et en conséquence être perdues.
À l'inverse, la contamination de la cassette de prélèvement et du couvercle (le cas échéant) peut être une
source d'erreur significative si la cassette (y compris le couvercle) fait partie du substrat.
Lorsqu'aucun couvercle n'est fourni, des poussières peuvent être perdues suite à une migration de la
cassette vers le récipient de transport.
De la poussière peut migrer de la cassette vers le substrat.
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NOTE Les pertes durant le transport ont fait l'objet d'une étude dans les références [12] et [13].
6.2 Recommandations concernant l'emballage
6.2.1 Tout substrat non contenu dans une cassette de prélèvement doit être transporté dans une boîte de
Petri, un récipient métallique ou un récipient clos similaire.
6.2.2 Il convient, de préférence, que les cassettes de prélèvement (c'est-à-dire dont les filtres sont installés)
soient munies de couvercles durant le transport. Si l'échantillon est constitué de poussières déposées à
l'intérieur de la cassette de prélèvement (équipée d'un filtre), les poussières qui ont migré de la cassette sur le
couvercle durant le transport doivent également être pesées.
6.2.3 Les substrats scellés doivent être transportés dans un récipient ou un emballage approprié. Il
convient de protéger le fond, le sommet et les parois du récipient avec un matériau spongieux (de préférence
conducteur) capable d'absorber une partie des chocs mécaniques, et donc de protéger les échantillons durant
le transport.
6.2.4 Les échantillons doivent durant le transport être protégés des excès de chaleur et de froid.
NOTE 1 Des procédures particulières sont requises pour le transport de particules instables ou de matières
biologiques.
NOTE 2 Lorsque des pertes de poussières peuvent avoir lieu sur le substrat, ces pertes peuvent être récupérées en
transportant le substrat dans un récipient pouvant lui-même être pesé.
7 Équipement et mode opératoire de pesée
7.1 Balance
Il convient d'utiliser une balance adaptée à la tâche exécutée. Le choix de la balance dépend des limites de
quantification souhaitées pour l'application (voir l'Article 8), ainsi que de la tare maximale des échantillons à
peser.
En règle générale, le prélèvement d'air sur les lieux de travail nécessite une balance offrant une précision à
cinq ou six chiffres. La balance doit être réétalonnée régulièrement, à l'aide de masses de référence traçables
par des Normes internationales.
NOTE Les performances de différentes balances ont été comparées et documentées dans la référence [5]. Au cours
de l'une des expériences, des pesées répétées de filtres de 25 mm ont été effectuées avec des filtres entreposés, entre
chaque pesée, dans des boîtes métalliques ventilées, dans des conditions strictement contrôlées. Les résultats d'une
balance précise à 1 µg (précision à six chiffres) ont été comparés à ceux d'une balance précise à 10 µg (précision à cinq
chiffres). Il a été constaté qu'une balance précise à 1 µg présentait un écart type sur les pesées successives égal à
environ la moitié de celui mesuré avec une balance précise à 10 µg. L'écart type mesuré sur une journée est inférieur à
l'écart interjournalier et est supposé augmenter lorsque des blancs sont utilisés pour corriger l'écart type interjournalier
dans la salle de balances. (Voir également référence [11].)
7.2 Contrôles environnementaux recommandés
7.2.1 L'équilibrage et la pesée doivent être effectués dans les mêmes conditions, c'est-à-dire dans la même
pièce ou la même chambre. Le contrôle environnemental peut être effectué de plusieurs manières:
dans une salle contenant la balance, les échantillons et le personnel qui effectue la pesée;
dans une chambre à environnement contrôlé contenant la balance et les échantillons, sise dans un
laboratoire propre.
NOTE L'obtention d'un niveau de contrôle environnemental adéquat ne nécessite parfois aucun conditionnement
actif de l'air. Toutefois, la qualité des analyses gravimétriques dépend fortement de la qualité du contrôle environnemental.
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7.2.2 Dans le cas d'échantillons sensibles (c'est-à-dire hygroscopiques), il est important de contrôler la
température et l'humidité de la chambre de pesée ou de la salle de balances. Il convient, dans ce cas, de
maintenir une température constante à ± 2 °C par rapport à la valeur de consigne, ainsi qu'une humidité
constante à ± 5 % d'humidité relative (HR) à la température cible. Il convient que les valeurs cibles de
température et d'humidité soient comprises dans les limites des conditions d'utilisation préconisées par le
fabricant de la
...
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