ASTM D6552-06(2011)
(Practice)Standard Practice for Controlling and Characterizing Errors in Weighing Collected Aerosols
Standard Practice for Controlling and Characterizing Errors in Weighing Collected Aerosols
SIGNIFICANCE AND USE
The weighing of collected aerosol is one of the most common and purportedly simple analytical procedures in both occupational and environmental atmospheric monitoring (for example, Test Method D4532 or D4096). Problems with measurement accuracy occur when the amount of material collected is small, owing both to balance inaccuracy and variation in the weight of that part of the sampling medium that is weighed along with the sample. The procedures presented here for controlling and documenting such analytical errors will help provide the accuracy required for making well-founded decisions in identifying, characterizing, and controlling hazardous conditions.
Recommendations are given as to materials to be used. Means of controlling or correcting errors arising from instability are provided. Recommendations as to the weighing procedure are given. Finally, a method evaluation procedure for estimating weighing errors is described.
Recommendations are also provided for the reporting of weights relative to LOD (see 3.2.6) and LOQ (see 3.2.7). The quantities, LOD and LOQ, are computed as a result of the method evaluation.
SCOPE
1.1 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 possibly hazardous material in the air. The uncertainty in such estimates depends on several factors, one of which relates to the specific type of analysis employed. The most commonly applied method for analysis of aerosols is the weighing of the sampled material. Gravimetric analysis, though apparently simple, is subject to errors from instability in the mass of the sampling medium and other elements that must be weighed. An example is provided by aerosol samplers designed to collect particles so as to agree with the inhalable aerosol sampling convention (see ISO TR 7708, Guide D6062, and EN 481). 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 practice covers such potential errors and provides solutions for their minimization.
1.2 The values given in SI units are to be regarded as standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D6552 − 06(Reapproved 2011)
Standard Practice for
Controlling and Characterizing Errors in Weighing Collected
Aerosols
This standard is issued under the fixed designation D6552; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope D4096Test Method for Determination of Total Suspended
ParticulateMatterintheAtmosphere(High–VolumeSam-
1.1 Assessment of airborne aerosol hazards in the occupa-
pler Method)
tional setting entails sampling onto a collection medium
D4532Test Method for Respirable Dust in Workplace At-
followed by analysis of the collected material. The result is
mospheres Using Cyclone Samplers
generally an estimated concentration of a possibly hazardous
D6062GuideforPersonalSamplersofHealth-RelatedAero-
material in the air. The uncertainty in such estimates depends
sol Fractions
on several factors, one of which relates to the specific type of
2.2 International Standards:
analysis employed. The most commonly applied method for
EN 481WorkplaceAtmospheres—Size Fraction Definitions
analysis of aerosols is the weighing of the sampled material.
for Measurement of Airborne Particles in the Workplace
Gravimetric analysis, though apparently simple, is subject to
EN 482Workplace Atmospheres—General Requirements
errorsfrominstabilityinthemassofthesamplingmediumand
for Performance of Procedures for the Measurement of
other elements that must be weighed. An example is provided
Chemical Agents
by aerosol samplers designed to collect particles so as to agree
prEN 13205Workplace Atmospheres—Assessment of Per-
with the inhalable aerosol sampling convention (see ISO TR
formance of Instruments for Measurement of Airborne
7708, Guide D6062, and EN 481). For some sampler types,
Particle Concentrations
filter and cassette are weighed together to make estimates.
Therefore, if the cassette, for example, absorbs or loses water
2.3 ISO Standards:
betweentheweighingsrequiredforaconcentrationestimation,
ISO TR 7708Air Quality—Particle Size Fraction Defini-
thenerrorsmayarise.Thispracticecoverssuchpotentialerrors
tions for Health-related Sampling
and provides solutions for their minimization.
ISO GUMGuide to the Expression of Uncertainty in Mea-
surement (1993)
1.2 The values given in SI units are to be regarded as
ISO 20988Air Quality—Guidelines for Estimating Mea-
standard.
surement Uncertainty
1.3 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
3. Terminology
responsibility of the user of this standard to establish appro-
3.1 Definitions:
priate safety and health practices and determine the applica-
3.1.1 For definitions of terms used in this practice, refer to
bility of regulatory limitations prior to use.
Terminology D1356.
2. Referenced Documents
3.2 Definitions of Terms Specific to This Standard:
3.2.1 blank substrate—a collection medium or substrate
2.1 ASTM Standards:
coming from the same batch as the sampling medium, but
D1356Terminology Relating to Sampling and Analysis of
unexposed.
Atmospheres
3.2.2 equilibrationtime—Forthepurposesofthispractice,a
time constant (seconds) characterizing an approximate expo-
1 nentiallydampedapproachofthemassofanaerosolcollection
ThispracticeisunderthejurisdictionofASTMCommitteeD22onAirQuality
and is the direct responsibility of Subcommittee D22.04 on WorkplaceAir Quality. mediumtoaconstantvalue.Theconstantcanbedefinedasthe
Current edition approved Oct. 1, 2011. Published October 2011. Originally
approved in 2000. Last previous edition approved in 2006 as D6552-06. DOI:
10.1520/D6552-06R11.
2 3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available from CEN Central Secretariat: rue de Stassart 36, B-1050 Brussels,
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Belgium.
Standards volume information, refer to the standard’s Document Summary page on Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
the ASTM website. 4th Floor, New York, NY 10036, http://www.ansi.org.
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D6552 − 06 (2011)
mean difference of the mass from equilibrium per mean mass
N = number of blanks per substrate set
b
loss or gain rate as measured over a finite time interval.
ν = number of degrees of freedom in method
3.2.2.1 Discussion—There may be important instances in
evaluation
Φ = cumulative normal function
which several time constants are required to describe the
χ = chi-square random variable
approach to equilibrium.
χ = chi-square quantile (that is, a fixed number
γ,ν
3.2.3 estimated overall uncertainty (U)—2×estimatedstan-
that exceeds the random variable χ at prob-
dard deviation of estimated mass, in the case of negligible
ability γ)
uncorrectable bias (see EN 482).
RH = relative humidity
3.2.4 field blank—a blank substrate that undergoes the same u (µg) = uncertainty component in two balance
handling as the sample substrate, generally including condi-
readings, an estimate of σ
u (µg) = weighing uncertainty, estimate of σ
tioningandloadingintothesamplersortransportcontainers,as
w w
σ (µg) = uncorrectable (for example, by way of blank
well as transportation to the sampling site, but without being
correction) standard deviation in (single)
exposed.
mass-change measurement
3.2.4.1 Discussion—If blanks are not actually loaded into
σ (µg) = confidence limit on σ
1-γ
samplers, losses due to handling could be underestimated.
σ (µg) = standard deviation in collected mass deter-
w
3.2.5 lab blank—a blank substrate that undergoes the same
mination
handling as the sample substrate in the laboratory, including
U = overall uncertainty
conditioning and loading into the samplers or transport con-
tainers when this is done in the laboratory.
4. Significance and Use
3.2.6 limit of detection (LOD)—a value for which ex-
4.1 The weighing of collected aerosol is one of the most
ceedence by measured mass indicates the presence of a
common and purportedly simple analytical procedures in both
substance at given false-positive rate: 3 × estimated standard
occupational and environmental atmospheric monitoring (for
deviation of the measured blank substrate mass (see Annex
example, Test Method D4532 or D4096). Problems with
A2).
measurement accuracy occur when the amount of material
collected is small, owing both to balance inaccuracy and
3.2.7 limit of quantitation (LOQ)—a value for which ex-
variationintheweightofthatpartofthesamplingmediumthat
ceedence by measured mass indicates the quantitation of a
is weighed along with the sample. The procedures presented
substanceatgivenaccuracy:10×estimatedstandarddeviation
here for controlling and documenting such analytical errors
of the measured blank substrate mass (see Annex A2).
will help provide the accuracy required for making well-
3.2.8 substrate—sampling filter, foam, and so forth together
founded decisions in identifying, characterizing, and control-
with whatever mounting is weighed as a single item.
ling hazardous conditions.
3.2.8.1 Discussion—The 25 or 37-mm plastic filter cassette
4.2 Recommendations are given as to materials to be used.
often used for total dust sampling in either its closed-face or
Means of controlling or correcting errors arising from insta-
open-face version is NOTpart of the substrate in the definition
bility are provided. Recommendations as to the weighing
above, since it is not weighed.
procedure are given. Finally, a method evaluation procedure
3.3 Symbols:
for estimating weighing errors is described.
α = detection error rate 4.3 Recommendations are also provided for the reporting of
B = number of substrate batches in method weights relative to LOD (see 3.2.6) and LOQ (see 3.2.7). The
evaluation
quantities, LOD and LOQ, are computed as a result of the
b = batch index (1, ., B)
method evaluation.
β = mean substrate mass change during evalua-
tion experiment 5. Weight Instability, Causes, and Minimization
CV = maximum relative error acceptable in quan-
max
5.1 Weight instability of sampling substrates may be attrib-
tifying collected mass
uted to several causes. The following subclauses address the
∆m (µg) = substrate mass change
fb
more important of these.
ε (µg) = substrate weight-change random variable
b
5.1.1 Moisture Sorption:
representing inter-batch variability
5.1.1.1 Moisture sorption is the most common cause of
ε (µg) = substrate weight change residual random
fb
2 weight instability. Water may be directly collected by the filter
variable with variance σ
or foam or other substrate material that is weighed. Water
f = substrate index (1, ., F)
sorption by any part of the sampling system that is weighed
F = number of substrates (for example, filters) in
must be suspected as well. For example, the sampling cassette
each batch tested in method evaluation
itself,ifweighed,maybethecauseofsignificanterror (1) (see
γ = method evaluation error rate
also 8.2.2).
LOD (µg) = limit of detection:3×s
w
LOD (µg) = LOD confidence limit
1-γ
LOQ (µg) = limit of quantitation: 10 × s
w
The boldface numbers in parentheses refer to the list of references at the end of
LOQ (µg) = LOQ confidence limit
1-γ
this standard.
D6552 − 06 (2011)
5.1.1.2 The effects of water sorption can be reduced by 5.1.4.1 The air sampling equipment should be designed so
using nonsorptive materials. However, there may exist specific that the substrate is not damaged during assembly and disas-
sampling needs for which a hydrophobic material is not sembly.
feasible. Table 1 presents a list of common aerosol sampling 5.1.4.2 Flat tipped forceps are recommended for handling
substrates with different water adsorption features. filters. Nonoxidizing metal tins may be used to weigh delicate
substrates without direct handling.
NOTE 1—Gonzalez-Fernandez, Kauffer et al, and Lippmann (2-4)
5.1.4.3 Parts to be weighed shall not be touched with the
provide further details. Also, Vaughan et al (5) report that filters of
hands, unless gloved.
evidently the same material, but originating from different manufacturers,
may have widely differing variabilities.
5.1.4.4 Handling shall take place in a clean environment to
NOTE 2—There is generally a trade-off between hydrophobicity and
avoid contamination.
conductivity in many materials (6). Therefore, one must be aware of the
5.1.4.5 Gloves, if used, shall leave no residue on what is
possibility of creating sampling problems while reducing hygroscopicity.
weighed.
NOTE 3—Pretreatments of substrates, such as greasing, may also affect
5.1.5 Buoyancy Changes—Corrections (9) for air buoyancy,
water sorption.
equal to the density of air multiplied by the air volume
5.1.2 Electrostatic Effects—Electrostatic effects are a com-
displaced,arenotnecessaryforsmallobjects,suchasa37-mm
monsourceofweighingproblems.Theseeffectscanusuallybe
diameter membrane filter. However, there may exist circum-
minimized by discharging the substrate through the use of a
stances (for example, if an entire sampling cassette were
plasma ion source or a radioactive source. Using conductive
weighed without the use of correcting blanks) in which the
materials may reduce such problems. Lawless and Rodes (7)
object to be weighed is so large that buoyancy must be
present details on electrostatic effects and their minimization
corrected. For example, if the volume weighed exceeds 0.1
(see also Engelbrecht et al (8)).
cm , then correction would be required to weigh down to 0.1
5.1.3 Effects of Volatile Compounds (other than water)—
mgifpressurechangesoftheorderof10%betweenweighings
Volatilecompoundsmaybepresentinunusedcollectionmedia
are expected. If such a correction is necessary, the atmospheric
(3) or may be adsorbed onto media during sampling.
pressure and temperature at the time of weighing should be
5.1.3.1 Desorption of volatiles from unused media may be
recorded.
controlled,forexample,byheatingoroxygenplasmatreatment
prior to conditioning and weighing. Alternatively, losses may
6. Correcting for Weight Instability
be compensated by the use of blanks (see Section 6).
6.1 Recommended Method for Correction by Use of
5.1.3.2 When volatile materials collected during sampling
Blanks—The use of blanks is the most important practical tool
form part of the intended sample, standardized written proce-
for reducing errors due to weight instability. Correction for
duresarerequiredtoensurethatanylossesareminimizedorat
weight instability depends on the specific application and
least controlled, for example, by conditioning under tightly
should follow a written procedure. The general principles are
specified conditions.
as follows. Blank sampling media are exposed, as closely as
NOTE4—Whenvolatilematerialscollectedduringsamplingarenotpart possible, to the same conditions as the active sampling media,
of the intended sample, it may be difficult to eliminate them if weighing
without actually drawing air through. Correction is effected by
istheonlyformofanalysis.Preferablynonsorptivemediashouldbeused.
subtractingtheaverageblankweightgainfromtheweightgain
5.1.4 Handling Damage—Lawless and Rodes (7) give rec- of the active samples. Of course, if the atmosphere to be
ommendations on minimizing balance-operator effects. If fri-
sampledcontainswater(orothervolatile)droplets,thentheuse
able substrates are used, procedures are needed to avoid of blanks alone cannot correct. Kauffer et al (3) note that
mechanical damage during gravimetric analysis.
blanks may also offer correction for filter material losses.
Blanks shall be matched to samples, that is, if the sample
consists of a filter within a cassette that is weighed, the blank
shallbethesametypeoffilterwithinthesametypeofcassette.
6.1.1 An alternative procedure employs matched weight
TABLE 1 Water Sorption Characteristics of Some Aerosol
Sampling Media
filters consisting of two nearly equal-weight filters, one placed
in front of the other, with the sampler following employed as
Substrate or Cassette Type Very Low Low High Very High
Cellulose fiber filter *
blank. The collected mass is estimated simply by subtracting
Glass fiber filter *
thefiltermassesfollowingsampling.Analysisofuncertaintyis
Quartz fiber filter *
Cellulose ester membrane filter * similartothepresentationhere,butalsoinvolvesestimationof
Polytetrafluoroethylene filter *
the uncertainty of the filter matching.
PVC membrane filter *
Polycarbonate filter *
6.2 Minimum Number of Blanks—Generally, at least one
Silver membrane filter
...
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