ASTM D6552-00
(Practice)Standard Practice for Controlling and Characterizing Errors in Weighing Collected Aerosols
Standard Practice for Controlling and Characterizing Errors in Weighing Collected Aerosols
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 accuracy of 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 D 6062M, 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 the standard.
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:D6552–00
Standard Practice for
Controlling and Characterizing Errors in Weighing Collected
Aerosols
This standard is issued under the fixed designation D 6552; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope D 4532 Test Method for Respirable Dust in Workplace
Atmospheres
1.1 Assessment of airborne aerosol hazards in the occupa-
D 6062M Guide for Personal Samplers of Health-Related
tional setting entails sampling onto a collection medium
Aerosol Fractions (Metric)
followed by analysis of the collected material. The result is
2.2 International Standards:
generally an estimated concentration of a possibly hazardous
EN481 WorkplaceAtmospheres—SizeFractionDefinitions
material in the air. The accuracy of such estimates depends on
for Measurement of Airborne Particles in the Workplace
several factors, one of which relates to the specific type of
EN 482 Workplace Atmospheres—General Requirements
analysis employed. The most commonly applied method for
for Performance of Procedures for the Measurement of
analysis of aerosols is the weighing of the sampled material.
Chemical Agents
Gravimetric analysis, though apparently simple, is subject to
prEN 13205 Workplace Atmospheres—Assessment of Per-
errors from instability in the mass of the sampling medium and
formance of Instruments for Measurement of Airborne
other elements that must be weighed. An example is provided
Particle Concentrations
by aerosol samplers designed to collect particles so as to agree
2.3 ISO Standards:
with the inhalable aerosol sampling convention (see ISO TR
ISO TR 7708 Air Quality—Particle Size Fraction Defini-
7708, Guide D 6062M, and EN 481). For some sampler types,
tions for Health-related Sampling
filter and cassette are weighed together to make estimates.
Therefore, if the cassette, for example, absorbs or loses water
3. Terminology
between the weighings required for a concentration estimation,
3.1 Definitions:
thenerrorsmayarise.Thispracticecoverssuchpotentialerrors
3.1.1 For definitions of terms used in this practice, refer to
and provides solutions for their minimization.
Terminology D 1356.
1.2 The values given in SI units are to be regarded as the
3.2 Definitions of Terms Specific to This Standard:
standard.
3.2.1 blank substrate—a collection medium or substrate
1.3 This standard does not purport to address all of the
coming from the same batch as the sampling medium, but
safety concerns, if any, associated with its use. It is the
unexposed.
responsibility of the user of this standard to establish appro-
3.2.2 equilibration time—For the purposes of this practice,
priate safety and health practices and determine the applica-
a time constant (seconds) characterizing an approximate expo-
bility of regulatory limitations prior to use.
nentially damped approach of the mass of an aerosol collection
2. Referenced Documents medium to a constant value.The constant can be defined as the
mean difference of the mass from equilibrium per mean mass
2.1 ASTM Standards:
loss or gain rate as measured over a finite time interval.
D 1356 Terminology Relating to Sampling and Analysis of
3.2.2.1 Discussion—There may be important instances in
Atmospheres
which several time constants are required to describe the
D 4096 Test Method for Determination of Total Suspended
approach to equilibrium.
Particulate Matter in the Atmosphere (High-Volume Sam-
pler Method)
This test method is under the jurisdiction of ASTM Committee D22 on
Sampling and Analysis of Atmospheres and is the direct responsibility of Subcom- Available from CEN Central Secretariat: rue de Stassart 36, B-1050 Brussels,
mittee D22.04 on Workplace Atmospheres. Belgium.
4 nd th
Current edition approved May 10, 2000. Published July 2000. Available from American National Standards Institute, 11 W 42 Street, 13
Annual Book of ASTM Standards, Vol 11.03. Floor, New York, NY 10017.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6552
3.2.3 estimated overall uncertainty (U)—2 3 estimated
x = chi-square quantile (that is, a fixed number
g,n
standard deviation of estimated mass, in the case of negligible
that exceeds the random variable x at
uncorrectable bias (see EN 482).
probability g)
RH = relative humidity
3.2.4 field blank—a blank substrate that undergoes the same
s (µg) = estimate of s
handling as the sample substrate, generally including condi-
s (µg) = estimate of s
w w
tioningandloadingintothesamplersortransportcontainers,as
s (µg) = uncorrectable (for example, by way of
well as transportation to the sampling site, but without being
blank correction) standard deviation in
exposed.
(single) mass-change measurement
3.2.4.1 Discussion—If blanks are not actually loaded into
s (µg) = confidence limit on s
1-g
samplers, losses due to handling could be underestimated.
s (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-
4. Significance and Use
tainers when this is done in the laboratory.
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 3 estimated standard occupational and environmental atmospheric monitoring (for
example, Test Method D 4532 or D 4096). Problems with
deviation of the measured blank substrate mass (see Annex
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 3estimatedstandarddeviation
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 NOT part 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.
4.3 Recommendations are also provided for the reporting of
weights relative to LOD (see 3.2.6) and LOQ (see 3.2.7). The
a = detection error rate
quantities, LOD and LOQ, are computed as a result of the
B = number of substrate batches in method
method evaluation.
evaluation
b = batch index (1, ., B)
5. Weight Instability, Causes, and Minimization
b = mean substrate mass change during evalu-
5.1 Weight instability of sampling substrates may be attrib-
ation experiment
CV = maximum relative error acceptable in uted to several causes. The following subclauses address the
max
quantifying collected mass more important of these.
Dm (µg) = substrate mass change
5.1.1 Moisture Sorption:
fb
e (µg) = substrate weight-change random variable
5.1.1.1 Moisture sorption is the most common cause of
b
representing inter-batch variability
weight instability. Water may be directly collected by the filter
e (µg) = substrate weight change residual random
fb or foam or other substrate material that is weighed. Water
variable with variance s
sorption by any part of the sampling system that is weighed
f = substrate index (1, ., F)
must be suspected as well. For example, the sampling cassette
F = number of substrates (for example, filters) 5
itself,ifweighed,maybethecauseofsignificanterror(1) (see
in each batch tested in method evaluation
also 8.2.2).
g = method evaluation error rate
5.1.1.2 The effects of water sorption can be reduced by
LOD (µg) = limit of detection: 3 3 s
w
using nonsorptive materials. However, there may exist specific
LOD (µg) = LOD confidence limit
1-g
sampling needs for which a hydrophobic material is not
LOQ (µg) = limit of quantitation: 10 3 s
w
feasible. Table 1 presents a list of common aerosol sampling
LOQ (µg) = LOQ confidence limit
1-g
substrates with different water adsorption features.
N = number of blanks per substrate set
b
n = number of degrees of freedom in method
evaluation
F = cumulative normal function
2 The boldface numbers in parentheses refer to the list of references at the end of
x = chi-square random variable
this standard.
D6552
TABLE 1 Water Sorption Characteristics of Some Aerosol
5.1.4.3 Parts to be weighed shall not be touched with the
Sampling Media
hands, unless gloved.
Substrate or Cassette Type Very Low Low High Very High
5.1.4.4 Handling shall take place in a clean environment to
Cellulose fiber filter *
avoid contamination.
Glass fiber filter *
5.1.4.5 Gloves, if used, shall leave no residue on what is
Quartz fiber filter *
weighed.
Cellulose ester membrane filter *
Polytetrafluoroethylene filter * 5.1.5 Buoyancy Changes—Corrections(9) for air buoyancy,
PVC membrane filter *
equal to the density of air multiplied by the air volume
Polycarbonate filter *
displaced, are not necessary for small objects, such as a 37-mm
Silver membrane filter *
Polyurethane foam *
diameter membrane filter. However, there may exist circum-
Greased polyester film impaction *
stances (for example, if an entire sampling cassette were
substrate
weighed without the use of correcting blanks) in which the
Greased aluminum foil impaction *
substrate
object to be weighed is so large that buoyancy must be
Carbon-filled resin *
corrected. For example, if the volume weighed exceeds 0.1
Aluminum cassette *
cm , then correction would be required to weigh down to 0.1
Stainless steel cassette *
mgifpressurechangesoftheorderof10 %betweenweighings
are expected. If such a correction is necessary, the atmospheric
pressure and temperature at the time of weighing should be
NOTE 1—Gonzalez-Fernandez, Kauffer et al, and Lippmann (2-4)
recorded.
provide further details. Also, Vaughan et al (5) report that filters of
evidently the same material, but originating from different manufacturers,
6. Correcting for Weight Instability
may have widely differing variabilities.
NOTE 2—There is generally a trade-off between hydrophobicity and
6.1 Recommended Method for Correction by Use of
conductivity in many materials (6). Therefore, one must be aware of the
Blanks—The use of blanks is the most important practical tool
possibility of creating sampling problems while reducing hygroscopicity.
for reducing errors due to weight instability. Correction for
NOTE 3—Pretreatments of substrates, such as greasing, may also affect
weight instability depends on the specific application and
water sorption.
should follow a written procedure. The general principles are
5.1.2 Electrostatic Effects—Electrostatic effects are a com-
as follows. Blank sampling media are exposed, as closely as
monsourceofweighingproblems.Theseeffectscanusuallybe
possible, to the same conditions as the active sampling media,
minimized by discharging the substrate through the use of a
without actually drawing air through. Correction is effected by
plasma ion source or a radioactive source. Using conductive
subtracting the average blank weight gain from the weight gain
materials may reduce such problems. Lawless and Rodes (7)
of the active samples. Of course, if the atmosphere to be
present details on electrostatic effects and their minimization
sampledcontainswater(orothervolatile)droplets,thentheuse
(see also Engelbrecht et al (8)).
of blanks alone cannot correct. Kauffer et al (3) note that
5.1.3 Effects of Volatile Compounds (other than water)—
blanks may also offer correction for filter material losses.
Volatile compounds may be present in unused collection media
Blanks shall be matched to samples, that is, if the sample
(3) or may be adsorbed onto media during sampling.
consists of a filter within a cassette that is weighed, the blank
5.1.3.1 Desorption of volatiles from unused media may be
shall be the same type of filter within the same type of cassette.
controlled,forexample,byheatingoroxygenplasmatreatment
6.2 Minimum Number of Blanks—Generally, at least one
prior to conditioning and weighing. Alternatively, losses may
blank is recommended for each ten samples. Measurement
be compensated by the use of blanks (see Section 6).
schemesincurrentuserequirebetweenoneandfourblanksper
5.1.3.2 When volatile materials collected during sampling
batch. See A1.1 for advantages of multiple blanks.
form part of the intended sample, standardized written proce-
6.3 Weighing Times and Sequence—Blanks shall be inter-
dures are required to ensure that any losses are minimized or at
spersed with samples, before and after use, so as to detect
least controlled, for example, by conditioning under tightly
systematic variations in mass (for example, due to sorption or
specified conditions.
evaporation of a contaminant during weighing).
6.4 Conditioning Times—Conditioning times for reaching
NOTE 4—When volatile materials collected during sampling are not
part of the intended sample, it may be difficult to eliminate them if equilibrium with the weighing environment may vary from a
weighing is the only form of analysis. Preferably nonsorptive media
fewhourstoseveralweeks,dependingonthespecificsampling
should be used.
media. Typically, for workplace sampling applications, over-
5.1.4 Handling Damage—Lawless and Rodes (7) give rec- night conditioning is satisfactory. For sampling media with
ommendations on minimizing balance-operator effects. If fri- longer conditioning times, error correction through the use of
able substrates are used, procedures are needed to avoid blank substrates is particularly important. Charell and Hawley
mechanical damage during gravimetric analysis. (10) indicate that extremely short equilibration times exist for
5.1.4.1 The air sampling equipment should be desi
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