1、Designation: D6552 06 (Reapproved 2011)Standard Practice forControlling and Characterizing Errors in Weighing CollectedAerosols1This standard is issued under the fixed designation D6552; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revisi
2、on, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 Assessment of airborne aerosol hazards in the occupa-tional setting entails sampling onto a collection
3、 mediumfollowed by analysis of the collected material. The result isgenerally an estimated concentration of a possibly hazardousmaterial in the air. The uncertainty in such estimates dependson several factors, one of which relates to the specific type ofanalysis employed. The most commonly applied m
4、ethod foranalysis of aerosols is the weighing of the sampled material.Gravimetric analysis, though apparently simple, is subject toerrors from instability in the mass of the sampling medium andother elements that must be weighed. An example is providedby aerosol samplers designed to collect particle
5、s so as to agreewith the inhalable aerosol sampling convention (see ISO TR7708, 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 waterbetween the weighings required for a concentratio
6、n estimation,then errors may arise. This practice covers such potential errorsand provides solutions for their minimization.1.2 The values given in SI units are to be regarded asstandard.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is t
7、heresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D1356 Terminology Relating to Sampling and Analysis ofAtmospheresD4096 Test Method fo
8、r Determination of Total SuspendedParticulate Matter in the Atmosphere (HighVolume Sam-pler Method)D4532 Test Method for Respirable Dust in WorkplaceAtmospheres Using Cyclone SamplersD6062 Guide for Personal Samplers of Health-RelatedAerosol Fractions2.2 International Standards:3EN 481 WorkplaceAtmo
9、spheresSize Fraction Definitionsfor Measurement of Airborne Particles in the WorkplaceEN 482 Workplace AtmospheresGeneral Requirementsfor Performance of Procedures for the Measurement ofChemical AgentsprEN 13205 Workplace AtmospheresAssessment of Per-formance of Instruments for Measurement of Airbor
10、neParticle Concentrations2.3 ISO Standards:4ISO TR 7708 Air QualityParticle Size Fraction Defini-tions for Health-related SamplingISO GUM Guide to the Expression of Uncertainty inMeasurement (1993)ISO 20988 Air QualityGuidelines for Estimating Mea-surement Uncertainty3. Terminology3.1 Definitions:3.
11、1.1 For definitions of terms used in this practice, refer toTerminology D1356.3.2 Definitions of Terms Specific to This Standard:3.2.1 blank substratea collection medium or substratecoming from the same batch as the sampling medium, butunexposed.3.2.2 equilibration timeFor the purposes of this pract
12、ice,a time constant (seconds) characterizing an approximate expo-nentially damped approach of the mass of an aerosol collectionmedium to a constant value. The constant can be defined as the1This practice is under the jurisdiction ofASTM Committee D22 onAir Qualityand is the direct responsibility of
13、Subcommittee D22.04 on Workplace Air Quality.Current edition approved Oct. 1, 2011. Published October 2011. Originallyapproved in 2000. Last previous edition approved in 2006 as D6552 - 06. DOI:10.1520/D6552-06R11.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM C
14、ustomer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from CEN Central Secretariat: rue de Stassart 36, B-1050 Brussels,Belgium.4Available from American National Standards Institute (ANSI),
15、25 W. 43rd St.,4th Floor, New York, NY 10036, http:/www.ansi.org.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.mean difference of the mass from equilibrium per mean massloss or gain rate as measured over a finite time interval.3.2.
16、2.1 DiscussionThere may be important instances inwhich several time constants are required to describe theapproach to equilibrium.3.2.3 estimated overall uncertainty (U)2 3 estimatedstandard deviation of estimated mass, in the case of negligibleuncorrectable bias (see EN 482).3.2.4 field blanka blan
17、k substrate that undergoes the samehandling as the sample substrate, generally including condi-tioning and loading into the samplers or transport containers, aswell as transportation to the sampling site, but without beingexposed.3.2.4.1 DiscussionIf blanks are not actually loaded intosamplers, loss
18、es due to handling could be underestimated.3.2.5 lab blanka blank substrate that undergoes the samehandling as the sample substrate in the laboratory, includingconditioning and loading into the samplers or transport con-tainers when this is done in the laboratory.3.2.6 limit of detection (LOD)a valu
19、e for which ex-ceedence by measured mass indicates the presence of asubstance at given false-positive rate: 3 3 estimated standarddeviation of the measured blank substrate mass (see AnnexA2).3.2.7 limit of quantitation (LOQ)a value for which ex-ceedence by measured mass indicates the quantitation of
20、 asubstance at given accuracy: 10 3 estimated standard deviationof the measured blank substrate mass (see Annex A2).3.2.8 substratesampling filter, foam, and so forth togetherwith whatever mounting is weighed as a single item.3.2.8.1 DiscussionThe 25 or 37-mm plastic filter cassetteoften used for to
21、tal dust sampling in either its closed-face oropen-face version is NOT part of the substrate in the definitionabove, since it is not weighed.3.3 Symbols:a = detection error rateB = number of substrate batches in methodevaluationb = batch index (1, ., B)b = mean substrate mass change during evalu-ati
22、on experimentCVmax= maximum relative error acceptable inquantifying collected massDmfb(g) = substrate mass changeb(g) = substrate weight-change random variablerepresenting inter-batch variabilityfb(g) = substrate weight change residual randomvariable with variance s2f = substrate index (1, ., F)F =
23、number of substrates (for example, filters)in each batch tested in method evaluationg = method evaluation error rateLOD (g) = limit of detection: 3 3 swLOD1-g(g) = LOD confidence limitLOQ (g) = limit of quantitation: 10 3 swLOQ1-g(g) = LOQ confidence limitNb= number of blanks per substrate setn = nu
24、mber of degrees of freedom in methodevaluationF = cumulative normal functionx2= chi-square random variablexg,n2= chi-square quantile (that is, a fixed numberthat exceeds the random variable x2atprobability g)RH = relative humidityu (g) = uncertainty component in two balancereadings, an estimate of s
25、uw(g) = weighing uncertainty, estimate of sws (g) = uncorrectable (for example, by way ofblank correction) standard deviation in(single) mass-change measurements1-g(g) = confidence limit on ssw(g) = standard deviation in collected mass deter-minationU = overall uncertainty4. Significance and Use4.1
26、The weighing of collected aerosol is one of the mostcommon and purportedly simple analytical procedures in bothoccupational and environmental atmospheric monitoring (forexample, Test Method D4532 or D4096). Problems withmeasurement accuracy occur when the amount of materialcollected is small, owing
27、both to balance inaccuracy andvariation in the weight of that part of the sampling medium thatis weighed along with the sample. The procedures presentedhere for controlling and documenting such analytical errorswill help provide the accuracy required for making well-founded decisions in identifying,
28、 characterizing, and control-ling hazardous conditions.4.2 Recommendations are given as to materials to be used.Means of controlling or correcting errors arising from insta-bility are provided. Recommendations as to the weighingprocedure are given. Finally, a method evaluation procedurefor estimatin
29、g weighing errors is described.4.3 Recommendations are also provided for the reporting ofweights relative to LOD (see 3.2.6) and LOQ (see 3.2.7). Thequantities, LOD and LOQ, are computed as a result of themethod evaluation.5. Weight Instability, Causes, and Minimization5.1 Weight instability of samp
30、ling substrates may be attrib-uted to several causes. The following subclauses address themore important of these.5.1.1 Moisture Sorption:5.1.1.1 Moisture sorption is the most common cause ofweight instability. Water may be directly collected by the filteror foam or other substrate material that is
31、weighed. Watersorption by any part of the sampling system that is weighedmust be suspected as well. For example, the sampling cassetteitself, if weighed, may be the cause of significant error (1)5(seealso 8.2.2).5.1.1.2 The effects of water sorption can be reduced byusing nonsorptive materials. Howe
32、ver, there may exist specific5The boldface numbers in parentheses refer to the list of references at the end ofthis standard.D6552 06 (2011)2sampling needs for which a hydrophobic material is notfeasible. Table 1 presents a list of common aerosol samplingsubstrates with different water adsorption fe
33、atures.NOTE 1Gonzalez-Fernandez, Kauffer et al, and Lippmann (2-4)provide further details. Also, Vaughan et al (5) report that filters ofevidently the same material, but originating from different manufacturers,may have widely differing variabilities.NOTE 2There is generally a trade-off between hydr
34、ophobicity andconductivity in many materials (6). Therefore, one must be aware of thepossibility of creating sampling problems while reducing hygroscopicity.NOTE 3Pretreatments of substrates, such as greasing, may also affectwater sorption.5.1.2 Electrostatic EffectsElectrostatic effects are a com-m
35、on source of weighing problems. These effects can usually beminimized by discharging the substrate through the use of aplasma ion source or a radioactive source. Using conductivematerials may reduce such problems. Lawless and Rodes (7)present details on electrostatic effects and their minimization(s
36、ee also Engelbrecht et al (8).5.1.3 Effects of Volatile Compounds (other than water)Volatile compounds may be present in unused collection media(3) or may be adsorbed onto media during sampling.5.1.3.1 Desorption of volatiles from unused media may becontrolled, for example, by heating or oxygen plas
37、ma treatmentprior to conditioning and weighing. Alternatively, losses maybe compensated by the use of blanks (see Section 6).5.1.3.2 When volatile materials collected during samplingform part of the intended sample, standardized written proce-dures are required to ensure that any losses are minimize
38、d or atleast controlled, for example, by conditioning under tightlyspecified conditions.NOTE 4When volatile materials collected during sampling are notpart of the intended sample, it may be difficult to eliminate them ifweighing is the only form of analysis. Preferably nonsorptive mediashould be use
39、d.5.1.4 Handling DamageLawless and Rodes (7) give rec-ommendations on minimizing balance-operator effects. If fri-able substrates are used, procedures are needed to avoidmechanical damage during gravimetric analysis.5.1.4.1 The air sampling equipment should be designed sothat the substrate is not da
40、maged during assembly and disas-sembly.5.1.4.2 Flat tipped forceps are recommended for handlingfilters. Nonoxidizing metal tins may be used to weigh delicatesubstrates without direct handling.5.1.4.3 Parts to be weighed shall not be touched with thehands, unless gloved.5.1.4.4 Handling shall take pl
41、ace in a clean environment toavoid contamination.5.1.4.5 Gloves, if used, shall leave no residue on what isweighed.5.1.5 Buoyancy ChangesCorrections (9) for air buoyancy,equal to the density of air multiplied by the air volumedisplaced, are not necessary for small objects, such as a 37-mmdiameter me
42、mbrane filter. However, there may exist circum-stances (for example, if an entire sampling cassette wereweighed without the use of correcting blanks) in which theobject to be weighed is so large that buoyancy must becorrected. For example, if the volume weighed exceeds 0.1cm3, then correction would
43、be required to weigh down to 0.1mg if pressure changes of the order of 10 % between weighingsare expected. If such a correction is necessary, the atmosphericpressure and temperature at the time of weighing should berecorded.6. Correcting for Weight Instability6.1 Recommended Method for Correction by
44、 Use ofBlanksThe use of blanks is the most important practical toolfor reducing errors due to weight instability. Correction forweight instability depends on the specific application andshould follow a written procedure. The general principles areas follows. Blank sampling media are exposed, as clos
45、ely aspossible, to the same conditions as the active sampling media,without actually drawing air through. Correction is effected bysubtracting the average blank weight gain from the weight gainof the active samples. Of course, if the atmosphere to besampled contains water (or other volatile) droplet
46、s, then the useof blanks alone cannot correct. Kauffer et al (3) note thatblanks may also offer correction for filter material losses.Blanks shall be matched to samples, that is, if the sampleconsists of a filter within a cassette that is weighed, the blankshall be the same type of filter within the
47、 same type of cassette.6.1.1 An alternative procedure employs matched weightfilters consisting of two nearly equal-weight filters, one placedin front of the other, with the sampler following employed asblank. The collected mass is estimated simply by subtractingthe filter masses following sampling.A
48、nalysis of uncertainty issimilar to the presentation here, but also involves estimation ofthe uncertainty of the filter matching.6.2 Minimum Number of BlanksGenerally, at least oneblank is recommended for each ten samples. Measurementschemes in current use require between one and four blanks perbatc
49、h. See A1.1 for advantages of multiple blanks.6.3 Weighing Times and SequenceBlanks shall be inter-spersed with samples, before and after use, so as to detectsystematic variations in mass (for example, due to sorption orevaporation of a contaminant during weighing).6.4 Conditioning TimesConditioning times for reachingequilibrium with the weighing environment may vary from afew hours to several weeks, depending on the specific samplingTABLE 1 Water Sorption Characteristics of Some AerosolSampling MediaSubstrate or Cassette Type Very Low Low
