1、Designation: D 6552 06Standard Practice forControlling and Characterizing Errors in Weighing CollectedAerosols1This standard is issued under the fixed designation D 6552; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of
2、last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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 mediumfollowed
3、 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 method foranalys
4、is 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 particles so as to agre
5、ewith the inhalable aerosol sampling convention (see ISO TR7708, 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 waterbetween the weighings required for a concentration estimation,
6、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 theresponsibil
7、ity 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:2D 1356 Terminology Relating to Sampling and Analysis ofAtmospheresD 4096 Test Method for Determina
8、tion of Total SuspendedParticulate Matter in the Atmosphere (HighVolume Sam-pler Method)D 4532 Test Method for Respirable Dust in WorkplaceAtmospheresD 6062M Guide for Personal Samplers of Health-RelatedAerosol Fractions Metric2.2 International Standards:3EN 481 WorkplaceAtmospheresSize Fraction Def
9、initionsfor 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 AirborneParticle Concentration
10、s2.3 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:3.1.1 For definitions of t
11、erms used in this practice, refer toTerminology D 1356.3.2 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 practice,a time constant (se
12、conds) characterizing an approximate expo-nentially damped approach of the mass of an aerosol collectionmedium to a constant value. The constant can be defined as themean difference of the mass from equilibrium per mean massloss or gain rate as measured over a finite time interval.3.2.2.1 Discussion
13、There may be important instances inwhich several time constants are required to describe theapproach to equilibrium.1This practice is under the jurisdiction ofASTM Committee D22 onAir Qualityand is the direct responsibility of Subcommittee D22.04 onWorkplaceAtmospheres.Current edition approved April
14、 1, 2006. Published May 2006. Originallyapproved in 2000. Last previous edition approved in 2000 as D 6552 - 00.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the
15、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), 25 W. 43rd St.,4th Floor, New York, NY 10036.1Copyright ASTM International, 100 Barr Harbor Drive, PO
16、Box C700, West Conshohocken, PA 19428-2959, United States.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 blank substrate that undergoes the samehandling as the sample substrate, gen
17、erally 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, losses due to handling could be underestimated.3.2.5 lab blanka blank substr
18、ate 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 value for which ex-ceedence by measured mass indicates the presence of asubs
19、tance 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 asubstance at given accuracy: 10 3 estimated standard deviationof the m
20、easured 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 total dust sampling in either its closed-face oropen-face version is NOT p
21、art 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-ation experimentCVmax= maximum relative error acceptable inquantifying coll
22、ected massDmfb(g) = substrate mass changeeb(g) = substrate weight-change random variablerepresenting inter-batch variabilityefb(g) = substrate weight change residual randomvariable with variance s2f = substrate index (1, ., F)F = number of substrates (for example, filters)in each batch tested in met
23、hod 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 = number of degrees of freedom in methodevaluationF = cumulative normal fu
24、nctionx2= 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 suw(g) = weighing uncertainty, estimate of sws (g) = uncorrectable (for
25、 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 The weighing of collected aerosol is one of the mostcommon and purport
26、edly simple analytical procedures in bothoccupational and environmental atmospheric monitoring (forexample, Test Method D 4532 or D 4096). Problems withmeasurement accuracy occur when the amount of materialcollected is small, owing both to balance inaccuracy andvariation in the weight of that part o
27、f 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, characterizing, and control-ling hazardous conditions.4.2 Recommend
28、ations 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 estimating weighing errors is described.4.3 Recommendations are also provided
29、 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 sampling substrates may be attrib-uted to several causes. The following
30、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 weighed. Watersorption by any part of the sampling system that is we
31、ighedmust 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. However, there may exist specificsampling needs for which a hydrophobic
32、material is notfeasible. Table 1 presents a list of common aerosol samplingsubstrates with different water adsorption features.5The boldface numbers in parentheses refer to the list of references at the end ofthis standard.D6552062NOTE 1Gonzalez-Fernandez, Kauffer et al, and Lippmann (2-4)provide fu
33、rther 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 hydrophobicity andconductivity in many materials (6). Therefore, one must be awa
34、re 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-mon source of weighing problems. These effects can usually beminimized by dis
35、charging 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(see also Engelbrecht et al (8).5.1.3 Effects of Volatile Compounds (other tha
36、n 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 plasma treatmentprior to conditioning and weighing. Alternatively, losses maybe
37、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 minimized or atleast controlled, for example, by conditioning under tightlyspecified
38、 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 used.5.1.4 Handling DamageLawless and Rodes (7) give rec-ommendations on minimi
39、zing 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 damaged during assembly and disas-sembly.5.1.4.2 Flat tipped forceps are recom
40、mended 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 place in a clean environment toavoid contamination.5.1.4.5 Gloves, if used, sh
41、all 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 membrane filter. However, there may exist circum-stances (for example, if an e
42、ntire 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 be required to weigh down to 0.1mg if pressure changes of the order of 10 %
43、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 Use ofBlanksThe use of blanks is the most important practical toolfor reduc
44、ing 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 closely aspossible, to the same conditions as the active sampling media,without
45、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) droplets, then the useof blanks alone cannot correct. Kauffer et al (3) note thatbl
46、anks 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 same type of cassette.6.1.1 An alternative procedure employs matched weight
47、filters 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.Analysis of uncertainty issimilar to the presentation here, but also involves
48、 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 perbatch. See A1.1 for advantages of multiple blanks.6.3 Weighing Times and Sequenc
49、eBlanks 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 samplingmedia. Typically, for workplace sampling applications, over-night conditioning is satisfactory. For sampling media withlonger conditioning times, error correction through the use ofTABLE
