1、Designation: D 6246 08Standard Practice forEvaluating the Performance of Diffusive Samplers1This standard is issued under the fixed designation D 6246; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A nu
2、mber in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers the evaluation of the performanceof diffusive samplers of gases and vapors for use over samplingperiods from 4 to 1
3、2 h and for wind speeds less than 0.5 m/s.Such sampling periods and wind speeds are the most commonin the indoor workplace setting. This practice does not apply tostatic or area sampling in wind speeds less than 0.1 m/s, whendiffusion outside the sampler may dominate needed convectionfrom the ambien
4、t air to the vicinity of the sampler. Given asuitable exposure chamber, the practice can be extended tocover sampler use for other sampling periods and conditions.The aim is to provide a concise set of experiments forclassifying samplers primarily in accordance with a singlesampler accuracy figure.
5、Accuracy is defined (3.2.1) in thisstandard so as to take into account both imprecision anduncorrected bias. Accuracy estimates refer to conditions ofsampler use which are normally expected in a workplacesetting. These conditions may be characterized by the tempera-ture, atmospheric pressure, humidi
6、ty, and ambient wind speed,none of which may be constant or accurately known when thesampler is used in the field. Futhermore, the accuracy accountsfor the effects of diffusive loss of analyte on the estimation oftime-weighted averages of concentrations which may not beconstant in time. Aside from a
7、ccuracy, the samplers are testedfor compliance with the manufacturers stated limits on capac-ity, possibly in the presence of interfering compounds.1.2 This practice is an extension of previous research ondiffusive samplers (1-14)2as well as Practices D 4597, D 4598,D 4599, and MDHS 27. An essential
8、 advance here is theestimation of sampler accuracy under actual conditions of use.Futhermore, the costs of sampler evaluation are reduced.1.3 Knowledge gained from similar analytes expedites sam-pler evaluation. For example, interpolation of data character-izing the sampling of analytes at separated
9、 points of ahomologous series of compounds is recommended. At presentthe procedure of (9) is suggested. Following evaluation of asampler in use at a single homologous series member accord-ing to the present practice, higher molecular weight memberswould receive partial validations considering sampli
10、ng rate,capacity, analytical recovery, and interferences. The test fordiffusive analyte loss can be omitted if the effect is foundnegligible for a given sampler or analyte series.1.4 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstanda
11、rd.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility 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. Reference
12、d Documents2.1 ASTM Standards:3D 1356 Terminology Relating to Sampling and Analysis ofAtmospheresD 4597 Practice for Sampling Workplace Atmospheres toCollect Gases or Vapors with Solid Sorbent DiffusiveSamplersD 4598 Practice for Sampling Workplace Atmospheres toCollect Gases or Vapors with Liquid S
13、orbent DiffusionalSamplers4D 4599 Practice for Measuring the Concentration of ToxicGases or Vapors Using Length-of-Stain Dosimeters2.2 International Standards:CEN EN 838 European Standard, Workplace atmospheres -Diffusive samplers for the determination of gases orvapours - Requirements and test meth
14、ods5MDHS 27 Protocol for assessing the performance of adiffusive sampler, Health and Safety Laboratory, UnitedKingdom6MDHS 80 Volatile organic compounds in air, Health andSafety Laboratory, United Kingdom63. Terminology3.1 Definitions:1This practice is under the jurisdiction of ASTM Committee D22 on
15、 Air Qualityand is the direct responsibility of Subcommittee D22.04 on Workplace Air Quality.Current edition approved April 1, 2008. Published May 2008. Originallyapproved in 1998. Last previous edition approved in 2002 as D 6246 - 02.2The boldface numbers in parentheses refer to the list of referen
16、ces at the end ofthis standard.3For 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 standards Document Summary page onthe ASTM website.4Withdrawn5Available from CEN
17、Central Secretariat, rue de Stassart 36, B-1050 Brussels,Belgium.6Available from HMSO Books, PO Box 276, London, England, SW8 5DT.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.1.1 For definitions of terms used in this practice, r
18、efer toTerminology D 1356.3.2 Definitions of Terms Specific to This Standard:3.2.1 Symmetric Accuracy Range Athe fractional range,symmetric about the true concentration c, within which 95 %of sampler measurements are to be found (14-19). In terms ofthe bias D relative to true concentrations and the
19、total (true)relative standard deviation RSD (sometimes designated asTRSD), the accuracy range A is closely approximated (19) by:A 5H1.960 3 =D21 RSD2, if |D| , RSD / 1.645|D| 1 1.645 3 RSD, otherwise(1)3.2.1.1 DiscussionIn the case that bias is corrected, leav-ing only an uncorrectable residual bias
20、 due to uncertainty in thecorrection, 95 %-confidence limits on A play the role of theexpanded uncertainty in (20). As described in (14), such aninterpretation is an extension of (20) for measurement, as inoccupational hygiene, of concentrations which are neitherspatially nor temporally constant. Ra
21、ther than continuallyre-evaluating a method through estimate replicates, the accu-racy provides confidence intervals bracketing (true) concentra-tions at greater than a given probability (95 %) for a fixedconfidence (95 %) in the initial sampler evaluation. Suchintervals with double confidence level
22、s (in both measurementand evaluation) are related to a branch of statistics known asthe theory of tolerance or prediction intervals.3.2.2 diffusive samplera device which is capable of takingsamples of gases or vapors from the atmosphere at a ratecontrolled by a physical process such as gaseous diffu
23、sionthrough a static air layer or permeation through a membrane,but which does not involve the active movement of air throughthe sampler. As such, direct-reading dosimeters, as well assamplers requiring lab analysis, are considered diffusive sam-plers within this practice.3.3 Symbols:A = symmetric a
24、ccuracy range as defined in termsof bias and imprecision = estimated symmetric accuracy range AA95 %= 95 % confidence limit on the symmetric ac-curacy range Ac (mg/m3) = true or reference analyte concentrationc (mg/m3) = mean of (four) concentration estimates (in-cluding (p, T)-corrections) obtained
25、 in accor-dance with instructions of sampler manufac-turerh = humidity (expressed as partial pressure)n = number of diffusive samplers tested for mea-suring sampler capacityp = atmospheric pressureRSD = overall (true) relative standard deviation ofconcentration estimates (dependent on as-sumed envir
26、onmental variability) expressedrelative to a “true” concentrationRSDrun= relative standard deviation characterizinginter-run chamber variabilityRSDs= inter-sampler imprecision (relative to the ref-erence concentration)RSDs= estimated inter-sampler imprecision RSDsRSDt= pulse-induced imprecisionRSD =
27、 estimated overall relative standard deviationRSDRSD95 %= 95 % confidence limit on the overall relativestandard deviation RSDs = estimated standard deviation characterizinginter-sampler imprecisiont0.95(y) = value which, at probability 95 %, exceedsrandom variables distributed according to thestuden
28、tized t-distribution with y degrees offreedomT = temperaturev (m/s) = ambient wind speedax= concentration estimate dependence on envi-ronmental variable x (T, h, v, or c).D = bias relative to reference concentration cD= estimated bias DD95 %= 95 % confidence limit on the bias DDt= bias associated wi
29、th concentration pulsey = degrees of freedom in determining RSDsyeff= effective number of degrees of freedom indetermining RSDsc= assumed concentration variabilitysh= assumed humidity variabilitysT= assumed temperature variabilitysv= assumed ambient wind speed variability4. Summary of Test Method4.1
30、 Bias, Inter-sampler Imprecision and the Effects of En-vironmental Uncertainty:4.1.1 This practice gives a procedure for assessing theeffects of variability in the following workplace variables:temperature T, humidity h (expressed in terms of the watervapor partial pressure to minimize interaction w
31、ith the tem-perature), the ambient wind speed v across the sampler face(see 4.7 regarding wind direction), and concentration c.Anexperiment is carried out which provides information about theconcentration estimates dependencies on these variables nearconditions of intended sampler use (T0, h0, v0, a
32、nd c0). Testingis required at the concentration c0of intended use, as well as atconcentrations reduced at least to c0/2. Furthermore, thesampler bias and the inter-sampler standard deviation aremeasured. Finally, the effect of diffusion of material out of thesampler is measured. Pressure effects res
33、ult in correctable biasand are not evaluated in this practice (4.6).4.1.2 Using four samplers for each of five experimental runs(the minimum possible), the sensitivities aT, ah, av, andac(relative to the chamber reference concentration and targetenvironmental parameters) to changes in T, h, v, and c
34、 aremeasured, following the sampler manufacturers instructionsregarding p- and T- corrections (if any). These experimentsalso give a value for the estimated sampler bias D relative tothe chamber reference concentration (defined for the targetconditions). Two further runs describing time-effects (4.2
35、.5)from diffusive loss of analyte are also carried out. The chamberreference concentration must be traceable to primary standardsof mass and volume.D62460824.1.3 Error in the estimates of the sensitivities aT, ah, av,and acwill exist on account of inter-sampler relative standarddeviation RSDsand an
36、inter-run chamber standard deviationRSDrun. The latter results in part from uncertainty in thereference concentration. RSDsis obtained by pooling thevariance estimates from each run and therefore is estimatedwith 7 3 3 = 21 degrees of freedom (or 15 degrees of freedomif the reverse diffusion experim
37、ent is omitted (1.3). So as toavoid re-measurement at each sampler/analyte evaluation,RSDrunis obtained by a separate characterization of thechamber with several runs at (for example) fixed environmen-tal conditions. An example in which the sensitivities a andRSDs, are estimated is presented in the
38、Annex A1.NOTE 1It is up to the user as to how traceability is established. Within(12) the concentration estimate as calculated from the chambers analytegeneration parameters is regarded as the benchmark, although an inde-pendent estimate is required and must be within 5 % of the calculatedestimate.
39、If these estimates differ, then a third independent estimate isrequired to establish the reference concentration through agreement withone of the other independent estimates. One possibility for such anindependent estimate is the mean of at least five independent, activesampler estimates per run wit
40、hin the chamber. Experiment (12)ontheaccuracy of such reference measurements using sorbent tubes indicatesthat a relative standard deviation of the order of 2 % can be achieved forthe individual measurements. Alternatively, (3) requires averaging of atleast two independent methods (possibly includin
41、g calculated estimates)with at least four samples per method. EN 838 has adopted the looserrequirement that calculated and independent measurements must agreewithin 10 %.4.1.3.1 A further consolidation of tests may be made byobserving that the dependence of concentration estimates onthe wind speed,
42、v, is only sampler specific, that is, does notdepend on the specific analyte. Therefore, after a singlemeasurement for a given sampler type, the set of tests can benarrowed.4.2 Reverse Diffusion:4.2.1 A potential problem with diffusive samplers is pre-sented by the possibility of reverse diffusion (
43、sometimesdenoted as back diffusion or off-gassing) of analyte. Reversediffusion is generally only significant in the case that an analyteis weakly bound to the sorbent (6). Therefore, inaccuracyassociated with these effects may generally be minimizedthrough proper sorbent selection and sampler desig
44、n.4.2.2 Because of reverse diffusion, estimates of a varyingconcentration may in some cases be biased. The worst-casesituation occurs with the concentration in the form of anisolated pulse at either the beginning or end of the samplingperiod. A pulse at the beginning of the period allows the entires
45、ampling period (4 to 12 h) for sample loss, possibly resultingin a low estimate relative to a pulse at the end of the period.4.2.3 In some cases, the time-dependence of a workplaceconcentration correlates strongly with the sampling period. Forexample, a cleanup operation at the end of a workday coul
46、dintroduce solvent only then. This could imply a positive bias inthe concentration estimates obtained from a days sampling.For simplicity, however, this practice is set up for assessingperformance of samplers for use in a concentration withstationary fluctuations, so that time-dependent effects aret
47、reated simply as components of sampler variance. Specifically,the effect of an isolated 0.5-h pulse occurring at random withinthe sampling period is estimated.4.2.4 Challenging samplers to 0.5-h pulses is similar to testssuggested by NIOSH (3) and CEN (EN 838).4.2.5 Let Dt(0) represent one-half the
48、bias between esti-mates from a 0.5-h pulse at the end versus the beginning of thesampling period, relative to the mean of the estimates.Assume,conservatively (see, for example, (6), that the bias in theestimates of 0.5-h pulse occurring at random within (forexample, an 8h sampling period ranges unif
49、ormly betweenDtand +Dt. Then the variance RSDt2associated with sam-pling a 0.5h pulse at random within the sampling period is asfollows:RSDt2513Dt2(2)4.3 Capacity; Control of Effects from Interfering Com-pounds:4.3.1 This practice provides a test for confirming a manu-facturers claimed sampler capacity under stated conditions ofuse. Such conditions would normally refer to a specificsampling period and to environmental extremes, such as 80 %relative humidity at a temperature equal to 30C. Additionally,a manufacturer may claim a value of capacity for sampling inthe
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