1、Designation: D6246 08 (Reapproved 2013)Standard Practice forEvaluating the Performance of Diffusive Samplers1This standard is issued under the fixed designation D6246; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of las
2、t 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 This practice covers the evaluation of the performanceof diffusive samplers of gases and vapors for use over samplingperi
3、ods from 4 to 12 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 convection
4、from the ambient 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 ac
5、curacy figure. 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 thetemperature, atmospheric pre
6、ssure, humidity, and ambientwind speed, none of which may be constant or accuratelyknown when the sampler is used in the field. Futhermore, theaccuracy accounts for the effects of diffusive loss of analyte onthe estimation of time-weighted averages of concentrationswhich may not be constant in time.
7、 Aside from accuracy, thesamplers are tested for compliance with the manufacturersstated limits on capacity, possibly in the presence of interferingcompounds.1.2 This practice is an extension of previous research ondiffusive samplers (1-14)2as well as Practices D4597, D4598,D4599, and MDHS 27. An es
8、sential 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 se
9、parated 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
10、 sampling 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 thi
11、sstandard.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. Re
12、ferenced Documents2.1 ASTM Standards:3D1356 Terminology Relating to Sampling and Analysis ofAtmospheresD4597 Practice for Sampling Workplace Atmospheres toCollect Gases or Vapors with Solid Sorbent DiffusiveSamplersD4598 Practice for Sampling Workplace Atmospheres toCollect Gases or Vapors with Liqu
13、id Sorbent DiffusionalSamplers (Withdrawn 1995)4D4599 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 - Requir
14、ements and test methods5MDHS 27 Protocol for assessing the performance of a1This practice is under the jurisdiction of ASTM Committee D22 on AirQualityand is the direct responsibility of Subcommittee D22.04 on Workplace AirQuality.Current edition approved April 1, 2013. Published April 2013. Origina
15、llyapproved in 1998. Last previous edition approved in 2008 as D6246 - 08. DOI:10.1520/D6246-08R13.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.3For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at
16、serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.4The last approved version of this historical standard is referenced onwww.astm.org.5Available from CEN Central Secretariat, rue de Stassart 36, B-1050 Brussels,Belgi
17、um.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1diffusive sampler, Health and Safety Laboratory, UnitedKingdom6MDHS 80 Volatile organic compounds in air, Health andSafety Laboratory, United Kingdom63. Terminology3.1 Definitions:3.1
18、.1 For definitions of terms used in this practice, refer toTerminology D1356.3.2 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 of
19、the bias relative to true concentrations and the total (true)relative standard deviation RSD (sometimes designated asTRSD), the accuracy range A is closely approximated (19) by:A 5H1.960 3 =21RSD2, if ?,RSD/1.645?11.645 3RSD, otherwise(1)3.2.1.1 DiscussionIn the case that bias is corrected, leav-ing
20、 only an uncorrectable residual bias 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 neithers
21、patially nor temporally constant. Rather 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. Suchin
22、tervals with double confidence levels (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 ph
23、ysical process such as gaseous diffusionthrough 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
24、practice.3.3 Symbols:A = symmetric accuracy range as defined in terms ofbias and imprecision = estimated symmetric accuracy range AA95 %= 95 % confidence limit on the symmetric accu-racy range Ac(mg/m3) = true or reference analyte concentration(mg/m3) = mean of (four) concentration estimates (includ
25、-ing (p, T)-corrections) obtained in accordancewith instructions of sampler manufacturerh = humidity (expressed as partial pressure)n = number of diffusive samplers tested for measur-ing sampler capacityp = atmospheric pressureRSD = overall (true) relative standard deviation of con-centration estima
26、tes (dependent on assumed en-vironmental variability) expressed relative to a“true” concentrationRSDrun= relative standard deviation characterizing inter-run chamber variabilityRSDs= inter-sampler imprecision (relative to the refer-ence concentration)RDs= estimated inter-sampler imprecision RSDsRSDt
27、= pulse-induced imprecisionRD = estimated overall relative standard deviationRSDRD95 %= 95 % confidence limit on the overall relativestandard deviation RSDs = estimated standard deviation characterizinginter-sampler imprecisiont0.95() = value which, at probability 95 %, exceeds ran-dom variables dis
28、tributed according to the stu-dentized t-distribution with degrees of free-domT = temperaturev (m/s) = ambient wind speedx= concentration estimate dependence on environ-mental variable x (T, h, v, or c). = bias relative to reference concentration c= estimated bias 95 %= 95 % confidence limit on the
29、bias t= bias associated with concentration pulse = degrees of freedom in determining RSDseff= effective number of degrees of freedom in de-termining RSDc= assumed concentration variabilityh= assumed humidity variabilityT= assumed temperature variabilityv= assumed ambient wind speed variability4. Sum
30、mary of Test Method4.1 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
31、minimize interaction with thetemperature), 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 sample
32、r use (T0, h0, v0, and 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.
33、Pressure effects result 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 T, h, v, and c(relative to the chamber reference concentration and targetenvironmental parameters) to changes
34、in T, h, v, and c aremeasured, following the sampler manufacturers instructions6Available from HMSO Books, PO Box 276, London, England, SW8 5DT.D6246 08 (2013)2regarding p- and T- corrections (if any). These experimentsalso give a value for the estimated sampler bias relative tothe chamber reference
35、 concentration (defined for the targetconditions). Two further runs describing time-effects (4.2.5)from diffusive loss of analyte are also carried out. The chamberreference concentration must be traceable to primary standardsof mass and volume.4.1.3 Error in the estimates of the sensitivities T, h,
36、v, andcwill exist on account of inter-sampler relative standarddeviation RSDsand an 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 estimatedwith73=2
37、1degrees of freedom (or 15 degrees of freedomif the reverse diffusion experiment 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 exa
38、mple in which the sensitivities andRSDs, are estimated is presented in the 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-pend
39、ent estimate is required and must be within 5 % of the calculatedestimate. 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
40、 the mean of at least five independent, activesampler estimates per run within 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, (
41、3) requires averaging of atleast two independent methods (possibly including 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 by
42、observing that the dependence of concentration estimates onthe wind speed, 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
43、 diffusive samplers is pre-sented by the possibility of reverse diffusion (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 m
44、ay generally be minimizedthrough proper sorbent selection and sampler design.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 th
45、e samplingperiod. A pulse at the beginning of the period allows the entiresampling 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 sa
46、mpling period. Forexample, a cleanup operation at the end of a workday couldintroduce 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 con
47、centration withstationary fluctuations, so that time-dependent effects aretreated 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 testssugg
48、ested by NIOSH (3) and CEN (EN 838).4.2.5 Let t(0) represent one-half the 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
49、 occurring at random within (forexample, an 8h sampling period ranges uniformly betweentand +t.Then the variance RSDt2associated with samplinga 0.5h pulse at random within the sampling period is asfollows:RSDt2513t2(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
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