1、Designation: D 6246 02Standard 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. Given a suitable exposurechamber, the practice can be extended to cover sampler use forother sampling periods and conditions. The aim is to provide aconcise set of expe
4、riments for classifying samplers primarily inaccordance with a single sampler accuracy figure. Accuracy isdefined (3.2.1) in this standard so as to take into account bothimprecision and uncorrected bias. Accuracy estimates refer toconditions of sampler use which are normally expected in aworkplace s
5、etting. These conditions may be characterized bythe temperature, atmospheric pressure, 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 estimati
6、on of time-weighted averages of concentrationswhich may not be constant in time. 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 ondiffu
7、sive samplers (1-14)2as well as Practices D 4597, D 4598,D 4599, and MDHS 27. An essential 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
8、. For example, interpolation of data character-izing the sampling of analytes at separated 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 pra
9、ctice, higher molecular weight memberswould receive partial validations considering 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 Units of the International
10、System of Units (SI) are usedthroughout this guide and should be regarded as standard.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 a
11、nd determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:3D 1356 Terminology Relating to Sampling and Analysis ofAtmospheresD 4597 Practice for Sampling Workplace Atmospheres toCollect Organic Gases or Vapor with Activated CharcoalDiffusive Sam
12、plersD 4598 Practice for Sampling Workplace Atmospheres toCollect Gases or Vapor with Liquid Sorbent DiffusionalSamplersD 4599 Practice for Measuring the Concentration of ToxicGases or Vapors Using Length-of-Stain Dosimeters2.2 International Standards:CEN EN 838 European Standard, Workplace atmosphe
13、res -Diffusive samplers for the determination of gases orvapours - Requirements and test methods4MDHS 27 Protocol for assessing the performance of adiffusive sampler, Health and Safety Laboratory, UnitedKingdom5MDHS 80 Volatile organic compounds in air, Health andSafety Laboratory, United Kingdom53.
14、 Terminology3.1 Definitions:3.1.1 For definitions of terms used in this practice, refer toTerminology D 1356.3.2 Definitions of Terms Specific to This Standard:1This practice is under the jurisdiction of ASTM Committee D22 on Samplingand Analysis of Atmospheres and is the direct responsibility of Su
15、bcommitteeD22.04 on Workplace Atmospheres.Current edition approved October 10, 2002. Published December 2002. Origi-nally published as D 6246 98. Last previous edition D 6246 01.2The boldface numbers in parentheses refer to the list of references at the end ofthis standard.3For referenced ASTM stand
16、ards, 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.4Available from CEN Central Secretariat, rue de Stassart 36, B-1050 Brussels,Belgium.5A
17、vailable 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.3.2.1 Symmetric Accuracy Range Athe fractional range,symmetric about the true concentration c, within which 95 %of sampler
18、 measurements are to be found (14-19). In terms ofthe bias D relative to true concentrations and the total relativestandard deviation RSD, the accuracy range A is closelyapproximated (19) by:A5 $1.960 3 D21 RSD2#1/2,|D| , RSD / 1.645|D| 1 1.645 3 RSD, otherwise(1)3.2.1.1 DiscussionIn the case that b
19、ias is corrected, leav-ing 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 concent
20、rations which are neitherspatially 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
21、sampler evaluation. Suchintervals with double confidence levels (in both measurementand evaluation) are related to a branch of statistics known asthe theory of tolerance intervals.3.2.2 diffusive samplera device which is capable of takingsamples of gases or vapors from the atmosphere at a ratecontro
22、lled by a physical 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
23、within this practice.3.3 Symbols:A = symmetric accuracy 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 es
24、timates (in-cluding (p, T)-corrections) obtained 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 relative standard deviation of concen-tra
25、tion estimates (dependent on assumed en-vironmental variability)RSDrun= 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 =
26、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 thestudent
27、ized 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 wit
28、h 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
29、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 wi
30、th 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, an
31、d 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 resu
32、lt 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
33、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.
34、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 aT, ah, av,and acwill exist on account of inter-sampler relative standarddeviation RSDsand an inter-run
35、 chamber standard deviationD6246022RSDrun. 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 experime
36、nt 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 A
37、nnex 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. I
38、f 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 with
39、in 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 including
40、 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, v
41、, 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 (s
42、ometimesdenoted 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 design
43、.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 entiresa
44、mpling 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 could
45、introduce 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 aretr
46、eated 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 b
47、ias 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 unifo
48、rmly 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 capaci
49、ty 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 presence of specific interferences at stated concentrations.4.3.2 Capacity is defined here as the sampled mass (orequivalently as the concentration at a specific sampling period)at which concentration estimates are 10 % low. Specifically,capacity is considered not exceeded if concentration estim
