ASTM D6246-08(2013)
(Practice)Standard Practice for Evaluating the Performance of Diffusive Samplers
Standard Practice for Evaluating the Performance of Diffusive Samplers
SIGNIFICANCE AND USE
5.1 Gas or vapor sampling is often accomplished by actively pumping air through a collection medium such as activated charcoal. Problems associated with a pump–inconvenience, inaccuracy, and expense–are inextricable from this type of sampling. The alternative covered by this practice is to use diffusion for moving the compound of interest onto the collection medium. This approach to sampling is attractive because of the convenience of use and low total monitoring cost.
5.2 However, previous studies have found significant problems with the accuracy of some samplers. Therefore, although diffusive samplers may provide a plethora of data, inaccuracies and misuse of diffusive samplers may yet affect research studies. Furthermore, worker protections may be based on faulty assumptions. The aim of this practice is to counter the uncertainties in diffusive sampling through achieving a broadly accepted set of performance tests and acceptance criteria for proving the efficacy of any given diffusive sampler intended for use.
SCOPE
1.1 This practice covers the evaluation of the performance of diffusive samplers of gases and vapors for use over sampling periods from 4 to 12 h and for wind speeds less than 0.5 m/s. Such sampling periods and wind speeds are the most common in the indoor workplace setting. This practice does not apply to static or area sampling in wind speeds less than 0.1 m/s, when diffusion outside the sampler may dominate needed convection from the ambient air to the vicinity of the sampler. Given a suitable exposure chamber, the practice can be extended to cover sampler use for other sampling periods and conditions. The aim is to provide a concise set of experiments for classifying samplers primarily in accordance with a single sampler accuracy figure. Accuracy is defined (3.2.1) in this standard so as to take into account both imprecision and uncorrected bias. Accuracy estimates refer to conditions of sampler use which are normally expected in a workplace setting. These conditions may be characterized by the temperature, atmospheric pressure, humidity, and ambient wind speed, none of which may be constant or accurately known when the sampler is used in the field. Futhermore, the accuracy accounts for the effects of diffusive loss of analyte on the estimation of time-weighted averages of concentrations which may not be constant in time. Aside from accuracy, the samplers are tested for compliance with the manufacturer's stated limits on capacity, possibly in the presence of interfering compounds.
1.2 This practice is an extension of previous research on diffusive samplers (1-14)2 as well as Practices D4597, D4598, D4599, and MDHS 27. An essential advance here is the estimation of sampler accuracy under actual conditions of use. Futhermore, the costs of sampler evaluation are reduced.
1.3 Knowledge gained from similar analytes expedites sampler evaluation. For example, interpolation of data characterizing the sampling of analytes at separated points of a homologous series of compounds is recommended. At present the procedure of (9) is suggested. Following evaluation of a sampler in use at a single homologous series member according to the present practice, higher molecular weight members would receive partial validations considering sampling rate, capacity, analytical recovery, and interferences. The test for diffusive analyte loss can be omitted if the effect is found negligible for a given sampler or analyte series.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: D6246 − 08(Reapproved 2013)
Standard Practice for
Evaluating the Performance of Diffusive Samplers
This standard is issued under the fixed designation D6246; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope homologous series of compounds is recommended.At present
the procedure of (9) is suggested. Following evaluation of a
1.1 This practice covers the evaluation of the performance
sampler in use at a single homologous series member accord-
ofdiffusivesamplersofgasesandvaporsforuseoversampling
ing to the present practice, higher molecular weight members
periods from 4 to 12 h and for wind speeds less than 0.5 m/s.
would receive partial validations considering sampling rate,
Such sampling periods and wind speeds are the most common
capacity, analytical recovery, and interferences. The test for
intheindoorworkplacesetting.Thispracticedoesnotapplyto
diffusive analyte loss can be omitted if the effect is found
static or area sampling in wind speeds less than 0.1 m/s, when
negligible for a given sampler or analyte series.
diffusion outsidethesamplermaydominateneededconvection
from the ambient air to the vicinity of the sampler. Given a 1.4 The values stated in SI units are to be regarded as
suitable exposure chamber, the practice can be extended to
standard. No other units of measurement are included in this
cover sampler use for other sampling periods and conditions. standard.
The aim is to provide a concise set of experiments for
1.5 This standard does not purport to address all of the
classifying samplers primarily in accordance with a single
safety concerns, if any, associated with its use. It is the
sampler accuracy figure. Accuracy is defined (3.2.1) in this
responsibility of the user of this standard to establish appro-
standard so as to take into account both imprecision and
priate safety and health practices and determine the applica-
uncorrected bias. Accuracy estimates refer to conditions of
bility of regulatory limitations prior to use.
sampler use which are normally expected in a workplace
setting. These conditions may be characterized by the
2. Referenced Documents
temperature, atmospheric pressure, humidity, and ambient
2.1 ASTM Standards:
wind speed, none of which may be constant or accurately
D1356Terminology Relating to Sampling and Analysis of
known when the sampler is used in the field. Futhermore, the
Atmospheres
accuracyaccountsfortheeffectsofdiffusivelossofanalyteon
D4597Practice for Sampling Workplace Atmospheres to
the estimation of time-weighted averages of concentrations
Collect Gases or Vapors with Solid Sorbent Diffusive
which may not be constant in time. Aside from accuracy, the
Samplers
samplers are tested for compliance with the manufacturer’s
D4598Practice for Sampling Workplace Atmospheres to
statedlimitsoncapacity,possiblyinthepresenceofinterfering
Collect Gases or Vapors with Liquid Sorbent Diffusional
compounds.
Samplers (Withdrawn 1995)
1.2 This practice is an extension of previous research on
D4599Practice for Measuring the Concentration of Toxic
diffusive samplers (1-14) as well as Practices D4597, D4598,
Gases or Vapors Using Length-of-Stain Dosimeters
D4599, and MDHS 27. An essential advance here is the
2.2 International Standards:
estimation of sampler accuracy under actual conditions of use.
CEN EN 838European Standard, Workplace atmospheres -
Futhermore, the costs of sampler evaluation are reduced.
Diffusive samplers for the determination of gases or
1.3 Knowledgegainedfromsimilaranalytesexpeditessam-
vapours - Requirements and test methods
pler evaluation. For example, interpolation of data character-
MDHS 27Protocol for assessing the performance of a
izing the sampling of analytes at separated points of a
1 3
This practice is under the jurisdiction of ASTM Committee D22 on Air For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Qualityand is the direct responsibility of Subcommittee D22.04 on Workplace Air contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Quality. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved April 1, 2013. Published April 2013. Originally the ASTM website.
approved in 1998. Last previous edition approved in 2008 as D6246-08. DOI: The last approved version of this historical standard is referenced on
10.1520/D6246-08R13. www.astm.org.
2 5
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof Available from CEN Central Secretariat, rue de Stassart 36, B-1050 Brussels,
this standard. Belgium.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6246 − 08 (2013)
diffusive sampler, Health and Safety Laboratory, United
RSD = overall (true) relative standard deviation of con-
Kingdom
centration estimates (dependent on assumed en-
MDHS 80Volatile organic compounds in air, Health and
vironmental variability) expressed relative to a
Safety Laboratory, United Kingdom
“true” concentration
RSD = relative standard deviation characterizing inter-
run
3. Terminology
run chamber variability
RSD = inter-sampler imprecision (relative to the refer-
3.1 Definitions:
s
ence concentration)
3.1.1 For definitions of terms used in this practice, refer to
RŜD = estimated inter-sampler imprecision RSD
Terminology D1356. s s
RSD = pulse-induced imprecision
t
3.2 Definitions of Terms Specific to This Standard:
RŜD = estimated overall relative standard deviation
3.2.1 Symmetric Accuracy Range A—the fractional range,
RSD
symmetric about the true concentration c, within which 95%
RŜD = 95% confidence limit on the overall relative
95%
of sampler measurements are to be found (14-19). In terms of
standard deviation RSD
the bias ∆ relative to true concentrations and the total (true)
s = estimated standard deviation characterizing
relative standard deviation RSD (sometimes designated as
inter-sampler imprecision
TRSD), the accuracy rangeAis closely approximated (19) by:
t (υ) = value which, at probability 95%, exceeds ran-
0.95
2 2
dom variables distributed according to the stu-
1.960 3=∆ 1RSD , if ?∆?,RSD/1.645
A 5 (1)
H
dentized t-distribution with υ degrees of free-
∆?11.645 3RSD, otherwise
?
dom
3.2.1.1 Discussion—In the case that bias is corrected, leav-
T = temperature
ingonlyanuncorrectableresidualbiasduetouncertaintyinthe
v (m/s) = ambient wind speed
correction, 95%-confidence limits on A play the role of the
α = concentration estimate dependence on environ-
x
expanded uncertainty in (20). As described in (14), such an
mental variable x (T, h, v, or c).
interpretation is an extension of (20) for measurement, as in
∆ = bias relative to reference concentration c
occupational hygiene, of concentrations which are neither
ˆ
= estimated bias ∆
∆
spatially nor temporally constant. Rather than continually
ˆ
= 95% confidence limit on the bias ∆
∆
95%
re-evaluating a method through estimate replicates, the accu-
∆ = bias associated with concentration pulse
t
racy provides confidence intervals bracketing (true) concentra-
υ = degrees of freedom in determining RSD
s
tions at greater than a given probability (95%) for a fixed
υ = effective number of degrees of freedom in de-
eff
confidence (95%) in the initial sampler evaluation. Such
termining RSD
intervals with double confidence levels (in both measurement
σ = assumed concentration variability
c
and evaluation) are related to a branch of statistics known as
σ = assumed humidity variability
h
the theory of tolerance or prediction intervals.
σ = assumed temperature variability
T
σ = assumed ambient wind speed variability
3.2.2 diffusive sampler—adevicewhichiscapableoftaking
v
samples of gases or vapors from the atmosphere at a rate
4. Summary of Test Method
controlled by a physical process such as gaseous diffusion
through a static air layer or permeation through a membrane,
4.1 Bias, Inter-sampler Imprecision and the Effects of En-
butwhichdoesnotinvolvetheactivemovementofairthrough
vironmental Uncertainty:
the sampler. As such, direct-reading dosimeters, as well as
4.1.1 This practice gives a procedure for assessing the
samplers requiring lab analysis, are considered diffusive sam-
effects of variability in the following workplace variables:
plers within this practice.
temperature T, humidity h (expressed in terms of the water
3.3 Symbols:
vapor partial pressure to minimize interaction with the
temperature),theambientwindspeed vacrossthesamplerface
A = symmetricaccuracyrangeasdefinedintermsof
(see 4.7 regarding wind direction), and concentration c.An
bias and imprecision
experimentiscarriedoutwhichprovidesinformationaboutthe
 = estimated symmetric accuracy range A
concentration estimates’ dependencies on these variables near
A = 95% confidence limit on the symmetric accu-
95%
conditions of intended sampler use (T , h , v , and c ). Testing
0 0 0 0
racy range A
isrequiredattheconcentrationc ofintendeduse,aswellasat
c(mg/m ) = true or reference analyte concentration 0
concentrations reduced at least to c /2. Furthermore, the
ĉ(mg/m ) = mean of (four) concentration estimates (includ- 0
sampler bias and the inter-sampler standard deviation are
ing (p, T)-corrections) obtained in accordance
measured. Finally, the effect of diffusion of material out of the
with instructions of sampler manufacturer
sampler is measured. Pressure effects result in correctable bias
h = humidity (expressed as partial pressure)
n = number of diffusive samplers tested for measur- and are not evaluated in this practice (4.6).
ing sampler capacity
4.1.2 Usingfoursamplersforeachoffiveexperimentalruns
p = atmospheric pressure
(the minimum possible), the sensitivities α , α , α , and α
T h v c
(relative to the chamber reference concentration and target
environmental parameters) to changes in T, h, v, and c are
Available from HMSO Books, PO Box 276, London, England, SW8 5DT. measured, following the sampler manufacturer’s instructions
D6246 − 08 (2013)
regarding p- and T- corrections (if any). These experiments introducesolventonlythen.Thiscouldimplyapositivebiasin
also give a value for the estimated sampler bias ∆ relative to the concentration estimates obtained from a day’s sampling.
the chamber reference concentration (defined for the target For simplicity, however, this practice is set up for assessing
conditions). Two further runs describing time-effects (4.2.5) performance of samplers for use in a concentration with
fromdiffusivelossofanalytearealsocarriedout.Thechamber stationary fluctuations, so that time-dependent effects are
reference concentration must be traceable to primary standards treatedsimplyascomponentsofsamplervariance.Specifically,
of mass and volume. theeffectofanisolated0.5-hpulseoccurringatrandomwithin
4.1.3 Errorintheestimatesofthesensitivities α , α , α ,and the sampling period is estimated.
T h v
α will exist on account of inter-sampler relative standard 4.2.4 Challengingsamplersto0.5-hpulsesissimilartotests
c
deviation RSD and an inter-run chamber standard deviation suggested by NIOSH (3) and CEN (EN 838).
s
RSD . The latter results in part from uncertainty in the 4.2.5 Let ∆ (>0) represent one-half the bias between esti-
run t
reference concentration. RSD is obtained by pooling the matesfroma0.5-hpulseattheendversusthebeginningofthe
s
variance estimates from each run and therefore is estimated samplingperiod,relativetothemeanoftheestimates.Assume,
with7×3=21 degrees of freedom (or 15 degrees of freedom conservatively (see, for example, (6)), that the bias in the
if the reverse diffusion experiment is omitted (1.3)). So as to estimates of 0.5-h pulse occurring at random within (for
avoid re-measurement at each sampler/analyte evaluation, example, an 8–h sampling period ranges uniformly between
RSD is obtained by a separate characterization of the –∆ and+∆.ThenthevarianceRSD associatedwithsampling
run t t t
chamber with several runs at (for example) fixed environmen- a 0.5–h pulse at random within the sampling period is as
tal conditions. An example in which the sensitivities α and
follows:
RSD , are estimated is presented in the Annex A1.
s
2 2
RSD 5 ∆ (2)
t t
NOTE1—Itisuptotheuserastohowtraceabilityisestablished.Within
(12) the concentration estimate as calculated from the chamber’s analyte
4.3 Capacity; Control of Effects from Interfering Com-
generation parameters is regarded as the benchmark, although an inde-
pounds:
pendent estimate is required and must be within 5% of the calculated
4.3.1 This practice provides a test for confirming a manu-
estimate. If these estimates differ, then a third independent estimate is
required to establish the reference concentration through agreement with
facturer’s claimed sampler capacity under stated conditions of
one of the other independent estimates. One possibility for such an
use. Such conditions would normally refer to a specific
independent estimate is the mean of at least five independent, active
sampling period and to environmental extremes, such as 80%
sampler estimates per run within the chamber. Experiment (12)onthe
relative humidity at a temperature equal to 30°C.Additionally,
accuracy of such reference measurements using sorbent tubes indicates
that a relative standard deviation of the order of 2% can be achieved for a manufacturer may claim a value of capacity for sampling in
the individual measurements. Alternatively, (3) requires averaging of at
the presence of specific interferences at stated concentrations.
least two independent methods (possibly including calculated estimates)
4.3.2 Capacity is defined here as the sampled mass (or
with at least four samples per method. EN 838 has adopted the looser
equivalentlyastheconcentrationataspecificsamplingperiod)
requirement that calculated and independent measurements must agree
at which concentration estimates are 10% low. Specifically,
within 10%.
capacity is considered not exceeded if concentration estimates,
4.1.3.1 A further consolidation of tests may be made by
corrected for correctable bias, are above 90% of the true
observing that the dependence of concentration estimates on
concentration at the 95% confidence level.
the wind speed, v, is only sampler specific, that is, does not
4.3.3 An example of the test follows. Eight diffusive and
depend on the specific analyte. Therefore, after a single
eightactivesamplerswithestimatedinter-samplerimprecision,
measurement for a given sampler type, the set of tests can be
s, are expose
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