ASTM D6246-08(2013)e1
(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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´1
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.
ε NOTE—The References Section was editorially corrected in July 2015.
1. Scope izing the sampling of analytes at separated points of a
homologous series of compounds is recommended.At present
1.1 This practice covers the evaluation of the performance
the procedure of (9) is suggested. Following evaluation of a
ofdiffusivesamplersofgasesandvaporsforuseoversampling
sampler in use at a single homologous series member accord-
periods from 4 to 12 h and for wind speeds less than 0.5 m/s.
ing to the present practice, higher molecular weight members
Such sampling periods and wind speeds are the most common
would receive partial validations considering sampling rate,
intheindoorworkplacesetting.Thispracticedoesnotapplyto
capacity, analytical recovery, and interferences. The test for
static or area sampling in wind speeds less than 0.1 m/s, when
diffusive analyte loss can be omitted if the effect is found
diffusion outsidethesamplermaydominateneededconvection
negligible for a given sampler or analyte series.
from the ambient air to the vicinity of the sampler. Given a
suitable exposure chamber, the practice can be extended to 1.4 The values stated in SI units are to be regarded as
cover sampler use for other sampling periods and conditions. standard. No other units of measurement are included in this
The aim is to provide a concise set of experiments for standard.
classifying samplers primarily in accordance with a single
1.5 This standard does not purport to address all of the
sampler accuracy figure. Accuracy is defined (3.2.1) in this
safety concerns, if any, associated with its use. It is the
standard so as to take into account both imprecision and
responsibility of the user of this standard to establish appro-
uncorrected bias. Accuracy estimates refer to conditions of
priate safety and health practices and determine the applica-
sampler use which are normally expected in a workplace
bility of regulatory limitations prior to use.
setting. These conditions may be characterized by the
2. Referenced Documents
temperature, atmospheric pressure, humidity, and ambient
wind speed, none of which may be constant or accurately
2.1 ASTM Standards:
known when the sampler is used in the field. Futhermore, the
D1356Terminology Relating to Sampling and Analysis of
accuracyaccountsfortheeffectsofdiffusivelossofanalyteon
Atmospheres
the estimation of time-weighted averages of concentrations
D4597Practice for Sampling Workplace Atmospheres to
which may not be constant in time. Aside from accuracy, the
Collect Gases or Vapors with Solid Sorbent Diffusive
samplers are tested for compliance with the manufacturer’s
Samplers
statedlimitsoncapacity,possiblyinthepresenceofinterfering
D4598Practice for Sampling Workplace Atmospheres to
compounds.
Collect Gases or Vapors with Liquid Sorbent Diffusional
1.2 This practice is an extension of previous research on Samplers (Withdrawn 1995)
diffusive samplers (1-14) as well as Practices D4597, D4598, D4599Practice for Measuring the Concentration of Toxic
Gases or Vapors Using Length-of-Stain Dosimeters
D4599, and MDHS 27. An essential advance here is the
estimation of sampler accuracy under actual conditions of use. 2.2 International Standards:
Futhermore, the costs of sampler evaluation are reduced. CEN EN 838European Standard, Workplace atmospheres -
Diffusive samplers for the determination of gases or
1.3 Knowledgegainedfromsimilaranalytesexpeditessam-
vapours - Requirements and test methods
pler evaluation. For example, interpolation of data character-
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
ThispracticeisunderthejurisdictionofASTMCommitteeD22onAirQuality contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
and is the direct responsibility of Subcommittee D22.04 on WorkplaceAir 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/D6246-08R13E01. 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
´1
D6246 − 08 (2013)
MDHS 27Protocol for assessing the performance of a
p = atmospheric pressure
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
4.1 Bias, Inter-sampler Imprecision and the Effects of En-
through a static air layer or permeation through a membrane,
vironmental Uncertainty:
butwhichdoesnotinvolvetheactivemovementofairthrough
4.1.1 This practice gives a procedure for assessing the
the sampler. As such, direct-reading dosimeters, as well as
effects of variability in the following workplace variables:
samplers requiring lab analysis, are considered diffusive sam-
temperature T, humidity h (expressed in terms of the water
plers within this practice.
vapor partial pressure to minimize interaction with the
3.3 Symbols:
temperature),theambientwindspeed vacrossthesamplerface
(see 4.7 regarding wind direction), and concentration c.An
A = symmetricaccuracyrangeasdefinedintermsof
experimentiscarriedoutwhichprovidesinformationaboutthe
bias and imprecision
concentration estimates’ dependencies on these variables near
 = estimated symmetric accuracy range A
conditions of intended sampler use (T , h , v , and c ). Testing
A = 95% confidence limit on the symmetric accu-
0 0 0 0
95%
isrequiredattheconcentrationc ofintendeduse,aswellasat
racy range A
c(mg/m ) = true or reference analyte concentration concentrations reduced at least to c /2. Furthermore, the
ĉ(mg/m ) = mean of (four) concentration estimates (includ- 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)
and are not evaluated in this practice (4.6).
n = number of diffusive samplers tested for measur-
4.1.2 Usingfoursamplersforeachoffiveexperimentalruns
ing sampler capacity
(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
´1
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) on the
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,
m
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
´1
Designation: D6246 − 08 (Reapproved 2013) 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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—The References Section was editorially corrected in July 2015.
1. 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-(21-14)) 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 ((39))
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.
2. Referenced Documents
2.1 ASTM Standards:
D1356 Terminology Relating to Sampling and Analysis of Atmospheres
D4597 Practice for Sampling Workplace Atmospheres to Collect Gases or Vapors with Solid Sorbent Diffusive Samplers
D4598 Practice for Sampling Workplace Atmospheres to Collect Gases or Vapors with Liquid Sorbent Diffusional Samplers
(Withdrawn 1995)
D4599 Practice for Measuring the Concentration of Toxic Gases or Vapors Using Length-of-Stain Dosimeters
This practice is under the jurisdiction of ASTM Committee D22 on Air Quality and is the direct responsibility of Subcommittee D22.04 on Workplace Air Quality.
Current edition approved April 1, 2013. Published April 2013. Originally approved in 1998. Last previous edition approved in 2008 as D6246 - 08.D6246 – 08. DOI:
10.1520/D6246-08R13.10.1520/D6246-08R13E01.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
D6246 − 08 (2013)
2.2 International Standards:
CEN EN 838 European Standard, Workplace atmospheres - Diffusive samplers for the determination of gases or vapours -
Requirements and test methods
MDHS 27 Protocol for assessing the performance of a diffusive sampler, Health and Safety Laboratory, United Kingdom
MDHS 80 Volatile organic compounds in air, Health and Safety Laboratory, United Kingdom
3. Terminology
3.1 Definitions:
3.1.1 For definitions of terms used in this practice, refer to Terminology D1356.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 Symmetric Accuracy Range A—the fractional range, symmetric about the true concentration c, within which 95 % of
sampler measurements are to be found (2-(414-19).). In terms of the bias Δ relative to true concentrations and the total (true)
relative standard deviation RSD (sometimes designated as TRSD), the accuracy range A is closely approximated ((419)) by:
2 2
1.960 3=Δ 1RSD , if ?Δ?,RSD/1.645
A 5 (1)
H
Δ?11.645 3RSD, otherwise
?
Available from CEN Central Secretariat, rue de Stassart 36, B-1050 Brussels, Belgium.
Available from HMSO Books, PO Box 276, London, England, SW8 5DT.
3.2.1.1 Discussion—
In the case that bias is corrected, leaving only an uncorrectable residual bias due to uncertainty in the correction, 95 %-confidence
limits on A play the role of the expanded uncertainty in ((520).). As described in ((214),), such an interpretation is an extension
of ((520)) for measurement, as in occupational hygiene, of concentrations which are neither spatially nor temporally constant.
Rather than continually re-evaluating a method through estimate replicates, the accuracy provides confidence intervals bracketing
(true) concentrations at greater than a given probability (95 %) for a fixed confidence (95 %) in the initial sampler evaluation. Such
intervals with double confidence levels (in both measurement and evaluation) are related to a branch of statistics known as the
theory of tolerance or prediction intervals.
3.2.2 diffusive sampler—a device which is capable of taking samples of gases or vapors from the atmosphere at a rate controlled
by a physical process such as gaseous diffusion through a static air layer or permeation through a membrane, but which does not
involve the active movement of air through the sampler. As such, direct-reading dosimeters, as well as samplers requiring lab
analysis, are considered diffusive samplers within this practice.
3.3 Symbols:
A = symmetric accuracy range as defined in terms of bias and imprecision
 = estimated symmetric accuracy range A
A = 95 % confidence limit on the symmetric accuracy range A
95 %
c(mg/m ) = true or reference analyte concentration
ĉ(mg/m ) = mean of (four) concentration estimates (including (p, T)-corrections) obtained in accordance with instructions of
sampler manufacturer
h = humidity (expressed as partial pressure)
n = number of diffusive samplers tested for measuring sampler capacity
p = atmospheric pressure
RSD = overall (true) relative standard deviation of concentration estimates (dependent on assumed environmental
variability) expressed relative to a “true” concentration
RSD = relative standard deviation characterizing inter-run chamber variability
run
RSD = inter-sampler imprecision (relative to the reference concentration)
s
RŜD = estimated inter-sampler imprecision RSD
s s
RSD = pulse-induced imprecision
t
RŜD = estimated overall relative standard deviation RSD
RŜD = 95 % confidence limit on the overall relative standard deviation RSD
95 %
s = estimated standard deviation characterizing inter-sampler imprecision
t (υ) = value which, at probability 95 %, exceeds random variables distributed according to the studentized t-distribution
0.95
with υ degrees of freedom
T = temperature
v (m/s) = ambient wind speed
α = concentration estimate dependence on environmental variable x (T, h, v, or c).
x
Δ = bias relative to reference concentration c
´1
D6246 − 08 (2013)
ˆ
= estimated bias Δ
Δ
ˆ
= 95 % confidence limit on the bias Δ
Δ
95 %
Δ = bias associated with concentration pulse
t
υ = degrees of freedom in determining RSD
s
υ = effective number of degrees of freedom in determining RSD
eff
σ = assumed concentration variability
c
σ = assumed humidity variability
h
σ = assumed temperature variability
T
σ = assumed ambient wind speed variability
v
4. Summary of Test Method
4.1 Bias, Inter-sampler Imprecision and the Effects of Environmental Uncertainty:
4.1.1 This practice gives a procedure for assessing the effects of variability in the following workplace variables: temperature
T, humidity h (expressed in terms of the water vapor partial pressure to minimize interaction with the temperature), the ambient
wind speed v across the sampler face (see 4.7 regarding wind direction), and concentration c. An experiment is carried out which
provides information about the concentration estimates’ dependencies on these variables near conditions of intended sampler use
(T , h , v , and c ). Testing is required at the concentration c of intended use, as well as at concentrations reduced at least to c /2.
0 0 0 0 0 0
Furthermore, the sampler bias and the inter-sampler standard deviation are measured. Finally, the effect of diffusion of material
out of the sampler is measured. Pressure effects result in correctable bias and 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 α , α , α , and α (relative
T h v c
to the chamber reference concentration and target environmental parameters) to changes in T,h,v, and c are measured, following
the sampler manufacturer’s instructions regarding p- and T- corrections (if any). These experiments also give a value for the
estimated sampler bias Δ relative to the chamber reference concentration (defined for the target conditions). Two further runs
describing time-effects (4.2.5) from diffusive loss of analyte are also carried out. The chamber reference concentration must be
traceable to primary standards of mass and volume.
4.1.3 Error in the estimates of the sensitivities α , α , α , and α will exist on account of inter-sampler relative standard deviation
T h v c
RSD and an inter-run chamber standard deviation RSD . The latter results in part from uncertainty in the reference concentration.
s run
RSD is obtained by pooling the variance estimates from each run and therefore is estimated with 7 × 3 = 21 degrees of freedom
s
(or 15 degrees of freedom if the reverse diffusion experiment is omitted (1.3)).(1.3)). So as to avoid re-measurement at each
sampler/analyte evaluation, RSD is obtained by a separate characterization of the chamber with several runs at (for example)
run
fixed environmental conditions. An example in which the sensitivities α and RSD , are estimated is presented in the Annex A1.
s
NOTE 1—It is up to the user as to how traceability is established. Within (612) the concentration estimate as calculated from the chamber’s analyte
generation parameters is regarded as the benchmark, although an independent estimate is required and must be within 5 % of the calculated estimate. If
these estimates differ, then a third independent estimate is required to establish the reference concentration through agreement with one of the other
independent estimates. One possibility for such an independent estimate is the mean of at least five independent, active sampler estimates per run within
the chamber. Experiment ((612)) on the accuracy of such reference measurements using sorbent tubes indicates that a relative standard deviation of the
order of 2 % can be achieved for the individual measurements. Alternatively, ((73)) requires averaging of at least two independent methods (possibly
including calculated estimates) with at least four samples per method. EN 838 has adopted the looser requirement that calculated and independent
measurements must agree within 10 %.
4.1.3.1 A further consolidation of tests may be made by observing that the dependence of concentration estimates on the wind
speed, v, is only sampler specific, that is, does not depend on the specific analyte. Therefore, after a single measurement for a given
sampler type, the set of tests can be narrowed.
4.2 Reverse Diffusion:
4.2.1 A potential problem with diffusive samplers is presented by the possibility of reverse diffusion (sometimes denoted as back
diffusion or off-gassing) of analyte. Reverse diffusion is generally only significant in the case that an analyte is weakly bound to
the sorbent ((86).). Therefore, inaccuracy associated with these effects may generally be minimized through proper sorbent
selection and sampler design.
4.2.2 Because of reverse diffusion, estimates of a varying concentration may in some cases be biased. The worst-case situation
occurs with the concentration in the form of an isolated pulse at either the beginning or end of the sampling period. A pulse at the
beginning of the period allows the entire sampling period (4 to 12 h) for sample loss, possibly resulting in a low estimate relative
to a pulse at the end of the period.
4.2.3 In some cases, the time-dependence of a workplace concentration correlates strongly with the sampling period. For
example, a cleanup operation at the end of a workday could introduce solvent only then. This could imply a positive bias in the
concentration estimates obtained from a day’s sampling. For simplicity, however, this practice is set up for assessing performance
of samplers for use in a concentration with stationary fluctuations, so that time-dependent effects
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