EN 14662-4:2005
(Main)Ambient air quality - Standard method for measurement of benzene concentrations - Part 4: Diffusive sampling followed by thermal desorption and gas chromatography
Ambient air quality - Standard method for measurement of benzene concentrations - Part 4: Diffusive sampling followed by thermal desorption and gas chromatography
This part of EN 14662 is in accordance with the generic methodology selected as the basis of the European Union reference method for the determination of benzene in ambient air [1] for the purpose of comparison of measurement results with limit values with a one-year reference period.
This part of EN 14662 gives general guidance for the sampling and analysis of benzene in air by diffusive sampling, thermal desorption and capillary gas chromatography.
This part of EN 14662 is valid for the measurement of benzene in a concentration range of approximately 0,5 mg/m3 to 50 mg/m3 in an air sample typically collected over a period of 14 days.
The upper limit of the useful range is set by the sorptive capacity of the sorbent and by the linear dynamic range of the gas chromatograph column and detector or by the sample splitting capability of the analytical instrumentation used. The lower limit of the useful range depends on the noise level of the detector and on blank levels of benzene and/or interfering artefacts on the sorbent. Artefacts are typically sub ng for sorbents such as graphitised carbon, but higher levels of aromatic hydrocarbons have been noted in other sorbents.
Außenluftbeschaffenheit - Standardverfahren zur Bestimmung von Benzolkonzentrationen - Teil 4: Diffusionsprobenahme mit anschließender Thermodesorption und Gaschromatographie
Dieser Teil von prEN 14662 ist in Übereinstimmung mit der allgemeinen Methode, die als Grundlage des
Referenzverfahrens der Europäischen Union zur Bestimmung von Benzol in Außenluft [ ] gewählt wurde, um Messergebnisse mit Grenzwerten auf Basis von Jahresmittelwerten zu vergleichen.
Dieser Teil von prEN 14662 gibt allgemeine Anleitungen für die Probenahme und Analyse von Benzol in Luft durch Diffusionsprobenahme, Thermodesorption und Kapillar-Gaschromatographie.
Dieser Teil von prEN 14662 ist anwendbar auf die Bestimmung von gasförmigem Benzol in einem Massenkonzentrationsbereich von etwa 0,5 µg/m³ bis 50 µg/m³ in einer Luftprobe, die üblicherweise über vier Wochen genommen wurde.
Die obere Grenze des anwendbaren Bereiches wird durch die Sorptionskapazität des verwendeten Sorbens und dem linearen dynamischen Bereich der gaschromatographischen Säule und des Detektors bzw. vom Trennvermögen der verwendeten Analysengeräte angegeben. Die untere Grenze des anwendbaren Bereiches hängt vom Rauschpegel des Detektors und von den Benzolblindwerten und/oder störenden Artefakten auf dem Sorbens ab. Der Gehalt an Artefakten liegt üblicherweise im unteren Nanogrammbereich für Sorbenzien wie graphitisiertem Kohlenstoff. Höhere Gehalte an aromatischen Kohlenwasserstoffen wurden in anderen Sorbenzien gefunden.
Qualité de l'air ambiant - Méthode normalisée pour le mesurage de la concentration en benzène - Partie 4: Prélèvement par diffusion suivi d'une désorption thermique et d'une analyse par chromatographie en phase gazeuse
La présente partie de l’EN 14662 est conforme à la méthodologie générique sélectionnée comme base pour la Méthode de référence de l’Union européenne de détermination du benzène dans l’air ambiant [1], en vue de la comparaison des résultats de mesurage avec des valeurs limites pour une période de référence d’un an.
La présente partie de l’EN 14662 fournit des indications pour le prélèvement et l’analyse du benzène dans l’air par le prélèvement par diffusion, désorption thermique et chromatographie en phase gazeuse sur colonne capillaire.
La présente partie de l’EN 14662 s’applique au mesurage du benzène présent dans un prélèvement d’air généralement prélevé pendant une période de 14 jours, pour une concentration comprise entre 0,5 µg/m3 et 50 mg/m3 environ.
La limite supérieure de la gamme utile est déterminée par la capacité d’adsorption du sorbant et la gamme linéaire dynamique de la colonne du chromatographe en phase gazeuse et du détecteur ou de la division appliquée (SPLIT). La limite inférieure de la gamme utile dépend du niveau de bruit du détecteur et des niveaux de blanc en benzène et/ou des artéfacts d’interférence du sorbant. Les artéfacts sont généralement inférieurs au nanogramme pour les sorbants, mais des niveaux plus élevés d’hydrocarbures aromatiques ont été notés dans d’autres sorbants.
Kakovost zunanjega zraka – Standardna metoda za določanje koncentracije benzena – 4. del: Difuzijsko vzorčenje in določanje s plinsko kromatografijo po termični desorpciji
General Information
Standards Content (Sample)
SLOVENSKI STANDARD
01-september-2005
.DNRYRVW]XQDQMHJD]UDND±6WDQGDUGQDPHWRGD]DGRORþDQMHNRQFHQWUDFLMH
EHQ]HQD±GHO 'LIX]LMVNRY]RUþHQMHLQGRORþDQMHVSOLQVNRNURPDWRJUDILMRSR
WHUPLþQLGHVRUSFLML
Ambient air quality - Standard method for measurement of benzene concentrations - Part
4: Diffusive sampling followed by thermal desorption and gas chromatography
Außenluftbeschaffenheit - Standardverfahren zur Bestimmung von
Benzolkonzentrationen - Teil 4: Diffusionsprobenahme mit anschließender
Thermodesorption und Gaschromatographie
Qualité de l'air ambiant - Méthode normalisée pour le mesurage de la concentration en
benzene - Partie 4: Prélevement par diffusion suivi d'une désorption thermique et d'une
analyse par chromatographie en phase gazeuse
Ta slovenski standard je istoveten z: EN 14662-4:2005
ICS:
13.040.20 Kakovost okoljskega zraka Ambient atmospheres
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
EUROPEAN STANDARD
EN 14662-4
NORME EUROPÉENNE
EUROPÄISCHE NORM
May 2005
ICS 13.040.20
English version
Ambient air quality - Standard method for measurement of
benzene concentrations - Part 4: Diffusive sampling followed by
thermal desorption and gas chromatography
Qualité de l'air ambiant - Méthode pour le mesurage des Luftbeschaffenheit - Standardverfahren zur Bestimmung
concentrations en benzène - Partie 4 - Echantillonnage par von Benzolkonzentrationen - Teil 4: Diffusionsprobenahme
diffusion suivi d'une désorption thermique et d'une mit anschließender Thermodesorption und
chromatographie en phase gazeuse Gaschromatographie
This European Standard was approved by CEN on 21 March 2005.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the Central Secretariat or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN 14662-4:2005: E
worldwide for CEN national Members.
Contents Page
Foreword .3
1 Scope .4
2 Normative references .4
3 Terms and definitions.4
4 Method description.6
4.1 Principle.6
4.2 Reagents and materials.7
4.3 Apparatus .9
4.4 Sample tube conditioning .10
4.5 Sampling.10
4.6 Procedure .11
4.7 Calculation of mass concentration of benzene .13
4.8 Report .14
5 Determination of measurement uncertainty .14
5.1 Introduction.14
5.2 Parameters contributing to measurement uncertainty .14
6 Recommendations for use .16
Annex A (informative) Diffusive uptake rates for benzene on sorbent tubes (without membrane) .17
Annex B (informative) Description of sorbent types.18
Annex C (informative) Guidance on sorbent selection.19
Annex D (informative) Guide on sorbent use .20
Annex E (informative) Assessment of performance indicators and uncertainty contributions .21
Annex F (informative) Performance characteristics .30
Bibliography.32
Foreword
This European Standard (EN 14662-4:2005) has been prepared by Technical Committee CEN/TC 264 “Air
Quality”, the secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of an identical
text or by endorsement, at the latest by November 2005, and conflicting national standards shall be withdrawn
at the latest by November 2005.
This document has been prepared under a mandate given to CEN by the European Commission and the
European Free Trade Association, and supports essential requirements of EU Directive 2000/69/EC and EU
Directive 96/62 EC.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following
countries are bound to implement this European Standard: Austria, Belgium, Cyprus, Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland
and United Kingdom.
1 Scope
This part of EN 14662 is in accordance with the generic methodology selected as the basis of the European
Union for the determination of benzene in ambient air [1] for the purpose of comparison of measurement
results with limit values with a one-year reference period.
This part of EN 14662 gives general guidance for the sampling and analysis of benzene in air by diffusive
sampling, thermal desorption and capillary gas chromatography.
This part of EN 14662 is valid for the measurement of benzene in a concentration range of approximately 0,5
3 3
µg/m to 50 µg/m in an air sample typically collected over a period of 14 days.
The upper limit of the useful range is set by the sorptive capacity of the sorbent and by the linear dynamic
range of the gas chromatograph column and detector or by the sample splitting capability of the analytical
instrumentation used. The lower limit of the useful range depends on the noise level of the detector and on
blank levels of benzene and/or interfering artefacts on the sorbent. Artefacts are typically sub ng for sorbents
such as graphitised carbon, but higher levels of aromatic hydrocarbons have been noted in other sorbents.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ENV 13005:1999, Guide to the expression of uncertainty in measurement
EN 13528-2:2002, Ambient air quality - Diffusive samplers for the determination of concentrations of gases
and vapours - Requirements and test methods. Part 2: Specific requirements and test methods
EN 13528-3:2003, Ambient air quality - Diffusive samplers for the determination of concentrations of gases
and vapours - Requirements and test methods - Part 3: Guide to selection, use and maintenance
EN ISO 16017-2, Indoor, ambient and workplace air – Sampling and analysis of volatile organic compounds
by sorbent tube/thermal desorption/capillary gas chromatography – Part 2: Diffusive sampling (ISO 16071-
2:2003)
EN ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories ( ISO/IEC
17025:1999)
ISO 5725-2:1994, Accuracy (trueness and precision) of measurement methods and results - Part 2: Basic
method for the determination of repeatability and reproducibility of a standard measurement method
ISO 5725-3:1995, Accuracy (trueness and precision) of measurement methods and results - Part 3:
Intermediate measures of the precision of a standard measurement method
3 Terms and definitions
For the purposes of this European Standard, the following definitions apply.
NOTE: Attention is drawn to the fact that the terms Ambient Air and Limit Value are defined in Directive 96/62/EC [2].
3.1
certified reference material
A reference material [3.7], accompanied by a certificate, one or more of whose property values are certified by
a procedure which establishes its traceability to an accurate realisation of the unit in which the property values
are expressed, and for which each certified value is accompanied by an uncertainty at a stated level of
confidence.
[ISO Guide 30:1992]
3.2
combined standard uncertainty
Standard uncertainty of the result of a measurement when that result is obtained from the values of a number
of other quantities, equal to the positive square root of a sum of terms, the terms being the variances or
covariances of these other quantities weighted according to how the measurement result varies with changes
in these quantities
[ENV 13005:1999]
3.3
desorption efficiency
Ratio of the mass of analyte desorbed from a sampling device to that applied
[EN 838:1995]
3.4
diffusive sampler
A device which is capable of taking samples of gases or vapours from the atmosphere at a rate controlled by
a physical process such as gaseous diffusion through a static air layer or a porous material and/or permeation
through a membrane, but which does not involve the active movement of air through the device
[EN 13528-1]
NOTE 1 Active normally refers to the pumped movement of air.
NOTE 2 This definition differs from that in EN 838:1995 by the addition of the words “or a porous material”.
3.5
diffusive uptake rate
The rate at which the diffusive sampler collects a particular gas or vapour from the atmosphere, expressed in
picograms per parts per billion per minute (pg/ppb/min) or cubic centimetres per minute (cm /min)
NOTE 1 pg/ppb/ min are equivalent to ng/ppm/ min.
NOTE 2 This definition differs from that in EN 838:1995 by the substitution of “picograms per parts per billion” for
“nanograms per parts per million”. The expression is numerically the same, but ambient concentrations are usually in the
ppb range.
3.6
expanded uncertainty
Quantity defining an interval about the result of a measurement that may be expected to encompass a large
fraction of the distribution of values that could reasonably be attributed to the measurand
[ENV 13005:1999]
NOTE 1 The fraction may be viewed as the coverage probability or level of confidence of the interval.
-9 -6
ppb is volume fraction, (φ)=10 ; ppm is volume fraction, (φ)=10 .
NOTE 2 To associate a specific level of confidence with the interval defined by the expanded uncertainty requires explicit
of implicit assumptions regarding the probability distribution characterised by the measurement result and its combined
standard uncertainty. The level of confidence that can be attributed to the interval can be known only to the extent to which
such assumptions may be justified.
NOTE 3 Expanded uncertainty is termed overall uncertainty in ENV 13005:1999.
3.7
reference material
A material or substance, one or more of whose property values are sufficiently homogeneous and well
established to be used for the calibration of an apparatus, the assessment of a measurement method, or for
assigning values to materials.
[ISO Guide 30:1992]
3.8
repeatability conditions
Conditions where independent test results are obtained with the same method on identical test items in the
same laboratory by the same operator using the same equipment within short intervals of time
[ISO 3534-1:1993]
3.9
standard uncertainty
Uncertainty of the result of a measurement expressed as a standard deviation
[ENV 13005:1999]
3.10
uncertainty (of measurement)
Parameter, associated with the results of a measurement, that characterises the dispersion of values that
could reasonably be attributed to the measurand
NOTE 1 The parameter may be, for example, a standard deviation (or given multiple of it), or the half width of an
interval having a stated level of confidence.
NOTE 2 Uncertainty of measurement comprises, in general, many components. Some of these components may be
evaluated from the statistical distribution of the results of a series of measurements and can be characterised by
experimental standard deviations. The other components, which can also be characterised by standard deviations, are
evaluated from assumed probability distributions based on experience or other information.
NOTE 3 It is understood that the result of a measurement is the best estimate of the value of a measurand, and that all
components of uncertainty, including those arising from systematic effects, such as components associated with
corrections and reference standards, contribute to this dispersion [ENV 13005:1999].
4 Method description
4.1 Principle
The diffusive sampler is exposed to air for a measured time period. [EN ISO 16017-2]. The rate of sampling is
determined by prior calibration in a standard atmosphere (4.2.5) The benzene vapour migrates down the tube
by diffusion and is collected on the sorbent. The collected vapour (on each tube) is desorbed by heat and is
transferred under inert carrier gas into a gas chromatograph equipped with a capillary column and a flame
ionization detector or other suitable detector, where it is analysed. The analysis is calibrated by means of
liquid or vapour spiking onto a sorbent tube or from comparative field trials.
Information on possible saturation of the sorbent bed, the effect of transients and the effect of face velocity is
given in EN 13528-3. This also explains the dependence of diffusion uptake rates on the concentration level
of pollutants and the time of diffusive sampling, for non-ideal sorbents, which results in different values being
given in Annex A. The theory of performance of diffusive samplers is also given in EN 13528-3.
4.2 Reagents and materials
During the analysis, use only reagents of recognised analytical reagent grade.
Use only volumetric glassware and syringes that are calibrated to ensure traceability of volume to primary
standards.
4.2.1 Benzene
Benzene is required as a reagent for calibration purposes, using either liquid spiking (4.2.7) or vapour spiking
(4.2.6) onto sorbent tubes.
4.2.2 Dilution solvent
A dilution solvent is required for preparing calibration blend solution for liquid spiking (4.2.7). This should be of
chromatographic quality. It shall be free from compounds co-eluting with benzene.
NOTE Methanol is frequently used. Alternative solvents may be used provided they do not interfere with the gas
chromatographic analysis, either by co-elution or by altering detector response.
4.2.3 Sorbents
A particle size 0,18 mm to 0,25 mm (60 to 80 mesh) is recommended. Sorbent particle sizes larger than 0,18
to 0,25 mm may be used. Smaller sorbent particle size ranges are not recommended because of back
pressure problems. Each sorbent should be preconditioned under a flow of inert gas by heating it at a
°
temperature at least 25 C below the published maximum for that sorbent overnight before packing the tubes.
To prevent recontamination of the sorbents, they shall be kept in a clean atmosphere during cooling to room
temperature, storage, and loading into the tubes. Wherever possible, analytical desorption temperatures
should be kept below those used for conditioning. Tubes prepacked by the manufacturer are also available for
most sorbents and as such only require conditioning.
NOTE A description of sorbents is given in Annex B and a guide for sorbent selection is given in Annex C. Equivalent
sorbents may be used. A guide to sorbent conditioning and analytical desorption parameters is given in Annex D.
4.2.4 Calibration standards
Calibration standards are preferably prepared by sampling a known volume from standard atmospheres
(4.2.6) as this procedure most closely resembles the practical sampling situation.
If this way of preparation is not practicable, standards may be prepared by a liquid spiking procedure (4.2.7,
4.2.8) provided that the accuracy of the spiking technique is either:
a) established by using procedures giving spiking levels fully traceable to primary standards of mass
and/or volume, or,
b) confirmed by comparison with certified reference materials if available, standards produced using
standard atmospheres, or results of reference measurement procedures.
To minimise matrix effects calibration tubes should be closely matched to the sample tubes with respect to
sorbent type, particle size and mass.
4.2.5 Standard atmospheres
Prepare standard atmospheres of known concentrations of benzene by a recognised procedure. Methods
described in ISO 6144 and ISO 6145 are suitable. If the procedure is not applied under conditions that will
allow the establishment of full traceability of the generated concentrations to primary standards of mass and/or
volume the concentrations need to be confirmed using an independent procedure.
4.2.6 Standard sorbent tubes loaded from standard atmospheres
Prepare loaded sorbent tubes by passing an accurately known volume of the standard atmosphere through
the sorbent tube e.g. by means of a pump. The volume of atmosphere sampled shall not exceed the
breakthrough volume of the sorbent. After loading the tube is disconnected and sealed. Prepare fresh
standards representing benzene levels in the samples corresponding to the concentration range of 0,5 µg/m
to 50 µg/m with each batch of samples, or use spiked control samples to determine consistency of detector
response.
4.2.7 Preparation of standard solutions for liquid spiking, gravimetric procedure.
4.2.7.1 Solution containing approximately 10 mg/ml benzene
Weigh a 100 ml volumetric flask and introduce approximately 75 ml of dilution solvent. Then accurately weigh
approximately 1 g benzene into the flask. Make up to 100 ml with dilution solvent, stopper or cap, weigh and
shake to mix.
4.2.7.2 Solution containing approximately 100 µµµµg/ml benzene
Weigh a 100 ml volumetric flask and introduce approximately 75 ml of dilution solvent. Then accurately weigh
approximately 1 ml of solution described in 4.2.7.1 into the flask. Make up to 100 ml with dilution solvent,
stopper or cap, weigh and shake to mix
4.2.7.3 Solution containing approximately 50 µµµµg/ml benzene
Introduce 30 ml of dilution solvent into a preweighed 100 ml volumetric flask. Weigh in 50 ml of solution
described in 4.2.7.2. Make up to 100 ml with dilution solvent, stopper or cap, weigh and shake to mix.
4.2.7.4 Solution containing approximately 10 µµµµg/ml benzene
Introduce 50 ml of dilution solvent into a preweighed 100 ml volumetric flask. Weigh in 10 ml of solution
described in 4.2.7.2. Make up to 100 ml with dilution solvent, stopper or cap, weigh and shake to mix
4.2.7.5 Solution containing approximately 5 µµµµg/ml benzene
Introduce 50 ml of dilution solvent into a preweighed 100 ml volumetric flask. Weigh in 5 ml of solution
described in 4.2.7.2. Make up to 100 ml with dilution solvent, stopper or cap, weigh and shake to mix.
4.2.7.6 Solution containing approximately 1 µµg/ml benzene
µµ
Introduce 50 ml of dilution solvent into a preweighed 100 ml volumetric flask. Weigh in 1 ml of solution
described in 4.2.7.2. Make up to 100 ml with dilution solvent, stopper or cap, weigh and shake to mix.
4.2.8 Preparation of standard solutions for liquid spiking, volumetric procedure
Alternatively, calibration solutions may be prepared by serial dilution of a stock solution of benzene in a
dilution solvent using volumetric glassware and syringes that are traceably calibrated. The calibration may be
performed by repeated weighing of the corresponding volume of water, using the appropriate specific density
to calculate the volume of the glassware. When preparing solutions in carbon disulphide by volumetry the
temperature in the working room shall be controlled to within ± 2 K in order to limit the effect of variations of
the specific density of the dilution solvent.
The dilution steps described in clause 4.2.7 may be used to prepare calibration standard solutions in the
appropriate benzene concentration range, using calibrated flasks, vials, syringes and pipettes.
4.2.9 Stability of standard solutions
Fresh standard solutions should be prepared weekly, or more frequently if evidence is noted of deterioration.
4.2.10 Standard sorbent tubes loaded by liquid spiking
Loaded sorbent tubes are prepared by injecting aliquots of standard solutions onto clean sorbent tubes as
follows. A sorbent tube is fitted to a T piece of which one end is fitted with a septum, or injection facility of a
gas chromatograph through which inert purge gas is passed at 100 ml/min. Inject an aliquot of an appropriate
standard solution through the septum and purge for 5 minutes. The tube is then disconnected and sealed.
Prepare fresh standards with each batch of samples, or use spiked control samples to determine consistency
of detector response.
NOTE In the case of methanol, a 5 µl aliquot, a purge gas flow of 100 ml/min and a 5 min purge time have been
found to be appropriate to eliminate most of the solution solvent from the tube if packed with Tenax. If other dilution
solvents and sorbents are used, the conditions should be determined experimentally.
NOTE A conventional gas chromatographic injection port may be used for preparing sample tube standards. This can
be used in situ, or it can be mounted separately. The carrier gas line to the injector should be retained. The back of the
injection port should be adapted if necessary to fit the sample tube. This can be done conveniently by means of a
compression coupling with a O-ring seal.
4.3 Apparatus
The following specific items of laboratory apparatus are required.
4.3.1 Sorbent tubes
These tubes shall be compatible with the thermal desorption apparatus to be used (4.3.8). Typically, they are
constructed of stainless steel tubing, 6,4 mm (1/4 inch) OD, 5 mm ID and 90 mm long. Tubes of other
dimensions may be used but the uptake rates given in Annex A are based on these tube dimensions. One
end of the tube is marked, for example by a scored ring about 10 mm from the (diffusion) sampling end. The
tubes are packed with preconditioned sorbents so that the sorbent bed will be within the desorber heated zone
and a consistent gap of about 14 mm is retained at the marked (diffusion) end of the tube, when measured
with the diffusion cap removed.
NOTE Uptake rates in Annex A are given for tubes with a nominal air gap (between sorbent bed and diffusion end
cap) of 15 mm. In practice, packed tube dimensions will vary and tubes should be rejected where the air gap (between
stainless steel screen retaining the sorbent bed and the end of the tube) is outside the range 14,0 mm to 14,6 mm, when
measured with the diffusion cap removed.
Tubes contain between 200 mg and 1000 mg sorbent, depending on sorbent density - typically about 250 mg
porous polymer, or 500 mg carbon molecular sieve or graphitised carbon. The sorbents are retained by a
stainless steel gauze at the diffusion end and an unsilanised glass wool plug and/or a second stainless gauze
at the other
4.3.2 Sorbent tube end caps
The tubes shall be sealed to prevent significant contamination, e. g. with metal screw cap fittings with PTFE
ferrules.
4.3.3 Diffusion end caps
The diffusion end cap is similar to 4.3.2, but allows the ingress of vapour through a metal gauze, the size of
the opening being the same as the cross-section of the tube.
Some versions of the end cap incorporate a silicone membrane next to the gauze.
4.3.4 Syringes
A precision 10 µl liquid syringe readable to 0,1 µl. The volume of the liquid delivered by the syringe shall be
calibrated by gravimetry.
4.3.5 Precipitation shield
A protective cover to prevent the entrance of particles or water droplets into the sampling tube during the
sampling.
NOTE EN 13528-3 describes various shields for diffusive samplers.
4.3.6 Support
A device capable of positioning the sorbent tube at the appropriate height and distance from obstacles to
warrant undisturbed sampling.
4.3.7 Gas chromatograph
A gas chromatograph fitted with a flame ionization, photo ionization, mass spectrometric or other suitable
detector, capable of detecting an injection of 1 ng benzene with a signal-to-noise ratio of at least 10:1.
A gas chromatograph column capable of separating benzene from other components.
4.3.8 Thermal desorption apparatus
Apparatus for the two-stage thermal desorption of the sorbent tubes and transfer of the desorbed vapour via
an inert gas flow into a gas chromatograph shall be required. A typical apparatus contains a mechanism for
holding the tubes to be desorbed whilst they are heated and purged simultaneously with inert carrier gas. The
desorption temperature and time is adjustable, as is the carrier gas flow rate. The apparatus should also
incorporate additional features, such as automatic sample tube loading, leak testing, and a cold trap in the
transfer line to concentrate the desorbed sample (4.6.2). The desorbed sample, contained in the purge gas, is
routed to the gas chromatograph and capillary column via a heated transfer line.
4.4 Sample tube conditioning
Prior to use, tubes shall be conditioned by desorbing them at a temperature at or just above the analytical
desorption temperature (see Annex D). Typical conditioning time is 10 min with a carrier gas flow of at least
100 ml/min. The carrier gas flow should be in a direction opposite to that used during sampling. Tubes should
then be analysed, using routine analytical parameters, to ensure that the thermal desorption blank is
sufficiently small. If the blank is unacceptable, tubes should be reconditioned by repeating this procedure.
Once a sample has been analysed, the tube may be reused to collect a further sample immediately. However,
it is advisable to check thermal desorption blank if the tubes are left for an extended period before reuse, or if
sampling for a different analyte is envisaged. Tubes should be sealed with metal screw caps with combined
PTFE ferrule fittings and stored in an airtight container when not sampling or being conditioned.
4.5 Sampling
Select a sorbent tube (or tubes) appropriate for sampling benzene. Guidance on suitable sorbents is given in
Annex C.
Immediately before sampling, remove the storage end cap from the marked end of the sample tube and
replace it with a diffusion end cap. Make sure the diffusion cap is properly seated and the other end cap is in
place.
For sampling ambient air recommendations for site selection and for the protection of samples from adverse
environmental conditions are given in EN 13528-3. Attention has to be paid to three main considerations: air
velocity, protection from precipitation and security.
Expose the sampling tubes only under conditions where the face velocity requirement can be expected to be
satisfied. For the tubes specified in 4.3.1 with end caps 4.3.3, wind speed (air velocity) has no influence. Other
devices may have different requirements, including also a minimum wind speed.
Instruments to measure wind speeds as low as 0.007 m/s are not commonly available, so the wind speed may
have to be measured indirectly. The user is also cautioned about the possible influence of very high wind
speeds (above 12 m/s) for which performance characteristics are currently unavailable.
The recommended exposure time is two weeks. Sampling over longer or shorter periods is possible, but the
working range will vary accordingly.
Knowledge of the average temperature and barometric pressure of the sampled air is required in order to
express concentrations reduced to standard conditions (4.7). This information may be obtained from
measurements on the site using a traceably calibrated thermometer and barometer. Alternatively information
from a nearby weather station may be used.
Replace the diffusion end cap with a storage end cap and tighten the seal securely. The tubes should be
uniquely labelled. Solvent containing paints and markers or adhesive labels should not be used to label the
tubes.
Field blanks shall be prepared by using tubes identical to those used for sampling and subjecting them to the
same handling procedure as the sample tubes except for the actual period of sampling. Label these as blanks.
4.6 Procedure
4.6.1 Safety precautions
This part of EN 14662 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 health and safety practices and
determine the applicability of regulatory limitations prior to use.
4.6.2 Desorption and analysis
The sorbent tube is placed in a compatible thermal desorption apparatus. Air is purged from the tube to avoid
chromatographic artefacts arising from the thermal oxidation of the sorbent or gas chromatographic stationary
phase. The tube is then heated to displace the organic vapours which are passed to the gas chromatograph
by means of a carrier gas stream. The gas flow at this stage should be the reverse of that used during
sampling, i.e. the marked end of the tube should point towards the gas chromatograph column inlet. The gas
flow through the tube should be typically 30 ml/min to 50 ml/min for optimum desorption efficiency.
NOTE 1 For the air purge described above it is usually necessary to use 10 x the tube volume (i.e. 20 ml to 30 ml) of
inert gas to completely displace the volume of air (2 ml to 3 ml) in the tube. However, if strongly hydrophilic sorbents are
used, it may be necessary to employ a larger purge volume to reduce sorbed air and water to prevent ice formation
blocking the cold trap. During the purge period care should be taken to minimise heating of the tube.
The desorbed sample occupies a volume of several millilitres of gas, so that pre-concentration is essential
prior to capillary GC analysis. This may be achieved using a small, cooled, secondary sorbent trap which can
be desorbed sufficiently rapidly at low flow rates (< 5 ml/min) to minimise band broadening and produce
capillary compatible peaks. Alternatively an empty secondary trap, or one containing an inert material such as
glass beads, may be used to pre-concentrate the sample, but such traps typically require cooling to below -
100°C.
NOTE 2 If a secondary sorbent trap is not available and if sub-zero capillary cryofocussing temperatures are used to
preconcentrate the analytes, water should be eliminated from the sample tube prior to desorption in order to prevent ice
formation blocking the capillary tubing and stopping the thermal desorption process.
Desorption conditions should be chosen such that desorption from the sample tube is complete, and no
sample loss occurs in the secondary trap. Typical parameters are:
Desorption temp 180°C - 325°C
Desorption time 5 min -15 min
Desorption flow rate 30 ml/min - 50 ml/min
Secondary trap low +20°C to -180°C, depending on type of cold trap
Secondary trap 250°C - 350°C
Secondary trap sorbent typically same as tubes, 25 mg -100 mg
Carrier gas helium, 99.9995 % minimum
Split ratios Split ratios between the sample tube and secondary trap and between the
secondary trap and analytical column (if applicable) should be selected
dependent on expected atmospheric concentration. (See guidance from
respective manufacturers of the thermal desorption apparatus.)
Set the sample flow path temperature (transfer line temperature) high enough to prevent analyte condensation
but not so high as to cause degradation, e.g. 20°C below desorption temperature.
Set up the gas chromatograph for the analysis of benzene. A variety of chromatographic columns may be
used for the analysis. The choice will depend largely on which compounds, if any, are present that might
interfere in the chromatographic analysis. Typical examples, are 50 m x 0,22 mm fused silica columns with
thick-film (1 µm to 5 µm), dimethyl polysiloxane or 14 % cyanopropylphenyl polysiloxane stationary phase.
Typical operating conditions for these columns are a temperature program from 50°C to 250°C at 5°C /min,
with an initial hold time of 10 min at 50°C.
The capillary column or, preferably, a length of uncoated, deactivated fused silica, should be threaded back
through the transfer line from the thermal desorption apparatus to the gas chromatograph such that it reaches
as close as possible to the sorbent in the cold trap. Internal tubing shall be inert and dead volumes shall be
minimised. A split valve(s) is conveniently placed at the inlet and/or outlet of the secondary trap. The split
valve on the outlet of the secondary trap may be located either at the inlet or the outlet of the transfer line.
Split ratios depend on the application.
Correspondence of retention time on a single column should not be regarded as proof of identity.
4.6.3 Calibration
A full calibration using each standard should be performed at the start of the analysis and the calibration curve
generated. A single point calibration should be performed at every tenth sample in the batch and at the end of
the batch. If the drift in these single point calibration standards is ≥ ± 5% of value then a full calibration should
be undertaken.
Prepare a calibration function from the responses of the calibration standards of benzene and the
corresponding masses of benzene in the sorbent tube standards. Various functions, such as linear,
exponential or polynomial, may be more or less suitable, depending on the linearity of the detector response.
Application of weighted regression may be necessary to obtain an appropriate goodness of fit of the function
over the entire concentration range.
The goodness of fit of the calibration function shall be determined by examining the residuals calculated for
each calibration standard concentration level as the difference between the mass of benzene calculated by
application of the calibration function and the actual mass of benzene in the standard. The requirements given
in clause 5.2 shall be fulfilled.
4.6.4 Determination of sample concentration
Analyse the samples and sample blanks as described for the calibration standards in 4.6.2. Determine the
peak area and read from the calibration graph the mass of the analyte in the desorbed sample.
4.6.5 Determination of desorption efficiency
For the analysis of benzene by thermal desorption conditions may usually be set such that a desorption
efficiency of 100% is obtained. The efficiency of desorption may be checked by comparing the
chromatographic response of a sorbent tube standard (4.6.3) with that obtained by injecting aliquots of the
standard solutions or the atmosphere directly into the gas chromatograph. Thus prepare a second calibration
graph of peak area against mass of analyte as in 4.6.3 but using solutions 4.2.7 or 4.2.8 or standard
atmosphere 4.2.6. This calibration should be the same or nearly the same as that in 4.6.3. The desorption
efficiency is the response of a tube standard divided by that of the corresponding liquid standard injected
directly. If the desorption efficiency is less than 95 %, change the desorption parameters accordingly.
4.6.6 Calibration of uptake rate
The uptake rates given in Annex A. are for tubes with the dimensions in 4.3.1 without a membrane in the
diffusion end cap. For other specifications, it may be necessary to follow EN 13528-2 to determine the uptake
rate.
4.7 Calculation of mass concentration of benzene
Calculate the concentration of the benzene in the sampled air, in µg/m , by means of the following equation:
m
meas
C = ⋅10 (1)
m
U ⋅t
where:
C is the concentration of benzene in the air sampled, in µg/m ;
m
m is the mass of benzene present in the actual sample as found in 4.6.4, in µg;
meas
U is the diffusive uptake rate in cm /min under operational conditions;
t is the exposure time in min.
The results are reported at 293K and 101.3 kPa using the following equation.
101.3 T + 273
C =C ⋅ .⋅ (2)
c m
P 293
where:
C is the concentration of the benzene in air sampled, reduced to specified conditions, in µg/m .
c
P is the average pressure of the air sampled, in kPa.
T is the average temperature of the air sampled, in °C.
4.8 Report
The test report shall contain at least the following information:
a) complete identification of the sample;
b) reference to this part of EN 14662;
c) sampling location, sampling time period and the diffusive uptake rate used;
d) average barometric pressure and average temperature;
e) test result;
f) any unusual features noted during the determination;
5 Determination of measurement uncertainty
5.1 Introduction
The measurement of the concentration of benzene in ambient air has to fulfil the requirement of a maximum
uncertainty in the measured values prescribed by Directive 2000/69/EC. In order to fulfil this requirement, the
measurement uncertainty has to be assessed by methods described in ENV 13005, ISO 5725-2 or equivalent
documents. In practice, input data for uncertainty assessment may be obtained from different experimental
sources, e.g. validation studies (comprising laboratory tests, field tests and/or inter-laboratory comparisons) or
QA/QC procedures (including replicate measurements of blank and control samples and certified reference
materials, and calibration procedures).
In this Standard the uncertainty assessment is based on results from laboratory tests that are used to
determine performance characteristics of the method used. The uncertainty assessment is based on Equation
(1) that - in general terms - describes the measurement problem under consideration.
This information is supplemented by results from experiments that were performed in support of the validation
of the Standard Method.
This approach is not meant to exclude evaluations based on data from ongoing QA/QC procedures, field
studies or inter-laboratory comparisons as long as these evaluations are consistent with ENV 13005 and/or
ISO 5725.
5.2 Parameters contributing to measurement uncertainty
5.2.1 Parameters to be assessed and minimum requirements
Based on Eq. (1) the parameters given in Table 1 have been identified to contribute to the uncertainty of
benzene concentrations measured by diffusive sampling and subsequent sample analysis by thermal
desorption and gas chromatography.
For each of these parameters minimum requirements are given; these serve as the basis for the
establishment of ongoing QA/QC programmes: when uncertainties based on these minimum requirements are
calculated, combined and expanded according to the rules given in Annex E, the uncertainties of the
measured concentrations will fulfil the uncertainty requirement of Directive 2000/69/EC.
Table 1 - Uncertainty parameters and minimum requirements
Uncertainty parameter Symbol Section Minimum requirement
Effective uptake rate U E.2 Relative uncertainty ≤ ± 5 %
eff
Sampling time t E.3 Relative uncertainty ≤ ± 0,1%
Conversion to standard E.4 Relative uncertainty ≤ ± 4%
temperature and pressure
Mass of benzene in sample m E.6
sam
• Analyte stability A E.6.2 No significant difference between results of
analysis of samples before and after storage
D E.5 ≥ 98 % at the limit value with a relative
• Desorption efficiency
uncertainty of ≤ ± 3%
Mass of benzene measured m
meas
E.6.3
m E.6.3.1 Relative uncertainty ≤ ± 2%
• Mass of benzene in calibration
CS
standards
F E.6.3.2 Relative residuals over the calibration range
• Lack-of-fit of calibration function
≤ ± 3%; at the limit value ≤ ±2%
• Response drift between d E.6.3.3 ≤ ± 5%
calibrations
• Analytical repeatability w E.5 ≤ ± 3%
anal
• Selectivity R E.6.3.4 Resolution factor > 1
Mass of benzene in sample blank m E.7 ≤ 2 ng with a relative uncertainty of ≤ ± 1 ng
bl
Between-laboratory uncertainty 5.2.2
5.2.2 Between-laboratory uncertainty
The procedures described in Clause 4 are not restrictive but allow variations in approaches between
laboratories. In a limited series of inter-laboratory comparisons that have been performed within the frame of
the evaluation of the above standard method it has been found that – even for laborator
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