Assessment of uncertainty of benzene measurements by Radiello diffusive sampler

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Atmospheric Environment 42 (2008) 2555–2568 www.elsevier.com/locate/atmosenv

Assessment of uncertainty of benzene measurements by Radiello diffusive sampler Herve´ Plaisancea,, Thierry Leonardisa, Michel Gerbolesb a

Laboratoire Central de Surveillance de la Qualite´ de l’Air—Ecole des Mines de Douai, De´partement Chimie et Environnement, 941 rue Charles Bourseul, B.P. 838, 59508 Douai, France b European Commission—DG JRC, Via E. Fermi 2749, I-21027 Ispra, Italy Received 5 October 2007; received in revised form 26 November 2007; accepted 3 December 2007

Abstract The uncertainty of benzene measurements obtained by the analysis of thermally desorbable Radiello diffusive samplers was evaluated according to the recent standard EN 14662-4 [EN 14662-4, 2005. Ambient air quality. Standard method for measurement of benzene concentrations. Part 4: diffusive sampling followed by thermal desorption and gas chromatography]. Considering the results of laboratory experiments, all the sources of uncertainty regarding the diffusive sampler method characteristics were accessed for the sampling times of 7 and 14 days. The major part of the uncertainty budget (479%) was explained by the variation of the sampling rate due to the environmental factors (temperature and concentration level). For weekly sampling, the diffusive sampler method satisfies the data quality objectives of the European Directive to supply the indicative measurements as well as the reference measurement, since the expanded uncertainty is found o25%. Using a model-predicted sampling rate which depends on the concentration and temperature, the expanded uncertainty is significantly decreased. The Radiello sampler was found to give correct results for a weekly sampling in a limited range of benzene concentrations between 0 and 10 mg m3, which is generally observed in environmental air monitoring. This narrow validation domain limits the application fields of the Radiello sampler exposed for 7 days to indoor air, personal exposure and ambient atmospheric monitoring excluding workplaces. For 2-week sampling, the expanded uncertainty of measurements exceeds 30%. However, this diffusive sampler can still be used to carry out an objective evaluation of benzene (minimum quality objective for the accuracy of 100%). Therefore, the performance of this diffusive sampler method appears to be suitable for the benzene monitoring in ambiant air. r 2007 Elsevier Ltd. All rights reserved. Keywords: Diffusive sampler; Benzene; Passive sampling; Quality assurance; Uncertainty; Environmental exposure

1. Introduction In the 2000/69/CE European Directive (2000), the European Union set up the limit value and upper and Corresponding author. Tel.: +33 3 27 71 26 14; fax: +33 3 27 71 29 14. E-mail address: [email protected] (H. Plaisance).

1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.12.009

lower assessment thresholds for the benzene concentration monitoring in ambient air as well the assessment methods and its levels of required accuracy expressed in term of uncertainty. The limit value is 5 mg m3 (annual mean concentration) to be reached in 2010; the corresponding upper and lower assessment thresholds are 3.5 and 2 mg m3, respectively. In the case of concentrations exceeding the upper

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assessment threshold (3.5 mg m3), the measurements obtained with a reference method are required and must comply with a quality objective for the accuracy of 25%. If the concentrations are between upper and lower assessment thresholds (from 2 to 3.5 mg m3), the reference measurements are necessary, but they can be combined with the data supplied by the indicative and modelling methods which respect the minimum quality objectives for the accuracy of 30% and 50%, respectively. Below lower assessment threshold (2 mg m3), both the measurements, modelling and objective evaluation are accepted. For the objective evaluation, a minimum quality objective for the accuracy of 100% is required. Benzene monitoring has conventionally been carried out at fixed stations using semi-continuous gas chromatographs. The diffusive samplers can be a low-cost alternative approach if it is demonstrated that their performances satisfies the objectives fixed by the European Directive for accuracy. The recent standard EN 14662-4 (2005) relative to the benzene measurements obtained by analysis of thermally desorbable diffusive sampler gives the whole list of performance characteristics to estimate for the qualification of a method. This standard also gives the procedures for determining these performance characteristics and a methodology for the uncertainty calculation. In accordance with this standard, the present paper reports on the evaluation of an optimized method for the Radiello diffusive sampler with an analysis by thermal desorption/gas chromatography/flame ionization detection. The uncertainty of benzene measurements obtained with this diffusive sampler method was assessed and the contributions of different parameters to the uncertainty budget were compared. The ability of the diffusive sampler method to meet the data quality objective of the European Directive was discussed. 2. Materials and method 2.1. Radiello diffusive sampler The Radiello sampler (Fig. 1) consists of a stainless-steel net coaxial cylindrical cartridge (60 mm long, 4.8 mm in diameter, 100 mesh hole size) filled with 400 mg of 40–60 mesh Carbograph 4 (a graphitized carbon) housed in a cylindrical diffusive body made of polycarbonate and microporous polyethylene (50 mm long, 16 mm diameter, 5 mm wall thickness and 10 mm pore size). Two

Fig. 1. Scheme of Radiello diffusive sampler.

cellulose acetate caps are soldered with an epoxy adhesive to the cylinder ends. In the case of ambient measurements, the Radiello sampler is screwed on a plane cellulose acetate equilateral triangle equipped with an attaching clip. All ready-to-use radial diffusive sampler components are commercially available to the Fondazione Salvatore Maugeri (http://www. radiello.com). This diffusive sampler is suitable for thermal desorption. The adsorbents used with thermal desorption are weaker than those used with solvent desorption to ensure the quantitative recovery of the target compounds at working temperatures of thermal desorber. Conversely, the use of a weak adsorbent can favour the back diffusion, a sampling artefact that is known to rise with the exposure time and the loading of compounds on the absorbent (Martin et al., 2005; Pennequin-Cardinal et al., 2005a). Recent studies have shown that back diffusion for benzene is limited on graphitized carbons (Strandberg et al., 2006). During sampling, the Radiello sampler is screwed on a plane cellulose acetate equilateral triangle equipped with an attaching clip. The benzene molecules diffuse through the cylindrical membrane towards the cartridge in which they are adsorbed by Carbograph 4. The diffusion of benzene molecules is controlled by the coefficient of molecular diffusion of compound in air, the geometry of the sampler, the porosity of the membrane and the gradient between the benzene concentration in ambient air and at the cartridge area where it is adsorbed on Carbograph 4. The concentration of pollutant in air is calculated by applying Eq. (1) derived from Fick’s first law: C ðmg m3 Þ ¼

m ðmgÞ  mb ðmgÞ  106 , 3 1 d  UR ðcm min Þ  t ðminÞ (1)

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where UR is the uptake rate, t the sampling time, d the desorption efficiency and m and mb the benzene masses measured in the exposed cartridge and in blank, respectively. Considering the results of previous works on the performances of Radiello diffusive sampler for Benzene, Toluene, Ethylbenzene and Xylenes (BTEX) measurements (Pennequin-Cardinal et al., 2005a, b), the sampling time of 7 days appears to be favoured for the estimation of the annual mean concentration of benzene in ambient air. Under standard conditions (20 1C, 50% relative humidity, 0.5 m s1 of wind velocity and concentration level below 10 mg m3), the nominal uptake rate was estimated to be 27.9 cm3 min1. The blank values (tubes conditioned at 290 1C for at least 24 h under a nitrogen flow of 15 mL min1), as evaluated by Pennequin-Cardinal et al. (2005a), was found to be 5.272.5 ng tube1. For another conditioning mode at 400 1C for 45 min under a helium flow of 35 mL min1, the mass of benzene in the blanks was lowered to 0.870.3 ng tube1 (based on a set of 12 tubes). These last results satisfy the EN protocol requirements (an average of blanks o2 ng with a standard deviation o1 ng). This blank is o0.2% of the mass sampled by a Radiello tube for 7-day sampling at 2 mg m3 (lower assessment threshold). Consequently, mb is neglected in Eq. (1). The concentration is then converted at 293 K (Ta) and 101.3 kPa (Pa) using the following Eq. (2): C P;T ¼

m Pa T  ,  d  UR  t P T a

(2)

where T is the mean ambient temperature in Kelvin and P is the mean atmospheric pressure in kPa over the exposure period. The uncertainty calculation was developed on this last expression of the concentration. 2.2. Analytical method The adsorbing cartridge is analysed with a thermal desorber (TD) (Turbomatrix from PerkinElmer) interfaced with a gas chromatograph (GC) and a flame ionization detector (FID). The cartridge is housed into a stainless-steel tube 89 mm long  6.3 mm OD  5 mm ID that can directly be desorbed and analysed with the thermal desorber coupled with GC. In a first step (primary desorption), purging the tube for 15 min with helium flow of 35 mL min1 at 350 1C allows to extract the analytes of sample. The desorbed analytes are

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focused on a cold trap filled with 80 mg of Carbopack B 60/80 mesh and maintained at 10 1C. In the second step (secondary desorption), the trap is quickly heated from 10 to 350 1C (at 40 1C s1) and purged simultaneously with helium at a flow rate of 13 mL min1. The target compounds are desorbed from the trap and transferred by flash injection to the GC. The transfer line is maintained at 250 1C. In these conditions (an outlet split of 10.8 mL min1 and a column flow of 1.8 mL min1), only 14% of sample reaches the column and detector. The analytical column is a 60 m  0.32 mm ID, 0.25 mm stationary phase thickness, CP-Sil5 CB. The temperature programming of the GC oven begins at 35 1C and then follows the temperature profile of 35 1C for 10 min, 5 1C min1 to 140 1C and 15 1C min1 to 250 1C which is held for 10 min. The thermal desorption and GC analysis last for 58 min. 2.3. Calibration standards Calibration is done analysing the known benzene masses loaded on blank Radiello cartridges. Various standard diluted solutions of benzene are gravimetrically prepared in methanol. Then, the required amount on each standard solution is vaporized in a heated injector at 250 1C. A flow of Helium carries the sample onto the Radiello cartridge. This procedure was validated by comparison to the calibration obtained by injecting directly the same standard solutions in GC–FID (Pennequin-Cardinal et al., 2005a). 2.4. Exposure chamber system To estimate the influences of environmental factors on the benzene uptake rate, batches of diffusive samplers were placed in a dynamic exposure chamber in which the BTEX concentrations, temperature, relative humidity and wind velocity were controlled. The whole system is presented in Fig. 2. It consists of a glass ring tube (size 74  88 cm2 and inner diameter: 15 cm). This exposure chamber has a capacity of 51 L. The air opening in the exposure chamber is produced by a compressor and is dried and chemically filtered in air purifier (AZ 2020 manufactured by Claind). The first airflow is produced by diluting a high BTEX concentration standard mixture contained in a compressed gas cylinder with purified air. The second airflow comes from a humidifier system consisted of two bubblers filled up with demineralized water and flushed with

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Fig. 2. Scheme of the exposure chamber system.

purified air. These two airflows are regulated by mass flow controllers. They are mixed at the chamber inlet, generating various test atmospheres controlled in BTEX concentrations and humidity. The open airflow is regulated between 15 and 20 L min1. Wind speed is controlled by a high power inductive fan (Papst P/N 5112 N) installed inside the exposure chamber. The ring tube shape of this exposure chamber favours the recirculation of air allowing to reach a large scale of wind velocity from 0.3 to 10 m s1. The exposure chamber is placed in a thermostatic enclosure of 990 L (M 54054 manufactured by Vo¨tsch) maintaining a constant temperature between 10 and +40 1C. The temperature, relative humidity and wind velocity conditions in the exposure chamber are continuously monitored and recorded by means of multifunction probes (Datalogger Testo term 400 and temperature, humidity,

wind velocity sensor 0635.1540). This sensor and diffusive samplers are introduced or removed through Scott 32 GL openings with caps (12 positions). The concentration of benzene in the exposure chamber is monitored by successive pumped 24- and 48-h samples onto a pair of Perkin-Elmer tubes placed in a row and filled with 400 mg of 40–60 mesh Carbograph 4. Preliminary tests showed that in this configuration, no breakthrough occurs for the conditions tested in exposure chamber. Dual active sampling devices were placed in parallel in the exposure chamber the nearest possible to the diffusive samplers. The sampling flow rates were adjusted to 50 mL min1 with a Tylan mass flow meter and checked before and after each pumped sample with a gas flow meter (DryCal DC-Lite). This flow meter was certified by a National Measurement Institute signatory of the CIPM-MRA (Laboratoire

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National d’Essais). These pumped tubes were analysed by TD–GC–FID using the same analytical method and calibration than the one applied to diffusive samplers (see Sections 2.2 and 2.3). 2.5. Methodology for the uncertainty calculation The uncertainty is calculated following the method described in standard 16662-4 (2005) derived from the ‘‘Guide to the Expression of Uncertainty in Measurement’’ (GUM ISO, 1995) and on the basis of the results obtained in various laboratory tests. Applying this method, the combined uncertainty can be estimated using Eq. (3) that is established by the derivation of Eq. (2). u2 ðC P;T Þ u2 ðmÞ u2 ðURÞ u2 ðdÞ u2 ðtÞ ¼ þ þ 2 þ 2 2 m2 t UR2 C P;T d u2 ðTÞ

u2 ðPÞ

. ð3Þ 2 T P This uncertainty estimation includes the effects of environmental factors (temperature, relative humidity, interferents, level and variation of concentration) on the uptake rate (u(UR)), the terms relative to analysis with the determination of mass (u(m)) and thermal desorption recovery (u(d)), the relative uncertainty of sampling time (u(t)) and the terms relative to the conversion to standard temperature and pressure (uðTÞ and uðPÞ). The definition of tests and the estimation of these uncertainty components are detailed in the following section. þ

2

þ

3. Results and discussion For each uncertainty parameter of Eq. (3), the recent standard EN 14662-4 (2005) gives minimum requirements. However, these requirements are not mandatory. They should rather be used as the basis for the establishment of ongoing QA/QC program. On the opposite, the expanded uncertainty of the measured concentration should fulfil the data quality objective of 25% set by the 2000/69/CE European Directive (2000). 3.1. Uncertainty of desorption efficiency, u(d) The desorption efficiency and its uncertainty were estimated analysing a set of seven Radiello cartridges prepared at Nmi (NL) and containing an amount of benzene (loaded from a certified mixture in cylinder). The reference mass loaded on these

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cartridges (mref) was about 1300 ng with an uncertainty (u(mref)) estimated by Nmi of 15 ng. This value is close to the amount of benzene collected by a Radiello diffusive sampler for a 7-day sampling to 5 mg m3. The desorption efficiency was calculated by computing the ratio of the recovered and the reference mass of benzene. The relative uncertainty was estimated by applying the Eq. (4): u2 ðdÞ u2 ðmref Þ þ ðs2 ðmd Þ=nÞ ¼ , m2ref d2

(4)

where mref is reference mass loaded on the Nmi cartridges, s(md) is standard deviation of the replicate measurement results on the Nmi cartridges and n is the number of loaded samplers that was analysed. The results relative to the desorption efficiency are shown in Table 1. The mean desorption efficiency and its uncertainty fulfils the EN protocol requirement (value 498% with a relative expanded uncertainty o3%). 3.2. Uncertainty of the measured mass, u(m) According to the standard EN 14662-4 (2005), the uncertainty of the measured mass of benzene (m) can be broken down into six components:

    

the the the the the

selectivity of the chromatographic system, analyte stability in the sample, analytical precision, response drift between calibrations, lack-of-fit of the calibration function,

Table 1 Data for the calculation of desorption efficiency and its uncertainty Nmi cartridges loaded by a reference mass: mref ¼ 1300715 ng

Measured mass (ng)

Ref. Ref. Ref. Ref. Ref. Ref. Ref.

1289 1281 1289 1292 1292 1289 1281

A82831 A82859 A83107 C09380 C010283 C011952 C012031

Mean, md (ng) Standard deviation, s(md) (ng) Desorption efficiency, d Uncertainty, U(d)/d

1288 0.66 99.1% 1.2%

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the uncertainty of the masses of calibration standards used,

the uncertainty contributions of these factors were evaluated on the basis of laboratory specific test results. 3.2.1. Selectivity The separation system was evaluated by the analysis of a Radiello cartridge loaded by the technique of vaporization with 1400 ng of benzene (amount of benzene collected by a Radiello sampler for a 7-day sampling to 5 mg m3) and amounts of interferents five times higher than the one of benzene (about 6500 ng). The potential benzene interferents were methylcyclopentane, 2,2,3-trimethylbutane, 2,4dimethylpentane, tetrachloromethane, cyclohexane, 2,3-dimethylpentane, 2-methylhexane, 3-ethylpentane, trichloroethene and n-heptane. As shown in Fig. 3, no co-elution was displayed. The resolution values calculated between benzene and interferent peaks were in all cases 41. So, no uncertainty contribution due to co-elution was taken into account.

3.2.2. Sample stability The storage stability of the cartridges in the glass containers after sampling was evaluated for benzene for up to 28 days storage at room temperature and at 4 1C. Some Radiello cartridges were loaded by a known amount of benzene from a dynamic sampling of standard atmosphere in cylinder. A first set of seven loaded cartridges was analysed the day of loading and the results taken as a reference. For the two temperatures of storage, three sets of seven loaded cartridges were analysed after storage durations of 1, 14 and 28 days. The results are reported in Table 2. Applying two-way analysis of variance (Saporta, 1990) on the measured masses of benzene, no significant influence of factors (temperature and duration of storage) could be evidenced in this experiment. In consequence, no uncertainty contribution due to storage was considered. 3.2.3. Analytical precision This component was evaluated by analysis under repeatability conditions of a set of seven cartridges loaded by vaporization of a known mass of benzene (around 1400 ng). The relative uncertainty due to

Fig. 3. Chromatogram of a cartridge loaded with 1400 ng of benzene and about 6500 ng of 10 potential interferents.

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Table 2 Influence of storage after sampling (temperature and duration) on sample integrity of benzene Mean mass of benzene analysed on Radiello cartridges7standard deviation (en ng)

Storage at room temperature Storage at 4 1C

Day 0

Day +1

Day +14

Day +28

12477 12477

12377 11775

12376 11976

11974 12373

analytical repeatability wrep was then calculated as follows:

180000000

2

s ðmrep Þ m2rep

3.2.4. Response drift between calibrations In the interval between two calibrations, a response drift may occur. The relative uncertainty due to response drift for the period between two successive calibrations (n and n1) was estimated from data on the relative differences in responses for a check value as follows: jrn  rn1 j wd ¼ pffiffiffi 3  ððrn þ rn1 Þ=2Þ

160000000

(5)

in which s(mrep) is standard deviation of the replicate analyses, mrep is mean mass of the replicate analyses. The mean mass of replicate measurements was 1489 ng with a standard deviation of 15.1 ng. The relative uncertainty was then estimated at 1.01% that is in accordance with the EN protocol requirement (value o3%).

(6)

in which rn is the peak area calculated from the regression equation of the n calibration for the mass of 1400 ng (amount of benzene sampled for 7-day sampling to 5 mg m3), rn1 is the peak area calculated from the regression equation of the n1 calibration for the mass of 1400 ng. Two calibrations of eight standard levels, in which the second calibration was carried out 8 days after the first one were considered (Fig. 4). Using these two calibrations, the drift value was estimated at 0.72% and satisfies the EN protocol requirement (value o5%). 3.2.5. Lack-of-fit calibration function A least-squares regression was applied to the whole of calibration standard points (masses of benzene versus peak areas) to obtain the calibration function. In order to evaluate the lack-of-fit of the

0.79 (po0.01) 1.91 (po0.01)

200000000

Peak area (u.a)

w2rep ¼

ANOVA test F-ratio (p-value)

140000000 120000000 06/09/2004 Calibration

100000000

y = 91689x + 413459

80000000

R2 = 0.999

60000000 06/17/2004 Calibration ie2 y = 92462x + 943971

40000000 20000000

R2 = 0.999

0 0

500 1000 1500 2000 Measured benzene mass (ng)

2500

Fig. 4. Two calibrations with eight standard levels set up after 8 days.

regression function, the relative residuals were calculated at each of the levels of the calibration standards as follows: q¼

jmreg  mcs j mcs

(7)

in which mreg is mass of benzene calculated from the regression equation at level of the calibration standard, mcs is mass of benzene present in the corresponding calibration standard. The relative uncertainty due to lack-of-fit of the calibration function was calculated using the maximum relative residual according to Eq. (8): qmax wF ¼ pffiffiffi 3

(8)

in which qmax is the maximum relative residual found. This calculation was performed using one typical calibration curve drawn using eight standard levels and shown in Fig. 4. According to these data, the relative uncertainty due to lack-of-fit was estimated at 1.5%. This value is consistent with the minimum requirement given in the EN protocol (value o2%).

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3.2.6. Uncertainty of mass of calibration standards This uncertainty was built up from various contributions comprising: the purity of benzene used in standards, the uncertainties in the masses of benzene and methanol estimated by gravimetry composing the intermediate and calibration solutions and the uncertainty in the volume of syringe used for the preparation of cartridges loaded by vaporization method (see in Section 2.3). The uncertainty was calculated following the method of GUM (1995) applied to the expression of mass in calibration standard. Some examples for the calculation of this uncertainty are given in Guide Eurachem/Citac (2000). In our case, this relative uncertainty due to the preparation of calibration standards was estimated at u(mcal)/mcal ¼ 1.4% for a mass of benzene in calibration standard of mcal ¼ 1400 ng. Its value is consistent with the minimum requirement given in the EN protocol for this uncertainty component (value o2%). 3.2.7. Combined uncertainty of the measured mass of benzene All the contributions previously described in this Section 3.2 were combined to give the uncertainty of the measured mass of benzene as follows: u2 ðmÞ u2 ðmcal Þ ¼ þ w2rep þ w2F þ w2d . m2 m2cal

(9)

Further to our estimations of these four contributions, this uncertainty was evaluated at 2.4%. 3.3. Uncertainty of the uptake rate, u(UR) For the estimation of this uncertainty component, sets of seven Radiello samplers were exposed for 7 and 14 days under two extreme conditions yielding low and high values of uptake rate. These conditions of exposure were generated and controlled in the exposure chamber. Temperatures ranged between 10 and 30 1C and relative humidities between 20% and 80% and BTEX concentrations from 1 to 10 mg m3 for benzene, from 3 to 33 mg m3 for toluene, from 0.6 to 5.4 mg m3 for ethylbenzene, from 1.2 to 13 mg m3 for m+p-xylene and from 0.7 to 6.5 mg m3 for o-xylene. The choice of the two sets of extreme conditions of exposure was based on the single effects of environmental factors found in some previous experiments in exposure chamber (Pennequin-Cardinal et al., 2005a, b). These first results showed that for benzene and for

7-day sampling, the uptake rate significantly decreases with rising temperature (0.6% per 1C) and more weakly with the level of concentration (o0.1% per mg m3 of benzene). No influence of humidity, chemical interferents could be evidenced (Pennequin-Cardinal et al., 2005b). Therefore, the lowest uptake rate values were estimated under high temperature, concentration and humidity levels (condition B). Low temperature, concentration and humidity levels (condition A) were chosen to obtain the highest uptake rate values. An additional test for 7-day sampling was carried out in exposure chamber (condition B0 ) under high temperature and humidity, thus similar to condition B, although with higher BTEX (benzene: 27 mg m3, toluene: from 86 mg m3; ethylbenzene: 13.7 mg m3; m+p-xylene: 33 mg m3; o-xylene: 16.8 mg m3). The results of this test are not taken into account in the calculation of the relative uncertainty of the uptake rate, but they are presented to inform about the applying limits of Radiello sampler for 7-day sampling. The results of the experiments under extreme conditions are reported in Table 3. Back diffusion was not tested here since it was already shown that no loss of collected benzene was observed when the Radiello sampler was exposed for 7 days to benzene varying every 12 h between a high concentration and clean air (PennequinCardinal et al., 2005b). The uptake rate decreased when changing the conditions of exposures from A to B. This decrease is slight for a sampling of 7 days (24%) and higher for the one of 14 days (60%). This phenomenon was often observed for the most volatile compounds like benzene and was assigned in literature to displacement and saturation effects at the vapour/ sorbent interface (Oury et al., 2006; Tolnai et al., 2001). The ratios of sampler responses out of the reference values for each condition were considered to evaluate the contribution of uptake rate to the measurement uncertainty. Assuming a rectangular distribution between these ratios, the relative standard uncertainty for uptake rate was then calculated as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi uðURÞ ðxA  xB Þ2 ¼ þ s2max , (10) UR 12 where xA is the ratio of mean uptake rate found under condition A out of the reference for condition A, with this standard deviation sxA, xB is the ratio

Table 3 Means and standard deviations of benzene uptake rate obtained for 7- and 14-day samplings under extreme conditions Wind speed (m s1)

Temperature (1C)

Relative humidity (%)

BTEX concentrations (mg m3)

Benzene uptake rate for each trial (cm3 min1) mean7standard deviation

Benzene uptake rates found under two extreme conditions and retained for the uncertainty calculation (cm3 min1) mean7standard deviation

A: 14/03/2006 to 28/03/2006

13.86

7

0.5

10.3

19.6

Benzene: 1.6 Toluene: 5.2 Ethylbenzene: 0.8 m+p-Xylene: 1.8 o-Xylene: 0.9

35.571.0

A: 35.571.0 (N ¼ 7)

B: 30/03/2006 to 13/04/2006

14.06

7

0.5

29.8

80.4

Benzene: 9.9 Toluene: 32.4 Ethylbenzene: 5.1 m+p-Xylene: 12.0 o-Xylene: 6.3

14.370.9

B: 14.370.9 (N ¼ 7)

A: 06/07/2006 to 13/07/2006

7.7

7

0.5

10.4

21.0

Benzene: 1.5 Toluene: 4.3 Ethylbenzene: 0.7 m+p-Xylene: 1.6 o-Xylene: 0.8

32.071.2

A: 33.371.7 (N ¼ 14)

A: 07/02/2006 to 15/02/2006

7.04

7

0.5

10.1

20.1

Benzene: 1.1 Toluene: 3.3 Ethylbenzene: 0.6 m+p-Xylene: 1.2 o-Xylene: 0.7

34.670.8

B: 16/01/2006 to 23/01/2006

6.81

7

0.5

29.9

81.9

Benzene: 9.9 Toluene: 32.9 Ethylbenzene: 5.4 m+p-Xylene: 12.7 o-Xylene: 6.5

25.671.8

B: 31/01/2006 to 07/02/2006

7.18

7

0.5

29.9

82.4

Benzene: 9.7 Tolue`ne: 32.0 Ethylbenzene: 5.2 m+p-Xylene: 12.2 o-Xylene: 6.3

24.970.9

B0 : 23/06/ 2005 to 30/ 06/2005

6.97

7

0.5

30.2

81.2

Benzene: 27 Toluene: 86 Ethylbenzene: 13.7 m+p-Xylene: 33 o-Xylene: 16,8

18.971.6

B: 25.371.5 (N ¼ 14)

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Number of samplers

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Exposure time (day)

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Condition and dates

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of mean uptake rate found under condition B out of the reference for condition B, with this standard deviation sxB, smax is the maximum standard deviation between sxA and sxB. For the 7-day sampling, two uptake rates were considered as references. The first one is the constant value equal to 27.9 cm3 min1 found by Pennequin-Cardinal et al. (2005a) estimated under standard conditions in exposure chamber. This nominal value is in agreement with those found in other works for 1-week benzene Radiello sampler, i.e., 27.8 cm3 min1 (Perez Ballesta et al., 2002) and 28 cm3 min1 (value of supplier listed in http:// www.Radiello.com). The second one is a modelpredicted uptake rate set up by Pennequin-Cardinal et al. (2005b) based on experimental results in exposure chamber under various conditions. This model takes into account the main factors affecting the uptake rate and is defined as followed: UR ¼ 0:18  T  0:02  C þ 31:9, 3

(11)

Table 4 Relative uncertainty of the uptake rate as a function of the sampling duration and sampling rate used Sampling duration (day)

Uptake rate used (mL min1)

7

A model-predicted uptake rate: UR ¼ 0.18  T0.02  C+3.19 A constant value: 27.9 A constant value: 24.9

7 14

Uncertainty of the uptake rate u(UR)/UR (%) 6.9 10.2 24.9

T: temperature (1C) and C: benzene concentration in air (mg m3).

due to the measurement of t was estimated as follows: DðtÞ uðtÞ ¼ 0:029%. uðtÞ ¼ pffiffiffi ¼ 2:9 min ) t 3

(12)

This relative uncertainty for a long sampling time as 7 and 14 days is very low in comparison to other terms.

1

where UR is the sampling rate (cm min ), T the temperature (1C) and C (mg m3) the benzene concentration in air. Eq. (11) is introduced in Eq. (3) and the resulting second-order equation on C is then resolved. For the 14-day sampling, a constant uptake rate equal to 24.9 cm3 min1 was considered as a reference. This value was determined by tests in exposure chamber under standard conditions (Pennequin-Cardinal et al., 2005a). The contribution of uptake rate to the measurement uncertainty is reported in Table 4. The use of diffusive Radiello sampler for a 7-day sampling using the model-predicted uptake rate allows to significantly reduce this contribution. Nevertheless, the relative standard uncertainty of uptake rate exceeds in any cases the requirement given in the EN protocol (value o5%). Under condition B0 , a high decrease of uptake rate was observed (close to 30%). The upper limit of benzene concentration range suitable for the Radiello sampler is clearly lower 27 mg m3. Consequently, the uncertainties of Radiello sampler measurements which were previously estimated below 25% cannot be extrapolated to a benzene concentration above 10 mg m3. 3.4. Uncertainty of the sampling time, u(t) The sampling time (t) can be measured to within maximum error of D(t) ¼ 75 min. For a sampling time of 7 days (10,080 min), the relative uncertainty

3.5. Uncertainty due to the conversion to standard temperature and pressure For the conversion of concentrations to standard temperature and pressure, it is required to know the mean values of temperature ðTÞ and pressure ðPÞ. The uncertainty associated with this conversion will differ from one sampling site to another depending whether meteorological data are available (as in the following field study) or not. Nevertheless, a gradient of temperature may always exist between the meteorological sensor and the Radiello sampler. Finally, the contribution of the conversion to the uncertainty can be evaluated using the extreme values of temperature and pressure (Tmin, Tmax, Pmin and Pmax) during sampling or equal to the unknown difference between the conditions applying to the sampler and the meteorological conditions. Assuming that these values of temperature and pressure are uniformly distributed, their uncertainties can be obtained from: u2 ðTÞ ¼ u2Tcal þ

ðT max  T min Þ2 12

and ðPmax  Pmin Þ2 , ð13Þ 12 where uTcal and uPcal are the uncertainties due to calibration of thermometer and manometer, respectively. The first term in two equations is usually u2 ðPÞ ¼ u2Pcal þ

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considered as negligible. Considering the ranges of temperature DT ¼ 20 K and pressure DP ¼ 4 kPa, the relative uncertainties were estimated to uðTÞ=T ¼ 2% and uðPÞ=P ¼ 1:1% for a total of 2.3%. 3.6. Expanded uncertainty of the benzene concentration The combined uncertainty of benzene measured using the Radiello sampler was calculated by summing all contributions given in the Sections 3.1–3.5, according to Eq. (3). The expanded uncertainty at the 95% confidence level was obtained by multiplying the combined uncertainty with a coverage factor of 2 as follows: UðC P;T Þ ¼ 2  uðC P;T Þ.

(14)

Table 5 presents the values of expanded uncertainty for benzene using the uptake rates previously defined and relative contributions of different parameters. In any cases, the variation of the uptake rate due to environmental factors is the main contribution to the uncertainty budget (479%). For 7-day sampling, using either the modelpredicted or the constant uptake rate allows meeting the data quality objectives of the European Directive both for indicative (expanded uncertainty o30%) and reference methods (expanded uncertainty o25%). The improvement of uncertainty given by the use of the model-predicted uptake rate is also clearly evidenced. For 14-day sampling, the overall uncertainty of measurements exceeds 30%, this diffusive sampler method can still be used to carry out an objective evaluation of benzene (minimum quality objective for the accuracy of 100%).

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3.7. Comparison between the Radiello sampler and the on-line gas chromatographic VOC analyser A field comparison between the Radiello sampler and on-line VOC monitor (Perkin-Elmer Turbo Matrix thermal desorber unit coupled with bidimensional capillary gas chromatography equipped with two flame ionization detectors) was carried out at three urban stations in France. Two stations were located in the north of France, Douai and Dunkerque and one in the south, Marseille. Continuous hourly measurements of benzene and 30 other C2–C9 non-methane hydrocarbons were performed with on-line VOC monitors in January 2004 and from May to July 2004 in Douai, from March to August 2003 in Dunkerque and from June to July 2003 in Marseille. The sampling and analytical parameters, calibration method, quality control procedure and data acquisition used for the on-line GC analysers were previously described in details by Badol et al. (2004). Seventeen sets of six samplers plus one blank were exposed in parallel with the on-line VOC monitor for 1 week. The samplers were mounted in shelters to protect them from high air speed and precipitation. The repeatability of diffusive sampler measurements was evaluated from the results of these sets of replicate measurements. The mean relative standard deviation (RSD) for benzene between 0.5 and 2.4 mg m3 was 7% on average with a maximum of 12% and a minimum of 3%. For the comparison between the Radiello sampler and on-line VOC monitor, 98 pairs of measurements were obtained. Two uptake rates (the nominal value: 27.9 cm3 min1 and the model-predicted uptake rate given by Eq. (11)) were applied to calculate the concentrations measured by diffusive

Table 5 Contributions of components to the uncertainty budget and expanded uncertainty according to the sampling duration and uptake rate used Relative contribution of each component to the uncertainty budget (%)

7-Day sampling and using the model-predicted uptake rate 7-Day sampling and using an uptake rate of 27.9 cm3 min1 14-Day sampling and using an uptake rate of 24.9 cm3 min1

Expanded uncertainty of the concentration U(CP,T)/CP,T (%)

u(m)/m

u(d)/d

u(UR)/UR

u(t)/t

Conversion to standard temperature and pressure

9.6

2.4

79.4

0.002

8.6

16

4.9

1.2

89.4

0.001

4.5

22

0.9

0.2

98.1

0.0002

0.8

51

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samplers (Fig. 5a). An orthogonal linear regression, as recommended in the Guide for the demonstration of equivalence (EC, 2005), was used. A good agreement was found between the Radiello sampler and on-line GC analyser measurements with the correlation coefficients above 0.9 and slopes close to 1.

Using the model-predicted uptake rate, the agreement between Radiello sampler and on-line GC analyser was better than using the nominal uptake rate. In the case of the model-predicted uptake rate, the slope of the regression line is nearer to 1 and lack-of-fit of the model is improved (r ¼ 0.96).

3.5

Benzene, Radiello sampler in µg. m-3

3.0 Constant UR: Radiello = (1.10 ±0.04) GC + (-0.14 ±0.06) r = 0.92

2.5

2.0

Model-predicted UR Constant UR

1.5

1.0 Model-predicted UR: Radiello = (1.02 ± 0.03) GC + (-0.13 ± 0.04) r = 0.96

0.5

0.0 0

0.5

1

1.5

2

2.5

3

3.5

Expanded uncertainty

Benzene, on-line GC in µg. m-3

100% 95% 90% 85% 80% 75% 70% 65% 60% 55% 50% 45% 40% 35% 30% 25% 20% 15% 10% 5% 0%

U% constant UR U% model-predicted UR

0

0.5

1 1.5 2 Benzene, on-line GC in µg/m3

2.5

3

Fig. 5. (a) One-week benzene concentration given by the Radiello diffusive sampler using the nominal uptake rate and a model-predicted uptake rate versus GC concentration (mg m3) and (b) relative expanded uncertainty versus benzene concentration using the constant and model-predicted uptake rates.

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The Guide for the demonstration of equivalence (EC, 2005) also proposes an evaluation of uncertainty based on the results of the orthogonal regression (Eq. (15) and Fig. 5a) between a candidate method (Radiello sampler) and a reference method (on-line GC analyser). The uncertainty of the candidate method for individual measurements is then calculated using Eq. (16) where x is the reference method, y the candidate method, y^ the estimation determined by orthogonal regression, RSS the sum of residuals resulting from the orthogonal regression (Eq. (17), u(xi) the uncertainty of the standard method, Sr is the standard deviation of repeatability and k the coverage factor (in our case, k ¼ 2). In this calculation, the random uncertainty of the standard method u(xi) was set to 2% based on the works of Badol (2005) which established all components of uncertainty for the on-line GC system. Considering 17 sets of six replicate measurements, the standard deviation of repeatability was found to be dependent on the benzene concentration according to the regression equation Srmg m3 ¼ 0:051½Benzenemg m3 þ 0:02 with r ¼ 0:7. Therefore, this equation was used to estimate the term Sr in Eq. (16). The improvement of uncertainty given by the use of the modelpredicted uptake rate is more clearly evidenced in Fig. 5b. Unfortunately, the range of benzene concentration in the field study is limited and does not reach the limit value of Directive. Even though Fig. 5b suggests that the relative expanded uncertainty would be o25% above 2 mg m3. The use of the model-predicted uptake rate has the effect of reducing the expanded uncertainty about 20% between 0.5 and 2.5 mg m3. y^ ¼ b0 þ b1 x,

(15)

UðyÞ ð%Þ ¼ k  uc ðyÞ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðRSS=ðn  2ÞÞ  u2 ðxi Þ þ ½b0 þ ðb1  1Þxi 2 þ s2r ¼k , y2

ð16Þ RSS ¼

X

ðyi  b0  b1 xi Þ2 .

(17)

4. Conclusions One of the key points regarding the monitoring criteria is to determine the uncertainty of the measurements for the used method. This study

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demonstrates the ability of an optimized diffusive sampler method to meet the data quality objective of the European Directive. For 7-day sampling, the diffusive sampler method satisfies the data quality objective of the European Directive for indicative and reference methods of measurements, since the expanded uncertainty is found to be o25%. An improvement in the uncertainty is recorded when a model-predicted uptake rate which depends on the benzene concentration and temperature is used. Nevertheless, the domain of benzene concentrations suitable for the Radiello sampler measurements with 7-day sampling appears to be limited (from 0 to 10 mg m3). It is noted that this narrow validation domain remains more limited than the concentration range of 0–50 mg m3 recommended in the standard EN 14662-4 (2005). For 14-day sampling, the expanded uncertainty of measurements exceeds 30%, this diffusive sampler method can still be used to carry out an objective evaluation of benzene (minimum quality objective for the accuracy of 100%). Completing the tests in field conditions for benzene concentration between 2.5 and over 5 mg m3 is still necessary in order to be able to draw a general conclusion about the equivalence of the Radiello sampler to EN standard methods, which consists of pumped sampling followed by sample analysis using gas chromatography. Acknowledgement The authors are grateful to the French Ministry of Environment and Environmental Agency (ADEME) for financial support towards this program as part of LCSQA (Laboratoire Central de Surveillance de la Qualite´ de l’Air) activities.

References Badol, C., 2005. Caracte´risation des Compose´s Organiques Volatils dans une atmosphere urbaine sous influence industrielle: de l’identification a` la contribution des sources. Thesis, Universite´ des Sciences et Technologies de Lille. Badol, C., Borbon, A., Locoge, N., Leonardis, T., Galloo, J.C., 2004. An automated monitoring system for VOC ozone precursors in ambient air: development, implementation and data analysis. Analytical and Bioanalytical Chemistry 378, 1815–1827. Directive 2000/69/EC relating to limit values for benzene and carbon monoxide in ambient air. Official Journal L313/12.

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EN 14662-4, 2005. Ambient air quality. Standard method for measurement of benzene concentrations. Part 4: diffusive sampling followed by thermal desorption and gas chromatography. European Commission, 2005. Demonstration of equivalence of ambient air monitoring methods. Report by an EC Working Group, /http://ec.europa.eu/environment/air/pdf/equivalence_ report3.pdfS. Fondazione Salvatore Maugeri. Instruction Manual for Radiello Sampler, version 1/2003. /http://www.radiello.comS. Guide Eurachem/Citac, 2000. Quantifier l’Incertitude dans les mesures analytiques. Technical Report. Available at: /www. lne.frS. GUM, 1995. Guide to the Expression of Uncertainty in Measurement Evaluation. International Organization for Standardization, Switzerland. Martin, N.A., Marlow, D.J., Henderson, M.H., Goody, B.A., Quincey, P.G., 2005. Studies using the sorbent Carbopack X for measuring environmental benzene with Perkin-Elmer-type pumped and diffusive samplers. Atmospheric Environment 37, 871–879. Oury, B., Lhuillier, F., Protois, J.C., Morele, Y., 2006. Behavior of the Gabie, 3M 3500, Perkin Elmer Tenax TA, and Radiello 145 diffusive samplers exposed over a long time to a low concentration of VOCs. Journal of Occupational and Environmental Hygiene 3, 547–557.

Pennequin-Cardinal, A., Plaisance, H., Locoge, N., Ramalho, O., Kirchner, S., Galloo, J.-C., 2005a. Dependence on sampling rates of Radiello diffusion sampler for BTEX measurements with the concentration level and exposure time. Talanta 65, 1233–1240. Pennequin-Cardinal, A., Plaisance, H., Locoge, N., Ramalho, O., Kirchner, S., Galloo, J.-C., 2005b. Performances of the Radiello diffusive sampler for BTEX measurements: influence of environmental conditions and determination of modelled sampling rates. Atmospheric Environment 39, 2535–2544. Perez Ballesta, P., Connolly, R., Cao, N., 2002. QAQC of BTEX analysis with thermal desorption diffusive samplers. Technical Note, Institute for Environment and Sustainability, JRC, Ispra, Italy. Saporta, G., 1990. Probabilite´s, Analyse des donne´es et Statistique. Technip Editions, Paris. Strandberg, B., Sunesson, A.-L., Sundgren, M., Levin, J.-O., Sa¨llsten, G., Barregard, L., 2006. Field evaluation of two diffusive samplers and two adsorbent media to determine 1,3butadiene and benzene levels in air. Atmospheric Environment 40, 7686–7695. Tolnai, B., Gelencser, A., Hlavay, J., 2001. Theoretical approach to non-constant uptake rates for tube-type diffusive samplers. Talanta 54, 703–713.