Diffusive Monitor

8/11/2019 Diffusive Monitor

1/19

The DiffusiveMonitor

Apri l 2008Issue 16

Health and Safety Executive

Committee of Analytical

Requirements - Working Group 5

Inside this Issue

1

By way of introduction

1Current News

4NO2and SO2in ambient

air by membrane-closed

Palmes tube

7Sample tube tagging

Enhanced tracking for

thermal desorption

10

Controlling GC carrier flow

through thermal desorb-

tion transfer line

15Uptake of occupational

BTX on Carbograph TD1

tube sampler

Conferences..p.17

Registering your interest

in Diffusive Monitor. p. 19e Monitor. p. 19

By way of IntroductionBy way of IntroductionWelcome to the sixteenth edition of The

Diffusive Monitor, which is a freepublication of the Health and SafetyExecutive CAR Committee (Committeeof Analytical Requirements), WorkingGroup 5. This working group isconcerned with workplace, indoor andenvironmental applications of diffusivesampling for assessing air quality.

Welcome to the sixteenth edition of TheDiffusive Monitor, which is a freepublication of the Health and SafetyExecutive CAR Committee (Committeeof Analytical Requirements), WorkingGroup 5. This working group isconcerned with workplace, indoor andenvironmental applications of diffusivesampling for assessing air quality.

The newsletter was started in May 1988as a consequence of the DiffusiveSampling Symposium held inLuxembourg the previous year and wasoriginally published about once a year.In recent years the frequency hasreduced for reasons described later inCurrent News. It contains articles ondiffusive monitoring techniques andapplications, and is a useful source ofinformation on European andinternational standardisation in this areaand of sampling rate data. Contributions

are mostly from members of theWorking Group, which has aninternational membership.

The newsletter was started in May 1988as a consequence of the DiffusiveSampling Symposium held inLuxembourg the previous year and wasoriginally published about once a year.In recent years the frequency hasreduced for reasons described later inCurrent News. It contains articles ondiffusive monitoring techniques andapplications, and is a useful source ofinformation on European andinternational standardisation in this areaand of sampling rate data. Contributions

are mostly from members of theWorking Group, which has aninternational membership.

Contributions to the newsletter are not,however, intended to be exclusivelyfrom CAR/WG 5, and any reader iswelcome to submit an item forconsideration. The only limitations arethat articles should have some diffusivesampling application and should not betoo obviously commercial.

Contributions to the newsletter are not,however, intended to be exclusivelyfrom CAR/WG 5, and any reader iswelcome to submit an item forconsideration. The only limitations arethat articles should have some diffusivesampling application and should not betoo obviously commercial.

The newsletter has a world circulation

of some 200 people, all of whom havespecifically requested the publication,so if you wish to contribute articles, youcan be assured of a wide and receptiveaudience. Articles are not peer-reviewed, but are subject to the Editorsdiscretion. A Word template for authorsis recommended and is available fromthe Editor on request.

The newsletter has a world circulation

of some 200 people, all of whom havespecifically requested the publication,so if you wish to contribute articles, youcan be assured of a wide and receptiveaudience. Articles are not peer-reviewed, but are subject to the Editorsdiscretion. A Word template for authorsis recommended and is available fromthe Editor on request.

Copies of this newsletter and previousissues back to no.12 (July 2001) may bedownloaded from the Health and SafetyLaboratory website athttp://www.hsl.gov.uk/publications/diffuse-

Copies of this newsletter and previousissues back to no.12 (July 2001) may bedownloaded from the Health and SafetyLaboratory website athttp://www.hsl.gov.uk/publications/diffuse-monitor.htm

No registration is necessary todownload a copy. However, if you wantto be placed on a list to be notified whena new issue is published contact theEditor (but see below for a change of

Editor in 2008). Copyright statement:Issues 1-11 ofThe Diffusive Monitorare available onrequest to the Editor. The early issuescan be supplied by e-mail as PDF fileson condition that they are for in-houseuse, private study and not fordistribution on a website or by othermeans. These particular copyrightrestrictions do not apply to issues 12and later, but in all cases the sourceshould be acknowledged if quoting.

Current NewsFirst some personal news. After 35years with HSL and its predecessors Iwill be retiring in May 2008 to pasturesnew. My successor as Editor andsecretary of CAR/WG5 will be NeilPlant ([email protected]) of HSLwho has been closely involved in the

practical side of workplace and ambientmonitoring campaigns for over 10years.

Committee of Analytical

Requirements (CAR) on the back

burner

Since the last issue in February 2006 theminutes of recent CAR meetings have

been placed on the HSL website athttp://www.hsl.gov.uk/publications/car.htmthe last one being held at HSL Buxtonon 16 May 2006. However, a meetingscheduled for October 2006 was

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postponed. The secretary of CAR hastaken into account the response ofmembers to further meetings in 2007,the fact that only one working group isactive (WG5) and it was agreed in HSLthat further proceedings of the mainCAR committee would be bycorrespondence only. The decision to

end face to face CAR meetings wastaken with reluctance but reflects whatis happening in specialist measurementforums elsewhere. CAR/WG5 continuesunaffected with Dr Kevin Saunders aschairman.

CEN air quality standards and

European news

Issue 15 of The Diffusive Monitordescribed the progress of various workitems in expert working groups (WGs)reporting to Technical Committees of

the Comit Europen de Normalisation(CEN) up to the end of 2005. CEN is alegal association, the members of whichare the national standards bodies of EUmember states and associate countries,supported by a management centre inBrussels. The programmes of TC137(workplace exposure) and TC264 (airquality) derive mainly from therequirements of the Chemical AgentsDirective (CAD)(98/24/EC) and theAmbient Air Directive (96/62/EC)respectively [1,2]. TC137 preparesstandards for the protection of workersagainst hazardous substances and

biological agents in workplaceatmospheres. Its work excludes the

proposal of limit values which areestablished by separate expertcommittees. TC264 has a similar rolefor ambient atmospheres.

An ad hocgroup of TC137/WG2completed its report on recommendedmethods for measuring prioritychemical substances in workplace air[3,4] and an on-line summary became

available in 2006 [5]. Following itspublication the convenor of the ad hocgroup Dr Dietmar Breuer ofBerufsgenossenschaftliches Institut frArbeitsschutz, St Augustin Germany(BGIA) proposed an internationaldatabase of occupational airborne limitvalues to be maintained by BGIA. Sincefirst going on line in late 2006 thisdatabase has now expanded in March

2008 to include the limit values ofAustria, Belgium, Canada (Qubec),Denmark, European Union, France,Germany (AGS), Germany (DFG),Hungary, Italy, Japan, Spain, Sweden,Switzerland, Netherlands, USA (OSHAPEL) and United Kingdom [6]. Irelandand Poland may be added in the near

future. For copyright reasons theACGIH TLVs have to be excluded.Between these regulatory authoritiesabout 1100 substances are listed,therefore coverage is more extensivethan that of any single country.However, for legal purposes somecaution is necessary. The BGIAdatabase is useful for comparing limitvalues of one country with another, buthas indicative status. Wherever possiblethere are linked citations that point tothe national source material (EH40Table 1in the case of the UK). Thenational source material will always bethe official controlled version.

The revised 'General requirements forthe performance of procedures' standardEN 482 [7] was published in 2006.

Numerical performance requirements(eg. relative expanded uncertainty30 % at 0.5 2 times limit value) areunchanged from the 1994 version. Oneof the changes has been the removal ofa statistical confusion over method'bias'. In accord with the Guide to the

Expression of Uncertainty inMeasurement (GUM) bias is only partof the uncertainty budget if it isunknown [8]. In EN 482:1994, becauseof the way bias was presented as part of'overall uncertainty', consistent andcorrectable bias was not clearlydistinguished from unknown anduncorrectable bias. Also EN 482:2006can now refer to tests in daughterstandards such as prEN 838 [9] and

prEN 1076 [10] that were yet to bedecided in 1994. Publication of therevised EN 838 (diffusive sampling)

and EN 1076 (active sampling)standards is expected not later than2009. For prEN 838 the working grouphas decided to exclude direct-readingstain length samplers, which reallyshould have their own different

performance requirements, but reagent-impregnated systems are included. Thestatistical treatment of uncertainty will

be quite different to EN 838:1995, beingpartly based on GUM and partly on theNordtest Handbook [11].

In the ambient air quality fieldTC246/WG11 (diffusive sampling) iswaiting for funding questions to beresolved before progressing further witha diffusive EN standard for NOx among

others. In TC264/WG12 (Referencemethods for SO2/NOx/O3/CO) the

publication of corrections to existingstandards has been delayed for similarreasons.

ISO air quality standards

The International Organisation forStandardisation (ISO) has a similarmanagement structure to CEN.Workplace, ambient and indoor airquality aspects in ISO are covered bysub-committees of TC146.

In TC146/ SC2 (Workplaceatmospheres) a revision of the diffusivesampling protocol ISO 16107 was

published in 2007 [12]. This standard iscomplementary in many ways to EN838 and there are no conflicting aspects.While ISO 16107 contains no

performance requirementsas such,examples of performance testsaredescribed in some detail. The revisedisocyanate by liquid chromatographystandard ISO 16702 usingmethoxyphenypiperazine reagent, was

published in December 2007 [13]. Anisocyanate standard usinganthracenylmethylpiperazine (MAP)reagent is at the draft stage and has been

balloted in ISO [14]. A guide forselection of isocyanate methods was

published in April 2006 as a TechnicalReport ISO/TR 17737 [15]. Since thelast issue of The Diffusive Monitorthefour generic ISO standards formeasurement of volatile organiccompounds (VOCs) by solvent andthermal desorption have been up for

systematic review and have all beenrenewed [16-19].

In TC/146/SC4 (General aspects) theISO 9169 standard that defines

performance tests for automaticmeasuring systems was published inJuly 2006 [20] and the ISO 20988 airquality guide to estimatingmeasurement uncertainty was published

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http://www.hse.gov.uk/coshh/table1.pdfhttp://www.hse.gov.uk/coshh/table1.pdf


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The Diffusive Monitor, 16

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in July 2007 [21]. Annex C.7 of ISO20988 includes a worked example ofuncertainty estimation in diffusivesampling of NO2compared with areference method.

In TC146/SC6 (Indoor air) the VOCmeasurement strategy standard ISO16000-5 was published in 2007 [22].

ISO 16000-2 (formaldehyde samplingstrategy) and ISO 16000-4(formaldehyde by diffusive sampling)were circulated for systematic review[23, 24].

UK Methods for the Determination of

Hazardous Substances (MDHS)

For a list of titles and revision history inthe UK MDHS series seehttp://www.hsl.gov.uk/publications/mdhs_list.htmAll MDHS titles currently in print areavailable for download from the HSE

site athttp://www.hse.gov.uk/pubns/mdhs/index.htm.For contractual reasons we have notmigrated old out-of-print MDHS titlesto the HSE website as promised in thelast CAR minutes, however, they will

be available by Email as searchablePDF files by application to the Editor.The only restriction is that they are forin-house study and not for onwarddistribution or uploading to a website.The criterion for inclusion in the"available" out of print group is that thetitles are not actually withdrawn astechnically deficient. In a few cases theyare cross-referenced by titles still in

print.Proficiency testing news

A training DVD aimed at WASPparticipants is available for purchasefrom HSL via the link below. If youhave Windows media player there is alink on the WASP information page to ashort video extract from the DVD.

http://www.hsl.gov.uk/proficiency-testing/wasp.htm#dvd

The topics covered include:

metals on filters by ICP-AES; VOCs on charcoal by solvent

desorption and gaschromatography;

VOCs on Tenax by thermaldesorption and gaschromatography;

isocyanate derivatives on glassfibre filters using liquidchromatography.

Other news f rom CAR/WG5 members

Downloadable English language reportsare available for two major studies oflow-level pollutants, the first being theFlemish Indoor Exposure Study(2005-2007) Amongst the substancesmeasured were benzene, toluene, xyleneformaldehyde, particulate matter andnitrogen dioxide with the objective ofdetermining the indoor/outdoor relation.

Recently more English languageversions of the latest German

Environmental Survey for Children

(2003/06 - GerES IV)have been issuedand are freely downloadable. Blood andurine monitoring results from GerES IVare also obtainable via the above link

although the biomonitoring study didnot involve VOC markers or othervolatiles, but rather heavy metals and

persistent organic pollutants.

References

1. Council Directive 98/24/EC on theprotection of the health and safety ofworkers from the risks related to chemicalagents at work (1998).

2. Council Directive 96/62/EC on ambient airquality assessment and management (1996).

3. Comit Europen de Normalisation (CEN):Project BC/CEN/ENTR/000/2002-16Analytical Methods for Chemical AgentsFinal Report, Sankt Augustin, Germany, 27June 2005. Brussels: CEN, 2005.

4. D. Breuer et al (2006). Journal ofOccupational and Environmental Hygiene,3: D126D136.

5. http://www.hvbg.de/e/bia/gestis/analytical_methods/

6. http://www.hvbg.de/e/bia/gestis/limit_values/index.html

7. EN 482:2006 Workplace atmospheres -General requirements for the performance of

procedures for the measurement of chemicalagents.

8. Guide to the Expression of Uncertainty inMeasurement (ISO, 1995, equivalent to EN13005:1998 equivalent to BS PD 6461 Part3:1995).

9. prEN 838 Workplace atmospheres Diffusive samplers for the determination ofgases and vapours Requirements and testmethods.

10. prEN 1076 Workplace atmospheres Pumped sorbent tubes for the determinationof gases and vapours Requirements andtest methods.

11. Practical Handbook for Calculation ofUncertainty Budgets for AccreditedEnvironmental Laboratories, TechnicalReport No. 537, February 2003, Nordtest

project 1589-02,http://www.nordicinnovation.net/nordtestfiler/tec537.pdf

12. ISO 16107:2007 Workplace air quality Protocol for evaluating the performance ofdiffusive samplers.

13. ISO 16702:2007 Workplace air quality -Determination of total organic isocyanategroups in air using 1-(2-methoxyphenyl)piperazine and liquidchromatography.

14. ISO/DIS 17735 Workplace atmospheres --Determination of total isocyanate groups inair using 1-(9-anthracenylmethyl)piperazine(MAP) reagent and liquid chromatography.

15. ISO/TR 17737 Workplace air quality Guide for the selection of isocyanatemeasuring methods.

16. ISO 16200-1:2001 Workplace air quality Sampling and analysis of volatile organiccompounds by solvent desorption/capillarygas chromatography Part 1: Pumpedsampling method.

17. ISO 16200-2:2000 Workplace air quality Sampling and analysis of volatile organiccompounds by solvent desorption/capillarygas chromatography Part 2: Diffusive

sampling method.18. ISO 16017-1:2000 Workplace air quality Sampling and analysis of volatile organiccompounds in ambient air, indoor air andworkplace air by sorbent tube/thermaldesorption/capillary gas chromatography Part 1: Pumped sampling.

19. ISO 16017-2:2003 Workplace air quality Sampling and analysis of volatile organiccompounds in ambient air, indoor air andworkplace air by sorbent tube/thermaldesorption/capillary gas chromatography Part 2: Diffusive sampling.

20. ISO 9169:2006 Air quality - Definition anddetermination of performance characteristicsof an automatic measuring system.

21. ISO 20988:2007 Air quality - Guidelines for

estimating measurement uncertainty.22. ISO 16000-5:2007 Indoor air - Part 5: :

Sampling strategy for volatile organiccompounds (VOCs).

23. ISO 16000-2:2004 Indoor air Part 2:Sampling strategy for formaldehyde.

24. ISO 16000-4:2004 Indoor air Part 4:Determination of formaldehyde -- Diffusivesampling method.
http://www.hsl.gov.uk/publications/mdhs_list.htmhttp://www.hsl.gov.uk/publications/mdhs_list.htmhttp://www.hse.gov.uk/pubns/mdhs/index.htmhttp://www.hsl.gov.uk/proficiency-testing/wasp.htm#dvdhttp://www.hsl.gov.uk/proficiency-testing/wasp.htm#dvdhttp://www.hsl.gov.uk/proficiency-testing/wasp.htm#dvdhttp://wwwb.vito.be/flies/flies_e.aspxhttp://www.umweltbundesamt.de/gesundheit-e/survey/us03/uprog.htmhttp://www.umweltbundesamt.de/gesundheit-e/survey/us03/uprog.htmhttp://www.umweltbundesamt.de/gesundheit-e/survey/us03/uprog.htmhttp://www.hvbg.de/e/bia/gestis/analytical_methods/http://www.hvbg.de/e/bia/gestis/analytical_methods/http://www.hvbg.de/e/bia/gestis/analytical_methods/http://www.hvbg.de/e/bia/gestis/limit_values/index.htmlhttp://www.hvbg.de/e/bia/gestis/limit_values/index.htmlhttp://www.hvbg.de/e/bia/gestis/limit_values/index.htmlhttp://www.nordicinnovation.net/nordtestfiler/tec537.pdfhttp://www.nordicinnovation.net/nordtestfiler/tec537.pdfhttp://www.nordicinnovation.net/nordtestfiler/tec537.pdfhttp://www.nordicinnovation.net/nordtestfiler/tec537.pdfhttp://www.nordicinnovation.net/nordtestfiler/tec537.pdfhttp://www.hvbg.de/e/bia/gestis/limit_values/index.htmlhttp://www.hvbg.de/e/bia/gestis/limit_values/index.htmlhttp://www.hvbg.de/e/bia/gestis/analytical_methods/http://www.hvbg.de/e/bia/gestis/analytical_methods/http://www.umweltbundesamt.de/gesundheit-e/survey/us03/uprog.htmhttp://www.umweltbundesamt.de/gesundheit-e/survey/us03/uprog.htmhttp://www.umweltbundesamt.de/gesundheit-e/survey/us03/uprog.htmhttp://wwwb.vito.be/flies/flies_e.aspxhttp://www.hsl.gov.uk/proficiency-testing/wasp.htm#dvdhttp://www.hsl.gov.uk/proficiency-testing/wasp.htm#dvdhttp://www.hse.gov.uk/pubns/mdhs/index.htmhttp://www.hsl.gov.uk/publications/mdhs_list.htm


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Stainless steel meshescoated with TEA

Teflon filter to stopaerosols and stabilize

diffusion path

Coloured capaccomodatingthe meshes

Acrylic plastic tube

Cap to beremoved during

sampling and replacedby a filter holder

Determination of NO2 and SO2 by ion chromatography in ambient air by use ofmembrane closed Palmes tube

Daniela Buzica, Michel Gerboles, DG-JRC, Institute for Environment and Sustainability, Joint Research Centre Via E. Fermi, I-

1027 Ispra VA, Italy E-mail: [email protected] , [email protected]

Introduction

Several authors have investigated the diffusive samplermethod for measuring NO2and SO2in ambient air. The two

pollutants are either independently [1,2,3,4] orsimultaneously [5,6,7] analysed using the diffusive sampler.Different types of diffusive samplers can be used e.g badgetype, radial type or open-ended longitudinal diffusion tubes.The Palmes tube, coated with triethanolamine (TEA), allowsthe simultaneous determination of NO2and SO2. However,field measurements showed that the open-ended diffusiontube is affected by a strong artefact on the SO2determinationarising from sulphate particulate matter. To avoid thisinterference, a Teflon membrane has been introduced at theopen end of the Palmes tube able to prevent contaminationcoming from the particulate matter. However theintroduction of the membrane creates an additive resistanceto the diffusion of molecules to the absorbent and thereforemodifies the uptake rate of the Palmes tube [8]. Theevaluation of NO2membrane-closed Palmes diffusion tubes(MCPTs) is already presented elsewhere [9]. Hereafter, anevaluation of the modified Palmes sampler with a membranefor the determination of SO2is presented.

Figure 1 Description of a Palmes tube

Principle

NO2and SO2are collected by molecular diffusion along theacrylic tube to the TEA where it is retained for subsequent

measurement. The first Ficks law describes diffusivetransport and allows after integration to determine theairborne concentration using an equation [10] of thefollowing form:

tU

mmC b

= (1)

where: U sampling rate, ng ppb-1min-1;m mass of the pollutant, ng;mb mass of the pollutant in the blank, ng;C concentration of the pollutant, ppb;t time of exposure, minutes.

Materials and methods

The modified Palmes sampler was described by Gerboles etal. [9]. The method of preparation of MCPTs is to cleantubes (Gradko DIF100), membranes (XDIF500F) and caps(XDIFCAP-001, XDIFCAP-003 and XDIFCAP-011) in anorbital shaker using Millipore water and changing the waterevery half hour for 3 hours. All samplers are then placed in

an oven, at 45 0C until they are completely dry. The stainlesssteel mesh discs (XDISC) are cleaned in an ultrasonic bath,at 60 0C for 5 hours, changing the water every half an hour.Then, they are placed in an oven, flushed with nitrogen, at125 0C until they are completely dry. Three clean and drydiscs are placed in the coloured cap (see Figure 1). 40 l of a10% v/v solution of TEA (Merck n8379) with 0.3 % of nonionic detergent (Brij-35, Merck n 1.01894) in deionisedwater is spread all over the mesh using a micropipette. Atube is placed immediately on the coloured cap while theother end is sealed immediately with a membrane forimmediate use. It is advised to check that the membrane iscorrectly placed to make sure that the pollutants diffuse onlythrough the membrane.

NO2 can be analysed using colorimetric method [8]. Eventhough this method gives good results it is still timeconsuming and since it is destructive it only allows

determination of one pollutant at a time. Conversely, usingion chromatography (IC) [11], it is possible tosimultaneously determine both pollutants.The tubes are analysed by adding 5 ml of MilliQ waterdirectly into the Palmes tube and then stirring them up withan orbital shaker creating a strong vortex for 5 minutes. The5 ml solution is then transferred into an IC cleaned vial and10 l of 30 % hydrogen peroxide (H2O2) is added to ensurecomplete oxidation of SO3

2-to SO42-. The vial is then closed

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Apr il 2008

average value of 0.00214 ng ppb-1min-1can be used for theSO2uptake rate.

Field experiments

In the framework of the AIRPECO project [ 14], 94duplicate pairs of membrane-closed Palmes tubes wereexposed during a measuring campaign over the city ofLjubljana (Slovenia) in February 2004. The samplers were

installed in protective boxes placed on lamp-posts at a height

ofplers, the NO2repeatability was 5.5 g m

-3.

PTs are installed in thee reference methods.

References

of 3 m.

The repeatability based on the duplicates has been evaluatedfor both NO2and SO2. For NO2, the repeatability was 9 gm-3(standard deviation of 3.2 g m-3) while for SO2it was 8g m-3 (standard deviation of 2.7 g m-3). During anothermeasuring campaign in June 2004 including 10 pairsduplicate sam

Conclusion

The work reported here highlights the possiblity ofdetermining both NO2 and SO2 in one run by ionchromatography using the MCPT. While for NO2the uptake

rate is already known, the uptake rate for SO2was found tobe independent of the conditions of exposure with a value of0.00214 ng ppb-1min-1. However, it is necessary to performsome field tests in which the MCvicinity of th

[1] Lin, J.M., Lin, T.S., A diffusive sampler for the ion-chromatogmeasurement of sulfur dioxide in ambient air, Toxicol

raphic

gry

er,

oekens, E., Keppens, V., Laboratory and field validation of a combined

ogical andEnvironmental Chemistry 39 (3-4), 1993, 229 236.[2] Tang, H., Brassard, B., Brassard, R., Peake, E., A new passive samplinsystem for monitoring SO2 in the atmosphere, Field analytical chemistand technology, 1 (5), 1997, 307 314.

[3] Cruz, L.P.S., Campos, V.P., Silva A.M.C., Tavares, T.M., A fieldevaluation of a SO2 passive sampler in tropical industrial and urban air,Atmospheric Environment 38, 2004, 6425 6429.[4] Buzica, D., Gerboles, M., Amantini, L., Prez Ballesta, P., De SaegE., Modelling of the uptake rate of nitrogen dioxide Palmes diffusivesampler based on the effect of environmental parameters, Journal ofEnvironmental Monitoring, 2005, 7, 169 174.[5] Plaisance, H., Sagnier, I., Saison, J.Y., Galloo, J.C, Guillermo, R.,Performances and application of a passive sampling method forsimultaneous determination of nitrogen dioxide and sulfur dioxide inambient air. Environmental Monitoring and Assessment 79, 2002, 301 315.[6] Kasper Giebl, A., Puxbaum, H., Deposition of particulate matter indiffusion tube samplers for the determination of NO2 and SO2, Technical

Note, Atmospheric Environment 33, 1999, 1323 1326.[7] Swaans, W., Goelen, E., De Fr, R., Damen, E., Van Avermaet, P.,R

NO2 SO2 Radiello passive sampler, Journal of EnvironmentalMonitoring, 2007, 9, 1231 1240.

[8] Passive Samplers for Nitrogen Dioxide, Agence de lEnvironnementet de la matrise de lEnergie, ADEME ditions, Rf. : 4414, Paris, 2002.[9] Gerboles, M., Buzica, D., Amantini, L., Modification of the Palmesdiffusion tube and semi-empirical modelling of the uptake rate formonitoring nitrogen dioxide, Atmospheric Environment 39, 2005, 2579 2592.

[10] European Committee for Standardization, Ambient air quality Diffusive samplers for the determination of concentrations of gases andvapours. Requirements and test methods, EN 13528:2002.[11] Miller, D.P., Ion chromatographic analysis of Palmes tubes for nitrite,Atmospheric Environment 1984, 18, 891-892.[12] Dionex, P/N 053891-16B, AS40 Automated sampler operatorsmanual[13] Buzica, D., Gerboles, M., Evaluation of the Palmes tube sampler withmembrane for the simultaneous determination of nitrogen dioxide andsulphur dioxide. Measurement campaign in the industrial area of Martigue(F), 2002, Technical Note No. I.02.110.[14] Field, R.A., Gerboles, M., Perez-Ballesta, P., Nikolova, I., Baeza-Caracena, A., Buzica, D., Connolly, R., Cao, N., Amantini, L., Lagler, F.,Stilianakis, N., Forcina, V., De Saeger, E., Air quality, Human exposureand Health impact assessment of air pollution in Ljubljana, Slovenia, 2005,EUR 21649.


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Tube-tagging Enhanced tracking of sample and tube-related information for thermaldesorption

Liz Woolfenden, Markes International Ltd, Llantrisant, UK, [email protected]

Wishful thinking?

Wouldnt it be good if there was a fail-safe way of linking

field sampling information with the relevant sorbent tubewithout relying on a chain of different people reading andrecording the individual number etched onto each tubewithout making any mistakes?

Wouldnt it also be good if TD users and field samplingpersonnel could immediately identify the sorbent-combination in the 6 tubes that have been rolling around onthe bench all week and when they were last used?

Wouldnt it be even better if there was a way ofautomatically tracking a thermal desorption tube throughoutits entire life recording what it is packed with, when it was

packed, how many times its been used and all the details

associated with its performance?This paper describes one possible answer to some of thesequestions

Historically, associating information with thermal desorption(TD) tubes has relied on manually reading and recording oftube serial numbers. Bar code technology has proveddifficult to apply to TD tubes because the high temperaturesrequired limit the lifetime of bar code labels. Bar codesetched onto curved tube surfaces also get increasinglydifficult to read electronically - especially after extensivehandling. Another limitation of bar codes is that they cant

be programmed to record sample or tube specific

information.A new RFID-tag based technology has recently beenintroduced for TD tubes which could overcome some ofthese limitations and offer a real step forward in sampletracking and analytical quality control for TD-GC(MS)users. The tags are re-usable, read/write programmable

RFID devices which can be attached to standard sorbenttubes (metal or glass) and may be applied in two ways:

Transit tagging used for tracking samples within a laband in transit between lab and field during airmonitoring projects. Available to all TD users

Tube tagging used both for sample tracking and tomonitor the history of each individual sample tubethroughout its life. Requires tag-compatibleinstrumentation.

The two modes of operation are illustrated in Figure 1.

Background

There are significant challenges in developing re-usableRFID tag technology for TD tubes not least the hightemperatures required for analysis. RFID tags are destroyedat temperatures above 140C and the associated read/writedevices dont work through the metal walls of most tubes.Tags must also be unobtrusive, resistant to environmentalfactors humidity, high particulate levels, etc. and stillallow a tube to be capped for long-term storage.

Developed by Markes, the new RFID TubeTAGs* seem tohave overcome these difficulties and provide a robust,

permanent and programmable tube labelling solution. Theyattach to the non-sampling ends of ordinary -inch (6.4mm) or 6 mm O.D. TD tubes and comprise a compactRFID-chip assembly mounted on a special tube clip. TheRFID chip itself is embedded in a protective, high

temperature, low-emission polymer to reduce the effect oftemperature and protect it from environmental factors seefigure 2. The clips are designed such that tube tags cant beattached to or removed from a TD tube without using aspecial tool.

* Patent number: US 6,446,515 B2

Data entry in f ield

Write project-specificinformation to tag in

the lab

Write tube specificdata to tag in the

laboratory

Figure 1 Using RFID tags for transit tagging and tube tagging.

Clear tube and sample data. Tag removed fromtube for re-use on another tube

Write sample start &end information to

tag in the field

Read tube and sampleinformation from tag in

the laboratory

Tube-history updated.Sample specific data cleared.Tagged tube ready for re-useTransit-tagging

Tube-tagging


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Tube tags in operation

When used for transit tagging, individual RFID tags areattached to every tube in a batch and programmed withrelevant information tube ID number, sorbent type, projectcode, etc. prior to dispatch. Once that batch of taggedtubes reaches the field, additional sample collection detailssuch as monitoring location, sampling method, samplingstart- and end-times, etc. can be entered onto each tag. When

the tubes are received back into the laboratory aftermonitoring, all of the stored information can be readilydownloaded from the tags and entered in the laboratorysinformation management system. The tags are then removedfrom the sampled tubes using the special tool as they are

placed into the automated thermal desorber for analysis.

Figure 2 Close up of tag assemblies and tagged tubes

From the moment a tube is tagged and programmed prior todispatch, in the relative calm of the lab environment (!!), nomanual re-entry of tube ID #, sorbent packing or project

number is required. Write-access to primary fields like thesecan be disabled by the system administrator if required.Subsequent reading and entering of other information ontothe tags in the field, for example monitoring location,sampling method and sample start & end times, simplyallow multiple opportunities for users to confirm the tube IDnumber programmed into the tag before dispatch.

Tags that have been removed from a batch of tubes justbefore analysis, can be cleared of information relating to thelast monitoring exercise and re-applied to the next batch oftubes going out for field sampling. Relevant new tube IDnumbers, sorbent details and project information can beentered onto the tags by the system administrator and the

whole cycle repeated. In this way, one RFID tag can beshared between several sampling tubes and costs can be keptdown to around 25 cents per tube per monitoring cycle.

When used for tube tagging, a given tag is linked to aspecific sorbent tube throughout its life or at least until thattube is re-packed (typically 200 or more sampling/analysiscycles.) This allows the history of that tube to be recordedand tracked. In this case, a tag is assigned to a tube as soonas it has been packed and conditioned and the tube ID

number, date of packing and combination of sorbents areentered only once. Each time a permanently-tagged tube isabout to be sent to the field, project information can beentered onto the tag in the lab before dispatch. As describedabove, sampling information (pumped or diffusive, flow rate/duration of exposure, start time, etc) can then be enteredonto the tag in the field. An example of the type of tube andsample data that can be recorded is shown in Figure 3 and a

typical field-portable tag read/write system is shown inFigure 4.

Operation in tube-tagging mode requires the use of tag-compatible TD instrumentation. Once the tagged tubes arereceived back into the laboratory they are placed into thetag-compatible automated TD system (see example in figure5) which automatically reads the recorded tube and entersthe relevant sample information into the automationsequence. Post run, the desorber can also write to the tags incrementing the number of thermal cycles, changing tubestatus (e.g. from sampled to desorbed) and clearing thesample collection information. Analytical anomalies such asleak test failures or unusually high back pressure can also be

recorded on the tag if required.Tags used in tube-tag mode i.e. permanently attached tothe same tube also last indefinitely. Tests have shownthem to be compatible with over a thousand thermal cycles even under extreme desorption conditions e.g. 400C for 30minutes. As above, this means that tagging costs areminimal Less than $0.25 per thermal cycle.

Figure 3 Tube and sample parameters recorded on tube tags

Tube conditioning in tube tag mode

The process of desorbing TD tubes is usually sufficient tocondition them. In other words, no additional cleaning isnecessary in most cases and analysed tubes can be re-usedstraight away. However, there are instances whereadditional, post-analysis conditioning is recommended forexample if tubes have been stored for extended periods (> 30days) or if the specific monitoring protocol requires the


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confirmation of tube blank levels before they can be used forfield sampling.

Figure 4 Example of portable device for programming tagsin the field or laboratory.

If additional tube cleaning is required it can be carried outeither using the TD-GC(MS) system or by using separateoff-line multi-tube conditioning rigs. The advantages ofusing tag-compatible TD analytical equipment for tubeconditioning is that the number of thermal cycles can beautomatically incremented and that a blank profile can beobtained automatically as part of the conditioning process.However, if ever / whenever multi-tube off-line equipmentis preferred for cost effective conditioning of an entire batchof tubes, tags can be readily removed from the tubes usingthe special tool and re-attached to the same tubes post-conditioning. The number of thermal cycles can be manuallyincremented as each tag is re-attached to its specificconditioned tube.

Data output and information sto rage

Users of tag-ready TD instrumentation record the status ofevery tagged tube whenever that tube is desorbed allowingthe information to be recorded as part of the sequence report.Moreover, a comma separated variable (CSV) file is createdevery time a tube tag is read or written to whether usingthe field portable tag-scribe device or via the desorber. Thisallows all tube- and sample-related data relevant to that tubeto be simply and easily entered into a database and accessedas and when required. Subsequent interrogation of thatdatabase could then be used to determine for example; whenthat tube or batch of tubes needs repacking or whether oneor more tubes have a history of leak test failures.

Summary

RFID tube tags such as those described have the potential togreatly enhance the analytical quality assurance of airmonitoring studies and TD-GC(MS) applications generally.

This is only the start. Future developments should allowtube-tags to be linked to TD methods allowing the analyticalsystem to generate its own automatic sequence for tubesloaded randomly into it. Tube tags also offer the potential

for intelligent interaction with GC(MS) data processingsystems. In the future, this should allow key analyticalfactors such as background levels or key artifacts to belinked with specific tubes and tracked over the lifetime ofthe tube.

Figure 5 Example of tag-compatible autosampler.


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Apr il 2008

Controlling GC carrier gas flow rate through a thermal desorption system transfer lineAndrew Tipler

Senior Scientist, GC Applications and Technology Group, PerkinElmer LAS, 710 Bridgeport Avenue, Shelton, Connecticut,06611, USA.

1. Introduction

Thermal desorption has become a popular technique for theextraction, concentration and injection of sample vaporscollected onto an adsorbent tube into a gas chromatographfor separation, identification and quantification. Figures 1and 2 illustrate the main steps involved in a typical 2-stagethermal desorption analysis.

GCDetector

Optionalinlet split Desorb flow

Cooledtrap

Carrier gas in

Heatedsample

tube

Analyti cal c olu mn

GCDetector

Optionalinlet split Desorb flow

Cooledtrap

Carrier gas in

Heatedsample

tube

Analyti cal c olu mn

Figure 1 First step in a 2-stage thermal desorption analysis

the primary (tube) desorption.

Carrier gas in

GCDetector

Analyti cal c olum n

Optional outlet split

heated trap

Carrier gas in

GCDetector

Analyti cal c olum n

Optional outlet split

heated trap

Figure 2 Second step in a 2-stage thermal desorptionanalysis the secondary (trap) desorption.

As can be seen in Figures 1 and 2, the sequence ofoperations involved in extracting and transferring the samplevapors into the column may have a dramatic effect on thegas about to enter the GC column there may be significantchanges in temperature, gas flows and gas pressures.

Throughout this whole process, we are trying to regulate theflow of carrier gas along the GC column from the thermaldesorption system.

The flow rate of carrier gas through the column is also

significantly affected by the column temperature as the gasincreases in temperature, it will become more viscous and, ifwe are using a pressure controller to supply the gas, the flowrate through the column will drop.

Modern GCs employ electronic systems to regulate carriergas supplies and users are now very familiar with concept ofconstant carrier gas flow control through the GC column.Such systems will provide better column efficiency and will

eliminate changes in response or background in a flow-sensitive detector such as a mass spectrometer.

This article describes systems and algorithms specificallydeveloped to overcome effects on the carrier gas justdescribed and to provide a constant flow rate of gascontrolled from the thermal desorption system, through atransfer line, through a column and into a detector.

2. Project requirements

This project was initiated to develop electronicprogrammable pneumatic control (PPC) systems capable ofproviding a level of flow control and performance notpossible with manual pneumatics systems. However, it was

also important that the stability and flexibility in using themanual pneumatic systems was not lost. Table 1 lists someof the key requirements for this project.

Table 1 Key requirements for PPC systems on a thermaldesorption system.

All control should come from the thermal desorption system

There should be no need for any additional external hardware (e.g.injectors or pneumatic controllers on the GC)

It should work with any GC (this implies the use of a flexible transferline)

There should be no transcription of column temperature programs

between the GC and the ATDThere is no need for the GC and ATD to communicate digitally witheach other

The flow rate will always track the current column temperatureautomatically without any additional input

The system will provide stable and precise operation over a widerange of flows and pressures

3. Electronic carrier gas control from a GC

To set the flow rate of gas through a GC column, we dontnormally control the flow rate directly. To precisely controla flow rate of 1mL/min through a capillary column is not

easy and we may wish to open split vents that will increasethe required flow rate by a factor of over 500 times. Also theslightest leak will represent a very significant lost portion ofthe gas that should be flowing through the column.

For these reasons, we normally apply the gas pressure that isexpected to deliver a required flow rate through the column.This approach makes the flow control through the columnlargely insensitive to changes in split flow rates and leaks.


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As a GC column is heated, the viscosity of the carrier gasflowing through it increases. In such cases the flow ratethrough the column will decrease with increasingtemperature. For most applications this will not have adetrimental effect on analytical results but with others, forinstance when a mass spectrometer is being used fordetection, the changing flow rate may have a dramatic effecton detector performance.

Most modern gas chromatographs are equipped withelectronic programmable pneumatic controls (PPC). Theseare able to compensate for the changing viscosity during atemperature program by increasing the inlet pressure at arate calculated to maintain a constant flow rate through thecolumn. To maintain a constant flow rate, the controllingsystem must have knowledge of the column temperature atall times and be able to calculate the gas viscosity at thattemperature and make the appropriate adjustments to theapplied pressure. The viscosity versus temperaturerelationships are well documented for all the popular carriergases used in GC and the Hagen-Poiseuille relationshipgiven in Equation 1 is used by the GC control systems to

perform these calculations.

( )

=

o

oico

PL

PPdF

256

224

Equation 1.

Where:Fo is the flow rate at the column outletdc is the internal diameter of the columnL is the length of the columnPi is the carrier gas pressure at the column inletPo is the carrier gas pressure at the column outlet is the viscosity of the carrier gas at the column

temperature

With a given column that is temperature programmed underisobaric conditions, the only variable that will alter will bethe viscosity, . Inspection of Equation 1 indicates that, asthe viscosity increases an appropriate increase in the inlet

pressure,Pi, can be applied to keep the column outlet flowrate, Fo, at a constant setting.

The oven temperature is known because it is controlled bythe GC. The viscosity of the carrier gas can be derived fromthis temperature. If the column dimensions are entered intothe system, then a specific flow rate may be controlled usingEquation 1.

4. Electronic carrier gas flow con trol through a transfer

line from a thermal desorption system

The situation becomes more complicated when the carriergas pressure is controlled on a system remote to the GC suchas a thermal desorption system.

Figure 3 summarizes the effects on the carrier shown inFigures 1 and 2 between where it exits the pressure regulatoron the thermal desorption system and where it enters thecolumn in the GC.

A. Regul ator con nect ed vi a tran sfer lin e to c olum n

B. Regulator connected via trap and transfer line to column

C. Regulator connected via trap, split and transfer line to column

pressureregulator

secondary

trap

split

transferline

column

B

B

B

detectorA

A

A

ATD GC

Figure 3 The various routes carrier gas can take betweenthe pressure regulator on the thermal desorption system andthe column inlet on the GC.

Both the secondary trap and the transfer line representrestrictions to gas flow and so the gas pressure delivered tothe column inlet indicated by point [B] will be less than the

pressure set by the pressure regulator at point [A].

Furthermore, with the trap inline, the pressure drop across itwill increase as the trap temperature increases (the gasviscosity increases with temperature) and as the split flowrate increases. The pressure drop will also change across the

transfer line if its temperature or the temperature of the GCcolumn is changed.

All this leads to the fact that the classic flow controlequation given in Equation1 cannot be used in this situationand some other approach must be used.

For the system to be effective, we must be able to control thepressure at point [B] shown in Figure 3. One significantdifference between a PPC system and a mechanical pressureregulator is that the pressure sensing device may be remotefrom the control valve. Figure 4 shows how a distributed

pressure control system could be applied to the worst-casescenario shown in Figure 3C.


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Apr il 2008

12

C. Pressure regulated at column inlet

BA

C T

V

A. Pres sur e regul ated pr ior to tr ap

BA

C T

V

B. Pressure regulated between trap and transfer line

BA

C T

V

Figure 4 Distributed PPC systems. (T = pressuretransducer, C = control system and V = control valve).

Figure 4A shows the PPC equivalent of a mechanicalpressure regulator. The control system (C) adjusts the

control valve (V) until the required pressure is seen at thepressure transducer (T). This configuration would perform ina very similar manner to a mechanical pressure regulator.

Figure 4C looks as if it would provide the ideal solution the pressure would be regulated directly at the column inlet.This means that Equation 1 could be used to provide carriergas flow control capabilities through the GC column.

Unfortunately this configuration also provides somepractical challenges. The first of these is that the pressuretransducer would need to be mounted on the GC this thenmakes the installation instrument specific. The major

problem, however, would be that the transducer would now

be very remote from the control valve and so the timeconstant of such a system would be very long which couldlead to instability (oscillation) or poor response to changesin upstream impedance or flows.

A variant on the configuration given in Figure 4C is to usean additional (and independent) pressure regulator on theGC which would be connected to a T-piece or inlet systemat the interface between the transfer line and the column.This was not considered as it would require additionalhardware and expense and would restrict the choice of GCsthat could be used. The additional carrier gas would alsodilute the sample entering the column.

The best solution appears to lie with Figure 4B. Tight andstable control of the gas pressure as it enters the transfer lineis achievable and the system responds well as the trap is

brought in and out of the gas stream and changes are madeto the trap temperature and split flow rate. Also, all thecontrolling hardware is now mounted within the thermaldesorption system facilitating operation with any GC.

The main issue with Figure 4B is that we still cannot useEquation 1 to control the flow rate of carrier gas through theGC column we still have a transfer line of (usually)different temperature and geometry to pass through first.

This matter is resolved by regarding the transfer line and theGC column as being two columns in series as shown inFigure 5.

PPii

PPxx

PPoo

Transfer lineTransfer line GC ColumnGC Column

( )xtt

xit

t

PL

PPdF

=

256

224 ( )xcc

oxc

i

PL

PPdF

=

256

224

FFii

FFtt

FFoo

PPii

PPxx

PPoo

Transfer lineTransfer line GC ColumnGC Column

( )xtt

xit

t

PL

PPdF

=

256

224 ( )xcc

oxc

i

PL

PPdF

=

256

224

FFii

FFtt

FFoo

Figure 5 Viewing the transfer line and GC column asserially connected columns.

In Figure 5, we have associated a different form of Equation1 to each of the two columns .

Where:Ft is the flow rate at the transfer line outletdt is the internal diameter of the transfer lineLt is the length of the transfer linePi is the carrier gas pressure at the transfer line inletPx is the carrier gas pressure at the transfer line outlet

and GC column inlett is the viscosity of the carrier gas at the transfer line

temperature

Fi is the flow rate at the GC column inletdc is the internal diameter of the GC columnLc is the length of the GC columnPo is the carrier gas pressure at the GC column outletc is the viscosity of the carrier gas at the GC column

temperature

Because the flow rate out of the transfer line, Ft, and theflow rate into the GC column, Fi, will be the same (oncecorrected for temperature), the two equations shown inFigure 5 may be solved simultaneously to produce arelationship between the outlet flow, Fo, and the appliedconditions to both the transfer line and the GC column.

Equation 2 gives the final relationship to describe how thecolumn output flow rate may be related to the appliedconditions applied to a column and transfer line of differingtemperatures and geometries connected in series.


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( )

+

=

44

22

256

c

ccc

t

ttt

oi

o

co

d

LT

d

LT

pp

p

TF

----- Equation 2

Where:Fo is the flow rate at the column outletTt is the transfer line absolute temperatureTc is the column absolute temperature

This approach needs information on the geometry of boththe column and the transfer line. This is easily addressed onthe thermal desorption instrument by the user entering boththeir geometries as inputs into the control system.

The temperature of the transfer line and the applied pressureare known as they are controlled from the thermaldesorption system so the only parameter not known is thetemperature of the GC column in the GC oven. To addressthis need, the transfer line has a thermocouple threadedthrough it as shown in Figures 6 and 7.

Gas ChromatographThermal Desorber

Detector

GCcolumn

GC oven

Heated

transfer linetubing

PPC

pressure

regulator

Signal cable

from

thermocouple

Thermocouple

T4

Gas ChromatographThermal Desorber

Detector

GCcolumn

GC oven

Heated

transfer linetubing

PPC

pressure

regulator

Signal cable

from

thermocouple

Thermocouple

T4T4

Figure 6 Using a thermocouple inside the transfer line to

monitor GC column temperature.The temperature sensor may be calibrated using the GCcolumn oven at one or more set-points to enable either asingle point or multi-point temperature calibration.

Thermocouple

GC Column

Thermocouple

GC Column

Figure 7 Thermocouple positioned against GC columninside GC oven.

5. Examples of PPC system operation

To evaluate the efficacy of the new control algorithm, aseries of tests was conducted using helium carrier gas dopedwith ~0.5% of methane using the apparatus shown in Figure8. This small concentration of methane was not expected tochange the behavior of the helium during these experiments.

Methane

Helium

PR1

PR2

MFC1

MFC2

BPR1(100psig)

2mL/min

400mL/min

Transferline

GC

FlameIonization

Detector

ATD

PP

Methane

Helium

PR1

PR2

MFC1

MFC2

BPR1(100psig)

2mL/min

400mL/min

Transferline

GC

FlameIonization

Detector

ATD

PP

Figure 8 System for delivering a stream of helium carriergas doped with 0.5% methane to a thermal desorptionsystem.

The back-pressure regulator (BPR1) in Figure 8 ensures thatthe upstream flows were unaffected by the gas demands onthe thermal desorption system (for example, as split vents

were opened) and so gas with a constant composition wasconsistently applied to the instrument. The flame ionizationdetector is a very linear mass-flow sensitive detector and isvery sensitive to methane. Consequently, the output signalfrom the detector was directly proportional to the flow ofdoped carrier gas flowing through it as shown by thecalibration plot given in Figure 9.


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Apr il 2008

14

0

100

200

300

400

500

600

700

800

900

1000

0 2 4 6

Measured Flow Rate (mL/min)

FID

Signal(mV)

8

r2=0.9996

0

100

200

300

400

500

600

700

800

900

1000

0 2 4 6


FID

Signal(mV)

8

r2=0.9996

0

100

200

300

400

500

600

700

800

900

1000

0 2 4 6


FID

Signal(mV)

8

r2=0.9996

Figure 9 Calibration plot of FID output signal versus flowrate of methane doped helium carrier gas produced using theapparatus shown in Figure 8.

1mL/min Flow Control

7.2psig Pressure Control

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36

min

mV



0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 360 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 340 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36

min

mV

Figure 10 Comparison between constant flow and pressurecontrols on a 15m x 0.25mm column programmed from40C for 1 minute, then 10C/min to 300C and held for 10minutes. The transfer line was 1.8m x 0.28mm and held at300C. The set flow rate of 1mL/min of the doped heliumhad an initial pressure of 7.2 psig this was used for theconstant pressure test. The test system was a PerkinElmerTurboMatrix 650 ATD and Clarus 500 GC.

This method of measuring gas flow is particularly suited tothis experiment as it allows the low flow rate of gas exitingfrom the column to be measured directly and under theconditions used for chromatography.

Figures 10 and 11 show comparisons between constant flowcontrol using the new PPC algorithm and constant pressurecontrol for two temperature programs.



0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0

min

mV



0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.00.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0

min

mV

Figure 11 Same conditions as Figure 10 but with aprogramming rate of 40C/min.

As can be seen from Figures 10 and 11, the new PPC controlalgorithm gave a very acceptable performance in the

constant flow control mode. The deviation was less than 2%throughout the whole temperature range applied to thecolumn oven.

6. Conclusions

A mathematical function has been developed that describesthe relationship between applied pressure and outlet flowrate from a GC column connected to a thermal desorptionsystem via a transfer line.

This function has been integrated into a programmablepneumatic control system to provide the effective control of

a set flow rate of carrier gas through a GC column.The PPC hardware has been implemented in such a way sothat changes in trap impedance or split flow rate do not

perturb the PPC control stability.

The system developed should be applicable to a wide rangeof analytical methods and should function with any GC.


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Diffusive uptake rates of aromatic hydrocarbons on Carbograph 1TD at workplaceconcentrations using a thermal desorption tube sampler

Neil Plant, Glen McConnachie, Kate Shrivastava and Mike Wright

Health and Safety Laboratory, Harpur Hill, Buxton, SK17 9JN UK, [email protected]

Introduction

Graphitized carbons suitable for thermal desorption fromtube samplers have been available for many years. HSL andothers have proved their utility in the diffusive sampling ofambient air for up to four weeks. However, for diffusivetube sampling in the workplace over 0.5 8 hours there is ashortage of published data for graphitized carbons. Whatfollows is an account of diffusive sampling/thermaldesorption methods applied to workplace air. Differentchoices and compromises will apply to ambient airsampling. Workplace validations, such as those published in

MDHS 80 [1], were mostly of porous polymers that wereconsistent from batch to batch and which did not generallycatalyse thermal decomposition. The earliest carbonaceoussorbents used in thermal desorption were of variable qualityand unsuitable for semi-volatile or thermally labilesubstances. This was not surprising since they were not

primarily intended for sample recovery by thermaldesorption. The situation improved when chromatographysuppliers started to make carbon sorbents with a variety ofclosely controlled properties. By this time so much work hadgone into measuring diffusive uptake rates on porous

polymers that there was little enthusiasm for duplicating thevalidations. We will briefly mention the pros and cons of

polymers versus carbons here. Out of one sorbent studyduring 1994-95 came the nomination of Chromosorb 106 asthe best compromise when compared with graphitizedcarbon and carbon molecular sieves [2]. The test substanceswere selected for a wide range of properties and possibly theoutcome of a repeated study with more sorbents using theoriginal criteria would have been the same. Nevertheless, amedium strength graphitized carbon is a good choice forcompliance monitoring of substances with low limit valuesor in diffusive sampling for short exposure times. Artefactlevels are much lower than those of porous polymers. HSLoriginally chose Carbograph 1TD for thermal desorption

because its performance with thermally labile substanceswas better than some other carbon sorbents. Carbopack B isvery similar for sorbent strength and wherever historicaldiffusive uptake rates exist for both sorbents there seems to

be no significant difference. Stability of aromatichydrocarbons at high temperatures is not at issue here.However, we have had more general experience withCarbograph 1TD than Carbopack B.

Experimental

Test atmospheres of benzene, toluene, m-xylene and 1,3,5-trimethylbenzene (TMB) as mixed vapour (1-100 ppm eachcomponent) were generated by a syringe infusion pump(Harvard model 22) injecting at a known rate into 30 l/min(nominal) dilution air via a heated block and glass exposurechamber. The reference concentrations and uptake rateswere determined by active and diffusive sampling onstainless steel tubes, 89 mm x 6.4 mm od., 5.0 mm id.,

packed with 300 mg Carbograph 1TD (Markes InternationalLtd), followed by thermal desorption and gas

chromatography with flame ionization detection (MarkesUnity/Agilent 6890 and PerkinElmer Turbomatrix 650/PEClarus). Within the exposure chamber diffusive tubes weremounted on a plate rotating at about 80 rpm. The effectiveair velocity experienced by the diffusive tubes was estimatedat about 50 cm/s. For determining reference concentrationsactive sampling conditions were 20 ml/min for 30 minutes.A number of sequential active samples (up to 12) were takento cover exposure periods up to 480 minutes. Flow rateswere controlled to within 0.5 % by electronic mass flowdevices (Brooks 5850S, 0-100 ml/min) traceable bycalibration to national standards. Equilibration time andstability of the test atmosphere was recorded by a total

hydrocarbon monitor (3000HM, Signal Instruments).Calibration of the gas chromatographs was by liquid spikingof thermal desorption tubes from methanol solutions

prepared gravimetrically. The delivery volume of amicrolitre syringe for liquid spiking (5 l, SGE Ltd.) wasdetermined by the gravimetric method of ISO 8655-6 usinga small weighing vessel with lid. [3]

Results

Diffusive uptake rates calculated as ng/ppm/min are given inTable 1. It was estimated that the combined expandeduncertainty of each mean value in Table 1 is not greater than5 % for 30-120 exposure and 7 % for 480 minutes exposure

(at 95 % confidence). The effect of exposure time appears tobe anomalous at 480 minutes exposure time, where uptakerates are significantly lower than those at 30-120 minutes.We do not believe that the use of mixed vapour atmospheresin place of single substances has any bearing. There is at themoment no good explanation other than some unknown biasin the measurement of the reference concentration or theamounts on the diffusive tubes that happened only on thetube sequences from the 480 minute exposure.
http://www.hse.gov.uk/pubns/mdhs/pdfs/mdhs80.pdfhttp://www.rsc.org/publishing/journals/AN/article.asp?doi=an9962101171http://www.rsc.org/publishing/journals/AN/article.asp?doi=an9962101171http://www.hse.gov.uk/pubns/mdhs/pdfs/mdhs80.pdf


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16

Table 1 Diffusive uptake rates (ng/ppm/min) for selectedaromatic compounds on Carbograph 1TD, PerkinElmer typetube sampler, effect of sampling time and concentration,mean values from 6-8 samplers, typical combined expandeduncertainty 5 % (30-120 mins), 7 % (480 mins) at 95 %confidence.

Exposure time (mins) 30 60 120 480

Concentrationrange ppm

1-3 2.02 2.14 2.08 1.36Benzene

100 2.01 - - 1.51

1-3 2.12 2.30 2.22 1.69Toluene

100 2.14 - - 1.71

1-3 2.11 2.28 2.22 1.78m-Xylene

100 2.23 - - 1.82

1-3 2.38 2.34 2.39 1.801,3,5-TMB

100 2.18 - - 1.90

We regard the results at 480 minutes as provisional and to beconfirmed or otherwise by a repeat determination. Thegreatest confidence is assigned to exposure times of 30-120minutes. Table 2 lists the mean uptakes rates over samplingtimes 30 -120 minutes and concentrations 1 ppm -100 ppm,compared with theoretical (ideal) uptake rates estimatedfrom diffusion coefficients taken from Lugg [4]. Theexperimental data of Lugg was temperature corrected from25C to 20C. For this estimation we have assumed that theArea/Length ratio of the tube sampler air gap is 0.121 cm.

Table 2 Estimated theoretical/ideal uptake rates comparedwith mean experimental uptake rates on Carbograph 1TD,averaged over the sampling conditions of Table 1(excluding 480 mins), combined expanded uncertainty at95 % confidence .

D20 Ud(ideal) Ud(exp.)

cm2/s ng/ppm/min

Benzene 0.0902 2.13 2.06 0.08

Toluene 0.0822 2.29 2.20 0.09

m-Xylene 0.0666 2.14 2.21 0.09

1,3,5-TMB 0.0641 2.32 2.32 0.09

Conclusions

Over the sampling times 30 480 minutes there was someevidence that at the longest time, corresponding to a full

shift, the uptake rate was significantly reduced. However,the reduction was a little more than was expected and needsfurther confirmation, particularly for benzene. There was noevidence of a concentration effect in the range 1-100 ppmand this would simplify the estimation of concentrationswhen not using a single mean value for uptake rate, butcorrecting for known bias as a function of sampling time.

References

1. Methods for the Determination of Hazardous Substances, MDHS 80,Volatile organic compounds in air: Laboratory method using diffusivesolid sorbent tubes, thermal desorption and gas chromatography,

Health and Safety Executive, August 1995, ISBN 0-7176-0913-8.2. R H Brown, What is the best sorbent for pumped sampling - thermaldesorption of volatile organic compounds? Experience with the ECsorbents project,Analyst, 1996, 121, 1171-1175.

3. BS EN ISO 8655-6:2002 Piston-operated volumentric apparatus Part 2: piston pipettes.

4. G A Lugg, Diffusion coefficients of some organic and other vapoursin air.Analytical Chemistry, 1968, 40, 1072-1077.


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Indoor Air 2008

The 11th International Conference on Indoor Air Quality and Climate

The 11thcongress is being organised by the Technical University of Denmark. At the the time of writing the Copenhagen venuehad not been announced.

17-22 August 2008, Copenhagen, Denmark

The series of Indoor Air and Climate conferences started in August 1978 in Denmark. The 11th congress in 2008 celebrates the30 year anniversary of the inaugural conference by revisiting Copenhagen. The 11th Indoor Air conference will be amultidisciplinary event involving participants from medicine, engineering, architecture and related fields. The conference willcover all aspects of Indoor Air Quality and Climate and the effects on human health, comfort and productivity. The conferencewill address a variety of indoor environments - residential, office, school, industrial, commercial and transport.

Topics: Indoor environmental exposure assessment in buildings and vehicles; Risk Characterization in the indoor environment; Control and Regulatory options; Socio-economic context of management of the indoor environment.

The published deadline for abstracts has expired

For further information see the conference website home page http://www.indoorair2008.org/


18/19

. . . . . . . . . . .

. . . . . . . . . . . . .

Apr il 2008

18

16th International Conference on

Modelling, Monitoring and Management

of Air Pollution

Organised by the Wessex Institute of Technology, UK;sponsored by The ASCE UK International Group andWIT Transactions on Ecology and the Environment.

22 - 24 September, 2008, Skiathos,

Greece

Topics: Air pollution modelling

Air quality management Urban air management Emission studies Monitoring and measuring Global and regional studies Aerosols and particles Climate change and air pollution Atmospheric chemistry Indoor air pollution Environmental health effects Remote sensing Policy studies Air Pollution Effects on Ecosystems

For further information see the conference website homepage http://www.wessex.ac.uk/conferences/2008/air08/

29th Triennial Congress of the

International Commission on

Occupational Health (ICOH2009)

International organisations participating are ILO and WHO.

22-27 March 2009, Cape Town

International Convention Centre,

South Africa

The Scientific Program has been posted on the website, andthe Call for Abstracts has been issued, with an end date of 21July 2008 for receipt of Abstracts. Early bird registration hasexpired (30 April, 2008). Grants are available for some

presenters from developing nations. Two page brochures

containing the scientific sessions and other key details can bedownloaded from the website for printing and distribution.Among the 160 listed topics in the Scientific Program are:

Toxicology Industrial hygiene Indoor air quality

For further information see the conference website homepage http://www.icoh2009.co.za


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THE DIFFUSIVE MONITOR

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