Introduction Concern is growing over emissions of VOCs and semi-VOCs from the materials used indoors and in vehicle cabins, due to their potential impact on human health/comfort. National and international regulations/protocols, such as the European Construction Products Directive, German protocol for fire-resistant floorings (AgBB 2004) and the Californian CHPS protocol for public school building programs (California 2004) require the determination of materials emissions using conventional test equipment such as chambers/cells (Methods EN 13419-1/- 2/-3, ISO/EN 16000-6/-9/-10/-11, etc.). This enables emissions to be evaluated under simulated real-use conditions and allows real- room concentrations to be estimated. However, conventional emissions testing requires considerable expertise to ensure production of meaningful/repeatable results, especially with respect to operation of emissions chambers. Although emission cells are much more compatible with industrial laboratories (See TDTS 72), tests still take over 24 hours if strict standard protocols are followed (ISI 16000-10, ENV 13419-2, etc.). If chambers are used, tests are expensive and lengthy to perform (typically 3-4 days per test) which makes them unsuitable for routine industrial QA/QC. One alternative way of minimising the risk from emissions is to ensure that materials do not contain significant concentrations of toxic/odorous compounds in the first place. A number of voluntary labelling schemes which promote 'Low VOC' products actually rely on product content testing rather than emissions testing in order to comply with the scheme (e.g. those using US EPA Method 311 or similar procedures for paint). The European automotive industry, adopts a similar approach to testing emissions from car trim components. Their Method VDA 278 specifies direct desorption of materials, at elevated temperatures, to assess both VOCs and SVOCs (fogging) components. Traditional methodology for product content testing has required complex, multi-step sample preparation, for example grinding of solids followed by solvent extraction, or steam distillation of resins, etc. Such sample preparation methods are inherently manual/time-consuming and subject to poor performance due to incomplete extraction, loss of volatiles and low sensitivity. Direct thermal desorption (TD) offers an alternative, readily- automated approach for the determination of (S-)VOCs in materials and has been applied to everything from fragrance in soap to solvent in pharmaceuticals (See TDTS 9). It can be applied to solids, resins, liquids and pastes and involves heating the material in a flow of inert gas. Vapours eluted from the sample are focused on a small electrically-cooled sorbent trap. This is subsequently heated at 100°C/sec, in a reverse flow of carrier gas, to transfer/inject the organics to the analyser (GC, MS or GC/MS) as a narrow, focused band of vapour thus maximising sensitivity. Key considerations to ensure optimum method performance include: simultaneous analysis of volatiles/semi-volatiles, quantitative recovery through the analytical system, repeatability and flexibility i.e. compatibility with multiple sample types. TDTS Thermal Desorption Technical Support Note 65: Automating the measurement of VOCs and semi- VOCs in building materials and car trim components using direct thermal desorption Keywords: emissions chambers, PVC foam, artificial leather, fogging compounds, leather discolouration, dried paint, standard methods www.markes.com Markes International Ltd. T: +44 (0) 1443 230935 F: +44 (0) 1443 231531 E: [email protected]
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IntroductionConcern is growing over emissions of VOCs andsemi-VOCs from the materials used indoors andin vehicle cabins, due to their potential impacton human health/comfort. National andinternational regulations/protocols, such as theEuropean Construction Products Directive,German protocol for fire-resistant floorings(AgBB 2004) and the Californian CHPS protocolfor public school building programs (California2004) require the determination of materialsemissions using conventional test equipmentsuch as chambers/cells (Methods EN 13419-1/-2/-3, ISO/EN 16000-6/-9/-10/-11, etc.). Thisenables emissions to be evaluated undersimulated real-use conditions and allows real-room concentrations to be estimated. However, conventional emissions testingrequires considerable expertise to ensureproduction of meaningful/repeatable results,especially with respect to operation ofemissions chambers. Although emission cellsare much more compatible with industriallaboratories (See TDTS 72), tests still take over24 hours if strict standard protocols arefollowed (ISI 16000-10, ENV 13419-2, etc.). Ifchambers are used, tests are expensive andlengthy to perform (typically 3-4 days per test)which makes them unsuitable for routineindustrial QA/QC. One alternative way ofminimising the risk from emissions is to ensurethat materials do not contain significantconcentrations of toxic/odorous compounds inthe first place. A number of voluntary labellingschemes which promote 'Low VOC' productsactually rely on product content testing ratherthan emissions testing in order to comply with
the scheme (e.g. those using US EPA Method311 or similar procedures for paint). TheEuropean automotive industry, adopts a similarapproach to testing emissions from car trimcomponents. Their Method VDA 278 specifiesdirect desorption of materials, at elevatedtemperatures, to assess both VOCs and SVOCs(fogging) components.Traditional methodology for product contenttesting has required complex, multi-stepsample preparation, for example grinding ofsolids followed by solvent extraction, or steamdistillation of resins, etc. Such samplepreparation methods are inherentlymanual/time-consuming and subject to poorperformance due to incomplete extraction, lossof volatiles and low sensitivity. Direct thermaldesorption (TD) offers an alternative, readily-automated approach for the determination of(S-)VOCs in materials and has been applied toeverything from fragrance in soap to solvent inpharmaceuticals (See TDTS 9). It can beapplied to solids, resins, liquids and pastes andinvolves heating the material in a flow of inertgas. Vapours eluted from the sample arefocused on a small electrically-cooled sorbenttrap. This is subsequently heated at 100°C/sec,in a reverse flow of carrier gas, totransfer/inject the organics to the analyser(GC, MS or GC/MS) as a narrow, focused bandof vapour thus maximising sensitivity. Keyconsiderations to ensure optimum methodperformance include: simultaneous analysis ofvolatiles/semi-volatiles, quantitative recoverythrough the analytical system, repeatability andflexibility i.e. compatibility with multiple sampletypes.
T D T S Thermal Desorption Technical Support
Note 65: Automating the measurement of VOCs and semi-VOCs in building materials and car trim components using
ExperimentalThe utility of direct thermaldesorption/extraction was evaluated for a rangeof materials. Various parameters were tried,including the exact procedure described inMethod VDA 278. The types of materials testedincluded: PVC foam sheet, artificial leather(polyurethane resin), real leather, and dried &liquid water-based paint.Samples of each material were weighed intoempty glass or stainless steel thermaldesorption sample tubes or into PTFE linerswhich were subsequently inserted into emptymetal tubes. Solid samples were supportedusing clean quartz or glass wool plugs toensure that the material stayed in the centralportion of the tube. Resins or pastes weresmeared around the inner walls of PTFE tubeliners and aliquots of liquid samples (typically2-10 µl) were deposited on clean quartz woolplugs inside PTFE tube liners (See figs 1-3). Inall cases, care was taken that the sample didnot block the gas flow through the sampletube.
In the event that the maximum temperature ofthe sample matrix was unknown, a short bed(1 cm) of conditioned Tenax was placed at thefront (desorption end) of the TD tube duringthe method development phase. This was toensure that sample matrix components did not
migrate into the flow path of the thermaldesorber and contaminate the system. Desorption parameters were selected such thatcomplete or representative extraction of (S-)VOCs was achieved while matrix compoundswere left behind in the sample tube. Focusingtrap parameters (sorbent, temperature, gasflow) were selected such that targetcompounds were quantitatively retained whilewater and other, unwanted volatile interferentswere purged to vent (See TDTS #s 26 and 51).Subsequent rapid (back-flush) desorption of thefocusing trap thus transfers/injects only thosevolatile and semi-volatile compounds ofinterest, free of interference from matrixartifacts, water and other unwanted volatiles.Desorption parameters were selected such thatcomplete or representative extraction of (S-)VOC target compounds was achieved withoutdecomposing or degrading the sample matrix.All experiments were carried out using aMarkes ULTRA-UNITY automated thermaldesorber (Figure 4) with GC/MS.
Quantitative recovery through the thermaldesorber was evaluated using SecureTD-Q -quantitative re-collection for repeat analysis(See TDTS 24 and the SecureTD-Q brochure)as described in ASTM Method D 6196-03.PVC foam and artificial leather (PUR) byVDA 278 (see also TDTS 59): Three ~30 mgsamples of a PVC foam sheet and an artificialleather were evaluated for VOCs and fogging
Figure 1: Solid samples weigheddirectly into empty TD tubes andsupported with clean glass wool
Figure 2: Liquid samples loadedonto glass wool plug. Shown with
optional Tenax bed
Figure 3: Resins or pastes smearedonto the inner walls of a PTFE tubeinsert. Shown with optional Tenax
bed
Figure 4: ULTRA-UNITYautomated thermal desorber for up
to 100 tubes
compounds respectively (Figures 5 and 6). Theprocedure followed method VDA 278 i.e.desorbing for 30 minutes at 90°C and for 60minutes at 120°C respectively followed byGC/MS analysis.
Key compounds desorbed during VOC analysisof the PVC sheet are:
Note that method VDA 278 does not attempt toachieve complete extraction, but generates arepresentative profile of VOCs or 'fogging'compounds allowing intercomparison of similarproducts.The Markes TD system offers quantitative re-collection of split flow for repeat analysis andvalidation of analyte recovery as described inASTM Method D6196. This procedure was usedto check for loss of the type of high boilinganalyte observed during the VDA 278 'fogging'test. A phthalate standard (di-ethyl to di-nonylphthalate) was loaded onto Tenax tubes thendesorbed, re-collected and re-analysed twice tocheck for bias (selective loss of one or otheranalytes) (Figure 7). Good recovery wasobserved across the volatility range.
Troubleshooting discoloration of leather(See Also TDTS 40): White leather upholsterywas found to be turning yellow in patches.Small sections of leather (~1.5 mm x 10 mm)from the discoloured and white areas werethermally desorbed (150°C for 5 mins) and theTD-GC/MS data compared. It was immediatelyevident that detergent residues, present on thewhite leather, were absent from the yellowedleather and that the yellowed leather had highlevels of natural oils (Figure 8).
Figure 6: Fog analysis of artificial leatherFigure 7: Original and repeat TD-GC-MSanalyses of phthalate standard to check
recovery
Complete desorption of (S-)VOCs fromdried and liquid water-based paint (Seealso TDTS 57): Direct thermal desorption wasused for complete extraction of the (S-)VOCcontent of dried paint (220°C for 10 minutes -Figure 9) and wet paint (200°C for 10 minutes- Figure 10). Repeat desorption demonstrated>99% recovery of all analytes across thevolatility range in one run.
Note that Markes TD technology is compatiblewith simultaneous VOC and SVOC analysisbecause the focusing trap is desorbed in areverse flow of carrier gas to that used duringthe trapping process i.e. in backflush mode(See TDTS #64). Semi-volatiles are thusquantitatively retained and released from weaksorbents in the front of the trap and volatilesare quantitatively retained and desorbed fromstronger sorbents at the rear of the trap.
DiscussionThese examples demonstrate that directthermal desorption is compatible with manymaterial types and can be used for bothcomplete (quantitative) extraction (Table 1)and representative profiling of the (S)VOCcontent of a material. Simultaneous (S)VOCanalysis is readily achieved provided that, ashere, the focusing trap is desorbed in backflushmode. By eliminating manual samplepreparation, automated thermal desorptionmakes it possible for (S-)VOC content testingto be carried out as part of a routine industrialQA/QC procedure. Recent developments, suchas quantitative re-collection for repeat analysishave also been shown to allow simple checkson method performance/analyte recovery.However, there are limitations to direct thermaldesorption/extraction, especially in relation toquality control of materials emissions. Keyissues include sample homogeneity (i.e.conventional sample tubes are only compatiblewith small sample sizes e.g. 100 mg of solids,or 5-10 µl of liquid. For many materials thismay not be representative of the whole). It isalso difficult to obtain a direct correlation
Figures 8(a) & (b): Direct desorption of leatherto determine cause of discolouration
Figure 9: Direct desorption of volatiles andsemi-volatiles from 2 mg of dry paint flakes
Figure 10: Direct desorption of volatiles from3.3 mg of liquid water-based paint
between (S-)VOC data obtained using directthermal desorption and conventional emissionstesting. This is partly due to materialheterogeneity and the small sample size, butother factors, such as the desorption of bulkmaterials (rather than surface only) plus theuse of elevated temperatures, also play theirpart.Markes has recently developed an automatedMicro-Chamber/Thermal Extractor (µ-CTE) (SeeFigure 11 and separate leaflet) to addressthese concerns. The new system comprises sixmicro-chambers (up to 25 mm deep and ~45mm in diameter) which allow surface or bulkemissions to be tested from up to 6 samplessimultaneously. A conditioned Tenax or othersorbent tube is attached to each micro-
chamber and a controlled flow of air passedthrough. A unique flow distribution system*maintains a constant flow of air through eachsample chamber, independent of sorbent tubeimpedance and whether or not a sorbent tubeis attached. No pump or mass flow controller isrequired. Tests can be carried out at ambient orelevated temperatures (up to 120°C) andmoderate temperatures (e.g. 40°C) can beused to approximate standard emissions testingand compensate for the relatively small samplesize without affecting correlation with data fromconventional chambers or cells). Total test time(equilibration and vapour sampling), for all 6samples, is normally between 30 and 60minutes, depending the temperature required.The µ-CTE does not fully comply withconventional emissions test methods (ISO/EN16000-9/-10) but provides industry with aneasy-to-use quality assurance tool forautomated emissions testing. By generatingdata that correlates with conventionalemissions tests it will also allow manufacturersto monitor product quality/uniformity inbetween formal certification tests by accreditedlaboratories. The µ-CTE also facilitates direct thermaldesorption/extraction of bulk materials therebyallowing accurate measurement orrepresentative profiling of the VOC/SVOCcontent of less homogenous materials.
SummaryThermal desorption is an invaluable analyticaltool for materials testing and its ability tosimplify and automate (S-)VOC content testinghas been shown in this application note. TD isalso used in specialist laboratories equippedwith conventional chambers/cells formeasurement of vapour-phase emissionstrapped on Tenax tubes per standard methods
Table 1: Content of propylene glycol, DOW DPnB and Texanol in liquid paint
such as ISO 16000-6, ASTM D6196-03 and ISO16017-1. The new 'µ-CTE' thermal desorptiontool should further provide manufacturingindustry with a means of carrying out cost-effective and automated in-house tests -generating data that correlates well withexternal product certification results producedusing conventional emissions chambers/cells.
References• Ausschuss zur gesundheitlichen Bewertung
von Bauproduckten (AgBB). July 2004.AgBB protocol: "Health-related evaluationprocedure for (S-)VOC emissions frombuilding products."
• California Dept. of Health Services. July2004. "Standard practice for the testing ofVOC emissions from various sources usingsmall-scale environmental chambers,CA/DHS/EHLB/R-174,(www.dhs.ca.gov/ps/deodc/ehlb/iaq/VOCS/Practice.htm)
• US EPA Method 311. "Analysis of hazardousair pollutants in paints and coatings."
• Verband der Automobilindustrie (VDA).November 2001. VDA Method 278"Thermodesorptionsanalyse organischerEmissionen."
• ASTM. 2003. ASTM Method D6196-03."Standard Practice for selection ofsorbents, sampling and thermal desorptionanalysis parameters for volatile organiccompounds in air.
* UK Patent Application No. 0501928.6
Applications were performed using the stated analytical conditions. Operationunder different conditions, or with incompatible sample matrices, may impact theperformance shown. www.m
