Thermal Desorption Technical Support · using low temperature thermal extraction of a few grammes of material. Automated headspace-trap4 or off-line thermal extraction are two typical

IntroductionFlavour and fragrance profiles typicallycomprise contributions from hundreds ofvolatile organic compounds (VOCs) and thoseat lowest concentration can be the mostsignificant olfactory components i.e. have themost profound effect on perceived aroma.Historically, this has made it difficult to carryout meaningful aroma tests using standardGC/MS instrumentation. Conventional sample preparation methods(solvent extraction, steam distillation, etc.)simply don’t offer the sensitivity required andmay also distort the vapour profile so that it isno longer representative of theflavour/fragrance perceived by consumers.Odour experts and aroma assessment panelshave therefore continued to play a significantrole in product testing with respect to flavourand fragrance.

These olfactometry procedures work well;however, they are to some extent subjective,are rarely able to identify the precise cause ofa problem and are also, by definition, manualand consequently expensive/time-consuming tocarry out.In recent years, analytical thermal desorption(TD) has provided a useful complement toGCMS enabling more aroma profilingapplications to be carried out usingquantitative, automatic instrumentation. TDcombines automated sample preparation withselective analyte enrichment allowing organiccompounds to be injected into the GCMS as anarrow concentrated band of vapour, free ofmost/all sample matrix effects. The technologyis available in on- and off-line configurationsand is now widely used for vapour profiling inthe food, flavour, fragrance and consumerproduct industries.

T D T S Thermal Desorption Technical Support

Note 84: Using thermal desorption to enhance aromaprofiling by GCMS – featuring example applications from

the tobacco industryKey Words:

Air sampling, sorbent tubes, sample identification, RFID, TubeTAG

AbstractFlavour and fragrance profiles typically comprise contributions from hundreds of volatile organic compounds(VOCs) and those at lowest concentration are often the most important i.e. have the most profound effect onperceived aroma. Historically, this has made it difficult to carry out meaningful aroma tests using standardGCMS instrumentation. Conventional sample preparation methods (solvent extraction, steam distillation, etc.)simply don’t offer the sensitivity required and may also distort the vapour profile so that it is no longerrepresentative of the flavour/fragrance perceived by consumers. Odour experts and aroma assessment panelshave therefore continued to play a significant role in product testing with respect to flavour and fragrance. This TDTS Note examines the potential of two recent GCMS-related technological developments for allowingmore odour profiling applications to be carried out by automated laboratory instrumentation thus reducingcosts. The first of these relates to Markes’ latest analytical thermal desorption technology for gas extractionand selective concentration of aroma constituents. The second is a complementary innovation in GCMSreprocessing software which further enhances the measurement of trace olfactory compounds in complexaroma profiles. The applications and potential advantages of both these technologies for automated aroma profiling aredescribed using applications from the tobacco industry as an example.

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TDTS84 May 2008Page 2 of 8

T D T S

Sampling options for thermaldesorptionOne of the strengths of thermal desorption forfood, flavour and fragrance profiling is that itoffers a versatile range of samplingmethodologies (Figure 1). TD sampling options include:

• Sorbent tubes/traps. Used for off-lineconcentration of organic vapours. Thetubes may be packed with multiplesorbents for collecting the completevapour profile or with a single sorbent thatretains key olfactory compounds butallows volatile interferences (e.g. water,ethanol and acetic acid) to be purged tovent. Example applications include;profiling the fragrance of consumerproducts, tracking taint in warehouse airor shipping containers, breath profilingand monitoring crop volatiles.

• On-line sampling i.e. discontinuoussampling/concentration and analysis -used for monitoring changes in odourprofile over time. Example applicationsinclude food shelf life studies, diurnalvariation in natural (biogenic) fragrances,process gas purity and monitoring theprofile of perfume products, such as airfresheners, as they decay with time.

• Direct, low-temperature desorption ofmaterials weighed into empty TD tubes –used for screening the odour profile of dry,homogeneous materials such as spices,instant coffee, soap powder, etc. Exampleapplications include quality control of spiceblends, identifying taint/off-odour inconsumer products, validating the qualityof natural products (checking for cheapsynthetics), QC of packaging andscreening the vapour profile of medicinalpastes/creams.

• Off-line thermal extraction or dynamicsampling of headspace vapours within-line sorbent trap. Used for monitoringthe vapour profile of a wide range ofinhomogeneous products and rawmaterials. Aroma profiling applicationscarried out this way include; tobacco andother vegetable products, shampoo andcleaning products, prepared foods (pizza,pet foods, meat products, etc.), dairyproducts, biogenic emissions (insectpheromones, plant volatiles, etc.),drinking water odour, taint frompackaging, potable spirits and GM foods.

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Figure 1: Overview of sample introduction methods compatible with thermal desorption analysis

on-line

directdesorption

sorbenttubes

headsapceorpurge & trap

focusingtrap

capillary GCMS

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Thermal desorption and repeatanalysisWhichever of these approaches is used tointroduce the sample to the TD-GCMS, thecompounds of interest end up separated fromthe sample matrix and focused on a small,electrically-cooled sorbent trap (see figure 1).The focusing trap is subsequently desorbed byheating it rapidly in a reverse flow of carriergas causing the organic compounds to beinjected/transferred into the GCMS analyticalsystem as a narrow band of vapour. Trapdesorption efficiency and system inertness arekey to ensure optimum sensitivity andquantitative recovery of the widest range ofcompounds – including odorous, reactivespecies like mercaptans. Series 2 TD systemsfrom Markes International incorporate a short,totally inert flowapth and a quartz focusing trapwhich is capable of heating at rates over100°C/sec. This ensures maximum desorptionefficiency and best possible detection limits.Markes TD systems also feature SecureTD-Q -the unique facility to quantitatively re-collectany/all split flow during primary (tube) orsecondary (trap) desorption. This allowsrepeat analysis and simple validation ofcomponent recovery through the analyticalsystem (Figure 2).

Pioneered by Markes International, theinnovation of SecureTD-Q™ is key for aromaprofiling by TD-GC/MS, both in the methoddevelopment phase and for monitoring systemstability/performance long term. Many keyolfactory constituents (i.e. compounds with thelowest odour thresholds) are very reactive e.g.amines, oxygenates and mercaptans. Theability to re-collect a portion of the sample,after it has been through the TD flow path, andthen repeat the analysis to check recovery,gives greatly enhanced confidence that targetcompounds are being quantitatively transferredthrough the analytical system. If there was aloss of one or more reactive compounds in theprofile, this would become apparent by achange in the relative responses in the repeatanalysis data. In both of the cases shown in figure 3successful recovery of every component testedwas demonstrated using the SecureTD-Q re-collection and repeat analysis technique.

Figure 3a: Vapour profile from boiling potatoes.Original data (black). Analysis of re-collectedsample (red) shows identical profile indicatingquantitative recovery across the analyte range

Figure 3b: Primary and repeat analysis of ethylmercaptan & benzene using SecureTD-Q shows

quantitative recovery of this highly reactivecompound through the Markes UNITY™ 2

thermal desorber

1-pe

ntan

al

2-pe

ntylf

uran

deca

nal α-co

paen

e

n-bu

tyl-n

-but

yrat

e2-

phen

oxye

than

ol

hexa

nal

pent

anol

Figure 2: SecureTD-Q: Two-stage thermaldesorption with quantitative re-collection ofany/all split flow onto a single sorbent tube.

Allows repeat analysis and validation of analyterecovery

benz

ene

ethyl

merca

ptan

UNITY 2

GC(MS)

Tube desorptionSample tube

Cold trap

Optional split

Bypass line

UNITY 2

GC(MS)

Trap desorptionSample tube

Cold trap

Optional split

re-collection

re-collection

Primary analysisSecondary analysis

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A fundamental advantage of thermal desorptionand its associated sampling methods is thatcompounds of interest are extracted straightinto the GC carrier gas stream. No manualsample preparation steps are required andproblems associated with the use of solventsare eliminated e.g. masking of peaks ofinterest, loss of volatiles and variable extractionefficiency. The vapour profile produced is thusmore likely to be representative of the aromaperceived by consumers. The latest Markes TD systems, as describedabove, are also capable of transferring thevapour profile constituents into the GC capillarycolumn in much smaller volumes of carrier gasthan was possible before. Transfer volumes canbe as low as 100 µL of carrier gas. This meansthat very significant concentrationenhancement factors can be achieved –typically from 103 to 106 depending on thenumber of concentration/desorption steps(Figure 4).

Thermal desorption also allows volatileinterferences such as water and ethanol to beselectively purged to vent prior to analysis thusmaking it easier to discriminate betweensamples according to the key olfactorycomponents (Figure 5).

Thermal desorption-GCMS in thetobacco industryAutomated thermal desorption was introducedto the tobacco industry in the early 1980’s inresponse to growing public awareness ofenvironmental tobacco smoke1-3. However, ascigarette manufacturers invested in TD-GCMSequipment for monitoring trace levels ofnicotine and other target compounds in theenvironment, it gave staff in the productdevelopment and quality control departmentsan additional, versatile analytical tool to applyto the challenges they faced every day. Therange of new applications successfullydeveloped included: A. ‘Fingerprinting’ the aroma of tobaccoraw materials and products. Tobacco fingerprinting is typically carried outusing low temperature thermal extraction of afew grammes of material. Automatedheadspace-trap4 or off-line thermal extractionare two typical sampling options in this case. Markes Micro-Chamber/Thermal Extractor(µ-CTE™) with 6 separate sample chambersand a temperature range of ambient to 120°Cis ideal for tobacco profiling. Vapours from thetobacco are collected on sorbent tubes attachedto each micro-chamber and are subsequentlyanalysed using 2-stage thermal desorption withGC/MS. Example data are shown in figure 6.


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Figure 4: TIC profile of an indoor air samplewith ppt level contaminants analysed by TD-

GCMS(scan)

Figure 5: Typical VOC profile from whiskyheadspace, with illustration (dotted line) of

how the ethanol peak would mask key aromacompounds if it was not selectively purged from

the trap prior to desorption

Whisky

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Direct thermal desorption of smaller samples oftobacco weighed into empty TD tubes is alsoused occasionally e.g. to measure flavouradditives such as menthol or vanillin or forscreening the total vapour profile (Figure 7).

Both of these approaches (figures 6 and 7) arecost-effective, easy-to automate and not proneto the limitations of conventional extractionmethods e.g. insensitivity and masking ofcompounds of interest with the solvent.

B. Identifying and tracking the cause ofoff-odour and taint complaints.Cigarette taint can be an expensive issue todeal with if large quantities of product areaffected and it is imperative that the cause ofthe contamination is identified and dealt withpromptly. In one actual case, a chemicalcontaminant causing a raft of consumercomplaints was identified and its sourcedetermined entirely using thermal desorptionwith GC/MS. First samples of the tobacco, filtertips and paper from contaminated and controlcigarettes were directly desorbed. When theGC/MS data obtained from contaminatedsamples was compared with that from thecontrols the chemical culprit was quicklyidentified. Then direct thermal desorption wasused to track the source of the contaminationthrough the cardboard of the cigarettepackaging until it was ultimately found that thecontaminant had originated from the woodenpallets used to ship the cardboard that madethe cigarette packets.

C Assessing the profile of VOCs inhaledby smokers. “Smoking rigs” are used in the tobacco industryto draw air through lit cigarettes and ontosorbent tubes. In this case the samplingprocess mimics the actual smoking process bydrawing several small volume “puffs” onto thesorbent tube, at specified time intervals, thusallowing the performance of cigarette filters tobe monitored. Example TD-GC/MS data areshown in figure 8.

Figure 8: VOCs sampled from a cigarette usinga “Smoking engine”

isopr

ene

tolue

ne

benz

ene

Figure 7: Analysis of tobacco by directdesorption

Triac

etin

n-he

xade

cano

ic ac

id

4-(3

-hyd

roxy

-1-b

uten

yl)3,

5,5-

trime

thyl-

2-cy

clohe

xen-

1-on

enico

tine

3-hy

drox

y-2,

3-dih

ydro

malto

l

Figure 6: Comparison of rolling tobacco andcigarette tobacco, sampled using the MicroChamber/Thermal Extractor. Inset shows

individual micro-chamber containing tobacco

aceti

c acid

prop

ylene

glyc

ol

butyr

olacto

ne

triac

etin

nicot

ine

2,6,

6-tri

meth

ylbic

ycloh

epta

ne

Rolling tobacco

Cigarette tobacco

This data enables researchers to refine filterdesign – minimizing the breakthrough of toxiccompounds while optimising consumerperception of aroma.

Enhancing automatic aromaprofiling using innovative (TD-)GCMS data reprocessing softwareWhile the latest innovations in analyticalthermal desorption technology have provided amajor breakthrough in allowing meaningful

aroma profiling using automated laboratoryinstrumentation, TD-GC/MS does not alwaysprovide the complete answer. Aroma profilingoften relates to natural products or complexcomposite manufactured products which canresult in a high background signal in the totalion chromatogram (TIC) produced (see figure7). Thick-film high-bleed columns may also berequired to separate volatile polar componentsin the profile and this can also compromisesubsequent analysis of trace components. Seetop chromatogram, figure 9.


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S/N = 3:1 S/N = 30:1

Figure 9: A chromatogram of trace-level landfill gas odour standard, reprocessed using ClearViewsoftware. This demonstrates the improved spectral quality and increase in signal to noise ratio for

trace level components

Original data - manualbackground subtraction

required beforecomponent can be

identified

After ClearViewreprocessing - automatic

library search of apexspectrum correctly

identifies component asthiophene

Original GCMS data from landfill odour standardSame data, reprocessed using ClearView software

A new dynamic background compensation(DBC) algorithm called ClearView has beendeveloped by Markes to address this issue.ClearView distinguishes and eliminates massions originating from the chromatographic‘background’ from those in real peaks, howeversmall. This has great potential for many real-world GC/MS applications including odour andaroma profiling. To summarise how it works;the software reprocesses stored GC/MS datafiles (singly or in batches) distinguishing massions from the background (column bleed,sample matrix, solvent tail, air/waterinterference, etc.) and eliminating theircontribution from the mass ion fragmentationpattern of chromatographic peaks. A separatereprocessed data file is then produced withlower interference, better signal to noise andenhanced spectral purity (see figure 9). Thisboosts sensitivity and aids automaticidentification of trace compounds. It alsoimproves integration and the repeatability ofTIC and extracted ion data.

NOTE 1: The original data file is left intact forseparate analysis if required. NOTE 2: The GCMS reprocessing software isdescribed in more detail elsewhere5. To evaluate the potential of ClearView forenhancing detection of trace olfactorycomponents in complex vapour profiles, theTD-GCMS aroma data from direct desorption oftobacco (Figure 7) was reprocessed using thenew software. The reprocessed data is shownin figure 10. The compounds highlighted in redwere not detected at first in the original datafile but were readily identified and measuredafter reprocessing using dynamic backgroundcompensation. Similarly, those compoundsidentified in blue gave a library match qualitysignificantly better after reprocessing whichwas the data using ClearView.


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Figure 10: Clearview reprocessing direct tobacco desorption data shown in figure 7. Significantremoval of unwanted background enables better qualification of trace aroma compounds present in

the tobaccoItems in blue displayed a greatly improved library match quality after ClearView reprocessing,

while those in red were not identified at all until ClearView reprocessing was applied

1

1615

14

1312

11109876

5

432

17

1. 1-(2-butoxyethoxy)-ethanol2. Menthol3. Benzeneacetic acid4. Triacetin5. Nicotine6. 4-hydroxy-benzeneethanol7. Myosmine8. 3-methyl-4-phenylpyrazole9. Butylated hydroxytoluene10. 3-Hydroxy-beta-damascone

11. Megastigmatrienone12. 4-(3-hydroxy-1-butenyl)-3,5,5-

trimethyl-2-cyclohexen-1-one13. Methyl-14-methylpentadeccanoate14. n-Hexadecanoic acid15. Ethyl palmitate16. Isopropyl palmitate17. 2-Ethylhexyl-trans-4-methoxy

cinnamate

Original GCMS data (black)and ClearView reprocesseddata (blue)

TD conditions for direct desorptionof tobaccoPrepurge: 1min, trap and split in-lineDesorption: 8 mins @120°C, split on.

desorb flow ~30 ml/min, split flow ~50 ml/min

Cold trap: -10°C to 300°C for 3 mins split on

Flow path 150°CGC programme: 40°C for 5 min, then

20°C/min up to 300°C. Column: HP1MS, 60 m x 0.25 mm x

0.25 µm

SummaryMarkes recent innovations in thermaldesorption technology have been shown tooffer a readily-validated, automatic and high-sensitivity alternative to conventional liquidextraction methods for enhanced aromaprofiling by GC/MS. TD allows vapour profileconstituents to be cleanly extracted from thesample matrix into the gas phase andfacilitates selective purging of volatileinterferences in many cases. This helps toensure that the vapour profile analyzed by theGC/MS system is as representative as possibleof the aroma perceived by consumers andmakes it easier to automate control of productquality. The complementary potential of Markes’ newClearView software for reprocessing GCMS datafiles and enhancing the detection andidentification of trace target olfactory analyteshas also been demonstrated.A combination of both of these technologies –thermal desorption to enhance sampleintroduction and ClearView reprocessing toenhance data analysis – should mean thatmore and more aroma profiling applications canbe transferred to automated laboratoryinstrumentation, thus reducing costs andenhancing routine quality control for a widerange of foods, drinks and consumer goods.

References1. Bell, R.E; Intern. J. Environ. Anal. Chem.

(1987), 33, pp219-2322. Proctor et al; Environ. Inter. (1991), 17,

pp287-2973. Burrefors, G & Petersson, G;

J. Chromatog., (1993), 643, PP71-764. Markes International TDTS note #785. Markes International TDTS note #83

TrademarksClearView™, SecureTD-Q™, UNITY™ andµ-CTE™ are trademarks of Markes InternationalLtd., UK

Applications were performed using the stated analytical conditions.Operation under different conditions, or with incompatible samplematrices, may impact the performance shown.


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Thermal Desorption Technical Support · using low temperature thermal extraction of a few grammes of material. Automated headspace-trap4 or off-line thermal extraction are two typical

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