Volatilizable Biogenic Organic Compounds

17 Pages·2012·2.01 MB·English

Atmos. Meas. Tech.,5,345–361,2012
Atmospheric
www.atmos-meas-tech.net/5/345/2012/
Measurement
doi:10.5194/amt-5-345-2012
©Author(s)2012. CCAttribution3.0License. Techniques
Volatilizable Biogenic Organic Compounds (VBOCs) with two
dimensional Gas Chromatography-Time of Flight Mass
Spectrometry (GC×GC-TOFMS): sampling methods, VBOC
complexity, and chromatographic retention data
J.F.Pankow1,2,W.Luo1,2,A.N.Melnychenko3,K.C.Barsanti2,L.M.Isabelle1,2,C.Chen1,A.B.Guenther4,and
T.N.Rosenstiel3
1DepartmentofChemistry,PortlandStateUniversity,Portland,OR97207,USA
2DepartmentofCivil&EnvironmentalEngineering,PortlandStateUniversity,Portland,OR97207,USA
3DepartmentofBiology,PortlandStateUniversity,Portland,OR97207,USA
4AtmosphericChemistryDivision,NationalCenterforAtmosphericResearch,Boulder,CO80305,USA
Correspondenceto: J.F.Pankow([email protected])
Received: 13May2011–PublishedinAtmos. Meas. Tech.Discuss.: 10June2011
Revised: 9December2011–Accepted: 4January2012–Published: 14February2012
Abstract.Twodimensionalgaschromatography(GC×GC) columnareprovidedfor21standardcompoundsandfor417
withdetectionbytime-of-flightmassspectrometry(TOFMS) tentatively identified VBOCs. 19 of the 21 authentic stan-
was applied in the rapid analysis of air samples contain- dard compounds were found in one of the Cedrus atlantica
ing highly complex mixtures of volatilizable biogenic or- SPME samples. In addition, easily quantifiable levels of at
ganic compounds (VBOCs). VBOC analytical method- least 13 sesquiterpenes were found in an ATD sample ob-
ologies are briefly reviewed, and optimal conditions are tained from a branch enclosure of Calycolpus moritzianus.
discussed for sampling with both adsorption/thermal des- Overall,theresultsobtainedviaGC×GC-TOFMShighlight
orption (ATD) cartridges and solid-phase microextraction anextreme,andlargelyuncharacterizeddiversityofVBOCs,
(SPME) fibers. Air samples containing VBOC emissions consistentwiththehypothesisthatsesquiterpenesandother
fromleavesoftwotreespecies(CedrusatlanticaandCaly- compounds beyond the current list of typically determined
colpusmoritzianus)wereobtainedbybothATDandSPME. VBOCanalytesmaywellbeimportantcontributorstoglobal
The optimized gas chromatographic conditions utilized a atmosphericlevelsoforganicparticulatematter.
45m,0.25mmI.D.low-polarityprimarycolumn(DB-VRX,
1.4µmfilm)anda1.5m, 0.25mmI.D.polarsecondarycol-
umn(StabilwaxTM,0.25µmfilm). Excellentseparationwas
1 Introduction
achieved in a 36min temperature programmed GC×GC
chromatogram. Thousandsof VBOCpeakswere presentin
Organic compounds volatilize to the atmosphere from both
thesamplechromatograms;hundredsoftentativeidentifica-
anthropogenic and biogenic sources. Anthropogenic emis-
tionsbyNISTmassspectralmatchingareprovided.Veryfew
sions of non-methane volatile organic compounds (VOCs)
of the tentatively identified compounds are currently avail-
have been estimated at 110 to 150TgCyr−1 (Muller, 1992;
able as authentic standards. Minimum detection limit val-
uesfora5lATDsamplewere3.5pptv(10ngm−3)foriso- Piccotetal.,1992). Incontrast,theglobalinputtotheatmo-
sphereofnon-methanebiogenicvolatileorganiccompounds
prene,methylvinylketone,andmethacrolein,and∼1.5pptv
(∼10ngm−3)formonoterpenesandsesquiterpenes. Kovats- (BVOCs) has been estimated at ∼1100TgCyr−1 (Guenther
et al., 1995), much of that amount being plant related. The
typechromatographicretentionindexvaluesontheprimary
non-methane BVOCs are: (1) important in the geochemi-
column and relative retention time values on the secondary
cal cycling of carbon (Guenther, 2002); (2) have significant
PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion.
346 J.F.Pankowetal.: VBOCswithtwodimensionalGC×GC-TOFMS
effects on tropospheric ozone levels (Williams et al., 1997; al., 2010). The complexity of the VBOC group is therefore
Starnetal.,1998);(3)affecthydroxylradicalconcentrations duebothto thevarietyofchemicalsub-classesrepresented,
(Tan et al., 2001; Lelieveld et al., 2008) and thus the life- and to the substantial numbers of compounds in many of
times of ozone-depleting and greenhouse gases (Kaplan et the sub-classes. Another complicating factor is that the re-
al., 2006); and (4) oxidized in the atmosphere to products activities of individual VBOCs vary widely. With regard to
thatcancondense(Haagen-Smit,1952)andtherebyformat- functionalizedcompounds,thetermVBOC,asusedhere,ex-
mospheric organic particulate matter (OPM) that can affect plicitly includes early oxidation products of plant-produced
theradiativeandcloudnucleationpropertiesofatmospheric compounds (caryophyllene oxide is an example of such an
particles(Goldsteinetal.,2009;Po¨schletal.,2010). oxidationproduct).
Thechemicalvariety,numbers,andreactivitiesofVBOCs
European Union Directive 1999/13/EC defines a VOC as
anyorganiccompoundhavingavaporpressureof≥10−2kPa have posed considerable challenges in efforts designed to
(10−4atm)at293.15K.Isopreneasapureliquidatthistem- develop quantitative understandings of important processes
perature has a vapor pressure of po=70kPa (0.7atm). A governing the VBOCs. First, large uncertainties remain re-
L
gardingsimplytheoverallmagnitudeofthetotalannualmass
semivolatileorganiccompound(SVOC)hasbeendefinedas
a compound with a po value in the range from 10−2kPa emissionsofVBOCsandwhetherunidentifiedorundetected
L
(10−4atm)downto10−9kPa(10−11atm)(Bidleman,1988). compoundscontributesignificantlytothoseemissions(e.g.,
GoldsteinandGalbally,2007).EvenwhenthelistofVBOCs
By definition, an SVOC has significant affinity for con-
considered is relatively “comprehensive” (e.g., isoprene, α-
densed phases, yet is sufficiently volatile that it can parti-
/β-pinene,α-phellandrene,camphene,13-carene,limonene,
tionsignificantlytotheatmosphere,particularlywhenother
myrcene, α-/γ-terpinene, terpinolene, linalool, nopinone,
mechanisms (e.g., oxidation) continually remove the com-
methyl-chavicol, α-bergamotene, β-caryophyllene, α-/β-
pound from the gas phase and thus maintain the driving
farnesene,longifolene),themeasuredfluxesandatmospheric
force for volatilization. De Gouw et al. (2011) have re-
concentrationsdonotappeartoaccountfortotalVBOCmass
ported observations that suggest that SVOCs can evaporate
emissions. For example, measured fluxes have not always
from spilled crude oil, be oxidized in the atmosphere to
been consistent with observed levels of OH reactivity (Di
lowervaporpressurecompounds,andtherebyleadtoforma-
tion of atmospheric OPM. At po≈10−3kPa (10−5atm) at Carlo et al., 2004), observed O3 fluxes (Goldstein et al.,
L 2004), or emissions data from co-located enclosure sam-
293K,sesquiterpenes(e.g.,farnesene)arerelativelyvolatile
ples (Goldstein et al., 2004; Bouvier-Brown et al., 2009a).
SVOCs,andareofsignificantinterestherefortheircontribu-
Moreover, observations of higher than expected secondary
tiontobothgas-andparticle-phaseprocesses. Therefore,in
organicaerosollevelshavebeencitedasevidenceof“miss-
placeofthetermBVOCs,weusethetermvolatilizablebio-
ing” VBOCs (e.g., Goldstein and Gallbally, 2007). There
genicorganiccompounds(VBOCs)torefertothefullspan
are numerous difficulties that complicate the acquisition of
ofbiogeniccompoundsofinteresthere. (TheVBOCgroup
accurateVBOCemissionestimates. First,massemissiones-
isnotintendedheretoincludemethane.)
timatesaredifficultforreactiveterpenoids,eveninlocations
Plants release VBOCs simply because in plant tissues
wheretheemissionratesarerelativelyhigh(Bouvier-Brown
these compounds tend to be lost to the surrounding envi-
etal.,2009a,b;Ortegaetal.,2007). Second,largeuncertain-
ronmentbydiffusivetransportmechanisms;acceleratedloss
tiesremainregardingthevariabilitiesinVBOCemissionsby
can occur because of mechanical/herbivore wounding (e.g.,
plantspecies,geographiclocation,andseason(e.g.,Helmig
Heiden et al., 2003). Of particular interest is the fact that
et al., 2007). For example, annual emissions in the US of
increased emissions of certain VBOCs are associated with
isoprene and a number of monoterpenes may be changing
plantsexperiencingenvironmentalstress(e.g.,Heidenetal.,
by amounts and for reasons that are not adequately under-
2003). Stress indicator compounds include ocimene, farne-
stood(Purvesetal.,2004),andtheresponsesofVBOCemis-
sene, methyl salicylate, salicylic acid, jasmonic acid, and a
sions to changing climate parameters remain highly uncer-
group of C aldehydes, alcohols, and esters referred to as
6 tain (Rosenstiel et al., 2003; Guenther et al., 2006; Chen et
“green-leaf volatiles” (Heil and Ton, 2008; Ka¨nnaste et al.,
al.,2009;PenuelasandStaudt,2010). Third,almostnothing
2008;StaudtandBertin;1998).
isknownconcerningeithertherelativeimportanceofchiral
Overall, VBOCs are highly diverse and include: (1) nu- variants (e.g. (+)- vs. (−)-α-pinene, (+)- vs. (−)-β-pinene,
merous terpenes including isoprene (C H ) which is etc.),orhowmuchimportantbiogeochemicalinformationis
5 8
a hemiterpene, monoterpenes (C H ), sesquiterpenes heldwithinchiralpatternsofVBOCemissions(Williamset
10 16
(C H ), and diterpenes (C H ); (2) terpenes that are al., 2007; Yassaaetal., 2010). Overall, despitethefactthat
15 24 20 32
functionalized(e.g,oxidized)innumerouswaysandatava- manyBVOCemissionstudiescurrentlyappeareachyearin
riety of positions; (3) alkanes and alkenes; (4) alkyl alde- the scientific literature, most have targeted only a few com-
hydes and ketones; (5) alkyl alcohols, ethers, acids, and es- poundsasanalytes. Incontrast,investigatorsmakingtheef-
ters; and 6) chiral variants such as (±)-α-pinene, (±)-β- fort to look for other compounds (e.g., Jardine et al., 2010)
pinene, and (±)-limonene (Williams et al., 2007; Yassaa et havefoundawiderangeofVBOCs.
Atmos. Meas. Tech.,5,345–361,2012 www.atmos-meas-tech.net/5/345/2012/
J.F.Pankowetal.: VBOCswithtwodimensionalGC×GC-TOFMS 347
Given the significant impact of VBOC emissions on e.g., cannot distinguish individual monterpenes, individual
biosphere-atmosphereinteractionsandairchemistry,thereis sesquiterpenes,etc.)
a great need for development and application of new ana-
lyticalmethodologiescapableofcharacterizingthecomplex 2.2 Determinationinthelaboratory
nature of VBOC emissions. Development of new method-
ologiescanbeobtainedby: (1)applicationofsuitablesam- ATD cartridges are easily transported to and from the field,
ple collection methods; (2) application of high-separation and provide a simple, sensitive, and quantitative approach
gas chromatography (GC) methods that can adequately ad- for determining a wide range of VOCs at ambient atmo-
dress VBOC complexity; and (3) accumulation of GC re- sphericlevels(Pankowetal.,1998,2003). Laboratory-based
tention index information based on as many authentic stan- ATD determinations of VBOCs in field samples have uti-
dard compounds as possible (many VBOCs of interest are lized GC/FID, GC/MS, and GC×GC-TOFMS (Saxton et
noteasilyobtainedinpureform). Herewedescribethecol- al., 2007). (FID=flame ionization detector.) Several con-
lection,tentativeidentificationanddetermination,andchro- siderationsareimportantintheimplementationofATDwith
matographic characterization of VBOCs in highly complex VBOCs. First, losses of reactive VBOCs may need to be
samples using two-dimensional gas chromatography/time- prevented by removal of oxidants, particularly ozone, prior
of-flight mass spectrometry (GC×GC/TOFMS), the latter to passage of the sample air through the cartridge (Cao and
being the most powerful separation+detection methodology Hewitt, 1994; Calogirou et al., 1996). Numerous different
currentlyavailableforVBOCs.Weprefacethedescriptionof ozoneremovalmethodshavebeenused(Helmig,1997;Fick
thelaboratorymeasurementswithabriefreviewofavailable et al., 2001; Pollmann et al., 2005). These have involved
methodsforsamplingandanalysis. sodiumthiosulfateonfilters(Helmigetal.,1998),potassium
iodideonglasswoolorfilters(HelmigandGreenberg,1994),
manganesedioxideonporousnetsorcopperscreens(Hoff-
2 Availablesamplingandanalysismethods mann, 1995; Calogirou et al., 1996), and titration of ozone
withnitrogenmonoxide(Komendaetal.,2003). Second,be-
2.1 Determinationwithfield-deployedinstrumentation causeatmosphericconcentrationsdecreasestronglywithde-
creasing VBOC volatility (viz., vapor pressure), if the goal
Analyticalinstrumentshaveoftenbeendeployedtothefield is the simultaneous quantitation of a wide range of com-
instudiesofplant-derivedVBOCsinambientairandasem- pounds, it may be necessary to use a large sample volume.
anatingfromplantswithinexperimentalenclosures. Though That choice usually leads to the need for multiple sorbents
oftencostlyandlogisticallydifficult,fielddeploymentofin- in the bed: weaker sorbent first, then the stronger sorbent.
strumentsisadvantageouswhenplantemissionsandambient Thelowpo compoundsareretainedontheweakersorbent,
L
concentrations are subject to short time variations: VBOCs andthehigherpo compoundspassontoandareretainedby
L
thataremoderately-to-highlyreactiveareofparticularinter- thestrongersorbent. Aftersampling,thethermaldesorption
est in this regard. Field-deployed analytical approaches are flowoccursinabackflushdirectionsothatthelowerpo com-
L
discussedinarecentreview(OrtegaandHelmig,2008)and pounds are not exposed to the stronger sorbent. Given the
include: (1) chemiluminescence for detection of isoprene largedynamicrangeinconcentrationusuallyaccompanying
(Guentheretal.,1996;SingsaasandSharkey,2000);(2)pro- such samples, currently it can be helpful to employ a unit
ton transfer reaction mass spectrometry (PTR-MS) for the such as the TurboMatrix 650 ATD (PerkinElmer, Waltham,
direct simultaneous detection of multiple compounds (Karl MA)forthethermaldesorption. WiththeATD,thedesorp-
et al., 2001; Bamberger et al., 2010; Mielke et al., 2010); tionflowcanbedividedsothatpart(e.g.,30%)goestothe
(3) solid phase microextraction (SPME) fiber collection of GC and the remainder goes to a pre-cleaned cartridge to be
analytesfollowedbythermaldesorptiontoafieldGCinstru- heldforapossiblesecondGCrun. ForthefirstGCrun,the
ment;and(4)adsorption/thermaldesorption(ATD)cartridge injector split ratio might be 10:1. If the injector split ratio
sampling of a known air volume (e.g., 1–10 l) followed by in the second GC run is lower (e.g., 5:1), then the method
thermaldesorptionoftheanalytestoafieldGCinstrument. sensitivity in the second run can be higher than in the first
PTR-MShasbeenemployedwhenVBOCconcentrationsare run(e.g. (70/30)(0.2/0.1)≈4×). Thiscanallowlessabun-
subjecttohightemporalvariability(e.g.,minutes)asinstud- dantcompoundstobecomequantifiable,thoughsomeofthe
iesofforestair(e.g.,Mielkeetal.,2010),fluxesbyeddyco- early-elutingcompoundsmaythenbeoverloaded.
variance measurements (e.g., Karl et al., 2001; Bamberger CommonATDadsorbentsforVBOCsincludetheporous
et al., 2010), and emission processes in dynamic enclosure organic polymer TenaxTM TA and several more strongly
studies (e.g., Grabmer et al., 2006; Bouvier-Brown et al., sorbing carbon-based materials (Table 1). Attractive char-
2007).Goodagreementhasbeenobtainedfortotalsesquiter- acteristics of these adsorbents are their good abilities to re-
penesinfield-deployedPTR-MSvs.SPME(Bouvier-Brown versibly sorb analytes, low water affinities (i.e., low break-
et al., 2007). (PTR-MS cannot distinguish different struc- throughvolume(BV)valuesforwater),andcomplementary
tural isomers because it only measures molecular masses, sorption strengths (Dettmer and Engewald, 2002; Arnts et
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348 J.F.Pankowetal.: VBOCswithtwodimensionalGC×GC-TOFMS
Table 1. Properties of adsorbent materials used in collection and analysis of volatilizable biogenic organic compounds (VBOCs) using
adsorption/thermaldesorption(ATD).
Sorbent Surfacearea Maximum Type Class Range Water Breakthroughvolume(BV)at20◦C(lg−1)
(m2g−1) temp.(◦C) capacity water pentane benzene limonene
(mgg−1)a
TenaxTMTA 35 350 porouspolymer weak C6-C30 <3.3b 0.039c 5d 70d 12000d
0.065d
TenaxTMGR 24 350 porouspolymer+ weak C6-C30 <2.0b 0.092c 1.7e 9e
graphitizedcarbon
CarbotrapTMB 100 >400 graphitizedcarbon medium/weak C5-C12 <1.2b 5.9f 11.7f
CarbopackTMB 100 >400 graphitizedcarbon medium/weak C5-C12
Carbograph1TDTM 100 >400 graphitizedcarbon medium/weak C5-C12
CarbotrapTMX 240 >400 graphitizedcarbon medium/strong C3-C9
CarbopackTMX 240 >400 graphitizedcarbon medium/strong C3-C9
Carbograph5TDTM 560 >400 graphitizedcarbon medium/strong C3-C7 24g
CarboxenTM569 485 >400 carbonmol.sieve strong C2-C5 403b 0.257c 200h 85h 16000h
CarboxenTM1000 1200 >400 carbonmol.sieve verystrong C2-C5 445g 0.418c
CarbosieveTMSIII 975 >400 carbonmol.sieve verystrong C2-C5 395b 0.378c 600i
aWatersorptioncapacitiesat20◦Candrelativehumidity=95–100%.
bHelmig,D.andVierling,L.: WateradsorptioncapacityofthesolidadsorbentsTenaxTA,TenaxGR,Carbotrap,CarbotrapC,CarbosieveSIII,andCarboxen569andwater
managementtechniquesfortheatmosphericsamplingofvolatileorganictracegases,Anal.Chem.,67,4380–4386,1995.
cDettmer,K.andEngewald,W.:Adsorbentmaterialscommonlyusedinairanalysisforadsorptiveenrichmentandthermaldesorptionofvolatileorganiccompounds,Anal.Bioanal.
Chem.,373,490–500,2002.
dhttp://www.sisweb.com/index/referenc/tenaxta.htm,lastaccess:6March2011.
ehttp://www.sisweb.com/index/referenc/tenaxgr.htm,lastaccess:6March2011.
fhttp://www.sigmaaldrich.com/etc/medialib/docs/Supelco/Bulletin/4501.Par.0001.File.tmp/4501.pdf,lastaccess:6March2011.
gFastyn,P.,Kornacki,W.,Gierczak,T.,Gawłowski,J.,andNiedzielski,J.: Adsorptionofwatervapourfromhumidairbyselectedcarbonadsorbents,J.Chromatogr.A,1078,
7–12,2005.
hhttp://www.sisweb.com/index/referenc/carbo569.htm,lastaccess:6March2011.
ihttp://www.sisweb.com/index/referenc/carbs111.htm,lastaccess:6March2011.
al.,2010). BVvaluesformonoterpenesonTenaxTM TAare Super-Q and HayeSep-Q adsorbent cartridges (Papiez et
high(Table1)sothatforsamplevolumesofafewlitersand al., 2009; Ormeno et al., 2010; Heuskin et al., 2012) and
∼100mg of TenaxTM TA, monoterpenes are quantitatively SPMEfibers(Bouvier-Brownetal.,2009b)providealterna-
retained. Thatisnotthecaseforisopreneandothervolatile tives to ATD cartridges. SPME, however, can be consider-
compounds, retentionofwhichrequirestheadditionalpres- ably less sensitive than ATD (as with highly volatile ana-
ence of a carbon-based sorbent. Many current ATD appli- lytesthatareweaklysorbedand/orwithlowvolatilitycom-
cations utilize 0.25 inch O.D.×3.5 inch cartridges packed pounds at low concentrations), does not provide as much
with ∼100mg of TenaxTM TA followed by ∼100mg of ei- overall sample stability (Bouvier-Brown et al., 2007; Baker
therCarbotrapTM BorCarbographTM 1TD(Komendaetal., andSinnott,2009),andislaborintensivetocalibrateinquan-
2001; Sartin et al., 2001; Hakola et al., 2006; Helmig et titative applications. Nevertheless, SPME becomes exceed-
al.,2007;OrtegaandHelmig,2008;Haapanalaetal.,2009; ingly attractive for its convenience when the analyte con-
Geron and Arnts, 2010). The TenaxTM TA prevents com- centrations are high, the work is proceeding in/near a lab-
pounds like the monoterpenes from substantively reaching oratory, and/or the desired determinations are only qualita-
the carbon-based sorbent whereon they would not only be tive or semi-quantitative in nature. SPME has been used in
strongly retained but also possibly chemically altered. Sig- laboratory-based measurements with GC-FID to determine
nificantinterconversionofseveralmonoterpeneshasbeenre- isopreneemissionratesfromenclosedtreeseedlingbranches
ported on CarbotrapTM 200 and CarbotrapTM 300 (Green- (Tsui et al., 2009), and with GC/FID, GC/MS and 100 l
berg et al., 1999a). Similar results have been reported by Tedlarbagstomeasuresesquiterpeneemissionsfromwhole
CaoandHewitt(1993). trees(PinussabinianaandPinusponderosa)(BakerandSin-
nott, 2009). Super-Q and HayeSep-Q are most appropriate
Atmos. Meas. Tech.,5,345–361,2012 www.atmos-meas-tech.net/5/345/2012/
J.F.Pankowetal.: VBOCswithtwodimensionalGC×GC-TOFMS 349
forlower-volatilityVBOCs(e.g.,mono-andsesquiterpenes); of forest canopy air and air from branch and leaf enclo-
samples typically are solvent extracted an analyzed using sures(Saxtonetal.,2007). Quantitativemeasurementswere
GC/MS(Papiezetal.,2009;Ormenoetal.,2010;Heuskinet reported for isoprene, α-pinene, β-pinene, camphene and
al.,2012). limonene,aswererelativepeakintensitiesfor11tentatively
“Whole-air” sampling with canisters or bags provides an identifiedcompounds(Saxtonetal.,2007).
alternativetoATDcartridgesandSPMEfibersasameansto
collect and transport sample analytes to a conventional lab-
3 Experimental
oratory for analysis. Whole air in a canister or bag can be
aliquottedwithasampleloop,cryofocused(e.g.,at−150◦C
3.1 Chemicalsandstandardmixtures
on a trap containing 60/80 mesh glass beads as in method
TO-14; US EPA, 1999a), sampled using ATD (Pressley et 21 VBOC chemicals (≥95% pure) were obtained from
al., 2004) or SPME (Bouvier-Brown et al., 2007), or cry-
Sigma-Aldrich (St. Louis, MO) (isoprene, α-pinene, cam-
ofocussed directly on the GC column (Pankow, 1986). For phene, myrcene, β-pinene, α-phellandrene, 13-carene,
canisters, inert internal surfaces are important, and both
limonene, terpinolene, p-cymene, nopinone, linalool, 4-
Summa-polishedTMstainlesssteelandSilcosteelTMcanisters
terpinenol,terpineol,eucalyptol,camphor,estragole(methyl
areused(USEPA,1999a,b).However,canistersarestillsub-
chavicol), α-cedrene, caryophyllene, aromadendrene, and
jecttolossesathighhumidity(Battermanetal., 1998), and
α-humulene). Optimally, the list of standard compounds
arecostlyandcomplicatedtoclean. Teflonbagssufferfrom
would have included many tens of C -and-higher VBOCs.
10
blankproblems,frequentlyleak,andaredifficulttocleanaf-
Unfortunately, authentic standard materials are not read-
ter use (Greenberget al., 1999a); Tedlar bagsare subject to
ily available for the vast majority of such compounds, and
similarproblems. Availabledataindicatethatmanyanalytes
evenjusthavingdoubledthenumberofstandardcompounds
aremorestablewhenstoredonATDcartridgesthaninbags,
wouldhaverequireduseofmulti-component“essentialoils”,
especiallywhenoxidantspeciesarepresentinthesampleair.
with concomitant prior quantitation of the VBOC compo-
Evidence of losses in Teflon bags for methacrolein, methyl
nents of interest. Such efforts are underway in our lab-
vinylketone,andα-pinenehavebeenreported(Greenberget
oratories, but were not applied in this study. Standard
al.,1999a). Asummaryofminimumdetectionlimit(MDL)
mixes of the 21 VBOCs in methanol were prepared at per-
values for VBOCs by various methodologies is provided in component concentrations from 0.5ngµl−1 to 20ngµl−1.
Table2.
A gas standard containing four internal standard (IS) com-
pounds (fluorobenzene, toluene-d , 4-bromofluorobenzene,
8
2.3 GC×GC-TOFMS and 1,2-dichlorobenzene-d ) at 80ngml−1 per component
4
was prepared in a stainless steel canister as described else-
Applications of GC×GC usually utilize a primary column
where(Pankowetal.,1998). TheIScompoundswereused:
witheitheranon-polarorlow-polaritystationaryphase. Co-
(1)tomonitortheoveralleffectivenessofthethermaltrans-
elutionofsomepeaksfromtheprimarycolumnisinevitable
ferfromeachsamplingcartridgetotheprimaryGCcolumn;
withhighlycomplexsamples. WithGC×GC(aka2DGC),
(2)asclearlyidentifiable(e.g.,non-biogenic)retentiontime
periodic slices of effluent from the primary column are in-
markers;and(3)asourceofconstantreferencesignalsduring
dividually: (1) cryofocused at the end of the column; then
thedeterminationoftheMDLvaluesreportedhere,asbased
(2) thermally desorbed to a secondary column with a more
onanalysescarriedoutatvaryingon-cartridgelevelsofiso-
polar stationary phase. Separation of compounds that co-
preneandtheothertargetanalytes. Isotopically-labelledsur-
existinagivenslicecanbeaccomplishedonthesecondary
rogatestandardcompoundswerenotnecessaryinthisstudy.
columnifthecompoundsdifferinpolaritiesand/orpolariz-
abilities. GC×GC-TOFMS has been applied with success 3.2 ATDcartridges–NCARpreparationand
in the analysis of diesel fuel (Arey et al., 2005), weathered proceduresforfieldsampling
petroleum(Frysingeretal.,2003),urbanaerosolparticulate
matter (Hamilton et al., 2004), tobacco smoke particulate Field samples discussed here were collected by the
matter (Cochran, 2008), and tissue extracts (Welthagen et Biosphere-Atmosphere Interactions Group of the National
al., 2005). GC×GC chromatographic retention index data CenterforAtmosphericResearch(NCAR).Detailsregarding
havebeentabulatedfordieselfuelhydrocarbons(Areyetal., sample collection are available (Greenberg et al., 1999a,b,
2005),butnotforVBOCs. 2004). Briefly, samples were obtained using stainless
The varying polarizabilty/polarity characteristics of the steel, 0.25 inch O.D.×3.5 inch, dual sorbent cartridges
VBOCs are imparted by varying numbers of rings, double (TenaxTM TA plus CarbotrapTM B, or TenaxTM GR plus
bonds, and polar functionalizations. GC×GC with a less- CarbographTM). Prior to sampling, cartridges were cleaned
polar primary column and a more-polar secondary column by heating for 8h at 275◦C with a 50mls−1 flow of ultra-
is well suited for separating VBOC mixtures. GC×GC- highpurityN gas. Cleancartridgeswerecappedandstored
2
TOFMShasbeenusedtodetermineVBOCsinATDsamples at 10◦C until used. The apparatus consisted of an O trap
3
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350 J.F.Pankowetal.: VBOCswithtwodimensionalGC×GC-TOFMS
b a T
p S a
ptv=partspertrillion IM=selectedionmon methylsalicylatesesquiterpenesmonoterpenesisoprenemethacroleinmethylvinylketone compound MDLdefinitiondetectioncollection ble2.Minimum
byvolume;conversion itoringmodeforthem 0.9–1.48–120.7–2.14–123.5103.5103.510 pptvngmb3−(S/N)=10signal/noiseGCxGC-TOFMSATD,5liter ThisWork detectionlimit(M
betweenngmandpp3−assspectrometer(i.e.,no 1106209326067200120350 pptvngm3−blanknoise3oftheσPTR-MSdirectinlet (2010)Mielkeetal. DL)valuesreporte
tvassumes=293KaT tfullscanningmode). 402262005667020470204 pptvngm3−(S/N)=2signal/noisePTR-MSdirectinlet (2010)Langfordetal. dinthisworkand
nd in
totalpressure=1atm 213 528 pptvngm3− notstatedPTR-MSdirectinlet (2008)Karletal. otherstudies.
.
400340050280100280 pptvngm3−blanknoise1.5oftheσGC/MSATD,1liter (2010)Misztaletal.
p
514 ptvngm3− notstatedGC/MSATD,3liter (2007)Saxtonetal.
921 p
79–1111–60132 ptvngm− notstatedGC/MSATD,3liter (2006)Hakolaetal.
3
1613 pptvngm3− notstatedGC/MSSIMaATD,6liter (2004)Greenbergetal.
p G
16131313 ptvngm3− notstatedGC-FIDATD,6liter (1999a)reenbergetal.
Atmos. Meas. Tech.,5,345–361,2012 www.atmos-meas-tech.net/5/345/2012/
J.F.Pankowetal.: VBOCswithtwodimensionalGC×GC-TOFMS 351
(KI impregnated filter), the sample cartridge, and a flow-
controlled pump. Samples were collected at a flow rate of
200cm3min−1 for 30min. After sampling, cartridges were
sealed, shipped at ambient temperatures back to the labora-
tory (maximum 1 week shipping time), then maintained at
10◦Cuntilanalyzed. Afterfourweeksatambienttempera-
tures,losseswere<10%forC toC compoundsofinterest
3 6
(Greenbergetal.,1999a).
3.3 ATDcartridgesandSPMEfibers–PSU
preparationandproceduresforsamples
obtainedinthelaboratory
In laboratory ATD sampling of air from vials holding plant
materials,eachglasscartridgecontained100mgofTenaxTM
TAplus120mgofCarbotrapTM B.(Priortopacking,5gof
TenaxTM TAwasplacedinaglasscolumnandcleanedwith Fig.1. DiagramofsystemusedtoobtainSPMEandATDsamples
a 250ml flow of 50/50 (v/v) acetone/hexane.) After pack- ofVBOCsinthelaboratorywithsamplesofCedrusatlantica.
ing, each cartridge was conditioned for 1h at 300◦C with
100mlmin−1 of He gas (precleaned using a U-shaped hy-
drocarbon trap in liquid N ). Each conditioned cartridge sampling event was carried out for a specific time period
2
wassealedwithbrass Swagelok endcaps thathadbeenpre- (10to60min)under“dynamic”conditions(=continuousair
cleanedbyrinsingin50:50acetone/hexanefollowedbybak- flow–50mlmin−1–throughthevialduringfiberexposure).
ing (90min, 150◦C). The endcaps were fitted with Teflon SPMEsampleswereanalyzedimmediately.
ferrules precleaned with methanol and water. Each sealed
cartridge was stored in a clean glass culture tube. Other 3.4 GC×GC-TOFMSmeasurements
cartridge handling procedures were as described elsewhere
(Pankow et al., 1998). The SPME assembly was obtained AllATDandSPMEsampleswereanalyzedusingaLecoPe-
fromSigma-Aldrich(St.Louis,MO)andutilizedwithfibers gasus 4D GC×GC-TOFMS (Leco, St. Joseph, Michigan).
coated with polydimethylsiloxane/divinylbenzene (coating Unlike the efforts using ATD, with SPME no calibration
thickness65µm). Priortosampling,thefiberswerecleaned standardrunsweremade: theprimarypurposeoftheSPME
for 30min at 250◦C in a GC injector through which pre- runs was to provide samples that could easily indicate the
cleanedHewasflowingat50mlmin−1. levelofsamplecomplexitythatistobeencounteredinanal-
Plant material samples (needles) were collected from a yses of VBOCs, and easily provide needed retention time
mature(>20m)CedrusatlanticatreelocatedonthePortland data. For all ATD cartridges (standard or sample or blank,
StateUniversity(PSU)campus.Threesamplesof0.5to1.0g NCARorPSU),priortoanalysiseachcartridgewasloaded
each were collected in October–December (2010) at ∼2m withtheIScompoundsbyinjecting0.2mloftheISgasstan-
above ground level from tips of branches with full-sun ex- dardintoa50mlmin−1,5minflowofprecleanedN2leading
posure. Eachsamplewasplacedinanindividualprecleaned to the sample inlet end of the cartridge. For a standard car-
40mlclearglassvialfittedwithaTeflon-linedseptum. Af- tridge,the21targetanalytecompoundswerethenaddedby
ter60minofexposureat∼20◦Ctoacoollightsource(air- injecting 4 µl of one of the methanolic standard mix solu-
portedhalogenlamp,300watt,1000µmolphotonsm−2s−1), tionsintotheinletofthecartridge;mostofthemethanolwas
sampling by SPME or ATD occurred as air was passing thenremovedandtheanalytesweremovedfurtherontothe
through the vials (Fig. 1). The air was either cleaned lab- cartridgebya50mlmin−1,5minflowofprecleanedN2.
oratoryair(CLA)oruncleanedlaboratoryair(ULA).Clean- TheATD400(Perkin-Elmer,Waltham,MA)thermaldes-
ing of the air was accomplished using a Perkin-Elmer hy- orption apparatus was connected to the GC injector by a
drocarbon trap. Use of the uncleaned laboratory air greatly fused silica transfer line (220◦C). Each ATD cartridge was
increased the complexity of the samples and thereby fa- thermally desorbed (270◦C, 10min) at 40mlmin−1 with
cilitated a more rigorous test of the ability of GC×GC- zero split to a focusing trap (45mg of TenaxTM TA, 10◦C)
TOFMS to adequately separate and detect VBOCs in the in the ATD 400. The focusing trap was desorbed (290◦C,
presenceofmyriadotherVOCs. ATDairsamplingoccurred 3min) with zero split (splitless) to the fused silica transfer
at 60mlmin−1 for 60min (3.6l). ATD cartridges were ei- line and thus the GC injector. Flow from the GC injec-
ther analyzed immediately or within three weeks after stor- tor to the primary GC column occurred with a 15:1 split.
ageat−15◦C;standardcartridgesanalyzedafterthreeweeks (Increased method sensitivity could have been achieved by
at−15◦Cshowednoevidenceofanalyteloss. EachSPME reducing the split to as low as 5:1 without reduction of
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352 J.F.Pankowetal.: VBOCswithtwodimensionalGC×GC-TOFMS
Table3.SummaryofGC×GC-TOFMSconditionsusedinVBOCdeterminations.
initial improved
◦ ◦
GCinjector 250 C;split20:1forATD 225 C;split15:1forATD,splitlessforSPME
columnflow 1.2mlmin−1(massflowcontroller) 1.0mlmin−1(massflowcontroller)
primarycolumn non-polar:Rxi-5ms,30m,0.25mmI.D., lowpolarity:DB-VRX,45m,0.25mmI.D.,
0.25µmfilm(Restek,Bellefonte,PA) 1.4µmfilm(Agilent,SantaClara,CA)
GC×GCmodulation 4speriod,0.8shotpulse 4speriod,0.9shotpulse
GC×GCmodulator trapwithcoldgasfromLN2,thenhotpulseat20◦aboveprimaryovenforreleasetosecondarycolumn
secondarycolumn polar:BPX-50,1.25m,0.10mmI.D., polar:StabilwaxTM,1.5m,0.25mmI.D.,
0.10µmfilm(SGE,Austin,TX) 0.25µmfilm(Restek,Bellefonte,PA).
GCprogram 40◦Cfor5min,15◦Cmin−1to300◦C,then 45◦Cfor5min,10◦Cmin−1to175◦C,hold
(primaryoven) holdat300◦Cfor5min at175◦Cfor2min,4◦Cmin−1to240◦C,
◦
thenholdat240 Cfor10min
◦
MSsource 200 C,electronimpact(70eV)
MSdetector 1550V
MSdataacquisition 150spectras−1;35to500amu
chromatographicperformance.) SPMEfibersweredesorbed stated otherwise. Figure 2a and b are GC×GC-TOFMS
splitless for 3min in the GC injector (225◦C) which con- chromatogramsobtainedbyATDforthe21-compoundstan-
tained an SPME liner (Restek, Bellefonte, PA). Thereafter, dardmix. Goodseparationwasachievedforallofthecom-
an injector purge flow of 100mlmin−1 was kept open for pounds except for α-cedrene and caryophyllene. The latter
4min. Each GC×GC run was initiated immediately upon two compounds possess sufficiently different EI mass spec-
beginningoftheheatingofthefocusingtrap(ATD),orafter tra that they are differentiable even when not fully resolved
theSPMEfiberwasinsertedintotheinjector(SPME).Inev- chromatographically.
erycase,TOFMSdatacollectionwasdelayed180s. Anini- In sample runs, the peak selection criteria (PSC) used
tialsetandanimprovedsetofGCcolumns/conditionswere todecideuponwhichchromatographicpeakswouldreceive
used(Table3). further consideration were applied on the Leco data system
asfollows: signaltonoiseratio>200,peakarea>100000,
and match similarity of >780 with an entry in the NIST
4 Resultsanddiscussion massspectrallibrary. Foreachsample,someofthosepeaks
corresponded to target compounds, and eluted at the proper
4.1 Detectionlimits retention times, and so were positively identified. Other
peaks passing the PSC but not corresponding to the target
The particular MDL values for the 21 target compounds compounds were considered only tentatively identified. Ta-
investigated here are provided in Table 4. For isoprene, ble S1 (Supplement) summarizes the information acquired
the MDL was 3.5pptv (10ngm−3). For the monoter- for438chromatographicpeaks(including21standardcom-
penes, theMDLvalueswereintherange0.7–2.1pptv(4to pounds)thatmetthePSCinthevariouschromatogramspre-
12ngm−3). For the sesquiterpenes, the MDL values were sentedhere.
∼1pptv (∼10ngm−3). No blank problems were expe-
rienced for any of the target compounds. As such, all – Calycolpus,ATD,ambientair:
MDLvalueswereassessedbyvaryingtheon-cartridgemass Figure 3 is a chromatogram for an ATD sample ob-
amounts of the target analytes, and determining which val- tainedduringanNCARenclosureexperimentusingthe
uesyieldedaninstrumentsignaltonoiseratioof10:1. MDL lower branch of a tree (Calycolpus moritzianus) grow-
valueswerethencalculatedasequalingthemassamountsat ing in a tropical forest in South America (Columbia,
10:1signal-to-noisedividedbythesamplevolumeof5l. 11.0704◦N, 74.0411◦W). For this sample, the initial
set of chromatographic conditions was used (Table 3).
4.2 Chromatograms The peaks for 15 of the 21 standard compounds met
the PSC and were positively identified based on their
Figures2–6showchromatogramsforsamplesrunusingthe chromatographicretentiontimesandmassspectra(Ta-
improvedsetofchromatographicconditions(Table3)unless bleS2,Supplement);fourofthese(α-pinene,limonene,
Atmos. Meas. Tech.,5,345–361,2012 www.atmos-meas-tech.net/5/345/2012/
J.F.Pankowetal.: VBOCswithtwodimensionalGC×GC-TOFMS 353
9
3.
Standard (~1 ng per component on column)
MT = monoterpenes
OOxxMMTT == ooxxyyggeennaatteedd mmoonnootteerrppeenneess
ST = sesquiterpenes
OxST = oxygenated sesquiterpenes
e
m
tion t92.9
nti
e
et
y r
dar isoprene 1,2-DCB-d4
n
eco9 toluene-d8 OxMT
s1.1

s) FB
t(2 BFB
ST
MT
9
0.
300 800 1300 1800 2100
t1(s) primary retention time (a)
Figure 2.a.
4
2. Standard (~1 ng per component on column)
e
m
ion t1.9 notteeprrippniionnneeeooll OOxxMMTT
nti estragole
e
et 1,2-DCB-d
y r 4 44tterpiinenoll
ar ST
nd BFB camphor cedrene
co linalool humulene
se.41. MT
s) pcymene aromadendrene
t(2 campmhyernceene limtoenrpeinneo,le enuecalyptol caryophyllene
phellandrene, 3carene
pinene
-pinene
9
0.
852 1352 1852
t1(s) primary retention time (b)
Figure 2.b.
Fig. 2. (a) GC×GC-TOFMS chromatogram of a standard containing 21 VBOCs at ∼1ng per component on-column; internal standard
compoundsarefluorobenzene(FB),toluene-d8,bromofluorobenzene(BFB),and1,2-DCB-d4(1,2-dichlorobenzene-d4);obtainedusingthe
improvedchromatographicconditions(Table3).(b)EnlargedregionofGC×GC-TOFMSchromatogramshowing20VBOCsat∼1ngper
componenton-column.
α-terpineol,andcaryophyllene)areknownconstituents Supplement) and included four monoterpenes and 10
of C. moritzianus essential oils (Valde´s et al., 2006; sesquiterpenes. In addition, at levels two to five times
Diazetal.,2008). Peaksforanothertwostandardcom- larger than blank levels, three of the 127 peaks were
pounds were positively identified based on their mass identifiedassalicylates(methylsalicylate,2-ethylhexyl
spectralandretentiontimedata,thoughweretoosmall salicylate, and homomenthyl salicylate). Two of the
to meet the PSC. The number of tentatively identified 127peaksmatchedwellasisomersofcumeneandtwo
peaksmeetingthePSCinFig.3totaled127(TableS2, matchedwellasisomersofaromadendrene. Overall,at
www.atmos-meas-tech.net/5/345/2012/ Atmos. Meas. Tech.,5,345–361,2012
354 J.F.Pankowetal.: VBOCswithtwodimensionalGC×GC-TOFMS
6
2
2. ATD –branch enclosure, Calycolpus moritzianus
MT = monoterpenes
OxMT = oxygenated monoterpenes
e ST = sesquiterpenes
m
tion t61.76
nti
e
et
y r
ar
d
n
o
sec.261 isopprene MT OOxxMMTT SSTT
s)
(
t2
aallkkaanneess
6
7
0.
182 682 1182
t (s) primary retention time
1
Figure 3.
Fig.3. GC×GC-TOFMSchromatogramofVBOCsfromCalycolpusmoritzianusbyATDusingabranchenclosureinthefield;obtained
usingthe“initial”chromatographicconditions(Table3).
9
3.
SPME -cleaned lab air (CLA) Cedrus atlantica
MT = monoterpenes
OxMT = oxygenated monoterpenes
e ST = sesquiterpenes
m OOxxSSTT = ooxxyyggeennaatteedd sseessqquuiitteerrppeenneess
tion t92.9 OxST
nti
e
et
ry
ar
d
on OxMT
c
e9
s1.1

s) isoprene ST
(
t2 alkanes
MT
9
0.
300 800 1300 1800 2100
t (s) primary retention time
1
Figure 4.
Fig. 4. GC×GC-TOFMS chromatogram of VBOCs from Cedrus atlantica by SPME and using cleaned laboratory air (CLA); obtained
usingthe“improved”chromatographicconditions(Table3).
least 13 sesquiterpene compounds were present at eas- – Cedrus,SPME,laboratoryair(cleaned):
ilydetectableconcentrations.Fortheseandthetentative Figure 4 is a chromatogram for VBOCs emitted from
identificationsdiscussedbelowforFigs.4–6,confirmed CedrusatlanticaneedlessampledinPortland(Decem-
identificationswillawaitacquisitionofsuitableauthen- ber 2010). Sampling took place with SPME using the
ticstandards. cleaned laboratory air (CLA) using dynamic sampling
conditionsasdescribedabove. Thepeaksfor10ofthe
Atmos. Meas. Tech.,5,345–361,2012 www.atmos-meas-tech.net/5/345/2012/