Kalberer, Markus

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Anal Bioanal Chem (2006) 385: 22–25DOI 10.1007/s00216-006-0354-0TRENDSMarkus KalbererAnalysis of oligomers in atmospheric aerosolparticles—analytical challengesPublished online: 28 March 2006# Springer-Verlag 2006IntroductionAmbient aerosol particles have a variety of importanteffects on many atmospheric processes, for examplescattering and absorbing solar radiation and affectingcloud formation. Adverse health effects, for examplerespiratory and cardiopulmonary diseases and mortality,have also been associated with atmospheric particles.Although a major fraction (up to 50%) of ambient aerosolparticles is organic material, little is known about thisfraction. Despite significant efforts to elucidate the chem-ical nature of organic particle components in recentdecades, often only 10–20% of the total organic mass canbe identified on a molecular level [1, 2] although hundredsof compounds have been identified, most of them at tracelevels [3].In recent years several authors have identified high-molecular-weight organic compounds in particles collectedin the ambient atmosphere or produced in laboratoryexperiments. These studies showed that a major part of thetotal organic aerosol mass (10–50%) can probably beattributed to this new class of compound and that many ofthese high-molecular-weight compounds pose new chal-lenges in the chemical analysis of atmospheric aerosolparticles.Ambient samplesHigh-molecular-weight compounds, often named HULIS(humic-like substances), were first detected in ambientatmospheric samples with characteristics similar to those ofhumic and fulvic acids, especially their ultraviolet (UV)and fluorescence spectra [4, 5]. Solid-phase extraction,size-exclusion chromatography (SEC), and ion chroma-tography have often been used to separate this class ofaerosol components from lower-molecular-weight com-pounds [5–7 ] and a variety of detection methods have beenused, for example UV, infrared (IR), fluorescence spec-troscopy, electrospray ionization (ESI) and laser-desorp-tion/ionization (LDI) mass spectrometry, and nuclearmagnetic resonance (NMR) [5–10]. No comprehensivestructural information is available for these high-molecu-lar-weight compounds, however.Because the chemical properties of HULIS are stilllargely unknown, determination of the molecular weight orconcentration of these compounds is difficult to achievewith most analytical methods. Electrospray ionization massspectrometry (ESI-MS) studies have indicated the maxi-mum molecular weight of HULIS is approximately 500 Da[9]; this compares well with results from a comparativestudy of size-exclusion chromatography and LDI-MS,which measured similar values for the molecular weight ofHULIS [7]. Concentration estimates using UV absorptionof Suwannee river fulvic acid as surrogate standard showedthat approximately 10% of the total organic carbon (OC)can be attributed to these high-molecular-weight com-pounds [7]. Zappoli et al. [5], using humic acid asquantification standard, showed that 10–30% of the totalOC was HULIS. Limbeck et al. [11] correlated the UVabsorption of HULIS with its organic carbon content (bycombustion of all organic material and subsequent quan-tification of CO2) and showed that use of humic acids as acalibration standard for UV detection underestimates theconcentration of HULIS by a factor of approximately two.They found that 5–40% of the total OC collected at anurban site is composed of HULIS.The sources and mechanisms of formation of this aerosolfraction are still highly speculative. Facchini et al. [12]suggested that biomass burning could be an importantsource of HULIS, at least in cold seasons, when woodburning for domestic heating is increased. This hypothe-sis was corroborated by laboratory studies showing thatoxidation products from the biomass burning had opti-cal properties and mass spectra similar to those ofHULIS [13].M. Kalberer (*)Department of Chemistry and Applied Biosciences,ETH Zurich,8093 Zurich, Switzerlande-mail: kalberer@org.chem.ethz.chThis suggests that HULIS are formed in the atmosphereby heterogeneous reactions, i.e. formation of these high-molecular-weight compounds by reaction of smallercompounds in aerosol particles. Similar reactions in fogor cloud droplets, i.e. in aqueous media, are also regardedas pathways to HULIS formation [13]. Besides theseatmospheric formation pathways, many high-molecular-weight compounds are emitted directly into the atmo-sphere; these are mainly products of degradation ofbiological material, for example proteins, cellulose, cellmembrane components, etc. A discussion of methods foranalysis of these compound classes goes beyond the scopeof this article.A very comprehensive recent review of carbonaceousaerosols in general and of high-molecular-weight com-pounds in particular is given by Gelencser [14].Laboratory measurementsOnly a few years ago evidence was presented first by Janget al. and Limbeck et al. [15, 16], that small volatile organiccompounds, usually not considered to partition from thegas phase into the particle phase, increase the mass ofacidic particles by forming low-volatility condensationproducts. Several mass-spectrometric studies [17–21]showed shortly after that high-molecular-weight com-pounds with masses up to approximately 1000 Da arepresent in secondary organic aerosol (SOA) particles, i.e.particles that are formed in the atmosphere by gas-to-particle conversion. These compounds have been identifiedin SOA formed from a variety of volatile precursors, forexample aromatic compounds or terpenes, which areregarded as the main anthropogenic and biogenic com-pound classes contributing to SOA mass. Acid seedparticles seem to increase the amount and the molecularweight of the high-molecular-weight compounds formed.Hypotheses have been made about the structure of thesehigh-molecular-weight compounds. Acid catalyzed acetalformation or aldol condensation reactions have beenproposed on the basis of mass spectrometric evidence[15, 17, 18, 20]; this suggests most of the high-molecular-weight compounds observed in laboratory experiments areoligomers or co-oligomers. Tolocka et al. [18] performedaccurate mass measurements of SOA components gener-ated from terpenes using a Fourier-transform ion cyclotronresonance (FTICR) mass spectrometer and assignedseveral peaks to small oligomers of carbonyl oxidationproducts. In contrast with the experimental results, firstmodel calculations showed that an oligomerization of mostof these carbonyl compounds would not be favorable underatmospheric conditions [22].Kalberer et al. found that possibly up to 50% of the totalSOA mass can be attributed to oligomers [17]. Similarly toambient samples, accurate quantification of the oligomermass fraction is complicated by their unknown structureand, therefore, the lack of accurate quantification stan-dards, which are needed for most analytical methods.Although at the current state of knowledge it seems that amajor fraction of organic aerosol particles can be attributedto high-molecular-weight compounds (or oligomers), theidentities of the precursors forming these compoundsremain unclear.Table 1 summarizes the analytical methods currentlyused to analyze different properties of HULIS or oligomersin organic aerosols from ambient atmospheric samples orlaboratory experiments.Table 1 Analytical methods and investigated properties of HULIS and oligomers in organic aerosolsInvestigated property Analytical method Sample type Ref.Molecular size ESI-MS Field, laboratory 9, 10, 18–20(MA)LDI-MSa Field, laboratory 7, 17, 21SEC Field 5, 7VPOa Field 9UFa Field 9Functional groups NMR Field 6FTIR Laboratory 15Structure LDI-MS Laboratory 17ESI-MS Laboratory 18–20IR Laboratory 15Concentration UV Field 5, 7OCa Field 11, 12VTDMAa Laboratory 17DMAa Laboratory 15, 19, 20Formation process ESI-MS Laboratory 13, 18–20LDI-MS Laboratory 17OCa Field, laboratory 12, 16aAbbreviations not explained in the text: OC: combustion methods used to determine the total organic carbon concentration, DMA:differential mobility analyzer, VTDMA: volatility tandem differential mobility analyzer, MALDI-MS: matrix assisted LDI-MS:VPO: vapor pressure osmometry: UF: ultra filtration23Analytical challengesChemical analysis of atmospheric particles is performed byuse of either on-line techniques, in which single particlesare directly analyzed from the air without time-integratingsampling, or off-line techniques, which require sampling ofparticles, usually on filters or by use of impactors, often forhours or days. The advantages of on-line methods are hightime-resolution and less interference from gaseous com-pounds that potentially interact with the particles duringsampling, which is often a serious issue in off-lineanalyses. In contrast, off-line methods often enable moreversatile analysis, for example chromatographic separa-tions. Both approaches have their advantages and should beused complementarily for identification of high-molecular-weight compounds in atmospheric particles. Among themany open questions in the analysis of high-molecular-weight compounds in atmospheric particles, two aspectsare discussed below.1 Chemical structure of oligomers/high-molecular-weightcompounds– There is an urgent need to show how the structuresof oligomers found in laboratory-generated particlescompare with those of HULIS (high-molecular-weight compounds) found in ambient samples. Thesmall amounts of sample usually available and thecomplex compound mixtures complicate suchanalyses. Tandem MS experiments complementedwith accurate mass determination would be idealtechniques to determine the structure of thesecompounds in both laboratory and ambient sam-ples. First steps towards such a detailed analysishave recently been described by Tolocka et al. [18].– Elucidation of the structures of high-molecular-weight compounds might potentially lead to newmarker compounds that distinguish SOA fromdirectly emitted organic aerosol mass and possiblyeven SOA from anthropogenic and biogenicsources, a source apportionment which is not yetpossible but urgently needed to assess the particleburden of the atmosphere as a result of humanactivity. Such marker compounds must be stable inthe atmosphere for several days. Mass spectrometricanalyses could reveal how stable these oligomersare in an oxidizing atmosphere in the presence ofOH radicals, ozone, and nitrogen oxides byobserving these compounds in time-dependentlaboratory experiments.2 Oligomer quantification– Accurate quantification of oligomers in aerosols iscertainly a major challenge. So far only a fewpreliminary estimates are available in the literature,all of them obtained with methods that have severedrawbacks. Several authors used humic substancesas surrogate calibration standards for quantificationof UVabsorption even though it is unknownwhetherhumic substances and the high-molecular-weightcompounds (or oligomers) in aerosols are structu-rally similar; this is crucial for such quantification.Oligomer quantification by thermal separation, asreported by Kalberer et al. [17], is only an approx-imate method for separation of monomers andoligomers and more detailed studies must beperformed to achieve accurate quantification byuse of this method. A liquid chromatographicmethod for separation of HULIS from other organicaerosol components followed by quantification ofthe organic carbon content, recently reported byLimbeck et al. [11], is a promising approach,because the detection method does not rely onchemical properties of a calibration compound.Similarly, evaporative light-scattering detectionwould enable quantification which is largely inde-pendent of the chemical properties of the calibrationcompounds and should be explored for analysis ofatmospheric samples. Both methods rely on accuratechromatographic separation of the high-molecular-weight compounds from other particle components,however.The discovery of high-molecular-weight compounds inaerosol particles challenges in many ways the analyticaltechniques currently used in the atmospheric sciences andopens a wide field for new and innovative analyticaldevelopments.References1. Rogge WF, Mazurek MA, Hildemann LM, Cass GR, SimoneitBRT (1993) Atmos Environ 27:1309–13302. Puxbaum H, Rendl J, Allabashi R, Otter L, Scholes MC (2000)J Geophys Res 105:20697–207063. Seinfeld JH, Pandis SN (1998) Atmospheric Chemistry andPhysics. Wiley, New York4. Havers N, Burba P, Lambert J, Klockow D (1998) J AtmosChem 29:45–545. Zappoli S, Andracchio A, Fuzzi S, Facchini MC, Gelencsér A,Kiss G, Krivácsy Z, Molnár A, Mészáros E, Hansson HC,Rosman K, Zebuhr Y (1999) Atmos Environ 33:2733–27436. Decesari S, Facchini MC, Fuzzi S, Tagliavini E (2000) JGeophys Res 105:1481–14897. Samburova V, Kalberer M, Zenobi R (2005) Atmos Chem Phys5:2163–21708. Krivácsy Z, Gelencsér A, Kiss G, Meszaros E, Molnár A,Hoffer A, Meszaros T, Sárvári Z, Temesi D, Varga B,Baltensperger U, Nyeki S, Weingartner E (2001) J AtmosChem 39:235–2599. Kiss G, Tombácz E, Varga B, Alsberg T, Persson L (2003)Atmos Environ 37:3783–379410. Feng J, Möller D (2004) J Atmos Chem 48:217–23311. Limbeck A, Handler M, Neuberger B, Klatzer B, Puxbaum H(2005) Anal Chem DOI:10.1021/ac050953l12. Facchini MC, Fuzzi S, Zappoli S, Andracchio A, Gelencser A,Kiss G, Krivacsy Z, Meszaros E, Hansson HC, Alsberg T,Zebuhr (1999) J Geophys Res 104:26821–2683213. Hoffer A, Kiss G, Blazso M, Gelencsér A (2004) Geophys ResLett 31; DOI:10.1029/2003GL01896214. Gelencsér A (2004) Carbonaceous Aerosol. Springer, Dor-drecht, The Netherlands15. Jang M, Czoschke NM, Lee S, Kamens RM (2002) Science298:814–8172416. Limbeck A, Kulmala M, Puxbaum H (2003) Geophys Res Lett30; DOI:10.1029/2003GL01773817. Kalberer M, Paulsen D, Sax M, Steinbacher M, Dommen J,Prevot ASH, Fisseha R, Weingartner E, Frankevich V, ZenobiR, Baltensperger U (2004) Science 303:1659–166218. Tolocka M, Jang M, Ginter J, Cox F, Kamens R, Johnston M(2004) Environ Sci Technol 38:1428–143419. Iinuma Y, Böge O, Gnauk T, Herrmann H (2004) AtmosEnviron 38:761–77320. Gao S, Ng NL, Keywood M, Varutbangkul V, Bahreini R,Nenes A, He JW, Yoo KY, Beauchamp JL, Hodyss RP, FlaganRC, Seinfeld JH (2004) Environ Sci Technol 38:6582–658921. Baltensperger U, Kalberer M, Dommen J, Paulsen D, AlfarraMR, Coe H, Fisseha R, Gascho A, Gysel M, Nyeki S, Sax M,Steinbacher M, Prevot ASH, Sjoren S, Weingartner E, ZenobiR (2005) Faraday Discuss 130:265–27822. Barsanti KC, Pankow JF (2004) Atmos Environ 38:4371–438225

Analysis of oligomers in atmospheric aerosol particles—analytical challenges

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Analysis of oligomers in atmospheric aerosol particles—analytical challenges

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