Environ. Sci. Technol. 2008, 42, 8224–8230
Two-Dimensional NMR Studies of Water-Soluble Organic Matter in Atmospheric Aerosols R E G I N A M . B . O . D U A R T E , * ,† ARTUR M. S. SILVA,‡ AND ARMANDO C. DUARTE† CESAM & Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal, and Department of Chemistry & QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal
Received May 10, 2008. Revised manuscript received September 5, 2008. Accepted September 12, 2008.
The potential and the limitations of combining two-dimensional nuclear magnetic resonance (2D NMR) techniques to deliver new qualitative information on the substructures present in watersoluble organic compounds (WSOC) from fine atmospheric aerosols collected during Winter and Spring/Summer seasons are highlighted in this study. The WSOC fractions were extracted from quartz filter samples and further separated into hydrophobicacidsandhydrophilicacidsfractionsbyusingaXAD8/XAD-4 isolation procedure. Despite different spectral resolution, it was found that the aliphatic material of both samples includes mostly long-chain (carbons greater than 3 or 4) and branched mono- and dicarboxylic acids, carbonyl, and ester structural types. The presence of such structural fragments could be associated to secondary organic aerosol formation. It has been further demonstrated that spectral signatures typical of anhydrosugars from cellulose and methoxyphenols from lignin can be identified among the carbohydrate and aromatic moieties of the Winter sample. Their presence highlights the major influence of biomass burning on atmospheric particulate matter collected during colder periods. The data reported heredemonstratethesuccessof2DNMRtechniquestoinvestigate aerosol WSOC composition, as well as to evaluate aerosol source fingerprints.
Introduction The water-soluble organic compounds (WSOC) in atmospheric particles have received considerable attention over the last two decades, mostly because of their importance in several atmospheric processes. They are ubiquitous constituents of airborne particulate matter, and they represent a major fraction (∼20-70%) of particulate organic matter in the atmosphere (1-3). The WSOC could play an important role on the hygroscopic growth (4), surface tension behavior (5), and effective density (6) of particles composed by these organic compounds and, eventually, on the radiative impact and possible climate forcing of atmospheric aerosols. Due to their complexity, however, a good understanding of the chemical composition of the aerosol WSOC is still lacking. Knowledge of their molecular structures is therefore manda* Corresponding author phone: +351 234370737; fax: +351 234 370084; e-mail:
[email protected]. † CESAM & Department of Chemistry. ‡ Department of Chemistry & QOPNA. 8224
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tory to better describe the global radiative effects and human health problems associated with atmospheric particles. Numerous off-line methodologies have been developed to study the chemical composition of aerosol WSOC. These analytical procedures are based on a combination of total organic carbon analysis, chromatographic separations, and organic functional group analysis (2, 3, 7-10). Among these studies, proton nuclear magnetic resonance (1H NMR) (2, 10) and solid-state 13C nuclear magnetic resonance (13C NMR) (3, 9) have proved to be very useful tools in revealing the functional group composition and source apportionment of WSOC from atmospheric aerosols. These studies have shown that the aerosol WSOC is a complex mixture likely composed of aliphatic structures, oxygenated alkyls, carboxylic acids, and aromatic structures. Despite their usefulness, these onedimensional (1D) NMR techniques have certain intrinsic drawbacks for the speciation of organic compounds. One of the factors that limits the information extracted from 1D NMR spectroscopy is the poor spectral resolution with broad overlapping signals due to sample structural heterogeneity. Multidimensional methods, on the other hand, have been shown to be valuable tools for partially resolving the spectral information that is superimposed in 1D spectra by dispersing the data into a second or third frequency dimension. One of the earliest reports on the application of twodimensional (2D) NMR methods to complex mixtures of organic compounds was a study on humic materials by Buddrus et al. (11). Following this pioneering work, the study of humic substances (HS) by 2D NMR suffered a hiatus of almost ten years, and only from 1998 there has been a serious effort to unravel the predominant substructures in HS by means of multidimensional NMR. Combinations of 2D NMR techniques, such as 1H-1H homonuclear covalent networks [in Correlation Spectroscopy (COSY) or Total Correlation Spectroscopy (TOCSY)], 1H-13C or 1H-15N linkages [in Heteronuclear Single Quantum Coherence (HSQC), Heteronuclear Multiple Quantum Coherence (HMQC), or Heteronuclear Multiple Bond Correlation (HMBC)], and spatial interactions and exchange dynamics [e.g., in Nuclear Overhauser Effect Spectroscopy (NOESY)] have been successfully applied for substructure elucidation of polydisperse HS from different origins (12-18). For an in-depth discussion on the use of multidimensional solution-state NMR techniques for the analysis of HS, readers should refer to Simpson et al. (19) where practical tips and useful guidelines on these methods are thoroughly described. In this paper, the potential and the limitations of combining gradient-selected COSY (gCOSY), HSQC (gHSQC), and HMBC (gHMBC) 2D NMR techniques to deliver new qualitative information on the substructures present in WSOC from fine atmospheric aerosols are discussed. This is the first study in which 2D NMR methods are used to investigate the molecular nature of heterogeneous aerosol WSOC samples. The functional group compositions of the studied WSOC samples have already been evaluated by means of Fourier transform infrared (FTIR) and solid-state 13C NMR spectroscopies (3). In the present study, it is shown that 2D NMR techniques improve functional group assignments by offering information on short- and long-range 1H-13C hetereonuclear connectivities and short-range 1H-1H homonuclear couplings with a high degree of confidence.
Experimental Section Aerosol Sampling, Extraction, and Isolation of WSOC Samples. The aerosol WSOC samples studied in this work were collected during the Winter (January 2-30) and Spring/ 10.1021/es801298s CCC: $40.75
2008 American Chemical Society
Published on Web 10/24/2008
Summer (May 29-June 26) seasons of 2003. Details on aerosol sampling, meteorological conditions and samples preparation are described elsewhere (3). Briefly, the fine aerosol (
