Structural Analysis of Oligomeric Molecules Formed from the Reaction

Environ. Sci. Technol. 2006, 40, 6674-6681

Structural Analysis of Oligomeric Molecules Formed from the Reaction Products of Oleic Acid Ozonolysis J. C. REYNOLDS,† D. J. LAST,‡ M. MCGILLEN,§ A. NIJS,† A. B. HORN,‡ C . P E R C I V A L , § L . J . C A R P E N T E R , * ,† A N D A . C . L E W I S * ,† Department of Chemistry, University of York, Heslington, York YO10 5DD, U.K., School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K., and Department of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.

The products arising from the ozonolysis of oleic acid (cis-9-octadecenoic acid) in solution have been studied using negative ion mode electrospray ionization ion trap mass spectrometry. Oleic acid is an important component of atmospheric organic aerosol and is a key model species in predicting aerosol physical and chemical characteristics. The four predicted reaction products, 1-nonanal, nonanoic acid, 9-oxononanoic acid, and azelaic acid, were all observed in roughly equal yields. In addition to these products a large number of higher molecular weight compounds were detected with m/z ratios of up to 1000 Daltons. Tandem mass spectrometry of these larger ions revealed that they represented a complex mixture of linear R-acyloxyalkyl hydroperoxides, secondary ozonides, and cyclic diperoxides, formed by reactions between ozonolysis products and Criegee intermediates. These comprise the first directly elucidated structures of large oligomeric species from oleic acid ozonolysis. The degree of oligomerization and hence molecular weight distribution was observed to increase with reaction time in solution.

Introduction Atmospheric particles have an important effect on climate. They scatter incoming and outgoing radiation and act as cloud condensation nuclei (CCN), and without them cloud formation could not take place (1-4). They can also have a negative effect on human health since epidemiological studies indicate damaging effects on respiratory and cardiovascular systems of people with heart and lung diseases (5-7). Up to 50% of atmospheric aerosol particle mass can be attributed to organic compounds emitted either by biogenic or anthropogenic sources (8-11). Organic compounds can react with oxidants in the gas phase of the atmosphere, such as the hydroxyl radical or ozone, to form polar oxygenated * Corresponding author e-mail: [email protected] (A.C.L.) and ljc4@[email protected] (L.J.C.). † University of York. ‡ School of Chemistry, The University of Manchester. § Department of Earth, Atmospheric and Environmental Sciences, The University of Manchester. 6674

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products. Once oxidized the vapor pressure of the resulting products is often lower than the starting material, and a partitioning of a wide range of species from gas to condensed phase can occur (12). The oxygenated products of the atmospheric reaction can once in aerosol act as surfactants and may change the hygroscopicity (13-15), increase the density of the particles (16), and may make them more effective CCN (2, 3). The oxygenated aerosol is also more hazardous to human health inducing oxidative stress and the catalytic generation of reactive oxygen species (6, 17). Oleic acid (C18H34O2) is a monounsaturated fatty acid that is present in meat, oils, and fossil fuels. It is released to the atmosphere within organic aerosol during cooking (18-20) and is a notable component of the organic fraction of atmospheric aerosol in many environments. The reaction between oleic acid and ozone has been studied extensively, important in both its own right and as a model system to improve understanding of the oxidation of organic particulates (13, 21-30). The initial reaction occurs when the double bond of the unsaturated fatty acid is attacked by an ozone molecule. This forms a high-energy molozonide (31) structure which can readily decompose into a number of different products. The main products reported from the ozonolysis of oleic acid in aerosol have been shown to be 1-nonanal and 9-oxononanoic acid. In addition, nonanoic acid and azelaic acid are formed as minor products. Other studies have shown that these products can subsequently recombine to form larger structures. Rebrovic (21) showed using 1H NMR that they can react together in solution to form peroxide species, and studies with aerosol mass spectrometry and electron capture have shown that high molecular weight species are present. Katrib et al. (22) demonstrated that these high molecular weight species also form when aerosol particles of oleic acid are ozonized. Zahardis et al. (23) used aerosol mass spectrometry and observed dimer structures which contained diperoxide and secondary ozonide linkages. These structures were assigned however based on the molecular weight of the observed compounds, and no detailed structural information was available to support the molecular weight characterization. Further work from the same group (24) also showed that reactions occur between intact oleic acid molecules and Criegee intermediates generated by the ozonolysis (32). The ions observed from these reactions suggest that polyanhydrides are also formed as well as peroxides and secondary ozonides; however, these could possibly result from dehydration of R-acyloxyalkyl hydroperoxides. We reproduce here the comprehensive reaction scheme from Ziemann (2005) (25) (Scheme 1) which indicates some of these reaction products. The stable Criegee intermediates (labeled CI 1 and CI 2) can react with carboxylic acid groups to form R-acyloxyalkyl hydroperoxides (e.g. labeled AAHP 1,2 etc.), carbonyl groups to form secondary ozonides (e.g. SOZ 1,2,3) and with other Criegee intermediates to form cyclic diperoxides. The carboxylic acid and carbonyl functionalities of these products can then further react with other Criegee intermediates to produce larger oligomeric species. The structures of these larger oligomeric species are of great interest since they may dominate the organic mass and hence chemical and optical aerosol properties. Although their presence has been observed in previous studies, their exact structures have not been confirmed. Several different pathways by which the oligomeric molecules are formed have been proposed. Ziemann (25) suggests that the oligomeric species can be formed by reactions between the monomers 10.1021/es060942p CCC: $33.50

 2006 American Chemical Society Published on Web 09/26/2006

SCHEME 1. Predicted Products from Oleic Acid Ozonolysisa

a

Adapted from ref 24. VOL. 40, NO. 21, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Ozonolysis of oleic acid: (A) ES-ITMS spectrum of 10 mM oleic acid in ethyl acetate and (B) ES-ITMS spectrum of ozonized 10 mM oleic acid in ethyl acetate.

FIGURE 2. Fragment ion spectrum of m/z 691. leading to linkage via an O-O bond. Hung et al. (15) propose a linear mechanism for polymerization where the 9-oxononanoic acid and the Criegee intermediate precursor to azelaic acid monomers act as polymerization propagators either by secondary ozonide or esterification pathways. Information obtained from infrared spectroscopy in this publication shows the presence of esters, inferring that esterification is a major polymerization pathway. Conversely nonanoic acid acts as a polymerization terminator. Once it is incorporated into an oligomer, the methyl group at the 6676

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SCHEME 2. Fragmentation of m/z 691

nonanoic acid terminus effectively blocks any further polymerization at that position. The fourth product of the ozonolysis, 1-nonanal, does not play a part in the oligo-

FIGURE 3. Fragment ion mass spectrum of m/z 533.

FIGURE 4. Fragment ion spectrum of m/z 563. merization. Further work by Katrib et al. (30) on oleic acid ozonolysis in the presence of stearic acid shows that the Criegee intermediate attacks the carboxylic acid group of the stearic acid, supporting the mechanism for the formation of R-acyloxyalkyl hydroperoxides. Katrib et al. (22). hypothesized another pathway that could possibly lead to a branching polymerization reaction. In this mechanism a Criegee intermediate reacts across the double bond of an intact oleic acid molecule. This would allow polymerization to occur at the termini of both the oleic acid and the Criegee intermediate. Here we present a structural analysis of the high molecular weight entities formed from the oleic acid ozonolysis reaction using tandem mass spectrometry and suggest the major oligomerization pathways proceeding from this reaction.

Methods Sample Preparation. Ozonized samples were prepared by taking a 10 mM mixture of oleic acid in ethyl acetate and then bubbling ozone through this mixture. The ozone concentration was measured using a Perkin-Elmer Lambda 25 UV/vis spectrometer, which gives a concentration of 1.5 × 104 ppmv in a flow of 10 sccm of O2. The reaction was stopped when it was observed to have gone to completion. This was accomplished using a potassium iodide scrubber connected to the effluent stream from the bubbler. When a red coloring was noted, then the reaction has gone to completion. This occurred at a reaction time of 30 min, which was used for all of the samples. Mass Spectrometry. Samples were analyzed by loop injection (20 µL) onto a Bruker HCT plus ion trap mass spectrometer. A flow of 150 µL min-1 of solvent (50:50 v:v

methanol:water) was infused into the source of the mass spectrometer. In order to generate the mass spectra 20 µL injections of the samples were used. All experiments were carried out using negative ion mode electrospray ionization. The source conditions were maintained with drying temperature of 200 °C, nebulizer gas flow of (N2) 30 L min-1, and drying gas flow of 4 L min-1 during all experiments. Fragment ion spectra were obtained using the auto MS/ MS feature of the Compass software (Bruker). Ions of interest were added to an include list for fragmentation, the maximum fragmentation voltage permitted was 2 V, and the collision gas used was helium. The ozonolysis of oleic acid described in this paper is a reaction occurring in bulk solution, with both reagents used in proportionally high concentrations. There are some obvious caveats that must be associated with any data and structures determined using this approach. First, both the concentrations of ozone and oleic acid are higher than would be typically found present in the atmosphere. While this situation is not ideal from a chemical perspective since differing kinetic orders of individual reaction pathways may result in an atypical distribution of products when compared to the atmosphere. Similarly our reactions in the bulk will not reflect physical mass transport of material in interfacial regions associated with gas and condensed phases. While these experimental conditions are far from ideal, they are entirely necessary, even when using the current analytical state of the art, to gain sufficient sensitivity to structurally characterize high molecular weight oligomeric species. A question naturally remains which is ‘are the species for which we determine structures the same as those found in atmospheric aerosol?’. This is impossible to prove directly VOL. 40, NO. 21, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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with analytical capabilities as they stand and remains a formidable challenge for the future. We comment however that the data we observe for dimeric species correlate with major molecular masses published in studies carried out using more realistic oleic acid in aerosol droplets, and we believe that the solution experiments performed here provide a valid starting point for droplet and in-aerosol molecular characterizations. The presence of the ethyl acetate in the reaction mixture may also have an effect on the ozonolysis reaction. An important uncertainty here is the degree to which components like oleic acid are ‘solvated’ within aerosol, and this not a problem that can be solved here. A gross excess of solvent may well produce intermediate stabilization, and caging effects have been reported by Zeimann (25). We can comment here only that any effects do not appear to be significant in terms of molecular distribution as the low molecular weight reaction products seen here are similar to those observed in studies where pure oleic acid is ozonized.

Results Standards. The mass spectra of the acid standards (azelaic acid, nonanoic acid, and oleic acid, 10 mM in ethyl acetate) and 1-nonanal gave the expected results with peaks at m/z 187 azelaic acid [M - H]-, 157 nonanoic acid [M - H]-, and 141 1-nonanal [M - H]-. The mass spectrum of the 10 mM oleic acid standard is shown in Figure 1a. This is a simple spectrum with the two peaks corresponding to the deprotonated oleic acid molecular ion (m/z 281) and an adduct of oleic acid [2M- H]- present at higher mass (m/z 563). Ozonized Samples. Comparison of the ozonized oleic acid mass spectrum (Figure 1b) with the standard oleic acid mass spectrum shows a number of differences. The molecular ion peak from oleic acid (m/z 281) is not present in the spectrum, indicating that all or almost all of the oleic acid has been ozonized. The expected reaction products, azelaic acid, 9-oxononanoic acid, nonanoic acid, and nonanal, are all observed at m/z 187, 171, 157, and 141, respectively. These ions are not present at similar intensities, suggesting that the reaction does not give equal yields of each product; however, this effect is due to the ionization efficiency of each of the products. For example azelaic acid with two carboxylic acid groups ionizes in negative ion mode much more effectively than nonanal with no acid groups. Based on the 6678

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ion intensities given by the analysis of the azelaic acid, nonanoic acid, and nonanal standards the intensities observed in the ozonolysed sample indicate that there are approximately equal amounts of these three products present, which is in contradiction to aerosol studies of oleic acid ozonolysis (15, 25) where nonanal and 9-oxononanoic acid are observed to be present in greater yields than azelaic acid and nonanoic acid. There is also an ion present in this spectrum at m/z 203 which we believe is a fragment from a larger molecule. There is a large distribution of ions at higher m/z ratios than that of oleic acid (m/z 281) which suggests that some polymerization is occurring between the classical reaction products and Criegee intermediate species to generate the oligomers. Notably there are no ions present that indicate that the addition of a Criegee intermediate across the double bond of an intact oleic acid molecule is taking place, following the mechanism hypothesized by Katrib et al. (22). Ions formed via this pathway would be expected to have mass-to-charge ratios in the range between 453 and 485 Da, which are absent in our ozonized oleic acid mass spectrum. The higher mass ions shown in Figure 1b, therefore, result from polymerization either by esterification to make R-acyloxyalkyl hydroperoxides or as secondary ozonides. It is also possible that some of these ions may be cyclic diperoxides (e.g. m/z 375) which are formed by a reaction between two Criegee intermediates. Tandem mass spectrometry of these ions can give some indication of what these structures are. Hung et al. (15) provides evidence (mass spectrometry and infrared spectroscopy) that a linear polymerization mechanism occurs with addition either by esterification (Criegee intermediate precursors to azelaic acid and nonanoic acid) or as a secondary ozonide (9-oxononanoic acid). The tandem mass spectrum of m/z 691 (Figure 2) from the ozonized oleic acid sample supports such a linear polymerization mechanism. This ion, based on its molecular weight, is formed from two azelaic acid and two nonanoic acid monomers. Because nonanoic acid is thought to act as a polymerization terminator there can be only one form of this ion, with the nonanoic acid monomers occupying both of the terminal positions. The MS/MS data obtained from m/z 691 supports this mechanism (Figure 2). The structure shown in Scheme 2 can explain all of the fragment ions present in the MS2 spectrum. The ion at m/z 549 corresponds to the loss of a terminal nonanoic acid monomer, and m/z 203 can be attributed to a nonanoic acid monomer with an additional carboxylic acid

FIGURE 5. Fragment ion spectrum of m/z 359.


group attached as an ester functionality. The base peak in this spectrum of m/z 345 can be assigned to the fragmentation of the bond between the two azelaic acid monomers in the middle of the oligomer. While all of the fragments in the MS/MS spectrum of m/z 691 can be assigned to a single R-acyloxyalkyl hydroperoxide structure, polymers may have multiple structures. For example the m/z 691 oligomer could have a diperoxide linkage between the two of the monomer units, although there is no evidence present from Figure 2 to suggest that this is occurring. The MS/MS spectrum of the ion at m/z 533 (Figure 4) is expected to give at least two different structures because the ion only contains a single nonanoic acid monomer which may be situated at either end of the oligomer (Scheme 3a+b). These two structures can between them explain almost all of the ions in the fragment ion spectrum with one notable exception. The ion at m/z 219 cannot be formed by fragmentation of either of these structures so in order for it to be present another structure must be present. The fragmentation of a diperoxide (Scheme 3c+d) can however explain the m/z 219 ion. Such diperoxide structures are formed via the reaction of two Criegee intermediates and are proposed by Ziemann (25) in Scheme 1. The presence of diperoxides is further supported by the data obtained from the fragmentation of the ion at m/z 563. This ion is formed from the polymerization of three azelaic acid monomers and their Criegee intermediates and should be expected to give a simplistic spectrum if formed only via R-acyloxyalkyl hydroperoxide linkages (Scheme 4a). However, the fragment ion spectrum (Figure 4) is more complex than is expected and also contains the m/z 219 fragment ion. The presence

of this ion indicates that there is at least one structure with a diperoxide linkage present. Three structures containing diperoxides are possible (Scheme 4b-d). Our data do not allow a firm conclusion to be drawn however on whether all three diperoxides are formed or only a subset. The fragmentation of an ion with a secondary ozonide linkage (m/z 359) gives the product ion spectrum shown in Figure 5. There are three possible isomers of this ion (Scheme 5) two of which are R-acyloxyalkyl hydroperoxides (AAHPs) (Scheme 5a+b) and one of which is a secondary ozonide (Scheme 5c). The evidence for the presence of the secondary ozonide structure comes from the fragment ions at m/z 157. The two linear R-acyloxyalkyl hydroperoxide structures cannot readily fragment to give this ion, which occurs through a rearrangement of the 5-membered ring structure resulting in the m/z 157 fragment and an unstable Criegee intermediate. The Criegee intermediate is a short-lived, unstable structure, so is not observed in the mass spectrum. The secondary ozonide structure cannot however fragment to give all of the ions seen in this fragment ion spectrum, unlike the cyclic diperoxide structures shown in the earlier examples. Study of the product ions in Figure 4 strongly suggests that the linear R-acyloxyalkyl hydroperoxide structures are formed, in addition to the cyclic structure. Therefore it is probable that the peak at m/z 359 represents three isobaric ions with the three structures shown in Scheme 5. Effect of Sample Aging on Ozonized Oleic Acid. Figure 6 shows the mass spectra of an ozonized oleic acid sample analyzed 1 week (Figure 6a) and 3 months (Figure 6b) after exposure to ozone. The samples were stored in ethyl acetate at 4 °C in the dark during this period. It is clear from these spectra that the higher m/z oligomers are more evident in the aged sample. This experiment is rather crude but does demonstrate that although the initial monomers are formed relatively quickly, a slow dark oligomerization process also appears to be occurring. This process may take place through a number of different routes and is unlikely to be a direct continuation of the early oligomerization reaction. Criegee intermediates are highly unstable structures and are unlikely to remain in the solution for a prolonged period of time. This suggests the existence of an alternative route for formation of the large oligomers which may involve decomposition and recombination of the smaller oligomers or further reaction with any dissolved ozone remaining in the sample from the reaction. The overall degree of oligomerization appears likely be dependent on the lifetime/oxidative exposure of the aerosol, and this has been suggested in several previous chamber studies (33, 34). The structures formed however through this ‘slow’ route show consistency in building block monomer units and indicate that larger VOL. 40, NO. 21, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Effect of aging on the ozonized sample: (A) 1 week after ozonolysis and (B) 3 months after ozonolysis. oligomers with masses up to at least one kiloDalton are possible. The data presented here determines the chemical structures of the major oligomeric components formed from the reaction of oleic acid with ozone in solution. The reaction is shown to produce on short timescales oligomers containing up to four precursor units and over time via dark reactions oligomers with up to six monomers in the chain and molecular weights approaching 1 kiloDalton. The major oligomers formed in this reaction are seen to be a combination of R-acyloxyalkyl hydroperoxides, secondary ozonides, and cyclic diperoxides. Tandem mass spectrometry demonstrates that the secondary ozonides and cyclic diperoxides are also present, in linear oligomeric structures. There is good agreement in terms of chemical structures seen here and predictions made in a number of previous papers. There is however no experimental information obtained from this study to support the hypothesis of branching polymerization pathways although this may be due to the high ozone levels and specific experimental conditions used in this experiment. Similarly a linkage mechanism suggested by Ziemann (25) where the monomers are joined by a single peroxide bond is likewise not observed in the experiment.

Acknowledgments The authors acknowledge financial support from the U.K. Natural Environment Research Council for this project (NE/ 6680

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B50582X/1). A.C.L. acknowledges the research support provided by a Philip Leverhulme award.

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Received for review April 19, 2006. Revised manuscript received June 28, 2006. Accepted August 23, 2006. ES060942P

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Structural Analysis of Oligomeric Molecules Formed from the Reaction

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