Infrared Ion Spectroscopy of Environmental Organic Mixtures: Probing

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Article Cite This: Environ. Sci. Technol. 2019, 53, 7604−7612

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Infrared Ion Spectroscopy of Environmental Organic Mixtures: Probing the Composition of α‑Pinene Secondary Organic Aerosol Emma Q. Walhout,† Shelby E. Dorn,§ Jonathan Martens,‡ Giel Berden,‡ Jos Oomens,‡,∥ Paul H.-Y. Cheong,*,§ Jesse H. Kroll,⊥ and Rachel E. O’Brien*,†,⊥ †

Department of Chemistry, College of William and Mary, Williamsburg, Virginia 23185, United States Radboud University, Institute for Molecules and Materials, FELIX Laboratory, Toernooiveld 7c, 6525ED Nijmegen, The Netherlands § Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331-4003, United States ∥ van’t Hoff Institute for Molecular Sciences, University of Amsterdam, 1098XH Amsterdam, Science Park 908, The Netherlands ⊥ Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States

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S Supporting Information *

ABSTRACT: Characterizing the chemical composition of organic aerosols can elucidate aging mechanisms as well as the chemical and physical properties of the aerosol. However, the high chemical complexity and often low atmospheric abundance present a difficult analytical challenge. Milligrams or more of material may be needed for speciated spectroscopic analysis. In contrast, mass spectrometry provides a very sensitive platform but limited structural information. Here, we combine the strengths of mass spectrometry and infrared (IR) action spectroscopy to generate characteristic IR spectra of individual, mass-isolated ion populations. Soft ionization combined with in situ infrared ion spectroscopy, using the tunable free-electron laser FELIX, provides detailed information on molecular structures and functional groups. We apply this technique, along with quantum mechanical modeling, to characterize organic molecules in secondary organic aerosol (SOA) formed from the ozonolysis of α-pinene. Spectral overlap with a standard is used to identify cis-pinonic acid. We also demonstrate the characterization of isomers for multiple SOA products using both quantum mechanical computations and analyses of fragment ion spectra. These results demonstrate the detailed structural information on isolated ions obtained by combining mass spectrometry with fingerprint IR spectroscopy.

1. INTRODUCTION

Analytical techniques such as infrared spectroscopy (IR) and nuclear magnetic resonance (NMR) are powerful tools used to characterize the functional group composition of organic aerosol as it ages in the atmosphere. By collecting timeresolved filters, insights into sources and atmospheric aging processes have been gained using ensemble IR and NMR analysis.24−33 The majority of this spectroscopy work has been carried out either directly on filters or on aerosol extracts with limited prior separation. With chromatography applied prior to these analyses, the chemical identities of the components can be determined.34 However, given the often low atmospheric mass loading of OA and the tens of micrograms of pure material needed, fully speciating a sample with chromatography plus IR or NMR will limit the time-resolution possible for sample collection.35

Organic aerosols (OA) impact the climate through both direct and indirect effects and are one of the biggest unknowns in our estimates of radiative forcing.1 Our ability to model and predict the full range of chemical and physical properties of OA is limited by the complexity of this organic mixture.2,3 In many locations, a large fraction of OA is secondary organic aerosol (SOA) which contains aerosol particles with organic components that are formed in the atmosphere from oxidation and condensation of gas-phase volatile organic compounds.4 The dominant, early generation of SOA oxidation products can often be predicted/determined from the measured molecular formulas, retention times in column separations, and ion fragmentation patterns.5−7 However, aerosol particles in the atmosphere have lifetimes of ∼1 week, and the organic mixture in OA can continue to oxidize and age through reactions such as heterogeneous oxidation,8−12 condensed/aqueous phase reactions,13−18 and photolysis,19−22 producing a very complex mixture of hundreds to thousands of different organic molecules.23 © 2019 American Chemical Society

Received: Revised: Accepted: Published: 7604

April 8, 2019 June 2, 2019 June 11, 2019 June 11, 2019 DOI: 10.1021/acs.est.9b02077 Environ. Sci. Technol. 2019, 53, 7604−7612

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Environmental Science & Technology

m long, polycarbonate flow tube with a 3 5/8′′ diameter. The flow tube had a spacer with small holes to create a laminar flow region located 1 foot from the two inlets. The first inlet introduced clean air from a zero air generator (Aadco 737−13 Pure Air Generator) at a flow rate of 1 L min−1 (RH < 5%) and had liquid α-pinene evaporated into the airflow fed via syringe pump (Harvard Apparatus) at 20 μL hr−1. The second inlet introduced 1.5 L min−1 of zero air from a pen-ray ozone generator (Model 600, Jelight Co., Inc.) which produced ∼15 ppm ozone in the flow tube. The tube was operated at room temperature and ambient pressure. Generated aerosol particles traveled through a 1.2 m long, black carbon denuder to remove excess (unreacted) ozone and VOCs before being collected on a Zefluor, 2 μm Teflon filter. Filters were loaded at flow rates of ∼2 L min−1 for 3 h per sample; excess flow was sent into exhaust through a carbon trap. Filters were weighed before and after collection, and approximately 3 mg of material was collected per filter. Filters were extracted with methanol using ultrasonication for ∼20 min, and the extract was dried using ultrapure N2. One filter extract was frozen and transported to the FELIX laboratory for analysis. Here, an excess of SOA material was generated to ensure successful transport of the sample to the FELIX laboratory. To generate all the spectra here, only 10−15 μg worth of SOA material was used. Mass spectrometric analysis was carried out using electrospray ionization on a Bruker Amazon ion trap mass spectrometer in the FELIX laboratory.58 SOA extracts were first diluted with 5 mL of 50/50 Milli-Q water/methanol to a concentration of ∼10 mM. Immediately prior to running, this stock SOA solution was further diluted to ∼20−40 μM with methanol and ∼0.1% ammonium hydroxide was added to aid ion formation in the negative ion mode. The syringe flow rate of sample to the source was 120 μL hr−1 with a spray voltage of −4500 V and dry N2 nebulizing gas. Precursor ions were isolated in the ion trap and irradiated with two infrared laser pulses from the FEL (repetition rate 10 Hz, pulse energies up to 60 mJ). The recorded mass spectra were used to determine the IRMPD yield at each wavelength, which is defined as the ratio of the summed fragment ion intensities divided by the total ion intensity. After measuring the intensities of the parent and fragment ions at a given wavelength of irradiation, the IR frequency was changed in steps of ∼3 cm−1. For each IR frequency, new packets of ions were loaded into the ion trap and irradiated. The intensities of the parent and fragment ions were measured and this was repeated eight times (8 mass spectra averages per IR step). The whole process continued across the fingerprint spectral region (600−1900 cm−1). One full IRMPD spectrum took approximately 30 min to collect. IRMPD spectra were linearly corrected for variations in laser power as a function of photon energy. At wavelengths with abundant ion fragmentation, an attenuation was applied to the beam to minimize saturation effects. Calculations of molecular structure and vibrational energies were carried out using PCModel 9.0 and Gaussian09. Molecules included cis-pinonic acid, terpenylic acid, norpinic acid, norpinonic acid, pinalic-3-acid, and pinalic-4-acid. Anions for each molecule were created by deprotonating at the most acidic site. For each compound studied, manual, extensive conformational searches were performed to locate all relevant structures. Ground state geometry and vibrational frequency computations were performed at the B3LYP/6-311++G(d,p)

A complementary analytical technique, mass spectrometry, has both very high sensitivity and high mass resolving power, making it a powerful tool to study both the composition and rapid chemical reactions that occur in OA.36−43 Unfortunately, by itself, mass spectrometry is unable to resolve isomers, which are often present in complex organic mixtures, and provides only limited information on chemical structure via fragmentation (MS/MS) analysis. Coupling mass spectrometry with advances in chromatography, including online derivatization,44,45 two-dimensional gas chromatography,46 liquid chromatography,47 and ion mobility separations,48 has expanded the range of SOA compounds that can be separated and characterized. Still, gaps remain in our ability to fully speciate organic mixtures, especially for components that are unresolved in chromatographic separations. Techniques that enhance organic structure characterization in mass spectrometry will further improve analyses of these mixtures. IR action spectroscopy using a mass spectrometer probes the intensities of fragment ions produced after irradiation of the parent ions at tunable wavelengths. This technique generates spectra similar to IR absorption spectroscopy as well as MS/ MS fragmentation spectra at each wavelength. Using ion trap mass spectrometers, infrared multiphoton dissociation (IRMPD) has been applied previously to analyze the gasphase structures of biomolecules49−55 and to identify molecules found in body fluids.56,57 For IRMPD, high intensity, tunable IR radiation is directed through an IRtransparent window into the ion trap in order to induce wavelength dependent photodissociation. This IR radiation can be either in the fingerprint region (∼600−1900 cm−1), typically generated using free electron lasers (FEL), or in the hydrogen stretching region (2800−4000 cm−1), generated using tunable OPO/OPA lasers. While it would be timeconsuming to obtain IRMPD spectra for all molecules in an individual sample, this combined analytical platform provides enhanced structural characterization for molecules of specific interest in complex environmental samples. Here we present a case study of IRMPD applied to a complex environmental organic mixture. Select chemicals in αpinene SOA were investigated with IRMPD in the fingerprint region. The tunable laser light from the FEL55,58 at the FELIX user facility at Radboud University, The Netherlands, was used to irradiate mass-isolated ion populations in an ion trap mass spectrometer. The resulting data set consists of a series of mass spectra showing the isolated precursor ion and the fragment ions produced as a function of IR wavelength (comparable to MS/MS products), from which we reconstruct the IRMPD spectrum of the selected ion. Good spectral overlap between results for a commercial standard and an expected, first generation α-pinene ozonolysis product shows IRMPD can be used to help identify the molecular structure of an unknown ion. Given the complexity of the sample, isomers at many m/z values are expected. The presence of isomers in ion populations at isolated m/z values is demonstrated here using quantum mechanical computations and fragment ion spectra. These examples show that the combination of mass spectrometry with IRMPD spectroscopy provides a platform for enhanced structural characterization of unknown organic molecules in complex environmental mixtures.

2. MATERIALS AND METHODS Chemicals (α-pinene, cis-pinonic acid, and methanol) were obtained from Sigma-Aldrich. Aerosols were generated in a 1.2 7605

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Environmental Science & Technology

distinct IR fingerprint characterized by both asymmetric and symmetric stretches. The lower frequency band at ∼1300 cm−1 corresponds to the symmetric stretch and the higher frequency band at ∼1600 cm−1 to the asymmetric stretch. In solution or on surfaces, these bands tend to be closer together and the spacing between them, Δνa‑s, can provide information on the strength of the counterion bonding interaction.67 Here, these IRMPD spectra provide a starting reference point, or “free” gas-phase Δνa‑s. In Figure 1b, absorption bands at frequencies higher than 1600 cm−1 are also observed. These carbonyl frequencies can shift depending on whether the CO group is a ketone, aldehyde, or lactone.68 Bands observed at lower frequencies correspond to other stretching modes and coupled vibrations. The frequencies of all the vibrations are very sensitive to the chemical environment. Thus, by comparing the location of these experimental absorption bands to computations and/or standards, insights into the structure of the isolated ion population can be gained. The full data sets obtained with this method contain information on the molecular weight, MS/MS fragmentation products, and chemical structures of the isolated ion population. In the following sections, we present a detailed characterization of selected mass-to-charge ratios to demonstrate the range and depth of information available with this technique. 3.2. Comparison with Standards. Comparing the IRMPD spectrum of an isolated ion in the SOA mixture to a known standard of the same expected identity demonstrates the utility of this technique to help identify and characterize the structure of unknowns using synthesized standards. Previous studies have determined that cis-pinonic acid is a major product formed during α-pinene ozononlysis.5,34,64 Figure 2 shows both of the IRMPD spectra for the anion

level of theory using the Gaussian09 software package.59 All computations were completed in the gas-phase at 300 K to match experimental conditions. The four lowest free-energy conformations for each compound were selected for further analyses. Vibrational frequencies were extracted for these conformers and adjusted by a scaling factor of 0.978; a 20 cm−1 fwhm Gaussian line shape convolution was applied for facile comparison with experimental spectra.52,60 All images were generated with CYLview 1.0.61

3. RESULTS AND DISCUSSION 3.1. Soft Ionization and Photodissociation. Electrospray ionization in negative ion mode (Figure 1a) generated

Figure 1. An overview of the data sets collected for α-pinene SOA: (a) full negative ion mode mass spectrum; (b) electrospray ionization mass spectrum (blue) with IRMPD spectra of select m/z values (yellow and red, for visual clarity). The y-axis scale for both data sets in (b) are arbitrary units.

ions corresponding to monomer units of α-pinene ozonolysis products as well as dimers and trimers formed by oligomerization reactions.62,63 The identities of some monomers in α-pinene SOA have been well characterized in the literature.5,6,34,64−66 A selection of ions at m/z values corresponding to some of these known monomers were chosen for isolation and IR irradiation. Some ion populations are expected to be dominated by a single SOA product, others have more than one isomer reported in the literature. In Figure 1b, the IRMPD spectra for ion populations isolated across the mass range of 157−197 m/z are shown. The IRMPD spectra show absorbance features in the CO stretching region (∼1600−1800 cm−1) due to the formation of ketones, aldehydes, carboxylic acids, and lactones during the reaction of ozone with α-pinene. For these anions, a carboxylic acid group is deprotonated forming a carboxylate group with a

Figure 2. IRMPD spectra for m/z 183 (black) and cis-pinonic acid standard (blue).

isolated at m/z 183 from α-pinene SOA (black) and from the standard, cis-pinonic acid (blue). The high degree of overlap between the locations and relative intensities of the major absorption bands in the two spectra strongly support the identification of the SOA product as cis-pinonic acid. In Figure 2, IR absorption peaks at 1617 and 1704 cm−1 are assigned to the asymmetric carboxylate stretch and the ketone carbonyl stretch, respectively. The absorption peak at 1326 cm−1 is assigned to the symmetric carboxylate stretch, and the bands at 1218 and 1170 cm−1 are assigned to coupled CH vibrational modes. These assignments are based on previous analyses of IRMPD spectra for molecules containing carboxylate and 7606

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Environmental Science & Technology carbonyl groups52,69 and quantum mechanical computations (Figure S1 of the Supporting Information, SI). Here, the ion population isolated at m/z 183 has an IRMPD spectrum that is very similar to that of the cis-pinonic acid standard. However, small variations in band intensities are observed between the standard and the SOA sample. Some of this variability is likely due to the fact that the samples were run 2 days apart and minor variations in IR beam intensity can impact the observed IRMPD yield.51 It is recommended that samples for spectral comparison be run on the same day. However, Figure 2 demonstrates that good agreement is still observed when spectra are collected days apart. Conclusions on spectral comparisons with standards should be primarily based on the frequencies of the major bands with lower emphasis on the observed intensities and on minor peaks; these peaks may disappear below a dissociation threshold at lower laser powers. Developments are underway to better control day-to-day power variations.70 Good spectral matches between standards and unknowns, as shown here, do not necessarily positively identify the structure of the unknown ion. However, this technique provides solid support for assignments, and, if known isomers have differing IRMPD spectra, then this technique can eliminate the possibility of certain isomeric structures. Hence, even if the exact structure cannot be determined, the IRMPD spectrum will be able to significantly reduce the number of possible candidates. 3.3. Quantum Chemical Analysis. Figure 3a shows the IRMPD spectrum for the ion population at m/z 171 in the αpinene SOA mass spectrum. This spectrum is very different from the one shown in Figure 2, confirming the expectation that spectral differences can be observed between components in this mixture despite the likely presence of similar functional groups in the product molecules. The broader absorption features observed in Figure 3a may be due to contributions from multiple conformers/isomers. These broad features have also been observed when protons are shared between nucleophilic functional groups.52 Computational modeling aids in spectral interpretation of gas-phase ion conformations.50,51,55,56,71,72 These calculations can also provide insights for structural interpretation when synthesized standards are not available. Such comparisons must be carried out with some caution as theory may deviate from experiment for various reasons such as imperfections in the underlying theory, differences between computed absorption spectra and recorded dissociation spectra, and noise in the experimental data. The anion population with m/z 171 has at least two known potential isomers: terpenylic acid and norpinic acid.6,34,65 Computed spectra (Figure 3b,c) based on density functional theory (DFT) at the B3LYP/6-311+ +G(d,p) level of theory for the lowest energy isomers are shown as colored spectra (see SI Figures S2 and S3 for higherenergy conformations). The computed line spectra have been broadened by Gaussian peak convolution with widths of 5 cm−1 (solid lines) and 20 cm−1 (shaded regions) fwhm. When comparing experimental IRMPD spectra to computed model spectra, the locations of the absorption bands are informative for structural characterization. The intensity in Figure 3a is the IRMPD yield (fragment ion intensity divided by total ion intensity). The intensities in Figure 3b,c shows the calculated, linear absorption intensities based on the DFT calculations. The IRMPD intensities can be influenced by experimental factors including saturation and band broadening.

Figure 3. (a) Experimental IRMPD spectra for m/z 171; computed spectra with 20 cm−1 (shaded) and 5 cm−1 (solid colored lines) peak widths for (b) terpenylic acid and (c) norpinic acid.

Shifts in the vibrational bands due to H-bonding are generally captured by computations, but the extent of possible broadening is not well represented.52 Thus, more reliable spectral interpretations are provided by comparing peak position and spacings.51,71 The computed spectrum for terpenylic acid (Figure 3b, red) matches well with the experimental spectrum, especially for the major carbonyl (1780 cm−1) and carboxylate (1644 cm−1) CO stretches. For norpinic acid (3c, blue), the computed spectrum also matches well for the carboxylic acid and carboxylate but lacks a band that correspond to the measured band near 1250 cm−1. The lack of computed peaks in this region indicates that norpinic acid may be present, but cannot be the only isomer at this m/z. The conformation of the modeled ion in Figure 3c is based on a structure for norpinic acid assuming ozone added across the double bond in α-pinene to form an ozonide followed by a ring opening. However, this is not the lowest-energy conformation possible for a four-membered ring with two methyl groups on one carbon and two carboxylic acid groups on the arms. If the carboxylic acid groups are flipped on the ring, relative to the two methyl groups, then the hydrogen on one of the acids can interact with the carboxylate group on the deprotonated acid (Figure S4). This interaction is not available in the structure shown in Figure 3c due to the location of one of the methyl groups blocking the interaction. The 7607

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Environmental Science & Technology conformation with the shared proton is 12 kcal mol−1 lower in energy than the one shown in Figure 3c and produces a very different computed spectrum (Figure S4). Most striking is the intense band predicted near 2000 cm−1 due to OHO stretching. Here we should ignore this feature as many spectroscopic studies of shared-proton systems have shown this band is likely incorrectly predicted by the harmonic calculations, both in frequency and intensity. Even without this band, the lack of spectral overlap for this structure throughout the spectrum demonstrates that rotation around the fourmembered ring does not occur during the experiment (electrospray ionization, trapping, isolation, and irradiation). This highlights the importance of using the experimental spectra to inform the likely conformations of the molecule and demonstrates the sensitivity of the absorption bands to the electronic environment of the functional groups. 3.4. Fragment Ion Spectra. Figure 4a shows the IRMPD spectrum for the ion population at m/z 169. This spectrum is

For pinalic-4-acid and pinalic-3-acid (b and c), both of which have aldehydes, this peak is calculated at 1711 and 1725 cm−1, respectively. For norpinonic acid, which contains a ketone functional group, the absorption is red-shifted to 1684 cm−1. The absorption at ∼1330 cm−1 corresponds to the symmetric OCO stretch of the carboxylate. All three isomers have similar computed spectra, although there are features that appear to correspond to specific isomers such as the small absorption band at ∼1684 cm−1 which matches the calculated frequency for the ketone stretch in norpinonic acid (purple dotted line). The observed peak at 1711 cm−1 best matches the pinalic-4acid aldehyde stretch, although pinalic-3-acid is very close in frequency (blue and green dotted lines). Here, chromatography would aid in the identification of these two compounds. When the structures of the molecules are very similar, or when the identities of the molecules are not known, characterizations based on quantum chemical analysis are more challenging. However, IRMPD can still provide valuable insights into the identity of unknowns and the possible presence of multiple isomers by the examination of the individual, fragment ion appearance spectra as a function of wavelength. Because the IRMPD process depends on (usually unknown) details of the hyperdimensional potential energy surface and on the kinetics of the energy deposition, redistribution, and fragmentation, the fragment ions cannot be directly assigned to individual isomers.73 However, the existence of different isomers and the range of potential structures contributing to them can sometimes be inferred when significant spectral differences are observed between different IR-induced product ions.74 Figure 5a shows a zoomed-in region of the IRMPD spectrum for ions at m/z 169 with the IR laser power attenuated by 3 dB, using a neutral density filter, to minimize effects of saturation and sequential photodissociation. The appearance spectra of dominant fragment ions are shown in Figure 5b. The fragment ion spectrum at m/z 125 (black), corresponding to the loss of CO2, overlaps with all of the major features in the IRMPD yield spectrum and most of the features for the other fragment ions. This indicates CO2 loss is a common channel for all isomers present, suggesting a carboxylate group. The fragment ion at m/z 123 (red) corresponds to a loss of mass equivalent to CO + H2O and overlaps with the dominant absorption bands in the IRMPD spectrum. The peak at ∼1711 cm−1 is consistent with the peak at the same frequency in Figure 4, corresponding to either pinalic-3-acid or pinalic-4-acid, or both. The fragment ion at m/z 58 (green) has one peak in the same region as the others (∼1620 cm−1), but the next higher peak (∼1686 cm−1) is red-shifted by a small amount compared to the highest frequency peaks for the fragment ion spectra in black and red (see vertical green dashed line). These differences indicate that different isomers may contribute to the fragment ion appearance spectra. This is consistent with the trend observed in Figure 4, a red-shift in band position when the carbonyl is a ketone compared to an aldehyde. The presence of a ketone-containing isomer contributing to the ion at m/z 58 is also supported by the identity of the fragment itself: m/z 58 can be a tracer for ketone fragments formed via a McLafferty rearrangement during the fragmentation process.75 In Figure 5b, the fragment at 97 m/z (blue) shows a carbonyl absorption band centered at ∼1760 cm−1 that is blueshifted farther than those observed for the other fragments. The DFT calculations for pinalic-3-acid, pinalic-4-acid, and

Figure 4. IRMPD spectra for m/z 169 (a) and computed spectra for pinalic-4-acid (b), pinalic-3-acid (c), and norpinonic acid (d). Computed spectra have 5 cm−1 widths.

very similar to the spectrum observed for cis-pinonic acid at m/ z 183 (Figure 2). Given this, and the fact that the mass difference of 14 atomic mass units (amu) can be obtained by replacing a methyl group with a hydrogen, we can expect that possible structures for the dominant components are very similar to cis-pinonic acid. Possible isomers corresponding to m/z 169 that have been previously reported include pinalic-3acid, pinalic-4-acid, and norpinonic acid.64,66 Figure 4b−d shows computed spectra for each of these ions. All three spectra are characterized by two absorption bands above 1600 cm−1. The lower-frequency band (∼1620 cm−1) corresponds to the asymmetric carboxylate stretch in each structure. The higher-frequency band is the carbonyl stretch. 7608

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different amounts of attenuation applied to the FEL beam can be used to probe these effects. The novel application of IRMPD spectroscopic analysis to unresolved, complex organic mixtures from environmental samples demonstrates the information that can be obtained for these challenging samples by combining mass spectrometry with wavelength-selective IRMPD of isolated ion populations. Here, a case study of select, previously well characterized αpinene SOA products is presented. Excellent overlap between a standard and an unknown is observed, and we demonstrate that insights into the molecular structures of the ions can be gained through both computational modeling and fragment ion appearance spectra. The results here support previous work showing cis-pinonic acid and terpenylic acid as important products formed during α-pinene ozonolysis. Additionally, we find evidence for a previously unidentified, minor SOA product with an expected neutral mass of 170 amu that may contain a lactone group or a second carboxylic acid. Because experimental IRMPD as well as theoretical intensities can be influenced by several factors, in addition to isomer abundance, the relative population of different conformers/isomers cannot be accurately determined directly from the data shown in Figures 3 and 4. Different dissociation thresholds for each of the isomers may further increase such inaccuracies. However, if there are different isomers, or if the conformers are not interconverting rapidly, then kinetic measurements of dissociation on characteristic absorption frequencies can be used to quantify the contributions of the different isomers.71,76 For some systems, this may be used to provide insights into SOA formation mechanisms. In Figure 3, the presence of isomer specific bands in the computed spectra suggest that relative abundances of terpenylic acid and norpinic acid could be estimated from dissociation kinetics measurements at those wavelengths. Thus, IRMPD provides a potential method to identify and quantify unresolved isomer abundances from, for example, ion mobility separations. The addition of chromatographic separations prior to IRMPD analysis has recently been developed77 and will greatly increase the interpretative capabilities of this technique for the analysis of unknown samples. Future experiments will include similar analysis for HPLC and ion mobility separated78,79 mixtures of aged α-pinene SOA and other atmospherically relevant SOA precursors. Here the focus was on monomers in α-pinene SOA: future experiments will also target dimers and trimers. The work shown here demonstrates the capabilities of IRMPD spectroscopy to characterize both resolved and unresolved isomers found in complex organic mixtures and was carried out on an ion trap mass spectrometer. Similar analyses can also be carried out using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers which provide enhanced mass resolution helpful for characterizing complex mixtures. Data sets obtained using this technique will aid in the interpretation of SOA composition and atmospheric processing by providing multidimensional data sets which include retention time, mass, and functional group information for individual organic components.

Figure 5. Individual fragment ion appearance spectra for the anion at m/z 169 with 3 dB attenuation. (a) IRMPD scan for m/z 169. (b) Individual fragment ion appearance spectra. All spectra have a 3-point box-car smoothing applied. Vertical dotted lines are provided to highlight the overlap between specific fragment ions and absorption bands in the IRMPD yield spectrum.

norpinonic acid show no carbonyl stretching bands near 1760 cm−1 (Figure 4), indicating that the fragment ion at 97 m/z is possibly from a previously unidentified isomer formed during α-pinene ozonolysis. Supporting this conclusion is the other main absorption band at ∼1408 cm−1, which is observed as a small absorption feature in the IRMPD yield spectrum and is not observed in any of the other fragment ion appearance spectra. Absorptions at ∼1760 cm−1 can correspond to lactone and carboxylic acid stretches (Figure 3), suggesting that the structure for this isomer possibly contains either a ketone on a ring or a second carboxylic acid group. If individual isomers fragment via unique channels, then insights into the presence of these isomers can be obtained by analyzing their IRMPD appearance spectra, as demonstrated here. However, fragment ion appearance spectra should be interpreted carefully as they may be complicated by the formation of secondary fragments, ions generated by primary fragments absorbing additional IR photons and fragmenting further. This process may lead to significant differences in the IR spectra imprinted into different fragment channels, even if there is only one isomer present in the ion population. Attenuating the beam, as was done here, can help minimize these effects. Additionally, replicate IRMPD scans with

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b02077. 7609

DOI: 10.1021/acs.est.9b02077 Environ. Sci. Technol. 2019, 53, 7604−7612

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Spectra and model structures from computational analyses (Figures S1−S7) as well as a report of Gaussian input file parameters and XYZ coordinates for optimized structures (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shelby E. Dorn: 0000-0002-1485-1709 Jonathan Martens: 0000-0001-9537-4117 Giel Berden: 0000-0003-1500-922X Rachel E. O’Brien: 0000-0001-8829-5517 Author Contributions

The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.E.O. acknowledges NOAA grants NA13OAR4310072 and NA140AR4310132 for support collecting IRMPD data as well as William and Mary New Faculty Summer Research Grant and the Jeffress Memorial Trust Award in Interdisciplinary Research for supporting E.Q.W. R.E.O. also thanks Jeremy O’Brien for helpful discussions, and Britta Redlich and the FELIX staff for excellent support and advice. We gratefully acknowledge the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) for the support of the FELIX Laboratory. NWO-VICI (724.011.002) and NWO-TTW (15769) grants helped us build the MS infrastructure used in this study. P.H.Y.C. is the Bert and Emelyn Christensen professor of OSU, and gratefully acknowledges financial support from the Vicki & Patrick F. Stone family and the National Science Foundation (NSF, CHE-1352663).

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