Deborah S. Gross

On the maximum charge state and proton transfer reactivity of peptide and protein ions formed by electrospray ionization

A relatively simple model for calculation of the energetics of gas-phase proton transfer reactions and the maximum charge state of multiply protonated ions formed by electrospray ionization is presented. This model is based on estimates of the intrinsic proton transfer reactivity of sites of protonation and point charge Coulomb interactions. From this model, apparent gas-phase basicities (GBapp) of multiply protonated ions are calculated. Comparison of this value to the gas-phase basicity of the solvent from which an ion is formed enables a maximum charge state to be calculated. For 13 commonly electrosprayed proteins, our calculated maximum charge states are within an average of 6% of the experimental values reported in the literature. This indicates that the maximum charge state for proteins is determined by their gas-phase reactivity. Similar results are observed for peptides with many basic residues. For peptides with few basic residues, we find that the maximum charge state is better correlated to the charge state in solution. For low charge state ions, we find that the most basic sites Arg, Lys, and His are preferentially protonated. A significant fraction of the less basic residues Pro, Trp, and Gln are protonated in high charge state ions. The calculated GBapp of individual protonation sites varies dramatically in the high charge state ions. From these values, we calculate a reduced cross section for proton transfer reactivity that is significantly lower than the Langevin collision frequency when the GBapp of the ion is approximately equal to the GB of the neutral base.

Dissociation of heme-globin complexes by blackbody infrared radiative dissociation: molecular specificity in the gas phase?

The temperature dependence of the unimolecular kinetics for dissociation of the heme group from holo-myoglobin (Mb) and holo-hemoglobin α-chain (Hb-α) was investigated with blackbody infrared radiative dissociation (BIRD). The rate constant for dissociation of the 9 + charge state of Mb formed by electrospray ionization from a “pseudo-native” solution is 60% lower than that of Hb-α at each of the temperatures investigated. In solutions of pH 5.5–8.0, the thermal dissociation rate for Mb is also lower than that of HB-α (Hargrove, M. S. et al. J. Biol. Chem.1994, 269, 4207–4214). Thus, Mb is thermally more stable with respect to heme loss than Hb-α both in the gas phase and in solution. The Arrhenius activation parameters for both dissociation processes are indistinguishable within the current experimental error (activation energy 0.9 eV and pre-exponential factor of 108–10 s−1). The 9+ to 12+ charge states of Mb have similar Arrhenius parameters when these ions are formed from pseudo-native solutions. In contrast, the activation energies and pre-exponential factors decrease from 0.8 to 0.3 eV and 107 to 102 s−1, respectively, for the 9+ to 12+ charge states formed from acidified solutions in which at least 50% of the secondary structure is lost. These results demonstrate that gas-phase Mb ions retain clear memory of the composition of the solution from which they are formed and that these differences can be probed by BIRD.

Single Particle Characterization of Automobile and Diesel Truck Emissions in the Caldecott Tunnel

Individual aerosol particles emitted from light-duty vehicles (LDV) and heavy-duty vehicles (HDV) were sampled in the Caldecott Tunnel (Berkeley, CA) using an aerosol time-of-flight mass spectrometer (ATOFMS). This instru ment determines both size and composition information of individual particles in real time. From the composition of individual particles, in conjunction with knowledge of the traffic patterns in the Caldecott Tunnel, information about the source of the particles can be determined. Based upon chemical composition, three main types of particles were detected: particles with significant mass spectral signal due to polycyclic aromatic hydrocarbons (PAH), elemental carbon (soot) particles, and inorganic particles containing substantial signal due to ions includ ing Al+, Ca+, Fe+, Ba+ and BaO+. Preliminary analysis of these classes shows that they encompass 61.4%, 10.3%, and 11.0%, respectively, of the total number of particles sampled with the ATOFMS instrument in 3 h, heavy traffic sampling periods, in an LDV-only bore of the tunnel. They represent 57.4%, 11.8%, and 18.0%, respectively, of the total number of particles sampled with the ATOFMS instrument in a 3 h sampling period in a mixed traffic (HDV and LDV) bore of the tunnel.

Environmental chemistry through intelligent atmospheric data analysis

Here we present a new open-source software package designed to facilitate the analysis of atmospheric data, with emphasis on data mining applications applied to single-particle mass spectrometry data from aerosol particles. The software package, Enchilada (Environmental Chemistry through Intelligent Atmospheric Data Analysis), is designed to seamlessly handle large datasets, to allow for temporal aggregation of data from many instruments, and to integrate techniques such as clustering (K-means, K-medians, and Art-2a), labeling of peaks in mass spectra, and temporal correlations of multiple datasets from multiple instrument types. The software, which continues to be developed and improved, provides users with a single package to integrate data from multiple mass spectrometer systems (ATOFMS, PALMS, SPASS, Q-AMS) as well as any time-based data stream. A detailed description of the software and examples of analysis methods that are incorporated into it are described here.

A case study of the highly time-resolved evolution of aerosol chemical and optical properties in urban Shanghai, China

Characteristics of the chemical and optical properties of aerosols in urban Shanghai and their relationship were studied over a three-day period in October 2011. A suite of real-time instruments, including an Aerosol Time-Of-Flight Mass Spectrometer (ATOFMS), a Monitor for AeRosols and GAses (MARGA), a Cavity Ring Down Spectrometer (CRDS), a nephelometer and a Scanning Mobility Particle Sizer (SMPS), was employed to follow the quick changes of the aerosol properties within the 72 h sampling period. The origin of the air mass arriving in Shanghai during this period shifted from the East China Sea to the northwest area of China, offering a unique opportunity to observe the evolution of aerosols influenced by regional transport from the most polluted areas in China. According to the meteorological conditions and temporal characterizations of the chemical and optical properties, the sampling period was divided into three periods. During Period 1 (00:00–23:00 LT, 13 October), the aerosols in urban Shanghai were mainly fresh and the single scattering albedo varied negatively with the emission of elemental carbon, indicating that local sources dominated. Period 2 (23:00 LT on 13 October to 10:00 LT on 15 October) was impacted by regionally transported pollutants and had the highest particulate matter (PM) mass loading and the lowest particle acidity, characterized by large fractions of aged particles and high secondary ion (nitrate, sulfate and ammonium) mass concentrations. Comparison between ATOFMS particle acidity and quantitative particle acidity by MARGA indicated the significance of semi-quantitative calculation in ATOFMS. Two sub-periods were identified in Period 2 based on the scattering efficiency of PM 1 mass. Period 3 (from 10:00 LT on 15 October to 00:00 LT on 16 October) had a low PM1/PM10 ratio and a new particle formation event. The comparison of these sub-periods highlights the influence of particle mixing state on aerosol optical properties. We directly observed the influence of regionally transported pollutants on local aerosol properties and demonstrate that the PM mass extinction efficiency is largely determined by the mixing states of the aerosol.

The EDAM project: Mining atmospheric aerosol datasets

There is a great need to better understand the sources, dynamics, and compositions of atmospheric aerosols. The traditional approach for particle measurement, collecting bulk samples of particulates on filters, is not adequate for studying particle dynamics and real‐time correlations. This has led to the development of a new generation of real‐time instruments that provide continuous or semicontinuous streams of data about certain aerosol properties. However, these instruments have added a significant level of complexity to atmospheric aerosol data and dramatically increased the amounts of data to be collected, managed, and analyzed. Our ability to integrate the data from all of these new and complex instruments now lags far behind our data‐collection capabilities, and severely limits our ability to understand the data and act upon it in a timely manner. In this article, we present an overview of EDAM (Exploratory Data Analysis and Management), a joint project between researchers in Atmospheric Chemistry and Computer Science. Important objectives include environmental monitoring and data quality assurance, and real‐time data mining offers great potential. While atmospheric aerosol analysis is an important and challenging domain, our objective is to develop techniques that have broader applicability.

On the dissociation and conformation of gas-phase methonium ions

The dissociation pathways of both doubly and singly charged methonium ions, (CH(3))(3)N(+) -(CH(2))(n)-(+)N(CH(3))(3)·X(-) (n = 6,10; X = Br, I, and OAc), are measured using blackbody infrared radiative dissociation (BIRD) and SORI-CAD in a Fourier transform mass spectrometer. SORI-CAD of the doubly charged decamethonium ions results primarily in the formation of even electron ions by hydrogen rearrangements. In contrast, homolytic bond cleavage to form two odd electron ions is highly favored in the hexamethonium ion, presumably due to increased Coulomb repulsion in this ion. For BIRD of the singly charged salts, ions are mass selected and dissociated by heating the vacuum chamber to elevated temperatures. Under the low pressure conditions of our experiment, energy is transferred from the chamber walls to the ions by the absorption of blackbody radiation. From the temperature dependence of the unimolecular rate constants for dissociation, Arrhenius activation energies in the zero-pressure limit are obtained. The primary dissociation pathways correspond to counterion substitution reactions which result in loss of N(CH(3))(3) and CH(3)X. For hexamethonium and decamethonium with X = Br or I, the branching ratios for these pathways differ dramatically; the ratio of loss of N(CH(3))(3) and CH(3)Br is 3.8 and 0.4 for hexamethonium and decamethonium bromide, respectively. The hexamethonium acetate salt has a branching ratio of 0.1. The Arrhenius activation energies for hexamethonium (Br or I) and decamethonium (Br or I) are 0.9 and 1.0 eV, respectively. This value for hexamethonium acetate is 0.6 eV. Molecular dynamics simulations and Monte Carlo conformation searching are used to obtain the lowest energy structures of hexamethonium and decamethonium bromide. These calculations indicate that the methonium ion folds around the counterion to form a cyclic salt-bridge structure in which both quaternary nitrogens interact with the oppositely charged counterion. The significantly different branching ratios observed for these ions is attributed to the large change in orientation of the counterion with respect to the ammonium centers as the number of methylene groups in these ions increases. Similar ion conformational differences appear to explain the fragmentation for the OAc counter ion as well.

Online single particle analysis of chemical composition and mixing state of crop straw burning particles: from laboratory study to field measurement

Fresh straw burning (SB) particles were generated in the laboratory by the combustion of rice straw and corn straw. The chemical composition and mixing state of the fresh SB particles were investigated by an Aerosol Time-of-Flight Mass Spectrometer (ATOFMS). Based on the mass spectral patterns, the SB particles were clustered into four major types: Salt, Organic Carbon (OC), Elemental Carbon (EC), and internally mixed particles of EC and OC (EC-OC). In addition, particles containing ash, polycyclic aromatic hydrocarbons, heavy metals or nicotine were also observed. Physical and chemical changes of the SB particles immediately after the emission were analyzed with highly time-resolved data. During the aging processes, the average particle size increased steadily. Freshly emitted organic compounds were gradually oxidized to more oxygenated compounds in the OC-containing particles. Meanwhile, an important displacement reaction (2KCl + SO42− → K2SO4 + 2Cl−) was observed. The marker ions for SB particles were optimized and applied to identify the SB particles in the ambient atmosphere. The fluctuation of the number fraction of ambient SB particles sorted by ATOFMS agrees well with that of water soluble K+ measured by an online ion chromatography, demonstrating that the optimized marker ions could be good tracers for SB particles in field measurements.

Characterization of aerosol optical properties, chemical composition and mixing states in the winter season in Shanghai, China.

Physical and chemical properties of ambient aerosols at the single particle level were studied in Shanghai from December 22 to 28, 2009. A Cavity-Ring-Down Aerosol Extinction Spectrometer (CRD-AES) and a nephelometer were deployed to measure aerosol light extinction and scattering properties, respectively. An Aerosol Time-of-Flight Mass Spectrometer (ATOFMS) was used to detect single particle sizes and chemical composition. Seven particle types were detected. Air parcels arrived at the sampling site from the vicinity of Shanghai until mid-day of December 25, when they started to originate from North China. The aerosol extinction, scattering, and absorption coefficients all dropped sharply when this cold, clean air arrived. Aerosol particles changed from a highly aged type before this meteorological shift to a relatively fresh type afterwards. The aerosol optical properties were dependent on the wind direction. Aerosols with high extinction coefficient and scattering Ångström exponent (SAE) were observed when the wind blew from the west and northwest, indicating that they were predominantly fine particles. Nitrate and ammonium correlated most strongly with the change in aerosol optical properties. In the elemental carbon/organic carbon (ECOC) particle type, the diurnal trends of single scattering albedo (SSA) and elemental carbon (EC) signal intensity had a negative correlation. We also found a negative correlation (r=-0.87) between high mass-OC particle number fraction and the SSA in a relatively clean period, suggesting that particulate aromatic components might play an important role in light absorption in urban areas.

Recent developments in the mass spectrometry of atmospheric aerosols

Atmospheric aerosol particles consist of a highly complex mixture of thousands of different compounds. Mass spectrometric techniques are well suited for the analysis of these particles, with each method of analysis having specific advantages and disadvantages. On-line techniques offer high time resolution and thus allow for the investigation of rapidly changing signals. They typically measure either single particles or the average non-refractory submicrometer aerosol. Off-line techniques are often coupled to chromatography or another technique separating for a specific property, which enhances their resolving power. Ultra-high resolution mass spectrometry allows for an unambiguous assignment of the elemental composition throughout the majority of the mass range typically measured in ambient aerosol samples, i.e. up to about m/z 400–600. The quantitative determination of individual compounds, or of classes of compounds, remains an important, but often unresolved, topic. Examples of applications of various mass spectrometric techniques are presented, both from laboratory and field studies.