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Complementary methods of high-resolution mass spectrometry and microspectroscopy were utilized for molecular analysis of secondary organic aerosol (SOA) generated from ozonolysis of two structural monoterpene isomers: D-limonene SOA (LSOA) and α-pinene SOA (PSOA). The LSOA compounds readily formed adducts with Na(+) under electrospray ionization conditions, with only a small fraction of compounds detected in the protonated form. In contrast, a significant fraction of PSOA compounds appeared in the protonated form because of their increased molecular rigidity. Laboratory simulated aging of LSOA and PSOA, through conversion of carbonyls into imines mediated by NH3 vapors in humid air, resulted in selective browning of the LSOA sample, while the PSOA sample remained white. Comparative analysis of the reaction products in the aged LSOA and PSOA samples provided insights into chemistry relevant to formation of brown carbon chromophores. A significant fraction of carbonyl-imine conversion products with identical molecular formulas was detected in both samples. This reflects the high level of similarity in the molecular composition of these two closely related SOA materials. Several highly conjugated products were detected exclusively in the brown LSOA sample and were identified as potential chromophores responsible for the observed color change. The majority of the unique products in the aged LSOA sample with the highest number of double bonds contain two nitrogen atoms. We conclude that chromophores characteristic of the carbonyl-imine chemistry in LSOA are highly conjugated oligomers of secondary imines (Schiff bases) present at relatively low concentrations. Formation of this type of conjugated compounds in PSOA is hindered by the structural rigidity of the α-pinene oxidation products. Our results suggest that the overall light-absorbing properties of SOA may be determined by trace amounts of strong brown carbon chromophores.

Understanding molecular interaction pathways in complex biological systems constitutes a treasure trove of knowledge that might facilitate the specific, chemical manipulation of the countless microbiological systems that occur throughout our world. However, there is a lack of methodologies that allow the direct investigation of chemical gradients and interactions in living biological systems, in real time. Here, we report the use of nanospray desorption electrospray ionization (nanoDESI) imaging mass spectrometry for in vivo metabolic profiling of living bacterial colonies directly from the Petri dish with absolutely no sample preparation needed. Using this technique, we investigated single colonies of Shewanella oneidensis MR-1, Bacillus subtilis 3610, and Streptomyces coelicolor A3(2) as well as a mixed biofilm of S. oneidensis MR-1 and B. subtilis 3610. Data from B. subtilis 3610 and S. coelicolor A3(2) provided a means of validation for the method while data from S. oneidensis MR-1 and the mixed biofi...

Understanding the emergence of properties from the size-selective cluster regime to larger nanoparticles is one of the principal goals of nanoscience. We have measured the size-selective reactivity of aluminum cluster anions with alcohols. All clusters with more than 20 atoms are found to be reactive, while Al11(-), Al13(-), and Al20(-) show enhanced resistance to oxidation at smaller sizes. The reactivity of aluminum cluster anions with water, methanol, and tert-butyl alcohol all exhibit patterns that require complementary active sites (Lewis acid, Lewis base) on adjacent atoms. Theoretical investigations reveal that at small sizes, the location of reactive pairs occurs on specific active sites, but at larger sizes the reactive pairs begin to accumulate on the edges between facets, marking the transition from the nonscalable size-dependent regime to the scalable regime where the nanoparticles are universally reactive.

Higher-order mass defect analysis is introduced as a unique formula assignment and visualization method for the analysis of complex mass spectra. This approach is an extension of the concepts of Kendrick mass transformation widely used for identification of homologous compounds differing only by a number of base units (e.g., CH(2), H(2), O, CH(2)O, etc.) in complex mixtures. We present an iterative renormalization routine for defining higher-order homologous series and multidimensional clustering of mass spectral features. This approach greatly simplifies visualization of complex mass spectra and increases the number of chemical formulas that can be confidently assigned for given mass accuracy. The potential for using higher-order mass defects for data reduction and visualization is shown. Higher-order mass defect analysis is described and demonstrated through third-order analysis of a deisotoped high-resolution mass spectrum of crude oil containing nearly 13,000 peaks.

The reactivity of aluminum anion clusters with water was found to exhibit variations with size, with some clusters exhibiting negligible reactivity, others absorbing one or more water, while even others releasing H(2) with addition of multiple waters. (Roach, P.J., Woodward, W.H. et al. Science, 2009, 323, 492). Herein, we provide further details on the role of complementary active sites in the breaking of the O-H bond on the cluster. We examine the reactions of Al(n)(-) + H(2)O where n = 7-18, and show how the complementary active sites may be best identified. The clusters with active sites are found to be reactive, and clusters with barriers to reactivity have an absence of paired active sites. The role of charge in the reactivity is considered, which could account for the observed increase in reactivity at large sizes. The H(2) release in the reactivity of Al(17)(-) with multiple water molecules is also studied by comparing multiple reaction pathways, and the selective H(2) production is explained by the first water inducing a new active site. A mechanism for transferring hydroxyl groups on the surface of the cluster is also discussed.

Characterization of the chemical composition and chemical transformations of secondary organic aerosol (SOA) is both a major challenge and the area of greatest uncertainty in current aerosol research. This study presents the first application of desorption electrospray ionization combined with high-resolution mass spectrometry (DESI-MS) for detailed chemical characterization and studies of chemical aging of organic aerosol (OA) samples collected on Teflon substrates. DESI-MS offers unique advantages both for detailed characterization of chemically labile components in OA that cannot be detected using traditional electrospray ionization mass spectrometry (ESI-MS) and for studying chemical aging of OA. DESI-MS enables rapid characterization of OA samples collected on substrates by eliminating the sample preparation stage. In addition, it enables detection and structural characterization of chemically labile molecules in OA samples by minimizing the residence time of analyte in the solvent. In this study, DESI-MS and tandem mass spectrometry experiments (MS/MS) were used to examine chemical aging of SOA produced by the ozonolysis of limonene (LSOA) in the presence of gaseous ammonia. Exposure of LSOA to ammonia resulted in measurable changes in the optical properties of the sample observed using ultraviolet (UV)-visible spectroscopy. High-resolution DESI-MS analysis demonstrated that chemical aging results in formation of highly conjugated nitrogen-containing species that are most likely responsible for light-absorbing properties of the aged LSOA. Detailed analysis of the experimental data allowed us to identify several key aging reactions, including the transformation of carbonyls to imines, intramolecular dimerization of imines with other carbonyl compounds in SOA, and intermolecular cyclization of imines. This study presents an important step toward understanding the formation of light-absorbing OA (brown carbon) in the atmosphere.

The reactions of metal clusters with small molecules often depend on cluster size. The selectivity of oxygen reactions with aluminum cluster anions can be well described within an electronic shell model; however, not all reactions are subject to the same fundamental constraints. We observed the size selectivity of aluminum cluster anion reactions with water, which can be attributed to the dissociative chemisorption of water at specific surface sites. The reactivity depends on geometric rather than electronic shell structure. Identical arrangements of multiple active sites in Al 16 – , Al 17 – , and Al 18 – result in the production of H 2 from water.

It is shown that spin accommodation plays a determining role in the reactivity of aluminum based anion clusters with oxygen. Experimental reactivity studies on aluminum and aluminum-hydrogen clusters show variable reactivity in even electron systems and rapid etching in odd electron systems. The reactivity of even electron clusters is governed by a spin transfer to the singlet cluster through filling of the spin down antibonding orbitals on triplet oxygen. Theoretical investigations show that when the spin transfer cannot occur, the species is unreactive. When spin accommodation is possible, more subtle effects appear, such as the required spin excitation energy, which raises the total energy of the system, and the filling of the antibonding levels of the O2 molecule, which is stabilized by becoming an aluminum oxygen pi bond. This explanation is consistent with observed behavior in oxygen etching reactions with a variety of clusters including AlnHm-, Aln-, AlnIm-, and AlnC-. The proposed reaction mechanism lends a physical interpretation as to why the HOMO-LUMO gap successfully predicts oxygen etching behavior of the considered systems.

The formation and oxygen etching of Al n H m − clusters are characterized in a flow reactor experiment with first-principles theoretical investigations to demonstrate the exceptional stability of Al 4 H 7 − . The origin of the preponderance of Al 4 H 7 − in the mass spectra of hydrogenated aluminum anions and its resistance to O 2 etching are discussed. Al 4 H 7 − is shown to have the ability to bond with ionic partners to form stable hydrides through addition of an alkali atom [XAl 4 H 7 (X = Li-Cs)]. An intuitive model that can predict the existence of stable hydrogenated cluster species is proposed. The potential synthetic utility of the superatom assemblies built on these units is addressed.

We recently demonstrated that, in gas phase clusters containing aluminum and iodine atoms, an Al 13 cluster behaves like a halogen atom, whereas an Al 14 cluster exhibits properties analogous to an alkaline earth atom. These observations, together with our findings that Al 13 − is inert like a rare gas atom, have reinforced the idea that chosen clusters can exhibit chemical behaviors reminiscent of atoms in the periodic table, offering the exciting prospect of a new dimension of the periodic table formed by cluster elements, called superatoms. As the behavior of clusters can be controlled by size and composition, the superatoms offer the potential to create unique compounds with tailored properties. In this article, we provide evidence of an additional class of superatoms, namely Al 7 − , that exhibit multiple valences, like some of the elements in the periodic table, and hence have the potential to form stable compounds when combined with other atoms. These findings support the contention that there should be no limitation in finding clusters, which mimic virtually all members of the periodic table.

Soft ionization used with gas chromatography time-of-flight mass spectrometry (GC–TOFMS) allows for the selective ionization of aliphatic hydrocarbons and polycyclic aromatic sulfur heterocycles in certified standard reference oil (NIST SRM2721) and coal tar (NIST SRM1597a). Soft ionization is accomplished by photoionization from vacuum ultraviolet resonance lines produced by Xe, Kr, and Ar microplasmas. Molecular ion fragmentation of an aliphatic hydrocarbon C 24 H 50 in a TOF mass spectrometer and a QTOF mass spectrometer using Ar and Kr are compared. The advantage of using soft ionization, as compared with the more commonly used electron ionization, lies in the enhancement of the relative intensity of molecular ions, which can be controlled by the choice of plasma gas. Furthermore, the enhanced selectivity provided by the ability to select the ionization wavelength is demonstrated through the qualitative analysis of a series of polycyclic aromatic sulfur heterocycles including dibenzothiophene, naphthothiophene, benzonaphthothiophene, and various methylated derivatives of these compounds.

Reactive nanospray desorption electrospray ionization (nano-DESI) combined with high-resolution mass spectrometry was utilized for the analysis of secondary organic aerosol produced through ozonolysis of limonene (LSOA). Previous studies have shown that LSOA constituents are multifunctional compounds containing at least one aldehyde or ketone groups. In this study, we used the selectivity of the Girard's reagent T (GT) toward carbonyl compounds to examine the utility of reactive nano-DESI for the analysis of complex organic mixtures. In these experiments, 1-100 μM GT solutions were used as the working solvents for reactive nano-DESI analysis. Abundant products from the single addition of GT to LSOA constituents were observed at GT concentrations in excess of 10 μM. We found that LSOA dimeric and trimeric compounds react with GT through a simple addition reaction resulting in formation of the carbinolamine derivative. In contrast, reactions of GT with monomeric species result in the formation of both the carbinolamine and the hydrazone derivatives. In addition, several monomers did not react with GT on the time scale of our experiment. These molecules were characterized by relatively high values of the double bond equivalent and low oxygen content. Furthermore, because addition of a charged GT tag to a neutral molecule eliminates the discrimination against the low proton affinity compounds in the ionization process, reactive nano-DESI analysis enables quantification of individual compounds in the complex mixture. For example, we were able to estimate for the first time the amounts of dimers and trimers in the LSOA mixture. Specifically, we found that the most abundant LSOA dimer was detected at the ~0.5 pg level and the total amount of dimers and trimers in the analyzed sample was ~11 pg. Our results indicate that reactive nano-DESI is a valuable approach for examining the presence of specific functional groups and for the quantification of compounds possessing these groups in complex mixtures.

Integrating the governing chemistry with the genomics and phenotypes of microbial colonies has been a “holy grail” in microbiology. This work describes a highly sensitive, broadly applicable, and cost-effective approach that allows metabolic profiling of live microbial colonies directly from a Petri dish without any sample preparation. Nanospray desorption electrospray ionization mass spectrometry (MS), combined with alignment of MS data and molecular networking, enabled monitoring of metabolite production from live microbial colonies from diverse bacterial genera, including Bacillus subtilis, Streptomyces coelicolor, Mycobacterium smegmatis , and Pseudomonas aeruginosa . This work demonstrates that, by using these tools to visualize small molecular changes within bacterial interactions, insights can be gained into bacterial developmental processes as a result of the improved organization of MS/MS data. To validate this experimental platform, metabolic profiling was performed on Pseudomonas sp. SH-C52, which protects sugar beet plants from infections by specific soil-borne fungi [R. Mendes et al. (2011) Science 332:1097–1100]. The antifungal effect of strain SH-C52 was attributed to thanamycin, a predicted lipopeptide encoded by a nonribosomal peptide synthetase gene cluster. Our technology, in combination with our recently developed peptidogenomics strategy, enabled the detection and partial characterization of thanamycin and showed that it is a monochlorinated lipopeptide that belongs to the syringomycin family of antifungal agents. In conclusion, the platform presented here provides a significant advancement in our ability to understand the spatiotemporal dynamics of metabolite production in live microbial colonies and communities.

Nanospray desorption electrospray ionization (nano-DESI) combined with high-resolution mass spectrometry was used for the first time for the analysis of the polar constituents of liquid petroleum crude oil samples. The analysis was performed in both positive and negative ionization modes using three solvents, one of which (acetonitrile/toluene mixture) is commonly used in petroleomics studies while two other polar solvents (acetonitrile/water and methanol/water mixtures) are generally not compatible with petroleum characterization using mass spectrometry. The results demonstrate that nano-DESI analysis efficiently ionizes petroleum constituents soluble in a particular solvent. When acetonitrile/toluene is used as a solvent, nano-DESI generates electrospray-like spectra. In contrast, strikingly different spectra were obtained using acetonitrile/water and methanol/water. Comparison with the literature data indicates that these solvents selectively extract water-soluble constituents of the crude oil. Water-soluble compounds are predominantly observed as sodium adducts in nano-DESI spectra indicating that addition of sodium to the solvent may be a viable approach for efficient ionization of water-soluble crude oil constituents. Nano-DESI enables rapid screening of different classes of compounds in crude oil samples based on their solubility in solvents that are rarely used for petroleum characterization providing better coverage of the crude oil composition as compared to electrospray ionization (ESI). It also enables rapid characterization of water-soluble components of petroleum samples that is difficult to perform using traditional approaches.

Ambient ionization imaging mass spectrometry is uniquely suited for detailed spatially resolved chemical characterization of biological samples in their native environment. However, the spatial resolution attainable using existing approaches is limited by the ion transfer efficiency from the ionization region into the mass spectrometer. Here, we present a first study of ambient imaging of biological samples using nanospray desorption ionization (nano-DESI). Nano-DESI is a new ambient pressure ionization technique that uses minute amounts of solvent confined between two capillaries comprising the nano-DESI probe and the solid analyte for controlled desorption of molecules present on the substrate followed by ionization through self-aspirating nanospray. We demonstrate highly sensitive spatially resolved analysis of tissue samples without sample preparation. Our first proof-of-principle experiments indicate the potential of nano-DESI for ambient imaging with a spatial resolution of better than 12 μm. The significant improvement of the spatial resolution offered by nano-DESI imaging combined with high detection efficiency will enable new imaging mass spectrometry applications in clinical diagnostics, drug discovery, molecular biology, and biochemistry.

The effect of relative humidity (RH) on the composition and concentrations of gas-phase products and secondary organic aerosol (SOA) generated from the photooxidation of isoprene under high-NOx conditions was investigated. Experiments were performed with hydrogen peroxide as the OH precursor and in the absence of seed aerosol. The relative yields of most gas-phase products were the same regardless of initial water vapor concentration with exception of hydroxyacetone and glycolaldehyde, which were considerably affected by RH. A significant change was observed in the SOA composition, with many unique condensed-phase products formed under humid (90 % RH) vs. dry (

This paper reports a systemic mass spectrometry (MS) investigation of a novel strategy for labeling biological thiols, involving the cleavage of the Se-N bond by thiol to form a new Se-S bond. Our data show that the reaction is highly selective, rapid, reversible, and efficient. Among 20 amino acids, only cysteine is reactive toward Se-N containing reagents and the reaction occurs in seconds. With the addition of dithiothreitol, peptides derivatized by selenium reagents can be recovered. The high reaction selectivity and reversibility provide potential in both selective identification and isolation of thiols from mixtures. Also, with dependence on the selenium reagent used, derivatized peptide ions exhibit tunable dissociation behaviors (either facile cleavage or preservation of the formed Se-S bond upon collision-induced dissociation), a feature that is useful in proteomics studies. Equally importantly, the thiol derivatization yield is striking, as reflected by 100% conversion of protein beta-lactoglobulin A using ebselen within 30 s. In addition, preliminary applications such as rapid screening of thiol peptides from mixtures and identification of the number of protein free and bound thiols have been demonstrated. The unique selenium chemistry uncovered in this study would be valuable in the MS analysis of thiols and disulfide bonds of proteins/peptides.

Nanospray desorption electrospray ionization (nano-DESI) combined with high-resolution mass spectrometry (HR-MS) is a promising approach for the detailed, molecular-level chemical characterization of atmospheric organic aerosols (OA) collected in laboratory and field experiments. The nano-DESI technique possesses distinct advantages of technical simplicity, enhanced sensitivity, and signal stability. In nano-DESI, analyte is desorbed into a solvent bridge formed between two capillaries and the analysis surface, which enables fast and efficient characterization of OA collected on substrates without sample preparation. Stable signals achieved using nano-DESI make it possible to obtain high-quality HR-MS data both for laboratory-generated and field-collected OA using only a small amount of material (

Nanospray desorption electrospray ionization (nano-DESI) mass spectrometry is presented as an ambient pressure liquid extraction-ionization technique for analysis of organic and biological molecules on substrates. Analyte is desorbed into a solvent bridge formed between two capillaries and the analysis surface. One capillary supplies solvent to create and maintain the bridge, while the second capillary transports the dissolved analyte from the bridge to the mass spectrometer. A high voltage applied between the inlet of mass spectrometer and the primary capillary creates a self-aspirating nanospray. This approach enables the separation of desorption and ionization events, thus providing independent control of desorption, ionization, and transport of the analyte. We present analytical capabilities of the method and discuss its potential for imaging applications.

The limits and useful modifications of the jellium model are of great interest in understanding the properties of metallic clusters, especially involving bimetallic systems. We have measured the relative reactivity of ${\text{CuAl}}_{n}^{\ensuremath{-}}$ clusters $(n=11--34)$ with ${\text{O}}_{2}$. An odd-even alternation is observed that is in accordance with spin-dependant etching, and ${\text{CuAl}}_{22}^{\ensuremath{-}}$ is observed as a ``magic peak.'' The etching resistance of ${\text{CuAl}}_{22}^{\ensuremath{-}}$ is explained by an unusually large splitting of the $2{\text{D}}^{10}$ subshell that occurs because of a geometric distortion of the cluster that may also be understood as a crystal field splitting of the superatomic orbitals.

The electronic structure, stability, and reactivity of iodized aluminum clusters, which have been investigated via reactivity studies, are examined by first-principles gradient corrected density functional calculations. The observed behavior of Al13I(x)- and Al14I(x)- clusters is shown to indicate that for xor = 8, they consist of compact Al13- and Al14++ cores, respectively, demonstrating that they behave as halogen- or alkaline earth-like superatoms. For x8, the Al cores assume a cagelike structure associated with the charging of the cores. The observed mass spectra of the reacted clusters reveal that Al13I(x)- species are more stable for even x while Al14I(x)- exhibit enhanced stability for odd x(xor = 3). It is shown that these observations are linked to the formation and filling of "active sites," demonstrating a novel chemistry of superatoms.

Al(n)I(x)- are reacted with methyl iodide, and the reaction mechanisms and products are discussed. The relevance of previous studies of the reactions between bare aluminum clusters and methyl iodide is addressed, and the chemical differences reported herein are explained. Particular attention is given to parallels with the known chemistry of alkyl halides on aluminum surfaces, where kinetically mediated etching reactions are prominent. The emergence of Al7I- as the dominant product in the present reactions is addressed via electronic structure calculations, which reveal that the cluster can be described in terms of an electron bound to a "jellium compound". Other significant products of the etching reaction include I-, I3-, and, importantly, the polyhalide-like Al13I2x- clusters. In the Al13I(x)- series, clusters with odd values for x are found to be reactive, and those with even x are far more stable. This observation is explained in terms of the presence or absence of active sites.

Two classes of gas-phase aluminum-iodine clusters have been identified whose stability and reactivity can be understood in terms of the spherical shell jellium model. Experimental reactivity studies show that the Al 13 I – x clusters exhibit pronounced stability for even numbers of I atoms. Theoretical investigations reveal that the enhanced stability is associated with complementary pairs of I atoms occupying the on-top sites on the opposing Al atoms of the Al 13 – core. We also report the existence of another series, Al 14 I – x , that exhibits stability for odd numbers of I atoms. This series can be described as consisting of an Al 14 I – 3 core upon which the I atoms occupy on-top locations around the Al atoms. The potential synthetic utility of superatom chemistry built upon these motifs is addressed.