Resin and Magnetic Nanoparticle-Based Free Radical Probes for Glycan Capture, Isolation, and Structural Characterization.

By combining the merits of solid supports and free radical activated glycan sequencing (FRAGS) reagents, we develop a multifunctional solid-supported free radical probe (SS-FRAGS) that enables glycan enrichment and characterization. SS-FRAGS comprises a solid support, free radical precursor, disulfide bond, pyridyl, and hydrazine moieties. Thio-activated resin and magnetic nanoparticles (MNPs) are chosen as the solid support to selectively capture free glycans via the hydrazine moiety, allowing for their enrichment and isolation. The disulfide bond acts as a temporary covalent linkage between the solid support and the captured glycan, allowing the release of glycans via the cleavage of the disulfide bond by dithiothreitol. The basic pyridyl functional group provides a site for the formation of a fixed charge, enabling detection by mass spectrometry and avoiding glycan rearrangement during collisional activation. The free radical precursor generates a nascent free radical upon collisional activation and thus simultaneously induces systematic and predictable fragmentation for glycan structure elucidation. A radical-driven glycan deconstruction diagram (R-DECON) is developed to visually summarize the MS2 results and thus allow for the assembly of the glycan skeleton, making the differentiation of isobaric glycan isomers unambiguous. For application to a real-world sample, we demonstrate the efficacy of the SS-FRAGS by analyzing glycan structures enzymatically cleaved from RNase-B.

Mass Spectrometric Study of Acoustically Levitated Droplets Illuminates Molecular-Level Mechanism of Photodynamic Therapy for Cancer involving Lipid Oxidation.

Even though the general mechanism of photodynamic cancer therapy is known, the details and consequences of the reactions between the photosensitizer-generated singlet oxygen and substrate molecules remain elusive at the molecular level. Using temoporfin as the photosensitizer, here we combine field-induced droplet ionization mass spectrometry and acoustic levitation techniques to study the "wall-less" oxidation reactions of 18:1 cardiolipin and 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) mediated by singlet oxygen at the air-water interface of levitated water droplets. For both cardiolipin and POPG, every unsaturated oleyl chain is oxidized to an allyl hydroperoxide, which surprisingly is immune to further oxidation. This is attributed to the increased hydrophilicity of the oxidized chain, which attracts it toward the water phase, thereby increasing membrane permeability and eventually triggering cell death.

Subtle Changes in Lipid Environment Have Profound Effects on Membrane Oxidation Chemistry.

Nature carefully designs the components of amphiphile-composed monolayer and bilayer membranes to deliver specific functions. The compositions of these interfacial layered structures are so delicate that minute modifications can result in huge changes in function. Great effort has been expended to understand membrane physical properties, with only minimum attention given to associated chemical properties. Here we report the first examples of the delicate chemistry associated with membrane amphiphilic components by studying OH-mediated oxidation of six different unsaturated lipids/surfactants and their mixtures at the air-water interface using field-induced droplet ionization mass spectrometry (FIDI-MS). When the packing is loose or perturbed to be loose by other components or prior chemical modification, the double bond is oxidized without cleavage by adding oxygen functionality. In contrast, compact packing results in double bond cleavage through a Criegee intermediate mechanism. We postulate that constrained environments imposed by lipid packing limit the conformations of the reaction intermediates, controlling reaction pathways.

Probing the OH Oxidation of Pinonic Acid at the Air-Water Interface Using Field-Induced Droplet Ionization Mass Spectrometry (FIDI-MS).

Gas and aqueous phases are essential media for atmospheric chemistry and aerosol formation. Numerous studies have focused on aqueous-phase reactions as well as coupled gas/aqueous-phase mass transport and reaction. Few studies have directly addressed processes occurring at the air-water interface, especially involving surface-active compounds. We report here the application of field-induced droplet ionization mass spectrometry (FIDI-MS) to chemical reactions occurring at the atmospheric air-water interface. We determine the air-water interfacial OH radical reaction rate constants for sodium dodecyl sulfate (SDS), a common surfactant, and pinonic acid (PA), a surface-active species and proxy for biogenic atmospheric oxidation products, as 2.87 × 10-8 and 9.38 × 10-8 cm2 molec-1 s-1, respectively. In support of the experimental data, a comprehensive gas-surface-aqueous multiphase transport and reaction model of general applicability to atmospheric interfacial processes is developed. Through application of the model, PA is shown to be oxidized exclusively at the air-water interface of droplets with a diameter of 5 µm under typical ambient OH levels. In the absence of interfacial reaction, aqueous- rather than gas-phase oxidation is the major PA sink. We demonstrate the critical importance of air-water interfacial chemistry in determining the fate of surface-active species.

Cholesterol provides nonsacrificial protection of membrane lipids from chemical damage at air-water interface.

The role of cholesterol in bilayer and monolayer lipid membranes has been of great interest. On the biophysical front, cholesterol significantly increases the order of the lipid packing, lowers the membrane permeability, and maintains membrane fluidity by forming liquid-ordered-phase lipid rafts. However, direct observation of any influence on membrane chemistry related to these cholesterol-induced physical properties has been absent. Here we report that the addition of 30 mol % cholesterol to 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) or 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (POPG) monolayers at the air-water interface greatly reduces the oxidation and ester linkage cleavage chemistries initiated by potent chemicals such as OH radicals and HCl vapor, respectively. These results shed light on the indispensable chemoprotective function of cholesterol in lipid membranes. Another significant finding is that OH oxidation of unsaturated lipids generates Criegee intermediate, which is an important radical involved in many atmospheric processes.

Real-Time Studies of Iron Oxalate-Mediated Oxidation of Glycolaldehyde as a Model for Photochemical Aging of Aqueous Tropospheric Aerosols.

The complexation of iron(III) with oxalic acid in aqueous solution yields a strongly absorbing chromophore that undergoes efficient photodissociation to give iron(II) and the carbon dioxide anion radical. Importantly, iron(III) oxalate complexes absorb near-UV radiation (λ > 350 nm), providing a potentially powerful source of oxidants in aqueous tropospheric chemistry. Although this photochemical system has been studied extensively, the mechanistic details associated with its role in the oxidation of dissolved organic matter within aqueous aerosol remain largely unknown. This study utilizes glycolaldehyde as a model organic species to examine the oxidation pathways and evolution of organic aerosol initiated by the photodissociation of aqueous iron(III) oxalate complexes. Hanging droplets (radius 1 mm) containing iron(III), oxalic acid, glycolaldehyde, and ammonium sulfate (pH ∼3) are exposed to irradiation at 365 nm and sampled at discrete time points utilizing field-induced droplet ionization mass spectrometry (FIDI-MS). Glycolaldehyde is found to undergo rapid oxidation to form glyoxal, glycolic acid, and glyoxylic acid, but the formation of high molecular weight oligomers is not observed. For comparison, particle-phase experiments conducted in a laboratory chamber explore the reactive uptake of gas-phase glycolaldehyde onto aqueous seed aerosol containing iron and oxalic acid. The presence of iron oxalate in seed aerosol is found to inhibit aerosol growth. These results suggest that photodissociation of iron(III) oxalate can lead to the formation of volatile oxidation products in tropospheric aqueous aerosols.

Eradicating mass spectrometric glycan rearrangement by utilizing free radicals.

Mass spectrometric glycan rearrangement is problematic because it provides misleading structural information. Here we report on a new reagent, a methylated free radical activated glycan sequencing reagent (Me-FRAGS), which combines a free radical precursor with a methylated pyridine moiety that can be coupled to the reducing terminus of glycans. The collisional activation of Me-FRAGS-derivatized glycans generates a nascent free radical that concurrently induces abundant glycosidic bond and cross-ring cleavage without the need for subsequent activation. The product ions resulting from glycan rearrangement, including internal residue loss and multiple external residue losses, are precluded. Glycan structures can be easily assembled and visualized using a radical driven glycan deconstruction diagram (R-DECON diagram). The presence and location of N-acetylated saccharide units and branch sites can be identified from the characteristic dissociation patterns observed only at these locations. The mechanisms of dissociation are investigated and discussed. This Me-FRAGS based mass spectrometric approach creates a new blueprint for glycan structure analysis.

Mass spectrometric sampling of a liquid surface by nanoliter droplet generation from bursting bubbles and focused acoustic pulses: application to studies of interfacial chemistry.

The complex chemistry occurring at the interface between liquid and vapor phases contributes significantly to the dynamics and evolution of numerous chemical systems of interest, ranging from damage to the human lung surfactant layer to the aging of atmospheric aerosols. This work presents two methodologies to eject droplets from a liquid water surface and analyze them via mass spectrometry. In bursting bubble ionization (BBI), droplet ejection is achieved via the formation of a jet following bubble rupture at the surface of a liquid to yield 250 µm diameter droplets (10 nL volume). In interfacial sampling by an acoustic transducer (ISAT), droplets are produced by focusing pulsed piezoelectric transducer-generated acoustic waves at the surface of a liquid, resulting in the ejection of droplets of 100 µm in diameter (500 pL volume). In both experimental methodologies, ejected droplets are aspirated into the inlet of the mass spectrometer, resulting in the facile formation of gas-phase ions. We demonstrate the ability of this technique to readily generate spectra of surface-active analytes, and we compare the spectra to those obtained by electrospray ionization. Charge measurements indicate that the ejected droplets are near-neutral (<0.1% of the Rayleigh limit), suggesting that gas-phase ion generation occurs in the heated transfer capillary of the instrument in a mechanism similar to thermospray or sonic spray ionization. Finally, we present the oxidation of oleic acid by ozone as an initial demonstration of the ability of ISAT-MS to monitor heterogeneous chemistry occurring at a planar water/air interface.

Investigation of the Mechanism of Electron Capture and Electron Transfer Dissociation of Peptides with a Covalently Attached Free Radical Hydrogen Atom Scavenger.

The mechanisms of electron capture and electron transfer dissociation (ECD and ETD) are investigated by covalently attaching a free-radical hydrogen atom scavenger to a peptide. The 2,2,6,6-tetramethylpiperidin-l-oxyl (TEMPO) radical was chosen as the scavenger due to its high hydrogen atom affinity (ca. 280 kJ/mol) and low electron affinity (ca. 0.45 ev), and was derivatized to the model peptide, FQXTEMPOEEQQQTEDELQDK. The XTEMPO residue represents a cysteinyl residue derivatized with an acetamido-TEMPO group. The acetamide group without TEMPO was also examined as a control. The gas phase proton affinity (882 kJ/mol) of TEMPO is similar to backbone amide carbonyls (889 kJ/mol), minimizing perturbation to internal solvation and sites of protonation of the derivatized peptides. Collision induced dissociation (CID) of the TEMPO tagged peptide dication generated stable odd-electron b and y type ions without indication of any TEMPO radical induced fragmentation initiated by hydrogen abstraction. The type and abundance of fragment ions observed in the CID spectra of the TEMPO and acetamide tagged peptides are very similar. However, ECD of the TEMPO labeled peptide dication yielded no backbone cleavage. We propose that a labile hydrogen atom in the charge reduced radical ions is scavenged by the TEMPO radical moiety, resulting in inhibition of N-Cα backbone cleavage processes. Supplemental activation after electron attachment (ETcaD) and CID of the charge-reduced precursor ion generated by electron transfer of the TEMPO tagged peptide dication produced a series of b + H (bH) and y + H (yH) ions along with some c ions having suppressed intensities, consistent with stable O-H bond formation at the TEMPO group. In summary, the results indicate that ECD and ETD backbone cleavage processes are inhibited by scavenging of a labile hydrogen atom by the localized TEMPO radical moiety. This observation supports the conjecture that ECD and ETD processes involve long-lived intermediates formed by electron capture/transfer in which a labile hydrogen atom is present and plays a key role with low energy processes leading to c and z ion formation. Ab initio and density functional calculations are performed to support our conclusion, which depends most importantly on the proton affinity, electron affinity and hydrogen atom affinity of the TEMPO moiety.

Mechanisms and energetics of free radical initiated disulfide bond cleavage in model peptides and insulin by mass spectrometry.

We investigate the mechanism of disulfide bond cleavage in gaseous peptide and protein ions initiated by a covalently-attached regiospecific acetyl radical using mass spectrometry (MS). Highly selective S-S bond cleavages with some minor C-S bond cleavages are observed by a single step of collisional activation. We show that even multiple disulfide bonds in intact bovine insulin are fragmented in the MS2 stage, releasing the A- and B-chains with a high yield, which has been challenging to achieve by other ion activation methods. Yet, regardless of the previous reaction mechanism studies, it has remained unclear why (1) disulfide bond cleavage is preferred to peptide backbone fragmentation, and why (2) the S-S bond that requires the higher activation energy conjectured in previously suggested mechanisms is more prone to be cleaved than the C-S bond by hydrogen-deficient radicals. To probe the mechanism of these processes, model peptides possessing deuterated ß-carbon(s) at the disulfide bond are employed. It is suggested that the favored pathway of S-S bond cleavage is triggered by direct acetyl radical attack at sulfur with concomitant cleavage of the S-S bond (SH2). The activation energy for this process is substantially lower by ∼9-10 kcal mol-1 than those of peptide backbone cleavage processes determined by density functional quantum chemical calculations. Minor reaction pathways are initiated by hydrogen abstraction from the α-carbon or the ß-carbon of a disulfide, followed by ß-cleavages yielding C-S or S-S bond scissions. The current mechanistic findings should be generally applicable to other radical-driven disulfide bond cleavages with different radical species such as the benzyl and methyl pyridyl radicals.

Ion funnel augmented Mars atmospheric pressure photoionization mass spectrometry for in situ detection of organic molecules.

Laser desorption is an attractive technique for in situ sampling of organics on Mars given its relative simplicity. We demonstrate that under simulated Martian conditions (~2.5 Torr CO(2)) laser desorption of neutral species (e.g., polycyclic aromatic hydrocarbons), followed by ionization with a simple ultraviolet light source such as a discharge lamp, offers an effective means of sampling organics for detection and identification with a mass spectrometer. An electrodynamic ion funnel is employed to provide efficient ion collection in the ambient Martian environment. This experimental methodology enables in situ sampling of Martian organics with minimal complexity and maximum flexibility.

Hydrogen bonding constrains free radical reaction dynamics at serine and threonine residues in peptides.

Free radical-initiated peptide sequencing (FRIPS) mass spectrometry derives advantage from the introduction of highly selective low-energy dissociation pathways in target peptides. An acetyl radical, formed at the peptide N-terminus via collisional activation and subsequent dissociation of a covalently attached radical precursor, abstracts a hydrogen atom from diverse sites on the peptide, yielding sequence information through backbone cleavage as well as side-chain loss. Unique free-radical-initiated dissociation pathways observed at serine and threonine residues lead to cleavage of the neighboring N-terminal Cα-C or N-Cα bond rather than the typical Cα-C bond cleavage observed with other amino acids. These reactions were investigated by FRIPS of model peptides of the form AARAAAXAA, where X is the amino acid of interest. In combination with density functional theory (DFT) calculations, the experiments indicate the strong influence of hydrogen bonding at serine or threonine on the observed free radical chemistry. Hydrogen bonding of the side-chain hydroxyl group with a backbone carbonyl oxygen aligns the singly occupied π orbital on the ß-carbon and the N-Cα bond, leading to low-barrier ß-cleavage of the N-Cα bond. Interaction with the N-terminal carbonyl favors a hydrogen-atom transfer process to yield stable c and z(•) ions, whereas C-terminal interaction leads to effective cleavage of the Cα-C bond through rapid loss of isocyanic acid. Dissociation of the Cα-C bond may also occur via water loss followed by ß-cleavage from a nitrogen-centered radical. These competitive dissociation pathways from a single residue illustrate the sensitivity of gas-phase free radical chemistry to subtle factors such as hydrogen bonding that affect the potential energy surface for these low-barrier processes.

Biomimetic reagents for the selective free radical and acid-base chemistry of glycans: application to glycan structure determination by mass spectrometry.

Nature excels at breaking down glycans into their components, typically via enzymatic acid-base catalysis to achieve selective cleavage of the glycosidic bond. Noting the importance of proton transfer in the active site of many of these enzymes, we describe a sequestered proton reagent for acid-catalyzed glycan sequencing (PRAGS) that derivatizes the reducing terminus of glycans with a pyridine moiety possessing moderate proton affinity. Gas-phase collisional activation of PRAGS-derivatized glycans predominately generates C1-O glycosidic bond cleavages retaining the charge on the reducing terminus. The resulting systematic PRAGS-directed deconstruction of the glycan can be analyzed to extract glycan composition and sequence. Glycans are also highly susceptible to dissociation by free radicals, mainly reactive oxygen species, which inspired our development of a free radical activated glycan sequencing (FRAGS) reagent, which combines a free radical precursor with a pyridine moiety that can be coupled to the reducing terminus of target glycans. Collisional activation of FRAGS-derivatized glycans generates a free radical that reacts to yield abundant cross-ring cleavages, glycosidic bond cleavages, and combinations of these types of cleavages with retention of charge at the reducing terminus. Branched sites are identified with the FRAGS reagent by the specific fragmentation patterns that are observed only at these locations. Mechanisms of dissociation as well as application of the reagents for both linear and highly branched glycan structure analysis are investigated and discussed. The approach developed here for glycan structure analysis offers unique advantages compared to earlier studies employing mass spectrometry for this purpose.

Continuous flow ion mobility separation with mass spectrometric detection using a nano-radial differential mobility analyzer at low flow rates.

We describe a hybrid mass-mobility instrument in which a continuous-flow ion mobility classifier is used as a front-end separation device for mass spectrometric analysis of ions generated with an electrospray ionization source. Using nitrogen as a carrier gas, the resolving power of the nano-radial differential mobility analyzer (nRDMA) for nanometer-sized ions is 5-7 for tetraalkylammonium ions. Data are presented demonstrating the ability of the system to resolve the different aggregation and charge states of tetraalkylammonium ions and protonated peptides using a quadrupole ion trap (QIT) mass spectrometer to analyze the mobility-classified ions. Specifically, data are presented for the two charge states of the decapeptide Gramicidin S. A key feature of the new instrument is the ability to continuously transmit ions with specific mobilities to the mass spectrometer for manipulation and analysis.

A microfluidic-based bubble generation platform enables analysis of physical property change in phospholipid surfactant layers by interfacial ozone reaction.

The air-liquid interface filled with pulmonary surfactant is a unique feature of our lung alveoli. The mechanical properties of this interface play an important role in breathing and its malfunction induced by an environmental hazard, such as ozone, relates to various lung diseases. In order to understand the interfacial physics of the pulmonary surfactant system, we employed a microfluidic bubble generation platform with a model pulmonary surfactant composed of two major phospholipids: DPPC (1,2-dipalmitoyl-sn-phosphatidylcholine) and POPG (1-palmitoyl-2-oleoyl-sn-phosphatidylglycerol). With fluorescence imaging, we observed the ozone-induced chemical modification of the unsaturated lipid component of the lipid mixture, POPG. This chemical change due to the oxidative stress was further utilized to study the physical characteristics of the interface through the bubble formation process. The physical property change was evaluated through the oscillatory behaviour of the monolayer, as well as the bubble size and formation time. The results presented demonstrate the potential of this platform to study interfacial physics of lung surfactant system under various environmental challenges, both qualitatively and quantitatively.

Time-of-flight mass spectrometry of mineral volatilization: toward direct composition analysis of shocked mineral vapor.

We have developed an orthogonal-acceleration time-of-flight mass spectrometer to study the volatiles produced when a mineral's shock-compressed state is isentropically released, as occurs when a shock wave, driven into the mineral by an impact, reflects upon reaching a free surface. The instrument is designed to use a gun or explosive-launched projectile as the source of the shock wave, impact onto a flange separating a poor vacuum and the high vacuum (10(-7) Torr) interior of the mass spectrometer, and transmission of the shock wave through the flange to a mineral sample mounted on the high-vacuum side of the flange. The device extracts and analyzes the neutrals and ions produced from the shocked mineral prior to the possible occurrence of collateral instrument damage from the shock-inducing impact. The instrument has been tested using laser ablation of various mineral surfaces, and the resulting spectra are presented. Mass spectra are compared with theoretical distributions of molecular species, and with expected distributions from laser desorption.

Designer reagents for mass spectrometry-based proteomics: clickable cross-linkers for elucidation of protein structures and interactions.

We present novel homobifunctional amine-reactive clickable cross-linkers (CXLs) for investigation of three-dimensional protein structures and protein-protein interactions (PPIs). CXLs afford consolidated advantages not previously available in a simple cross-linker, including (1) their small size and cationic nature at physiological pH, resulting in good water solubility and cell-permeability, (2) an alkyne group for bio-orthogonal conjugation to affinity tags via the click reaction for enrichment of cross-linked peptides, (3) a nucleophilic displacement reaction involving the 1,2,3-triazole ring formed in the click reaction, yielding a lock-mass reporter ion for only clicked peptides, and (4) higher charge states of cross-linked peptides in the gas-phase for augmented electron transfer dissociation (ETD) yields. Ubiquitin, a lysine-abundant protein, is used as a model system to demonstrate structural studies using CXLs. To validate the sensitivity of our approach, biotin-azide labeling and subsequent enrichment of cross-linked peptides are performed for cross-linked ubiquitin digests mixed with yeast cell lysates. Cross-linked peptides are detected and identified by collision induced dissociation (CID) and ETD with linear quadrupole ion trap (LTQ)-Fourier transform ion cyclotron resonance (FTICR) and LTQ-Orbitrap mass spectrometers. The application of CXLs to more complex systems (e.g., in vivo cross-linking) is illustrated by Western blot detection of Cul1 complexes including known binders, Cand1 and Skp2, in HEK 293 cells, confirming good water solubility and cell-permeability.

Click chemistry facilitates formation of reporter ions and simplified synthesis of amine-reactive multiplexed isobaric tags for protein quantification.

We report the development of novel reagents for cell-level protein quantification, referred to as Caltech isobaric tags (CITs), which offer several advantages in comparison with other isobaric tags (e.g., iTRAQ and TMT). Click chemistry, copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), is applied to generate a gas-phase cleavable linker suitable for the formation of reporter ions. Upon collisional activation, the 1,2,3-triazole ring constructed by CuAAC participates in a nucleophilic displacement reaction forming a six-membered ring and releasing a stable cationic reporter ion. To investigate its utility in peptide mass spectrometry, the energetics of the observed fragmentation pathway are examined by density functional theory. When this functional group is covalently attached to a target peptide, it is found that the nucleophilic displacement occurs in competition with formation of b- and y-type backbone fragment ions regardless of the amino acid side chains present in the parent bioconjugate, confirming that calculated reaction energetics of reporter ion formation are similar to those of backbone fragmentations. Based on these results, we apply this selective fragmentation pathway for the development of CIT reagents. For demonstration purposes, duplex CIT reagent is prepared using a single isotope-coded precursor, allyl-d(5)-bromide, with reporter ions appearing at m/z 164 and 169. Isotope-coded allyl azides for the construction of the reporter ion group can be prepared from halogenated alkyl groups which are also employed for the mass balance group via N-alkylation, reducing the cost and effort for synthesis of isobaric pairs. Owing to their modular designs, an unlimited number of isobaric combinations of CIT reagents are, in principle, possible. The reporter ion mass can be easily tuned to avoid overlapping with common peptide MS/MS fragments as well as the low mass cutoff problems inherent in ion trap mass spectrometers. The applicability of the CIT reagent is tested with several model systems involving protein mixtures and cellular systems.

Studying interfacial reactions of cholesterol sulfate in an unsaturated phosphatidylglycerol layer with ozone using field induced droplet ionization mass spectrometry.

Field-induced droplet ionization (FIDI) is a recently developed ionization technique that can transfer ions from the surface of microliter droplets to the gas phase intact. The air-liquid interfacial reactions of cholesterol sulfate (CholSO(4)) in a 1-palmitoyl-2-oleoyl-sn-phosphatidylglycerol (POPG) surfactant layer with ozone (O(3)) are investigated using field-induced droplet ionization mass spectrometry (FIDI-MS). Time-resolved studies of interfacial ozonolysis of CholSO(4) reveal that water plays an important role in forming oxygenated products. An epoxide derivative is observed as a major product of CholSO(4) oxidation in the FIDI-MS spectrum after exposure of the droplet to O(3) for 5 s. The abundance of the epoxide product then decreases with continued O(3) exposure as the finite number of water molecules at the air-liquid interface becomes exhausted. Competitive oxidation of CholSO(4) and POPG is observed when they are present together in a lipid surfactant layer at the air-liquid interface. Competitive reactions of CholSO(4) and POPG with O(3) suggest that CholSO(4) is present with POPG as a well-mixed interfacial layer. Compared with CholSO(4) and POPG alone, the overall ozonolysis rates of both CholSO(4) and POPG are reduced in a mixed layer, suggesting the double bonds of both molecules are shielded by additional hydrocarbons from one another. Molecular dynamics simulations of a monolayer comprising POPG and CholSO(4) correlate well with experimental observations and provide a detailed picture of the interactions between CholSO(4), lipids, and water molecules in the interfacial region.

Switched ferroelectric plasma ionizer (SwiFerr) for ambient mass spectrometry.

We present the implementation of a switched ferroelectric plasma ionizer (SwiFerr) for ambient analysis of trace substances by mass spectrometry. The device utilizes the ferroelectric properties of barium titanate (BaTiO(3)) to take advantage of the high electric field resulting from polarization switching in the material. The source comprises a [001]-oriented barium titanate crystal (5 × 5 × 1 mm) with a metallic rear electrode and a metallic grid front electrode. When a high voltage AC waveform is applied to the rear electrode to switch polarization, the resulting electric field on the face of the crystal promotes electron emission and results in plasma formation between the crystal face and the grounded grid at ambient pressure. Interaction with this plasma and the resulting reagent ions effects ionization of trace neutrals. The source requires less than 1 W of power to operate under most circumstances, ionizes molecules with acidic and basic functional groups easily, and has proven quite versatile for ambient analysis of both vapor phase and aspirated powdered solid samples. Ionization of vapor phase samples of the organics triethylamine, tripropylamine, tributylamine, and pyridine results in observation of the singly protonated species in the positive ion mass spectrum with sensitivity extending into the high ppb range. With acetic acid, deprotonated clusters dominate the negative ion mass spectrum. Aerodynamic sampling of powdered samples is used to record mass spectra of the pharmaceuticals loperamide and ibuprofen. Chemical signatures, including protonated loperamide and deprotonated ibuprofen, are observed for each drug. The robust, low power source lends itself easily to miniaturization and incorporation in field-portable devices used for the rapid detection and characterization of trace substances and hazardous materials in a range of different environments.