Method and apparatus for chemical and topographical microanalysis

A scanning probe microscope is combined with a laser induced breakdown spectrometer to provide spatially resolved chemical analysis of the surface correlated with the surface topography. Topographical analysis is achieved by scanning a sharp probe across the sample at constant distance from the surface. Chemical analysis is achieved by the means of laser induced breakdown spectroscopy by delivering pulsed laser radiation to the sample surface through the same sharp probe, and consequent collection and analysis of emission spectra from plasma generated on the sample by the laser radiation. The method comprises performing microtopographical analysis of the sample with a scanning probe, selecting a scanned topological site on the sample, generating a plasma plume at the selected scanned topological site, and measuring a spectrum of optical emission from the plasma at the selected scanned topological site. The apparatus comprises a scanning probe, a pulsed laser optically coupled to the probe, an optical spectrometer, and a controller coupled to the scanner, laser and spectrometer for controlling the operation of the scanner, laser and spectrometer. The probe and scanner are used for topographical profiling the sample. The probe is also used for laser radiation delivery to the sample for generating a plasma plume from the sample. Optical emission from the plasma plume is collected and delivered to the optical spectrometer so that analysis of emission spectrum by the optical spectrometer allows for identification of chemical composition of the sample at user selected sites

Ion Mobility-Mass Spectrometry with a Radial Opposed Migration Ion and Aerosol Classifier (ROMIAC)

The first application of a novel differential mobility analyzer, the radial opposed migration ion and aerosol classifier (ROMIAC), is demonstrated. The ROMIAC uses antiparallel forces from an electric field and a cross-flow gas to both scan ion mobilities and continuously transmit target mobility ions with 100% duty cycle. In the ROMIAC, diffusive losses are minimized, and resolution of ions, with collisional cross-sections of 200–2000 Å^2, is achieved near the nondispersive resolution of ~20. Higher resolution is theoretically possible with greater cross-flow rates. The ROMIAC was coupled to a linear trap quadrupole mass spectrometer and used to classify electrosprayed C2–C12 tetra-alkyl ammonium ions, bradykinin, angiotensin I, angiotensin II, bovine ubiquitin, and two pairs of model peptide isomers. Instrument and mobility calibrations of the ROMIAC show that it exhibits linear responses to changes in electrode potential, making the ROMIAC suitable for mobility and cross-section measurements. The high resolution of the ROMIAC facilitates separation of isobaric isomeric peptides. Monitoring distinct dissociation pathways associated with peptide isomers fully resolves overlapping peaks in the ion mobility data. The ability of the ROMIAC to operate at atmospheric pressure and serve as a front-end analyzer to continuously transmit ions with a particular mobility facilitates extensive studies of target molecules using a variety of mass spectrometric methods

Host-Guest Complexation of Amphiphilic Molecules at the Air-Water Interface Prevents Oxidation by Hydroxyl Radicals and Singlet Oxygen

The oxidation of antioxidants by oxidizers imposes great challenges to both living organisms and the food industry. Here we show that the host–guest complexation of the carefully designed, positively charged, amphiphilic guanidinocalix[5]arene pentadodecyl ether (GC5A‐12C) and negatively charged oleic acid (OA), a well‐known cell membrane antioxidant, prevents the oxidation of the complex monolayers at the air–water interface from two potent oxidizers hydroxyl radicals (OH) and singlet delta oxygen (SDO). OH is generated from the gas phase and attacks from the top of the monolayer, while SDO is generated inside the monolayer and attacks amphiphiles from a lateral direction. Field‐induced droplet ionization mass spectrometry results have demonstrated that the host–guest complexation achieves steric shielding and prevents both types of oxidation as a result of the tight and “sleeved in” physical arrangement, rather than the chemical reactivity, of the complexes

Host-Guest Complexation of Amphiphilic Molecules at the Air-Water Interface Prevents Oxidation by Hydroxyl Radicals and Singlet Oxygen

The oxidation of antioxidants by oxidizers imposes great challenges to both living organisms and the food industry. Here we show that the host–guest complexation of the carefully designed, positively charged, amphiphilic guanidinocalix[5]arene pentadodecyl ether (GC5A‐12C) and negatively charged oleic acid (OA), a well‐known cell membrane antioxidant, prevents the oxidation of the complex monolayers at the air–water interface from two potent oxidizers hydroxyl radicals (OH) and singlet delta oxygen (SDO). OH is generated from the gas phase and attacks from the top of the monolayer, while SDO is generated inside the monolayer and attacks amphiphiles from a lateral direction. Field‐induced droplet ionization mass spectrometry results have demonstrated that the host–guest complexation achieves steric shielding and prevents both types of oxidation as a result of the tight and “sleeved in” physical arrangement, rather than the chemical reactivity, of the complexes

Continuous flow mobility classifier interface with mass spectrometer

A continuous flow mobility classifier provide the ability to perform two-dimensional separation in mass spectrometry. An ionization system is used to ionize a sample. A differential mobility analyzer (DMA) (e.g., a nano-radial DMA) is coupled to the ionization system and to a mass spectrometer. The nano-RDMA is configured to separate the ionized sample by mobility for subsequent mass analysis by the mass spectrometer

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) cm^2 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

Field induced droplet ionization: Investigations of complex chemical reactions at the air-​water interface employing mass spectrometry

Every new method for sampling complex mols. from liqs. and solids into the gas phase has fueled the expanding application of mass spectrometry to mol. biol. and other fields of science. Recently developed, field induced droplet ionization (FIDI), combined with mass spectrometric (MS) sampling, holds promise for a broad spectrum of applications. FIDI-MS is based on the physics of nanoliter droplets in time dependent elec. fields, leading to formation of dual Taylor cones from which streams of pos. and neg. charged submicron droplets are emitted in opposite directions, forming what is essentially a dual electrospray ion source. In multi-component systems ionic species with high surface activity are detected with high sensitivity, along with mol. clusters indicative of specific ion pair interactions and mol. aggregation at the air-water interface. The air-water interface represents one of the most ubiquitous chem. environments in nature, present on the surface of oceans and lakes, on atm. aerosols, and even in the human respiratory system. Yet in spite of their prevalence, chem. reactions occurring at these interfaces remain poorly characterized. FIDI-MS has proven itself to be ideally suited for such studies. Several examples of these investigations will be presented, including org. reaction mechanisms involving acid and base catalysis, atm. aerosol chem., and processes that involve biol. interfaces such as the pulmonary surfactant layer, where we have unraveled the myriad chem. and phys. processes assocd. with oxidants such as ozone interacting with lipids and proteins in this complex interfacial environment. Theor. studies involving state of the art mol. dynamics and quantum chem. calcns. play an important role in in these investigations. Several new approaches demonstrating selective sampling of complex mol. species from planar liq. surfaces will also be discussed

The unusually high proton affinity of Aza-18-crown-6 ether: Implications for the molecular recognition of lysine in peptides by lariat crown ethers

Recent studies have shown that 18-crown-6 ether (18C6) will selectively form adducts in the gas phase with small, lysine containing peptides. The present study extends this work by investigating the ability of aza-18-crown-6 ether (A18C6) and L1 (a simple lariat crown ether derivative of A18C6) to form similar noncovalent adducts with the side chain of lysine in model peptides in the gas phase. The substitution of nitrogen for oxygen greatly increases the proton affinity of A18C6 relative to 18C6 and inhibits the formation of noncovalent adducts with small lysine containing peptides. The proton affinity of A18C6 is determined by the kinetic method to be 250 ± 1 kcal/mol. This value is much higher than that for diethanolamine (228 kcal/mol) or for 18C6 (231 kcal/mol). This unusually high basicity is rationalized by semi-empirical calculations that suggest a highly symmetrical structure for protonated A18C6 in which the three most distant oxygens are able to fold back and hydrogen bond with the protonated nitrogen. In the case of L1, the lariat side chain is attached by an amide bond, lowering the proton affinity of L1 relative to that of A18C6. This allows L1 to form noncovalent adducts with lysine despite the fact that steric repulsion within the cavity of the crown is increased to some extent. The relative ammonium ion affinities of these various crown ethers are shown to serve as qualitative predictors for the molecular recognition of lysine. The order of the relative ammonium ion affinities is 18C6≫L1>A18C6 as determined by the kinetic method. These results suggest that the substitution of nitrogen for oxygen in the crown ether is not beneficial for the molecular recognition of lysine

Relative Signs of the ^(31)P-^1H and ^(31)P-C-^1H Nuclear Magnetic Resonance Coupling Constants

The possibility that terms other than Fermi contact interaction make significant contributions to nuclear spin-spin coupling with heavy nuclei has stimulated considerable interest in the n.m.r. spectra of compounds containing heteroatoms. In particular, relative signs and magnitudes of J_(XCH) and J_(XCCH) have be en reported for ethyl derivatives of several heavy isotopes of spin 1/2. In order to understand better the factors responsible for spin coupling with a representative heavy nucleus, we have analyzed the spectra of (CH_3)_3P, (CH_3)_2PH, and (CH_3)PH_2 and have obtained relative signs and magnitudes of J_(PH) and J_(PCH)