Molecular SIMS at surfaces: Thiophene on silver

Molecular SIMS, an important development in mass spectrometry of complex organic molecules, has application to the analysis of reactive adiayers on surfaces. We report here the discovery by SIMS of thiophene self-hydrogenation on silver foil.     

Direct analysis of mixtures by double quadrupole mass spectrometry

The double quadrupole mass spectrometer is shown to be applicable to differentiation of isomeric C₅ ketones and for the detection of camphor in a drug and a cosmetic. Detection limits in the picogram range are demonstrated. The similarity between the low-energy QQ collision-induced dissociations and those of high-energy MIKE spectrometry facilitates use of both techniques for mass spectrometry/mass spectrometry.     

Resonant ac dipolar excitation for ion motion control in the Orbitrap mass analyzer

A dipolar ac signal applied to the split outer electrode of an Orbitrap mass spectrometer at the axial resonance frequency causes excitation of ion axial motion and either eventual ion ejection from the trap, if applied in phase with ion motion, or de-excitation, if applied 180 degrees out of phase. Both de-excitation and excitation may be achieved mass-selectively. The extent of ion axial de-excitation depends on the ac amplitude and on the number of cycles applied; sufficient de-excitation can be accomplished such that the ion signal cannot be observed above baseline noise. After de-excitation, the ions remain trapped and in rapid orbital (but not axial) motion, which allows them to be re-excited coherently by application of a second ac waveform allowing the signal again to be observed. Both broad-band and narrow-band waveforms have been used to de-excite and to re-excite ion motion. Using narrow-band waveforms, selective de-excitation and re-excitation can be performed with unit mass selection, leaving an adjacent 13C isotopic peak unaffected. The origin and potential applications of these new capabilities is delineated.     

Extractive electrospray ionization for direct analysis of undiluted urine, milk and other complex mixtures without sample preparation

On-line droplet-droplet extraction occurs when a sample spray intersects a reagent electrospray; this allows continuous analysis of trace amounts of compounds directly in complex matrices including undiluted urine, milk and polluted water over extended periods of time.     

Serine octamers: cluster formation, reactions, and implications for biomolecule homochirality

The emergence of homochirality continues to be one of the most challenging topics associated with the origin of life. One possible scenario is that aggregates of amino acids might have been involved in a sequence of chemical events that led to chiral biomolecules in self-replicating systems, that is, to homochirogenesis. Serine is the amino acid of principal interest, since it forms "magic-number" ionic clusters composed of eight amino acid units, and the clusters have a remarkable preference for homochirality. These serine octamer clusters (Ser8) can be generated under simulated prebiotic conditions and react selectively with other biomolecules. These observations led to the hypothesis that serine reactions were responsible for the first chiral selection in nature which was then passed through chemical reactions to other amino acids, saccharides, and peptides. This Review evaluates the chemistry of Ser8 clusters and the experimental evidence that supports their possible role in homochirogenesis.     

Gas Phase Ion Chemistry to Determine Isoaspartate in a Peptide Backbone

Proof of concept evidence is presented for a new method for the determination of isoaspartate, an important post-translational modification. Chemical derivatization is performed using common reagents for the modification of carboxylic acids and shown to yield suitable diagnostic information with regard to isomerization at the aspartate residue. The diagnostic gas phase chemistry is probed by collision-induced dissociation mass spectrometry, on the timescale of the MS experiment and semi-quantitative calibration of the percentage of isoaspartate in a peptide sample is demonstrated.

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Graphical Abstract ?

     

Uncatalyzed,Regioselective Oxidation of Saturated Hydrocarbons in an Ambient Corona Discharge

Atmospheric pressure chemical ionization (APCI) in air or in nitrogen with just traces of oxygen is shown to yield regioselective oxidation, dehydrogenation, and fragmentation of alkanes. Ozone is produced from ambient oxygen in situ and is responsible for the observed ion chemistry, which includes partial oxidation to ketones and C?C cleavage to give aldehydes. The mechanism of oxidation is explored and relationships between ionic species produced from individual alkanes are established. Unusually, dehydrogenation occurs by water loss. Competitive incorporation into the hydrocarbon chain of nitrogen versus oxygen as a mode of ionization is also demonstrated.     

Surface effects and electrochemical cell capacitance in desorption electrospray ionization

Time resolved measurements show that during a desorption electrospray ionization (DESI) experiment, the current initially rises sharply, followed by an exponential decrease to a relatively steady current. When the high voltage on the spray emitter is switched off, the current drops to negative values, suggesting that the direction of current flow in the equivalent DESI circuit is reversed. These data demonstrate that the DESI source behaves as a dc capacitor and that the addition of a surface between the sprayer and the counter electrode in DESI introduces a new electrically active element into the system. The charging and discharging behavior was observed using different surfaces and it could be seen both by making current measurements on a plate at the entrance to the mass spectrometer as well as by measuring ion current in the linear ion trap within the vacuum system of the mass spectrometer. The magnitude of the steady state current obtained without analyte present on the surface is different for different surface materials, and different capacitor time constants of the equivalent RC circuits were calculated for different DESI surfaces. The PTFE surface has by far the greatest time constant and is also able to produce the highest DESI currents. Surface properties play a crucial role in charge transfer during DESI in addition to the effects of the chemical properties of the analyte. It is suggested that surface energy (wettability) is an important factor controlling droplet behavior on the surface. The experimental data are correlated with critical surface tension values of different materials. It is proposed, based on the results presented, that super-hydrophobic materials with extremely high contact angles have the potential to be excellent DESI substrates. It is also demonstrated, using the example of the neurotransmitter dopamine, that the surface charge that develops during a DESI-MS experiment can cause electrochemical oxidation of the analyte.     

RESTRICTED LIE ALGEBRAS WITH MAXIMAL 0-PIM

In this paper it is shown that the projective cover of the trivial irreducible module of a finite-dimensional solvable restricted Lie algebra is induced from the one dimensional trivial module of a maximal torus. As a consequence, the number of the isomorphism classes of irreducible modules with a fixed p-character for a finite-dimensional solvable restricted Lie algebra L is bounded above by p ^MT(L), where MT(L) denotes the maximal dimension of a torus in L. Finally, it is proved that in characteristic p > 3 the projective cover of the trivial irreducible L-module is induced from the one-dimensional trivial module of a torus of maximal dimension, only if L is solvable.