Physics of the aurora and airglow

A new matrix-assisted laser desorption/ionization (MALDI) time-of-flight/time-of-flight (TOF/TOF) high-resolution tandem mass spectrometer is described for sequencing peptides. This instrument combines the advantages of high sensitivity for peptide analysis associated with MALDI and comprehensive fragmentation information provided by high-energy collision-induced dissociation (CID). Unlike the postsource decay technique that is widely used with MALDI-TOF instruments and typically combines as many as 10 separate spectra of different mass regions, this instrument allows complete fragment ion spectra to be obtained in a single acquisition at a fixed reflectron voltage. To achieve optimum resolution and focusing over the whole mass range, it may be desirable to acquire and combine three separate sections. Different combinations of MALDI matrix and collision gas determine the amount of internal energy deposited by the MALDI process and the CID process, which provide control over the extent and nature of the fr...

The characteristics of peptide collision-induced dissociation using a high-performance MALDI-TOF/TOF tandem mass spectrometer.

The current state of the theory and its application to low-energy electron-molecule collisions is reviewed. The emphasis is on elastic scattering and vibrational and rotational excitation of small diatomic and polyatomic molecules. New and traditional theoretical approaches are described, and the results of calculations are compared with existing experimental measurements.

The theory of electron-molecule collisions

Momentum-transfer and inelastic-collision cross sections for electrons in ${\mathrm{O}}_{2}$, CO, and C${\mathrm{O}}_{2}$ are calculated from measured values of the electron drift velocity, characteristic energy, attachment coefficient, and ionization coefficient. The experimental data for ${\mathrm{O}}_{2}$ are most consistent with vibrational excitation cross sections consisting of a series of resonances located at the vibrational energy levels of the negative ion and having values of cross section times energy half-width of the order of ${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$ eV. The calculated effective dipole moment for rotational excitation of CO is in very good agreement with values obtained by other techniques. The vibrational excitation cross section for CO at electron energies below 1 eV is in agreement with theoretical predictions. The vibrational excitation cross sections required for consistency with the C${\mathrm{O}}_{2}$ data are very large [(2-5) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$] and include a peak very close to the vibrational threshold of 0.083 eV.

Momentum-Transfer and Inelastic-Collision Cross Sections for Electrons in O-2, CO, and C O-2

Traditionally, mass spectrometry has been a powerful analytical method enabling the structural analysis of small molecules, and later on peptides and proteins. With the advent of native mass spectrometry, using a combination of electrospray ionisation and time of flight analysis, mass spectrometry could also be applied to the mass determination of large protein complexes such as ribosomes and whole viruses. More recently, ion mobility has been coupled to mass spectrometry providing a new dimension in the analysis of biomolecules, with ion mobility separating ions according to differences in size and shape. In the context of native mass spectrometry, ion mobility mass spectrometry opens up avenues for the detailed structural analysis of large and heterogeneous protein complexes, providing information on the stoichiometry, topology and cross section of these assemblies and their composite subunits. With these characteristics, ion mobility mass spectrometry offers a complementary tool in the context of structural biology. Here, we critically review the development, instrumentation, approaches and applications of ion mobility in combination with mass spectrometry, focusing on the analysis of larger proteins and protein assemblies (185 references).

Ion mobility mass spectrometry of proteins and protein assemblies.

A new method is described for the measurement of electron mobilities in gases. An electrical shutter method is employed in which the shutters take the form of two fine wire grids, alternate wires of which are connected to a high frequency alternating potential. Electrons pass through the grids only when the potential between adjacent grid wires is zero, and only electrons which cross the gas space in one half-cycle are received at the collecting electrode. A sharp maximum is thus observed in the electrometer current when the drift velocity of the electron multiplied by the time of one half-cycle is equal to the distance between the grids. The theoretical shape of the current curve is compared with experiment and good agreement observed. Measurements have been made between $\frac{X}{p}$ of 0.03 and 20, and the values obtained are compared with the Compton mobility equation.

Absolute Values of the Electron Mobility in Hydrogen

The formation of negative ions by electron capture in gases in which a dissociation process does not occur is explained by a unimolecular process involving the excitation of molecular vibrational levels and subsequent loss of energy by collision or resonance. In order to obtain a proper order of magnitude to agree with experimental observations, one must assume a change of only one vibrational quantum number. This sets an upper limit on the electron affinity. For the case of ${\mathrm{O}}_{2}$, this limit is 0.17 volt consistent with other observations. The theory also yields a dependence of the phenomenon on the average energy of the electrons which is in agreement with experiment.

On the Mechanism of Unimolecular Electron Capture

Electron and Negative Ion Mobilities in Oxygen, Air, Nitrous Oxide and Ammonia

Electron Attachment and Negative Ion Formation in Oxygen and Oxygen Mixtures

The ionization in the ionosphere is discussed in the light of present theories of recombination and negative-ion formation. It is suggested that the E- and F1-regions are formed as the result of the absorption of special bands of solar radiation and that negative-ion formation is the predominant process in these regions. A sufficient number of electrons exists, however, to cause the reflection of electromagnetic waves. The F2-region is probably not due to the absorption of any special band of solar radiation but owes its formation to the preponderance at this elevation of electron-positive-ion recombination over negative-ion formation during the day. The characteristics of the E- and F1-regions are discussed quantitatively and compared with experimental observations. A qualitative discussion is given for the F2-region since it seems probable that significant changes in level and density take place at these elevations.

Norris E. Bradbury

Papers

Absolute Values of the Electron Mobility in Hydrogen

On the Mechanism of Unimolecular Electron Capture

Electron and Negative Ion Mobilities in Oxygen, Air, Nitrous Oxide and Ammonia

Electron Attachment and Negative Ion Formation in Oxygen and Oxygen Mixtures

Ionization, negative-ion formation, and recombination in the ionosphere