Oszillographische Messungen der Fowler-Nordheim-Geraden

A method for rapid measurement of Fowler-Nordheim curves is described. By means of this method the change of the work function due to gas adsorption in the field electron microscope can be investigated. Our results are in good agreement with the theoretical prediction. Furthermore, we examined the adsorption at constant voltage and with constant emission current and compared these with the undisturbed adsorption. It has been found that the accelerated adsorption is due to the emission current.     

Electron transfer dissociation in the hexapole collision cell of a hybrid quadrupole-hexapole Fourier transform ion cyclotron resonance mass spectrometer

Electron transfer dissociation (ETD) of proteins is demonstrated in a hybrid quadrupole-hexapole Fourier transform ion cyclotron resonance mass spectrometer (Qh-FTICRMS). Analyte ions are selected in the mass analyzing quadrupole, accumulated in the hexapole linear ion trap, reacted with fluoranthene reagent anions, and then analyzed via an FTICR mass analyzer. The hexapole trap allows for a broad fragment ion mass range and a high ion storage capacity. Using a 3 T FTICRMS, resolutions of 60 000 were achieved with mass accuracies averaging below 1.4 ppm. The high resolution, high mass accuracy ETD spectra provided by FTICR obviates the need for proton transfer reaction (PTR) charge state reduction of ETD product ions when analyzing proteins or large peptides. This is demonstrated with the ETD of ubiquitin and apomyoglobin yielding sequence coverages of 37 and 20%, respectively. We believe this represents the first reported successful combination of ETD and a FTICRMS.     

X-Band Elektronenspinresonanz mit Blochscher meßanordnung

The use of Bloch-type arrangements (induction principle) in ESR spectrometry is superior over conventional reflection arrangements. The advantage results from the better decoupling between transmitter and receiver, resulting in a higher stability of the overall system. A cylindrical cavity sustaining the dual degenerate TE ₁₁₂ ⁰ modes showed best performance. With such cavities, decoupling-values of more than—110 dB are possible due to special devices. The spectrometer has an overall noise-figure of 8.3 dB, and 6 dB with an additional parametric amplifier. The decoupling device allows to couple circularly polarized waves into the cavity. Thus the sign of the spectroscopic splitting constant may be determined.

     

Automation of a Fourier transform ion cyclotron resonance mass spectrometer for acquisition, analysis, and e-mailing of high-resolution exact-mass electrospray ionization mass spectral data

A system has been designed to automatically acquire high-resolution (>50,000 FWHM), exact-mass (mass measurement error ≤3 mmu) electrospray ionization mass spectra with a commercial Fourier transform ion cyclotron resonance mass spectrometer equipped with a high-field (9.4 tesla) superconducting magnet and a commercial autosampler. Upon the injection of each individual sample, the autosampler transmits a contact closure signal to the previously tuned and calibrated mass spectrometer to initiate data acquisition. A software package was designed to run off-line and to accept a sample list with input information for each of the samples. Then for each of the samples, the software automatically processes the acquired data, interprets the exact-mass data by correlating the observed masses with predicted masses computed from proposed elemental formulas, and then finally prints the spectra, peak lists, and exact-mass reports, and e-mails the exact-mass reports to the submitting chemists. With this automation package, large numbers of samples can be run unattended while obtaining exact masses for all the abundant ions in the spectra. Sample turnaround times are reduced with a corresponding increase in sample throughput. The performance of the system was evaluated with nearly 700 samples with a precalibrated instrument, without the presence of an internal standard. The system was found to be reliable and robust with a fitted standard deviation of 0.32 mmu and a small average systematic mass error of ?0.28 mmu. Typical data acquired with the system have resolving powers >50,000 (FWHM) and mass errors <1.0 mmu.     

Pulsed Field Gradient Selection in Two-Dimensional Magic Angle Spinning NMR Spectroscopy of Dipolar Solids

The utility of gradient selection in MAS spectroscopy of dipolar solids is explored in two examples. In the first, rotor-synchronized gradients of appropriate strength and duration are applied to select¹H double-quantum coherences. The resulting DQ MAS spectrum of adamantane is compared with that acquired by the corresponding phase-cycling technique. As a second example, a¹H 2D exchange MAS experiment is performed on an elastomer sample. In this experiment, a gradient is applied to remove undesired coherences that would otherwise distort the spectrum for short mixing times. The diagonal-peak intensities in the resulting spectrum show a linear decrease with increasing mixing time indicating cross-relaxation by slow chain motions as the relevant process. Both types of experiments demonstrate the potential of gradient-selection techniques for MAS spectroscopy of dipolar solids.     

Untersuchung der Gasadsorption im Feldelektronenmikroskop in Abhängigkeit von Strom und Spannung

In studies of gas adsorption on metals with the aid of the field emission microscope influences of the electric field or the current on the adsorption have been reported. In the present paper we succeed in seperating the effects of field and current by using pulsed field emission with a variable voltage in the interval between pulses. We find that the electric field is of no influence, that larger currents, however, result in an increase in the speed of adsorption. Possible mechanisms are discussed. A new method for measuring the change in work function on adsorption is reported. With this method the change in work function during the adsorption of O₂ on tungsten has been measured.     

Magnetic field gradients in solid state magic angle spinning NMR.

Magnetic field gradients have proven useful in NMR for coherence pathway selection, diffusion studies, and imaging. Recently they have been combined with magic angle spinning to permit high-resolution measurements of semi-solids, where magic angle spinning averages any residual dipolar couplings and local variations in the bulk magnetic susceptibility. Here we show the first examples of coherence pathway selection by gradients in dipolar coupled solids. When the gradient evolution competes with dipolar evolution the experiment design must take into account both the strength of the dipolar couplings and the means to refocus it. Examples of both homonuclear and heteronuclear experiments are shown in which gradients have been used to eliminate the need for phase cycling.