Molecular weight distributions on an external injection matrix-assisted laser desorption/ionization Fourier transform mass spectrometer are subject to time-of-flight distortions as different ion velocities are probed with varied delay times between ionization and trapping (i.e., the trapping time). This phenomenon is used to advantage to reject low-mass matrix ions which would otherwise saturate the trapped ion cell; however, for accurate determination of molecular weight distributions of complex samples like polymeric systems, several mass spectra must be acquired at a series of different trapping times to compensate for this distortion. The spectra acquired should be superimposed (not summed) on the same m/z axis to yield the correct molecular weight distribution as summation of these spectra merely causes further distortions and can cause loss of signal/noise. Distortions due to TOF effects are probed with a calibration compound, poly(ethylene glycol) of peak mass (M p ) ∼1000 Da, as well as the more difficult to ionize polystyrene, which was obtained as a chromatographic molecular weight standard (MW 950). This polystyrene reference material was determined to have a ∼20% error in M p .Matrix-assisted laser desorption and ionization (MALDI) has rapidly become a routine tool for analysis of polymeric systems, 1-3 is currently used extensively for determination of molecular weight distributions, copolymerization structure, and homogeneity, 4 and is beginning to be used for branching studies. 5 The rapid, lowcost analysis provided by MALDI time-of-flight (TOF) instruments has allowed mass accuracies of 0.1% with oligomeric mass resolution, and the advent of delayed extraction has extended this to isotopic resolving power for polymers up to ∼4000 Da with mass accuracies of ∼150 ppm 6-8 and, recently, 10-15 ppm mass accuracy for peptides from 1 to 4 kDa. 9
The Grid-based Virtual Laboratory AMsterdam (VLAM-G), provides a science portal for distributed analysis in applied scientific research. It offers scientists remote experiment control, data management facilities and access to distributed resources by providing cross-institutional integration of information and resources in a familiar environment. The main goal is to provide a unique integration of existing standards and software packages. This paper describes the design and prototype implementation of the VLAM-G platform. In this testbed we applied several recent technologies such as the Globus toolkit, enhanced federated database systems, and visualization and simulation techniques. Several domain specific case studies are described in some detail. Information management will be discussed separately in a forthcoming paper.
In this article we report on surface scattering experiments at graphite as to model the interactions at divertor surfaces in tokamaks and conversion electrodes in negative ion sources. The formation of negative and positive hydrogen ions on a graphite surface during positive hydrogenous ion scattering at a projectile energy of 400 eV is observed. A remarkably high H− and H+ ion fraction, both varying between 10% and 30% of all scattered particles, is found during scattering of positive hydrogenous ions.
Measurements on the formation of O- and C- by scattering a positive-ion beam from a cesiated (0.6 ML) W(110) surface are presented. The results, 66% of O- and 33% of C-, respectively, in the outgoing beam, are interpreted by means of a strong anisotropy of the transition of electrons between the metal conduction band and the 2p-shell. Symmetry considerations induce that only transitions towards 2p-states with m1 = 0 are possible.
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