Showing papers by "Nicholas Winograd published in 2003"

Enhancement of Sputtering Yields Due to C60 versus Ga Bombardment of Ag{111} As Explored by Molecular Dynamics Simulations

Abstract: The mechanism of enhanced desorption initiated by 15-keV C60 cluster ion bombardment of a Ag single crystal surface is examined using molecular dynamics computer simulations. The size of the model microcrystallite of 165 000 atoms and the sophistication of the interaction potential function yields data that should be directly comparable with experiment. The C60 model was chosen since this source is now being used in secondary ion mass spectrometry experiments in many laboratories. The results show that a crater is formed on the Ag surface that is ∼10 nm in diameter, a result very similar to that found for Au3 bombardment of Au. The yield of Ag atoms is ∼16 times larger than for corresponding atomic bombardment with 15-keV Ga atoms, and the yield of Ag3 is enhanced by a factor of 35. The essential mechanistic reasons for these differences is that the C60 kinetic energy is deposited closer to the surface, with the deeply penetrating energy propagation occurring via a nondestructive pressure wave. The number...

Proton transfer in time-of-flight secondary ion mass spectrometry studies of frozen-hydrated dipalmitoylphosphatidylcholine.

Abstract: A frozen water matrix, as found in freeze-fractured frozen-hydrated cellular samples, enhances the ionization of phosphatidylcholine lipids with static time-of-flight secondary ion mass spectrometry (TOF-SIMS). Isotopic profiles of the phosphocholine ion from deuterated forms of dipalmitoylphosphatidylcholine (DPPC) have been examined under various sample preparation conditions to show that ionization occurs through protonation from the matrix and is enhanced by the water present in freeze-fractured samples. The ionization of DPPC results in positively charged fragment ions, primarily phosphocholine, with a m/z of 184. Other ions include the M + H ion (m/z 735) and an ion representing the abstraction of the two palmitoyl fatty acid groups (m/z 224). Freeze-fracture techniques have been used to prepare frozen aqueous samples such as liposomes and cells to expose their membranes for static TOF-SIMS imaging. Due to the importance of surface water during SIMS analyses, sources of gas-phase water resulting fro...

Substrate-Assisted Laser-Initiated Ejection of Proteins Embedded in Water Films

Abstract: Molecular dynamics simulations have been employed to investigate laser initiated liftoff of the protein enkephalin embedded in a H2O film adsorbed onto a gold substrate. The laser energy is deposit...

Laser desorption and imaging of proteins from ice via UV femtosecond laser pulses.

Abstract: We have employed 200-fs, 400-nm laser pulses to desorb intact protein molecular ions directly from a frozen aqueous matrix. The resulting spectra obtained using a variety of proteins varying in molecular weight from 1060 (bradykinin) to 5778 Da (insulin) are compatible with those obtained with traditional matrix-assisted laser desorption/ionization experiments. High-quality spectra could be generated using a fluence of 4.0-9.0 J/cm2 to desorb proteins from an aqueous solution frozen onto metal substrates with a sensitivity in the femtomole range. Although the mechanism behind this effect is still not clear, we speculate that it involves explosive boiling of the ice layer due to rapid heating of the substrate. Imaging experiments conducted on the ice layer suggest that the yield of protein is approximately independent of the film thickness and is very reproducible from shot to shot. The results are particularly significant since they open the possibility of examining a range of biomaterials directly from the in vivo aqueous environment.

Molecular dynamic simulations of the sputtering of multilayer organic systems

Abstract: Sputtering of organic overlayers has been modeled using molecular dynamics computer simulations. The investigated systems are composed of benzene molecules condensed into one, two and three layers on an Ag{1 1 1} surface. The formed organic overlayers were bombarded with 4 keV Ar projectiles at normal incidence. The development of the collision cascade in the organic overlayer was investigated. The sputtering yield, mass, internal and kinetic energy distributions of ejected particles have been analyzed as a function of the thickness of the organic layer. The results show that all emission characteristics are sensitive to the variation of layer thickness. Although most of the ejected intact benzene molecules originate from the topmost layer, the emission of particles located initially in second and third layers is significant. The analysis indicates that the metallic substrate plays a dominant role in the ejection of intact organic molecules. 2002 Elsevier Science B.V. All rights reserved.

Applicability of Imaging Time-of Flight Secondary Ion MS to the Characterization of Solid-Phase Synthesized Combinatorial Libraries

Abstract: We employ imaging time-of-flight secondary ion mass spectrometry to perform high-throughput analysis of solid-phase synthesized combinatorial libraries by acquiring mass spectra from arrays of polymer resin particles. To generalize this procedure to various types of resins and their associated chemical linkers, it is necessary to understand the dynamics associated with the analyte molecules during chemical pretreatment steps. Using stearic acid as a model compound, we examine the influence of three classes of linkers-acid or base labile linkers, a thermally labile linker, and a photochemically cleavable linker- all of which are used to anchor one end of the analyte to the polymer resin. With data obtained using secondary ion mass spectrometry, scanning electron microscopy, and X-ray photoelectron spectroscopy, we conclude that an effective treatment of the resin needs to include cleaving the linker and extracting the unbound analyte to the resin surface. We also demonstrate that the hydrophilicity of the polymeric constituents of a resin particle affects the experiments by changing the location of the analyte molecules during resin treatment. With this information, it is possible to utilize imaging TOF-SIMS to assay a range of material supports with assurance that high-quality spectra can be acquired.