Direct laser vaporization method

A similar regularity in the mass distribution of the various Zr-substituted met-cars was observed for different Ti/Zr molar ratios and was used as an argument in favor of a pentagonal dodecahedron structure in which all metal sites are equivalent. In subsequent studies, binary metal metallocarbohedrenes of titanium and other metals Ti cM Ci2 (x -I- y = 8) have been obtained either by the standard method using pure metal powders of Ti and M as a target for laser vaporization and a mixture of helium with 10% methane as carrier gas (M = Nb, Ta, Y, Si), or by direct laser vaporization of a mixture of titanium carbide and pure M metal (M = Y, Nb, Mo, Ta, W). ... 

There are several methods in use for producing these clusters. Particle bombardment or laser vaporization of a graphite surface leads to direct formation of ions that can be detected by mass spectrometry. These are normally of relatively small size (n<30). By laser vaporization of graphite into a molecular beam neutral... 

The central idea in the laser-based method for detection of small quantities of H in restricted areas is that an intense laser strike can form a pothole in a metal. The metal of the pothole and the H it contains are vaporized. Both the H and some of the metallic atoms are withdrawn by a vacuum that pulls gaseous constituents out of the system. On the way, however, these constituents are made to fly through the space between the electrodes of a quadrupole mass spectrometer set to measure mass 2. Knowing the H2 produced from one laser strike, and the dimensions of the pothole, the H concentration in the metal—and within any restricted area to which the laser can be directed—can be measured. 

Metallofullerenes can be synthesized typically in two ways similar to the s)mthesis of empty fullerenes, which involves the generation of a carbon-rich vapor or plasma in He or Ar gas atmosphere. The two methods have been routinely used to date for preparing macroscopic amounts of metallofullerenes the high-temperature laser vaporization or "laser-furnace method (Chai et al., 1991 Haufler et al., 1991 Ying et al., 1994) and the standard direct current (DC) arc discharge method (Haufler et al., 1990). Both methods simultaneously generate a mixture of hollow fullerenes (Ceo, C70, C76, C78, Cs4,...) together with metallofullerenes. The production of metallofullerenes can be followed by procedures to extract from soot and to separate/purify the metallofullerenes from the hollow fullerenes (see Sections 2.2, 3.1, and 3.2). 

Last years considerable efforts have been directed to preparation of metal nanoparticles having a desired diameter and shape. A number of production techniques has been reported such as wet chemical processes, (co-precipitation, complexation, sol-gel), physical vapor deposition, sputtering, and laser ablation methods [1]. The ultimate goal of each technique is fabrication of monodisperse stmctures with a predetermined size, shape and arrangement. 

Several techniques have been used to investigate the reactivity of the metal carbide cluster ions formed in a laser vaporization source. The earliest investigations performed by Castleman s group relied on a preliminary mass selection of the desired cluster. The ion beam was then injected into a drift tube where the selected cluster encounters the reactant mixed with helium as a buffer gas. The FTICR (Fourier-transform ion cyclotron resonance) mass spectrometer studies reported by Byun, Freiser and co-workers basically rely on the same principle even though the total pressure of the reaction chamber is 10 torr, compared with 0.7 torr in Castleman s experiments. A new method of forming met-car ligand complexes was then reported by Castleman et al. this involved the direct interaction of the vaporized metal with mixtures of methane and selected reactant gases. ... 

SWNT were produced by laser ablation and subsequently purified via acid treatment. Single-wall nanotubes were manufactured by laser vaporization of carbon rods doped with Co, Ni and FeS in an atmosphere of Ar H2. Standard SWNT products made by HiPCO and other methods contain significant amount of sooth, graphite flakes, and remnants of the catalyst, which need to be removed prior to the assembly. The quality of the dispersion directly affects the mechanical performance of the resulting composite. 

The previous discussion has centered on how to obtain as much molecular mass and chemical structure information as possible from a given sample. However, there are many uses of mass spectrometry where precise isotope ratios are needed and total molecular mass information is unimportant. For accurate measurement of isotope ratio, the sample can be vaporized and then directed into a plasma torch. The sample can be a gas or a solution that is vaporized to form an aerosol, or it can be a solid that is vaporized to an aerosol by laser ablation. Whatever method is used to vaporize the sample, it is then swept into the flame of a plasma torch. Operating at temperatures of about 5000 K and containing large numbers of gas ions and electrons, the plasma completely fragments all substances into ionized atoms within a few milliseconds. The ionized atoms are then passed into a mass analyzer for measurement of their atomic mass and abundance of isotopes. Even intractable substances such as glass, ceramics, rock, and bone can be examined directly by this technique. 

The above direct process does not produce a high yield of ions, but it does form many molecules in the vapor phase. The yield of ions can be greatly increased by applying a second ionization method (e.g., electarn ionization) to the vaporized molecules. Therefore, laser desorption is often used in conjunction with a second ionization step, such as electron ionization, chemical ionization, or even a second laser ionization pulse. 

MALDI is a method that allows for vaporization and ionization of non-volatile biological molecules from a solid-state or liquid phase directly into the gas phase. The presence of a solid or liquid matrix (around 1 part in 100-50,000), which strongly absorbs the laser radiation, spares the smdied species from degradation, by protecting them from the harsh effects of the laser. It results in the detection of intact molecules... 

Production of Ions. Several methods are used (11 by bombardment with electrons from a heated filament (2 by application of a strong electrostatic field (field ionization, field desorption) Ot by reaction with an ionized reagent gas (chemical ionization) (4 by direct emission of ions from a solid sample that is deposited on a heated filament (surface ionization) (SI by vaporization from a crucible and subsequent electron bombardment (e.g.. Knudsen cell for high-lcmperalure sludies id solids and (6) by radio-frequency spark bomhardmenl of sample fur parts-per-biliion (ppb) elemental analysis of solids as encountered in metallurgical, semiconductor, ceramics, and geological studies. Ions also are produced by photoion izution and laser ionizalion. 

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