Resonant laser ablation as a selective metal ion source for gas-phase ion molecule reactions

Thermochemistry, bonding, and reactivity of Ni+ and Ni2+ in the gas phase

In this review, we present a general overview on the studies carried out on Ni(+-)- and Ni(2+)-containing systems in the gas phase since 1996. We have focused our attention in the determination of binding energies in parallel with an analysis of the structure and bonding of the complexes formed by the interaction of Ni(+) with one ligand, or in clusters where this metal ion binds several identical or different ligands. Solvation of Ni(2+) by different ligands is also discussed, together with the theoretical information available of doubly charged Ni-containing species. The final section of this review is devoted to an analysis of the gas-phase uni- and bimolecular reactivity of Ni(+) and Ni(2+) complexes.

Resonant laser ablation of metals detected by atomic emission in a microwave plasma and by inductively coupled plasma mass spectrometry

It has been shown that an increase in sensitivity and selectivity of detection of an analyte can be achieved by tuning the ablation laser wavelength to match that of a resonant gas-phase transition of that analyte. This has been termed resonant laser ablation (RLA). For a pulsed tunable nanosecond laser, the data presented here illustrate the resonant enhancement effect in pure copper and aluminum samples, chromium oxide thin films, and for trace molybdenum in stainless steel samples, and indicate two main characteristics of the RLA phenomenon. The first is that there is an increase in the number of atoms ablated from the surface. The second is that the bandwidth of the wavelength dependence of the ablation is on the order of 1 nm. The effect was found to be virtually identical whether the atoms were detected by use of a microwave-induced plasma with atomic emission detection, by an inductively coupled plasma with mass spectrometric detection, or by observation of the number of laser pulses required to penetrate through thin films. The data indicate that a distinct ablation laser wavelength dependence exists, probably initiated via resonant radiation trapping, and accompanied by collisional broadening. Desorption contributions through radiation trapping are substantiated by changes in crater morphology as a function of wavelength and by the relatively broad linewidth of the ablation laser wavelength scans, compared to gas-phase excitation spectra. Also, other experiments with thin films demonstrate the existence of a distinct laser-material interaction and suggest that a combination of desorption induced by electronic transition (DIET) with resonant radiation trapping could assist in the enhancement of desorption yields. These results were obtained by a detailed inspection of the effect of the wavelength of the ablation laser over a narrow range of energy densities that lie between the threshold of laser-induced desorption of species and the usual analytical ablation regime. Normal ablation employs high-power lasers in an attempt to create a vapor plume without selective vaporization, and with a stoichiometry that accurately represents the stoichiometry of species in the solid sample. RLA, as a method of selective vaporization, appears to provide an opportunity to exploit selective vaporization in new ways.

Silver cationization of thia fatty acids and esters in laser desorption/ionization time-of-flight mass spectrometry

A laser desorption/ionization (LDI) time-of-flight mass spectrometric (TOF-MS) technique was used for the molecular mass analysis of thia fatty acids and esters, samples without appreciable light absorption at the laser wavelength. After a sample overlayer is deposited by solvent evaporation on a thin silver film substrate, it is subjected to 355 or 532 nm Nd : YAG laser irradiation. Photoablation of the Ag film substrate occurs with sufficient laser fluence, producing silver cluster cations, which can react with the desorbed thia fatty acid or ester molecules in the gas phase. Silver cation attachment of thia fatty esters may produce a silver-cationized analyte and fragments of structural diagnostic value, whereas thia fatty acids would not. With oxygen(s) present on the sulfur in sulfoxy fatty acids and esters, a silver-cationized analyte and additional fragments are produced. Formation of these fragments is consistent with charge-remote mechanisms through simple cleavage and rearrangement pathways. The structural reactivity of these compounds with ablated silver cations is hence comprehensively analyzed.