The laser-induced desorption/ionization of organic compounds from etched carbon and silicon substrate surfaces was investigated. Two different etching procedures were used. Silicon surfaces were etched either by galvanostatic anodization to produce porous silicon or by a hyperthermal (∼5 eV) F-atom beam to produce nonporous silicon. Atomic force microscopy (AFM) images showed that both etching procedures yielded surfaces with sub-micrometer structures. Highly oriented pyrolytic graphite was etched with hyperthermal O atoms. A 337 nm ultraviolet (UV) laser and a 3.28 μm infrared (IR) laser were used for desorption. Analytes were deposited on the substrates either from the liquid or the gas phase. Mass spectra were obtained provided that three conditions were fulfilled. First, sufficient laser light had to be absorbed. When the IR laser was employed, a thin physisorbed solvent layer was required for sufficient laser light absorption to occur. Though the required fluence of IR and UV light differed by a factor of about 20, the calculated maximum surface temperatures were similar, about 1000 K. The second requirement was that the substrate had a “rough” surface. The third requirement, for the observation of protonated analytes, was that the aqueous pKa-value of the analyte be larger than about 4. These observations support the conclusion that the desorption–ionization mechanisms of analytes from porous and nonporous surfaces are very similar or essentially the same.

The generation of ions from silicon substrates in surface-assisted laser desorption ionization (SALDI) has been studied using silicon substrates prepared and etched by a variety of different methods. The different substrates were compared with respect to their ability to generate peptide mass spectra using standard liquid sample deposition. The desorption/ionization processes were studied using gas-phase analyte deposition. Mass spectra were obtained from compounds with gas-phase basicities above 850 kJmol and with molecular weights up to 370 Da. UV, VIS, and IR lasers were used for desorption. Ionization efficiencies were measured as a function of laser fluence and accumulated laser irradiance dose. Solvent vapors were added to the ion source and shown to result in fundamental laser-induced chemical and physical changes to the substrate surfaces. It is demonstrated that both the chemical properties of the substrate surface and the presence of a highly disordered structure with a high concentration of "dangling bonds" or deep gap states are required for efficient ion generation. In particular, amorphous silicon is shown to be an excellent SALDI substrate with ionization efficiencies as high as 1%, while hydrogen-passivated amorphous silicon is SALDI inactive. Based on the results, a novel model for SALDI ion generation is proposed with the following reaction steps: (1) the adsorption of neutral analyte molecules on the SALDI surface with formation of a hydrogen bond to surface Si-OH groups, (2) the electronic excitation of the substrate to form free electron/hole pairs (their relaxation results in trapped positive charges in near-surface deep gap states, causing an increase in the acidity of the Si-OH groups and proton transfer to the analyte molecules), and (3) the thermally activated dissociation of the analyte ions from the surface via a "loose" transition state.

Surface-assisted, laser desorption ionization (SALDI) time-of-flight mass spectra of proteins and peptides have been obtained from bulk frozen aqueous solutions by adding solid organic powders to the solutions before freezing. Abundant analyte ions were obtained with a 3.28 microgram Nd:YAG/OPO laser. 20 compounds were evaluated as solid additives, and 16 yielded protein mass spectra. Successful solids included compounds like pyrene, aspartic acid, and polystyrene. The best results were obtained with nicotinic acid and indole-2-carboxylic acid, which yielded protein mass spectra anywhere on the sample and with every laser shot. Compared with ultraviolet-matrix-assisted laser desorption ionization on the same instrument, cryo-IR-SALDI had a comparable detection limit (approximately equal to 1 micro M), a lower mass resolution for peptides, and a higher mass resolution for large proteins. Approximately 2500 cryo-IR-SALDI mass spectra were obtained from a single spot on a 0.3-mm-thick frozen sample before the metal surface was reached. About 0.1 nL of frozen solution was desorbed per laser shot. The extent of protein charging varied between the SALDI solids used. With thymine, myoglobin charge states up to MH12(+12) were observed. It is tentatively concluded that observed ions are performed in the frozen sample.

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The technique of frozen matrix-assisted laser ablation coupled with resonance-enhanced multiphoton ionization and reflectron time-of-flight mass spectrometry was used to detect intact organic molecules directly from solutions. When frozen at the temperature of liquid nitrogen, the matrices of interest were ablated by a pulsed C 0 2 single-mode laser. The analyte molecules emerging from the ablated plume were then ionized by a tunable XeCl excimer laser-pumped dye laser and analysed with a gridless reflectron time-of-flight mass spectrometer. The ablation process from an ice matrix was studied with the amino acids tryptophan and tyrosine dissolved in an aqueous ethanol solution to a concentration level of 5 X M. It was found that fragmentation of the analyte molecules is strongly dependent on the ablating laser fluence and that there is a laser fluence range just above the ablation threshold where the decomposition is negligible. The different fragmentation mechanisms are discussed and a cavitation under the liquid surface, causing the sonoluminescence signal from an ice matrix, was shown to be responsible for the decomposition of the analyte molecules ablated by a low photon energy IR laser. Under the appropriate conditions, the analyte molecules were found to have a low rotational temperature of about 150 K resulting from the jet-like cooling in multiple collisions with matrix molecules.