On the role of defects and surface chemistry for surface-assisted laser desorption ionization from silicon

Abstract

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.

A novel laser desorption/ionization method using through hole porous alumina membranes

“…To date a number of ionization techniques that belong to SALDI have been proposed,1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, ...…”

“…The techniques in the other category utilize planar solid substrates with fine structures on the uppermost surface layer as the target plate, so that the photon energies of the laser can reach analytes which are spread over and adsorbed onto the surface and achieve desorption and ionization of analytes 20, 23, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80. Desorption Ionization on Silicon (DIOS) is the representative technique in this category 46.…”

Laser-Induced Thermal Desorption Facilitates Postsource Decay of Peptide Ions

“…However, previous studies recognized the possible role of laser-induced surface heating, which has provided a reasonable account for energy transfer enabling intact desorption of molecules from nanostructured surfaces [3,10,11]. In the thermal mechanism, the surface nanostructures provide a thermal conduction barrier that tends to trap absorbed laser energy momentarily in the nano-dimension.…”

“…Without the surface porosity of a submicrometer dimension on c-Si, the LDI process was hardly observed to take place. Later on, the surfaces of c-Si tailored with various nanostructures were also demonstrated for their capabilities for efficient LDI, the examples of which include porous Si surfaces formed by various methods [3], Si nanowires [4,5], clathrate-structured Si surfaces [6], Si microcolumn and nanoposts arrays [7,8], as well as commercial sample plates such as NALDI (Bruker-Daltonics, Bremen, Germany) [9]. Accordingly, the utilization of surface nanostructures has been common and in fact considered to be indispensable in the previous developments of high-efficiency LDI surfaces.…”

Surface-assisted laser desorption/ionization-mass spectrometry using TiO2-coated steel targets for the analysis of small molecules

“…For DIOS-MS, porosity or roughness is presumed to be necessary for desorption/ionization [28,13,29,30]. In DIOS-MS, the dissolved analytes are deposited directly on the silicon surface without an additional matrix.…”