Oxidation of Ferrocene Derivatives in Desorption/Ionization on Porous Silicon

Abstract

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is currently an indispensable method for analyzing biomolecules and synthetic polymers. [1][2][3][4][5][6] While the mechanism of ionization is largely unknown, physical data on the ionization potentials and the gas-phase acidity or basicity have been accumulating, and the roles of the matrix-matrix, matrix-analyte and analyte-analyte reactions in both primary and secondary ionization events have been evaluated in some detail. [7][8][9][10][11][12][13][14][15][16] These studies have suggested the secondary ionmolecule reactions in the plume to be essential to understanding the process of ionization and the mass spectrum. 7,9,11 Some analytes are reduced by these reactions as follows: Copper(II) ions are reduced by laser desorption/ionization (LDI) with or without a chemical matrix. 16Flavin-containing compounds, such as riboflavin, riboflavin 5′-phosphate and flavin-adenine dinucleotide, are reduced in MALDI and fast atom bombardment (FAB) ionization, but not in electrospray ionization, as reported by Itoh et al., who also found that the reduction levels depend on the matrix molecule types. 17 In the case of nitrotyrosine, the NO2 moiety is partially converted into NH2, probably due to reduction after photo-induced dissociation by UV laser irradiation. 18,19 Similarly, the S-nitrosylated cysteine, Cys(SNO), in peptides is completely converted to cysteine, Cys(SH), in MALDI. 20,21 Different mechanisms, such as electron capture and charge exchange with the chemical matrix, have been proposed to account for the reductions in MALDI. 8,9 A line of studies has indicated that free electrons are formed by photoelectric emission from the metal/dielectric-substance interface. [22][23][24][25] The metal sample target is not the photoelectron source, because the work function of metals is greater than the photon energy. 23 The presence of matrix molecules or analytes placed on a metal target enhances electron emission via band bending and the associated reduction in work function. 22,23 Thus, electron transfer from the sample target would be essential for understanding the analyte reduction in LDI.In desorption/ionization on porous silicon, termed DIOS, 26 analytes are deposited on a silicon chip, which is prepared by electrochemical anodization or chemical etching of crystalline silicon, for LDI without the addition of a chemical matrix. The DIOS mass spectra are thus free of cluster ions of the matrix, facilitating identification of the ions of small analyte molecules. DIOS-MS has been applied to various kinds of compounds, such as peptides, natural products, small organic molecules and synthetic polymers. [26][27][28][29][30][31][32][33][34][35][36] The ionization efficiency of DIOS relies on a porous scaffold providing a sufficient surface area to retain analytes, and on the UV-absorptive property, which affords the transfer of laser energy to analytes. 26,27 In contrast to MALDI, chemical reactions in a plume composed of analytes and the sample ma...

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.…”

Ordered Porous Alumina Geometries and Surface Metals for Surface-Assisted Laser Desorption/Ionization of Biomolecules: Possible Mechanistic Implications of Metal Surface Melting

“…Porous silicon is a UV-absorbing semiconductor produced by electrochemical anodization or chemical etching of crystalline silicon providing a large surface area. Although the DIOS mechanism is largely unknown, electron transfer between the silicon and analytes participates in the ionization process as inferred from our studies on analyte reduction and oxidation. − DIOS applications cover a wide range of compounds including peptides, natural products, small organic molecules, and synthetic polymers. ,− DIOS sensitivity for peptides has been improved by pentafluorophenyl derivatization or functionalization of the porous silicon surface . Most DIOS MS applications involve small molecules, less than 3 kDa.…”

Nanomaterial-Assisted Laser Desorption Ionization for Mass Spectrometry-Based Biomedical Analysis

“…Afterwards, the reactants as well as the products could be extracted by the electric field and analyzed in the mass spectrometer. �rimary studies on in-source redox reactions during NALDI include oxidation of ferrocene derivatives and reduction of organic dyes during DIOS [91][92][93]. Recently, we employed TiO 2 nanoparticle modinanoparticle modiparticle modified substrates to perform on-line redox reactions on biomolecules coupled with MS analysis [30][31][32].…”