Formation of Metal-Related Ions in Matrix-Assisted Laser Desorption Ionization

Abstract

Abstract. In a study of the metal-related ion generation mechanism in matrix-assisted laser desorption ionization (MALDI), crystals of matrix used in MALDI were grown from matrix-and salt-containing solutions. The intensities of metal ion and metal adducts of the matrix ion obtained from unwashed crystals were higher than those from crystals washed with deionized water, indicating that metal ions and metal adducts of the matrix ions are mainly generated from the surface of crystals. The contributions of preformed metal ions and metal adducts of the matrix ions inside the matrix crystals were minor. Metal adducts of the matrix and analyte ion intensities generated from a mixture of dried matrix, salt, and analyte powders were similar to or higher than those generated from the powder of dried droplet crystals, indicating that the contributions of the preformed metal adducts of the matrix and analyte ions were insignificant. Correlation between metal-related ion intensity fluctuation and protonated ion intensity fluctuation was observed, indicating that the generation mechanism of the metal-related ions is similar to that of the protonated ions. Because the thermally induced proton transfer model effectively describes the generation of the protonated ions, we suggest that metal-related ions are mainly generated from the salt dissolution in the matrix melted by the laser.

Toward understanding the ionization mechanism of matrix‐assisted ionization using mass spectrometry experiment and theory

“…The powders were ground into fine particles, and then pressed using a stainless-steel rod to form thin disks (approximately 0.5-1 mm thick) similar to a previous procedure. 34 The second sample, dried droplet, was prepared by dissolving the first sample (dried powder mixture) in solvent (water and ACN 1:1 v/v), and forming the dried-droplet sample by air drying. The third sample was prepared by grinding the second sample into fine particles, and then pressing the powders using a stainless-steel rod to form thin disks (approximately 0.5-1 mm thick).…”

“…A detailed mechanism has not been fully developed for the new ionization processes. 3,20,21 Here, we apply well-established experimental and ab initio quantum chemistry calculations and high-vacuum TOF MS [33][34][35][36] to elucidate the ionization mechanism of 3-NBN and other MAI matrices with the use of laser ionization (LSI) from nearly collision-free vacuum TOF conditions. A commercial mass spectrometer was retrofitted with a Nd:YAG laser to be operated from 266 nm laser wavelength.…”

“…3-NBN samples were measured using a 266 nm frequency laser, and other matrix (2,5-DHB, CHCA, 2,5-DHAP, and 2-NPG) samples were measured using a 355 nm frequency laser [Color figure can be viewed at wileyonlinelibrary.com] Heat of reaction (kcal/mol) of protonation, deprotonation, and proton transfer reactions in the gas phase from ab initio quantum chemistry calculationsMatrix (M) Proton affinity Deprotonation energy Proton transfer reaction M + H + → [M + H] + M → [M − H] − + H + M + M → [M + H] + + [M − H] − 5731) obtained from a high-vacuumTOF mass spectrometer using a 266 nm laser when 3-NBN was used as the matrix.The microcopy photograph of sample I, which is a mixture of dried 3-NBN and insulin powders ground together in the solid state without using any solvent, is shown inFigure 5A. Unlike the dried powder mixture of the common MALDI matrix (2,5-DHB),34 in which insulin ion signals as large as that of the dried droplet prepared samples are observed, no analyte ion signal was observed in the mass spectrum in A1). Previous investigations using small peptides showed that relatively low abundance, multiply charged analyte ions can be observed after more extensive homogenization of the matrix and analyte in the solid state using mechanical force 2,5,20.…”

Clinical Mass Spectrometry in the Bioinformatics Era: A Hitchhiker’s Guide

“…A two-step framework for the mechanism of MALDI has been proposed: the primary ionization step where ions are first formed during the laser pulse and the secondary reactions that follow in the desorption/ablation plume, with the first step occurring in the nanosecond timescale and the secondary reaction in the microsecond timescale [ 64 ]. While multiple models have been proposed for the ion formation in MALDI in recent years, the mechanism itself remains controversial [ [65] , [66] , [67] , [68] ].…”

Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry: Mechanistic Studies and Methods for Improving the Structural Identification of Carbohydrates

“…A reaction model proposed by Lee et al suggests that the increase in thermal energy can e ectively induce the production of alkali ion adducts of carbohydrates. 103) ey suggested alkali metal ions were generated from the dissolution of salts at high-temperature MALDI condition. ose alkali metal ions attached to nearby matrices and analytes during desorption.…”