Efficiency of Gas-Phase Ion Formation in Matrix-Assisted Laser Desorption Ionization with 2,5-Dihydroxybenzoic Acid as Matrix

Abstract

Numbers of matrix-and analyte-derived ions and their sum in matrix-assisted laser desorption ionization (MALDI) of a peptide were measured using 2,5-dihydroxybenzoic acid (DHB) as matrix. As for MALDI with α-cyano-4-hydroxy cinnamic acid as matrix, the sum was independent of the peptide concentration in the solid sample, or was the same as that of pure DHB. This suggested that the matrix ion was the primary ion and that the peptide ion was generated by matrix-to-peptide proton transfer. Experimental ionization efficiencies of 10 -10 −3 for matrices speculated by Hillenkamp and Karas. Number of gas-phase ions generated by MALDI was unaffected by laser wavelength or pulse energy. This suggests that the main role of photo-absorption in MALDI is not in generating ions via a multi-photon process but in ablating materials in a solid sample to the gas phase.

Ion-to-Neutral Ratios and Thermal Proton Transfer in Matrix-Assisted Laser Desorption/Ionization

“…For example, a thermal ionization of electronically excited matrix model [18] and an energy pooling model [3] predicted the ion-to-neutral ratio of the matrix to be 10 . The ion-to-neutral ratios of the matrix predicted from these two models were approximately 10 4 times greater than those derived from numerous experimental measurements [4,9,17,19], The predicted values were only close to the ion-to-neutral ratios of the analyte for certain analytes but not the ion-to-neutral ratios of the matrix.…”

Fluorescence spectroscopy of UV-MALDI matrices and implications of ionization mechanisms

“…Based on the model proposed by Knochenmuss, which involved using a 355 nm laser pulse for molecular excitation, the ion yield of matrices is approximately 10 4 -fold more than the results of many experimental measurements. [15][16][17][18] Knochenmuss's prediction is close to only one experimental measurement, which involved using a 193 nm laser pulse. 19 Molecules must satisfy several properties to undergo S 1 -S 1 annihilation.…”