The Matrix Suppression Effect and Ionization Mechanisms in Matrix-assisted Laser Desorption/Ionization

Abstract

At appropriate matrix:analyte mixing ratios, small to moderate sized analyte ions (1000–20 000 u) can fully suppress positively charged matrix ions in matrix-assisted laser desorption/ionization (MALDI) mass spectra. This is true for all matrix species, including radical cations and adducts with protons or alkali-metal ions. Full matrix suppression is also observed, regardless of the preferred analyte ion form, be it protonated or an alkali adduct. These facts lead us to propose a mechanism for prompt, primary (not secondary gas-phase) MALDI ionization in which excited matrix molecules are the key species. At least two such excited molecules are believed to be necessary for free ion generation. This model is found to be consistent with the available data, as well as making several predictions which are confirmed by new observations. The model also predicts that the matrix suppression effect will not be observable with heavy analytes because their large excluded volume precludes desorption at the necessary mixing ratios.