THE MATRIX SUPPRESSION EFFECT AND IONIZATION MECHANISMS IN MATRIX-ASSISTED LASER DESORPTION IONIZATION/

At appropriate matrix:analyte mixing ratios, small to moderate sized a nalyte ions (1000-20 000 u) can fully suppress positively charged matr ix ions in matrix-assisted laser desorption/ionization (MALDI) mass sp ectra, 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 mech anism for prompt, primary (not secondary gas-phase) MALDI ionization i n which excited matrix molecules are the key species, At least two suc h excited molecules are believed to be necessary for free ion generati on, This model is found to be consistent with the available data, as w ell as making several predictions which are confirmed by new observati ons, The model also predicts that the matrix suppression effect will n ot be observable with heavy analytes because their large excluded volu me precludes desorption at the necessary mixing ratios.