| Abstract: | Matrix-assisted laser desorption ionization (MALDI) facilitates the formation of intact gas-phase ions of compounds (e.g., high molecular weight species such as proteins and polymers) that cannot be ionized using conventional ionization techniques (e.g., EI, CI, FAB). Desorption and ionization is achieved using a "matrix", usually an aromatic organic compound that absorbs at the wavelength of the incident laser. A better understanding of the role of the matrix in analyte ion formation in MALDI provides insight into the experimental parameters necessary to produce optimum analyte (M+H) $ sp{+}$ ion yield for a given analyte. In MALDI, analyte (M+H) $ sp{+}$ ions are formed by a proton transfer reaction between the matrix and analyte. Thus, the best framework to describe the formation of positive ions is in terms of an acid-base reaction where the matrix acts as the proton donor and the analyte acts as the proton acceptor. It is suggested that during the ion formation process in MALDI, UV photon absorption produces excited state "acidic" matrix molecules or ions (designated (matrix)*), that undergo proton transfer to a "basic" analyte to form (M+H) $ sp{+}$ ions. Experimental results from studies designed to probe the effects of matrix structure (and acidity) on ion formation are consistent with a mechanism of excited state proton transfer. Heavy atom substitution (Br, I, F, Cl) of aromatic organic molecules is known to alter excited state relaxation dynamics by promoting intersystem crossing to form triplet species. Experimental results show that when heavy atom matrices are used there is a decrease in matrix luminescence and a corresponding increase in analyte (M+H) $ sp{+}$ ion yield. The fact that perturbing the excited state chemistry of a matrix has a direct effect on the analyte (M+H) $ sp{+}$ ion yield is also consistent with the involvement of excited state processes in MALDI ion formation. |
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