PROTON AFFINITIES OF 8 MATRICES USED FOR MATRIX-ASSISTED LASER DESORPTION IONIZATION/

Protonated molecules of analytes in matrix-assisted laser desorption/i onization (MALDI) are frequently the most intense ions observed, espec ially when the concentration of alkali metal cations is low. Examinati on of the laser desorption mass spectra of MALDI matrices usually show s the presence of both molecular radical ions M(+.) and [M + H](+) ion s. With some matrices, the intensity of the [M + H](+) ion is greater than that of the molecular radical ion, e.g. with 2,5-dihydroxybenzoic acid. A logical source for the ions of protonated analyte in MALDI is proton donation from the [M + H](+) ions of the matrix, but donation could also occur from the radical molecular ions. A knowledge of the p roton affinities of the common MALDI matrices might be helpful in unde rstanding why some matrices are 'hotter' than others and lead to more post-source as well as prompt decay, The ground-state proton affinity of eight common MALDI matrices were determined, For each matrix, the [ M + H](+) ion was generated by methane chemical ionization, trapped an d isolated in a Fourier transform ion cyclotron resonance mass spectro meter, allowed to cool for 5 s and reacted with reference compounds of known proton affinities, In some cases, the matrix proton affinities are low enough that proton transfer can occur from the ground state [M + H](+) ion to MALDI analytes; in other cases, the matrix proton affi nities are so high that some other mechanism for proton transfer is re quired. (C) 1997 by John Wiley & Sons, Ltd.