Proton Affinities of Eight Matrices Used for Matrix-assisted Laser Desorption/Ionization

Protonated molecules of analytes in matrix-assisted laser desorption/ionization (MALDI) are frequently the most intense ions observed, especially when the concentration of alkali metal cations is low. Examination of the laser desorption mass spectra of MALDI matrices usually shows the presence of both molecular radical ions M+• and [M + H]+ ions. 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 proton affinities of the common MALDI matrices might be helpful in understanding 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 and isolated in a Fourier transform ion cyclotron resonance mass spectrometer, 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 affinities are so high that some other mechanism for proton transfer is required.