Internal Energy of Ions Generated by Matrix-Assisted Laser Desorption/Ionization

To provide an objective measure of the correlation between the internal energy content of ions generated by matrix-assisted laser desorption/ionization (MALDI) and the matrix properties, a series of well-characterized benzyl-substituted benzylpyridinium salts were used as thermometer molecules (TMs). To determine the internal energy variations of analyte ions, the survival yields of TM molecular ions were measured in three different matrixes, α-cyano-4-hydroxycinnamic acid (CHCA), 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid, SA), and 2,5-dihydroxybenzoic acid (DHB). Statistical analysis of extensive survival yield data indicated that there were discernible differences among the studied matrixes. The experimental survival yields of the TM ions were used to calculate the unimolecular decomposition rate coefficient. Corresponding theoretical reaction rate coefficients were calculated based on the Rice−Ramsperger−Kassel−Marcus (RRKM) theory for different internal energies of the TMs. The internal energies of the ions were obtained by projecting the experimental rate coefficient values onto the theoretical curves obtained by the RRKM calculations. Molecular ions of the analytes showed decreasing survival yields and consequently increasing internal energies in the three matrixes in the following order:  CHCA, SA, and DHB with “cold”, “intermediate”, and “hot” characteristics, respectively. Qualitatively, this could be interpreted as a significant departure from earlier observations suggesting an opposite trend. The classification as hot and cold matrixes should be further qualified by accounting for the influence of laser pulse energy and the nature of the analyte. Higher laser pulse energy led to an elevated level of energy transferred to the analyte, which in turn resulted in a diminished survival yield of the analyte molecular ion. It is quite possible that the assignment of hot and cold reverses as the analyte or the laser energy changes. These findings can help predict the outcome of postsource decay experiments and clarify the concept of hot and cold matrixes in MALDI mass spectrometry.