Quantitative reproducibility of mass spectra in matrix‐assisted laser desorption ionization and unraveling of the mechanism for gas‐phase peptide ion formation

Abstract

In a previous study on matrix-assisted laser desorption ionization (MALDI) of peptides using α-cyano-4-hydroxycinnamic acid (CHCA) as a matrix, we found that the patterns of single-shot spectra obtained under different experimental conditions became similar upon temperature selection. In this paper, we report that absolute ion abundances are also similar in temperature-selected MALDI spectra, even when laser fluence is varied. The result that has been obtained using CHCA and 2,5-dihydroxybenzoic acid as matrices is in disagreement with the hypothesis of laser-induced ionization of matrix as the mechanism for primary ion formation in MALDI. We also report that the total number of ions in such a spectrum is unaffected by the identity, concentration and number of analytes, i.e. it is the same as that in the spectrum of pure matrix. We propose that the generation of gas-phase ions in MALDI can be explained in terms of two thermal reactions, i.e. the autoprotolysis of matrix molecules and the matrix-to-analyte proton transfer, both of which are in quasi-equilibrium in the early matrix plume.

Ionization Mechanism of Matrix-Assisted Laser Desorption/Ionization

“…They suggested that [M + H] + was the primary ion and [A + H] + was formed through matrix-to-analyte thermal proton transfer reactions (32). In another study, however, they (29, p. 1,813) concluded, "it becomes difficult to find out which of the models is mainly responsible for the primary ion formation.…”

“…Ahn et al (32) observed that (a) the total number of ions was independent of the analyte, concentrations of the analyte, and laser fluence and (b) the mass spectral pattern strongly correlated with the temperature in the early plume. They suggested that [M + H] + was the primary ion and [A + H] + was formed through matrix-to-analyte thermal proton transfer reactions (32).…”

Ion Yields in the Coupled Chemical and Physical Dynamics Model of Matrix-Assisted Laser Desorption/Ionization

“…The simplest thermal model is perhaps the polar fluid model (PFM) [2,3], which was recently revived and cast into quantitative form by the Ni group [4]. Similar proposals have been made by the Kim group [5,6]. In the PFM, liquid matrix acts as a solvent to promote autoprotolysis of the matrix itself to form ion pairs.…”

Femtosecond Pumping Rate Dependence of Fragmentation Mechanisms in Matrix-Assisted Laser Desorption Ionization

“…A similar observation has also been made when studying liable biomolecules using nanosecond pulses. 46 Assuming unimolecular reactions, the internal energy becomes equivalent to the concept of an effective temperature. It is established that the effective temperature within the desorption plume can be deduced from the extent of matrix fragmentation.…”