Ion Intensity and Thermal Proton Transfer in Ultraviolet Matrix-Assisted Laser Desorption/Ionization

Abstract

The ionization mechanism of ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) was investigated by measuring the total cation intensity (not including sodiated and potasiated ions) as a function of analyte concentration (arginine, histidine, and glycine) in a matrix of 2,4,6-trihydroxyacetophenone (THAP). The total ion intensity increased up to 55 times near the laser fluence threshold as the arginine concentration increased from 0% to 1%. The increases were small for histidine, and a minimal increase occurred for glycine. Time-resolved fluorescence intensity was employed to investigate how analytes affected the energy pooling of the matrix. No detectable energy pooling was observed for pure THAP and THAP/analyte mixtures. The results can be described by using a thermal proton transfer model, which suggested that thermally induced proton transfer is crucial in the primary ion generation in UV-MALDI.

Comment on: “Energetics and Kinetics of Thermal Ionization Models of MALDI” by Richard Knochenmuss. J. Am. Soc. Mass Spectrom. 25, 1521–1527 (2014)

“…However, the ionization mechanism remains unclear. Recently we adopted the concept of the polar fluid model [1,2] and developed a thermal proton transfer model to quantitatively describe the generation of primary ions [3][4][5]. In the thermal proton transfer model, matrix molecules absorb photon energy and convert the majority into thermal energy.…”

“…He suggested that the dielectric constant of the matrix fluid must be close to that of the supercritical fluids. In fact, phase equilibrium does not exist during the MALD I desorption process, and supercritical fluid does not exist in a MALDI event [3][4][5]. The so called "liquid phase" in thermal proton transfer model is not the liquid phase in a phase equilibrium.…”

“…In thermal proton transfer model, we assume that proton disproportionation reactions reach equilibrium before desorption [3][4][5]. Knochenmuss cited [17] and [18] as evidence to demonstrate that these reactions does not reach equilibrium.…”

Formation of Metal-Related Ions in Matrix-Assisted Laser Desorption Ionization

“…Many studies on MALDI have discussed the procedures involved in generating these protonated peptides and proteins [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Carbohydrate ions are generated in MALDI as sodiated or potassiated carbohydrates.…”

“…Recently, we proposed a thermal proton transfer model to describe the generation of protonated ions in MALDI [10][11][12][13][14][15]. The absorption of laser light by matrix molecules results in an increased temperature, and the solid matrix melts and behaves in a similar manner to a polar solvent before the occurrence of desorption.…”