Ion-to-Neutral Ratios and Thermal Proton Transfer in Matrix-Assisted Laser Desorption/Ionization

Abstract

Abstract. The ion-to-neutral ratios of four commonly used solid matrices, α-cyano-4-hydroxycinnamic acid (CHCA), 2,5-dihydroxybenzoic acid (2,5-DHB), sinapinic acid (SA), and ferulic acid (FA) in matrix-assisted laser desorption/ionization (MALDI) at 355 nm are reported. Ions are measured using a time-of-flight mass spectrometer combined with a time-sliced ion imaging detector. Neutrals are measured using a rotatable quadrupole mass spectrometer. The ion-to-neutral ratios of CHCA are three orders of magnitude larger than those of the other matrices at the same laser fluence. The ion-to-neutral ratios predicted using the thermal proton transfer model are similar to the experimental measurements, indicating that thermal proton transfer reactions play a major role in generating ions in ultraviolet-MALDI.

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

“…The values of the dipole moment and the static polarizability used in these equations were calculated using an ab initio method. The Kirkwood-Fröhlich equation was replaced by the equation reported by Caillol et al [6,7] in our recent study [8,9], but the results were similar. The ion loss caused by ion-ion recombination after desorption was determined to be low; therefore, the ionto-neutral ratio in liquid is similar to that after desorption.…”

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

“…[1].^The low ion yields from Knochenmuss' calculations arise for two reasons. (1) He calculated the ion yield using a temperature of 450 K. As we showed in previous studies [5,6], the temperature of MALDI before desorption is much higher. Our calculated temperature is consistent with experiment.…”

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.…”