Ionization detection of neutral 2,5-dihydroxy benzoic acid molecules in the matrix-assisted laser desorption/ionization plume by ultraviolet laser post-ionization: Correlation between internal energy distribution and thermal decomposition rate

Abstract

We probed neutral 2,5-dihydroxy benzoic acid (DHB) crystals in the matrix-assisted laser desorption/ionization (MALDI) plume using ultraviolet (UV) laser post-ionization at 266 nm to investigate the initial MALDI process involving fast phase transitions. Mainly three ion signals, DHB+, [DHB–H2O]+ produced by the dissociative ionization of neutral DHB, and [DHB–CO2]+ formed by pyrolysis in the dense plume, were detected using time-of-flight mass spectrometry. The [DHB–H2O]+/DHB+ ratio was utilized as the fraction of highly excited neutral DHB molecules produced by photoexcitation and survived after phase explosion, and the [DHB–CO2]+/DHB+ ratio indicated the extent of pyrolysis of neutral DHB molecules in the thermally heated condensed phase before phase explosion. The two indexes showed an inverse relationship with respect to the laser delay corresponding to the position in the plume, i.e., the [DHB–H2O]+/DHB+ ratio revealed high values at the tip of the plume, whereas the [DHB–CO2]+/DHB+ ratio increased in the rear part of the plume, reflecting the molecular dynamics of solid-to-gas phase transition involving the production of most MALDI analyte ions.