Detailed Characterization of the Post-Ionization Efficiencies in MALDI-2 as a Function of Relevant Input Parameters

Potthoff A, Dreisewerd K, Soltwisch J

Abstract

A recently introduced technique based on MALDI with laser-induced post-ionization (PI), also named MALDI-2, increases the ion yields for numerous classes of lipids, metabolites, and carbohydrates in MALDI-MS imaging experiments under certain experimental conditions. Here we used a semi-automatic LabVIEW-based protocol to investigate and optimize the efficiency of the PI process in dependence of four relevant input parameters and a dense parameter grid: pulse energies of the two lasers, delay between the laser pulses, and buffer gas pressure in the ion source. All experiments were conducted with a modified MALDI-2 Synapt G2-S mass spectrometer (Waters) and use of a focal spot size on the sample of 15-17 µm. A wavelength-tunable optical parametric oscillator (OPO) laser served for PI at 260 nm or 280 nm. The investigated MALDI matrices were: 2,5-dihydroxybenzoic acid (pos. ion mode, +), 2,5-dihydroxyacetophenone (+), α-cyano-4-hydroxycinnamic acid (+), norharmane (neg. ion mode, -), and 1,5-diaminonapthalene (-). A porcine brain extract served as lipid standard. In the positive ion mode, a maximum boost for the generated [M + H]+ species was found with a N2 buffer gas pressure of ~2 mbar and a delay between the laser emissions of ~10 μs. Higher optimal delay settings of several 10 µs were registered for the two studied matrices in negative ion mode. With regard to the laser fluences, best PI efficiencies were reached using maximum available ablation and PI laser pulse energies of up to 25 µJ and 160 µJ, respectively. For analytes not profiting from MALDI-2, best ion signal yields were recorded for ablation laser pulse energies of around 7 µJ, depending on the MALDI matrix. At higher laser pulse energies, sizeable fragmentation is observed for these ions. The PI laser pulse energy did not have any influence on the ion signals of these species. For optimal ion yield of all analyte species, best results were obtained with an ablation laser pulse energy of ~7 μJ and a PI laser pulse energy of ~160 μJ. Our comprehensive data set provides valuable insight into the mechanisms underlying the MALDI-2 processes and could help to further optimize this emerging technique.