Compelling Evidence for Lucky Survivor and Gas Phase Protonation: The Unified MALDI Analyte Protonation Mechanism

Abstract

This work experimentally verifies and proves the two long since postulated matrix-assisted laser desorption/ionization (MALDI) analyte protonation pathways known as the Lucky Survivor and the gas phase protonation model. Experimental differentiation between the predicted mechanisms becomes possible by the use of deuterated matrix esters as MALDI matrices, which are stable under typical sample preparation conditions and generate deuteronated reagent ions, including the deuterated and deuteronated free matrix acid, only upon laser irradiation in the MALDI process. While the generation of deuteronated analyte ions proves the gas phase protonation model, the detection of protonated analytes by application of deuterated matrix compounds without acidic hydrogens proves the survival of analytes precharged from solution in accordance with the predictions from the Lucky Survivor model. The observed ratio of the two analyte ionization processes depends on the applied experimental parameters as well as the nature of analyte and matrix. Increasing laser fluences and lower matrix proton affinities favor gas phase protonation, whereas more quantitative analyte protonation in solution and intramolecular ion stabilization leads to more Lucky Survivors. The presented results allow for a deeper understanding of the fundamental processes causing analyte ionization in MALDI and may alleviate future efforts for increasing the analyte ion yield.

Negative Ion MALDI Mass Spectrometry of Polyoxometalates (POMs): Mechanism of Singly Charged Anion Formation and Chemical Properties Evaluation

“…On the other hand, POMs are known to precipitate and form crystalline structures with alkali or ammonium counter-ions. For this reason, it seems improbable to us that the adduct species observed in MS spectra are Bpre-formed^in the matrix crystal, as proposed by the BLucky Survivor^model [44]. We thus consider that the formation of the singly charged adduct molecular ions rather occurs by cation exchange in the plume (i.e., during decomposition of the initially formed aggregates).…”

Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry: Mechanistic Studies and Methods for Improving the Structural Identification of Carbohydrates

“…78,79) In this model, the laser energy overcomes ion-pair interactions to free analyte ions from their respective counter ions. It should be emphasized that a single ionization mechanism that is applied universally to elucidate all MALDI observations is highly unlikely owing to di erent photochemical properties of matrices as well as excitation conditions.…”

Influence of the Laser Spot Size, Focal Beam Profile, and Tissue Type on the Lipid Signals Obtained by MALDI-MS Imaging in Oversampling Mode

“…Although the MALDI technique was introduced already about three decades ago [14] and has evolved into an established method for the mass spectrometric analysis of many biomolecular classes (e.g., in the fields of proteomics and lipidomics [15,16]), details of the complex desorption and ionization processes remain in part still unclear [17][18][19][20]. Numerous fundamental studies were conducted to investigate the influence of relevant Binput parameters^on the MALDI ion yields, like the type and physicochemical properties of the MALDI matrix [21], the analyte-to-matrix ratio in the sample [22], or those of the laser irradiation parameters like wavelength [23][24][25], fluence [23,24,[26][27][28][29], and pulse width [30,31].…”