Solid-Phase Thermodynamic Interpretation of Ion Desorption in Matrix-Assisted Laser Desorption/Ionization

Abstract

This work demonstrates a quantitative interpretation of ion desorption in matrix-assisted laser desorption/ionization (MALDI). The theoretical modeling incorporates transition state theory for the desorption of surface ions, assuming chemical and thermal equilibrium in the solid state prior to desorption. It is distinct from conventional models that assume chemical equilibrium in the gas phase. This solid-state thermodynamic interpretation was used to examine the desorption of pure 2,4,6-trihydroxyacetophenone (THAP) and of angiotensin I mixed with THAP. It successfully described the changes in ion yield with the effective temperature under various laser fluence and initial temperature conditions. The analysis also revealed the key role played by ion concentration in the modeling to provide the best fit of the model to observations. On the other hand, divergence of the ion beam with laser fluence was examined using an imaging detection method, and the signal saturation normally seen at high fluence was appropriately reduced by ion focusing. Simplified but deceptive theoretical interpretations were obtained when the analysis was conducted without adequate calibration of the instrument bias.

Target Plate Material Influence on Fullerene-C60 Laser Desorption/Ionization Efficiency

“…The laser partly destroys the (at 337 nm slightly absorbing [39]) indium tin oxide layer at the highest applied laser fluence (dark ellipsoid spot). Fullerene-C 60 LDI experiments generally resulted in mass spectra as indicated in Figure 3d, which were used for relative ion signal intensity determination for the indicated m/z ranges. The spectral patterns did not show any significant m/z shifts when the extraction delay time was varied; only the resolution was somewhat affected.…”

“…In order to verify our results, a dual-TOF setup could prove beneficial. However, the target plate should be grounded and the detectors should have equal but oppositely charged potential differences [59,60]. Finally, working with fullerene-C 60 has the advantage that proton transfer reactions are absent.…”

“…All reactions could occur in principle, but species formation, ion directionality, and detection depend on the polarity, extraction delay time and potential, laser fluence, and the intrinsic unimolecular and bimolecular reactions. Small solid arrows indicate ionization reactions, whereas dashed arrows indicate neutralization reactions reduction, a more linear relationship is expected than when a trend had resulted from the lowering of the substrate material's work function, which in turn would be expected to affect the anion signal intensity due to the attachment of additionally produced photoelectrons to fullerene-C 60 . The presence of additional photoelectrons from the target plate would, based on the high electron attachment cross sections reported for fullerene-C 60 , have resulted in an overall higher anion yield.…”

Coupled Space- and Velocity-Focusing in Time-of-Flight Mass Spectrometry—a Comprehensive Theoretical Investigation

“…Assuming the ions have an initial temperature of 1700 K [12][13][14][15][16][17], the ion population with a certain average velocity in one dimensional space can be described by the Maxwell-Boltzmann distribution [16][17][18][19][20][21] as follows:…”

“…Table 1 shows the calculation results obtained for protonated 2,5-dihydroxybenzoic acid (DHB) with a m/z of 155 and a temperature of 1700 K. To mimic the conventional space-focusing condition, Equation 1 is used to obtain an s 0 of 13.5 mm. Notably, the selected temperature is only a rough estimate and must be adjusted according to the experimental conditions [12][13][14][15][16][17].…”