Desorption dynamics of neutral molecules in matrix-assisted laser desorption/ionization

Abstract

This work studies the expansion dynamics of molecules ejected from sample crystals in matrix-assisted laser desorption/ionization (MALDI) reaction. Measurements were made in two steps -sample desorption using a 355 nm pulsed laser beam and gas-phase postionization using a 266 nm laser at various delays. The estimated ion production yield is approximately 10 4 times better than that obtained using conventional MALDI methods. A model system of co-crystallized tryptophan (Trp) analyte and 2,4,6-trihydroxyacetophenone (THAP) matrix yielded overlapping time-of-arrival profiles. This result suggests that translational behaviours of gas-phase Trp and THAP are thermalized during expansion. The observed plume velocity depends strongly on the matrix and analyte composition: the most probable velocity is 214 m/s for pure Trp and 450 m/s for pure THAP. The incorporation of Trp into THAP makes both their velocities fall between the two extreme cases. Multiple half-space Maxwellian curves adequately describe the observed desorption velocity profile, implying that MALDI consists of multiple desorption components.

Desorption Dynamics, Internal Energies, and Imaging of Organic Molecules from Surfaces with Laser Desorption and Vacuum Ultraviolet (VUV) Photoionization

“…Tsai et al demonstrated that in their experiment the velocity distributions of tryptophan and 2,4,6-trihydroxyacetophenone (THAP) cannot be fitted by a single Maxwell-Boltzmann distribution. [39] Instead, they used three Maxwell-Boltzmann distributions to fit the experimental curve. The most probable velocities for these distributions were 61, 154, and 287 m s À1 for tryptophan and 127, 258, and 503 m s À1 for THAP.…”

Plume Expansion Dynamics of Matrix‐Assisted Laser Desorption Ionization

“…Keywords: desorption · ionization · laser chemistry · mass spectrometry · photochemistry Studies of neutral molecules are limited by complications in detection. The detection of desorbed neutral molecules in a defined volume above the sample surface by techniques like post-ionization, [17][18][19] gated laser-induced fluorescence (LIF), [20,21] or absorption spectroscopy [21] have been applied for the velocity distribution measurements. Compared to the ion velocity, lower mean velocity values have generally been found for the neutrals.…”

“…On the other hand, the velocity distribution obtained from 355 nm desorption of THAP and 266 nm multiphoton ionization at a position 4.5 mm above the surface shows a different shape. [19] A single half-space Maxwellian velocity distribution does not fit the velocity profile.…”

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

“…Therefore, the optimal s 1 value for heavy ions is inapplicable to light ions because, as reported in the literature, the desorbed materials from the MALDI reaction have roughly equal values for υ [17,22,23]. In such a case, ions that have different masses but are traveling at the same initial velocity result in different E 0 (i.e., higher mass ions exhibit higher E 0 ).…”