Enhanced MALDI ionization efficiency at the metal-matrix interface: Practical and mechanistic consequences of sample thickness and preparation method

Abstract

Electrosprayed spots of varying thickness were evaluated for use as reproducible, homogenous, high efficiency MALDI samples. Thin samples on stainless steel plates were found to give exceptionally strong signals, as did the last layers of thick samples, when ablated down to the steel substrate. A small enhancement was also observed for thin samples on a gold substrate, and with a few-nanometer gold coating on top of a thick sample. Ion yields and intensity ratios can be understood in the context of the previously described quantitative MALDI model including the matrix-metal interfacial ionization potential reduction effect (Knochenmuss, R.; Anal. Chem. 2004, 76, 3179 -3184). The absolute and relative stabilities of ion signals were found to be at least a factor of two better for the thin electrosprayed spots, , both the underlying ionization mechanisms and sample preparation methods remain objects of active study. These are coupled topics since mechanistic understanding can allow rational, rather than undirected, method development. Here, we make use of a recently developed quantitative model for UV-MALDI based on a two-step ionization process [2,3]. Primary laser-created matrix ions and analyte neutrals undergo ion-molecule reactions in the expanding ablation plume to yield secondary analyte ions. The observed mass spectrum is determined by the thermodynamics of these reactions. Because matrix and analyte are explicitly coupled, the model proved particularly suitable for understanding of practically relevant phenomena such as ion suppression effects [4 -6], which have been shown to be very common phenomena [7].A variant of this model has recently been developed for thin samples on metal substrates [8]. This was motivated by measurements of electron emission yields from MALDI samples on metallic versus insulating substrates [9], and suggestions that such electrons might neutralize (positive) analyte ions during the desorption process [10]. The model gave excellent agreement with the electron yield data, but this was not consistent with claims of higher analyte ion yields for samples on nonconductive versus conductive substrates [9]. The major difference between thick and thin samples is the interaction of matrix molecules with the substrate, which can lower the ionization potential of the combined system if the substrate is metal. It was, therefore, theoretically expected that thin samples on metal would yield higher, not lower, signals. The present study was undertaken to investigate this discrepancy and restore consistency to sample preparation strategy.The method selected here to prepare homogenous samples of variable thickness is electrospray (ES) deposition of analyte/matrix solution [11,12]. Electrospray produces more uniform, fine-grained sample spots than the more convenient and widespread dried droplet technique, which tends to form large matrix crystals in random locations. By varying the electrospray time, it is also possible to make samples as thin as a few hundred nanometers, but a few mm in diamete...

Comparison between vacuum sublimed matrices and conventional dried droplet preparation in MALDI-TOF mass spectrometry

“…The detected higher initial velocities of analytes in sublimed preparations probably lower the internal ion energy with the beneficial effect of higher stability of the desorbed analytes compared with standard DD preparations, which can be an additional reason for the detected increase in analyte intensity. Increased analyte ion yields from very thin electrosprayed matrix samples have been reported [31]. According to a model from Knochenmuss et al, this was explained by an interaction between matrix molecule and metal substrate electronic states facilitating a two-photon ionization process instead of three photons as needed for the bulk matrix [32].…”

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

“…A lack of counterions inhibits quantitative charge neutralization and leads to protonated analytes. Provided that these ions are not neutralized by absorption of photoelectrons or electrons from the metallic target [18,55,56], they can be detected as so-called Lucky Survivors.…”

Ion Yields for Some Salts in MALDI: Mechanism for the Gas-Phase Ion Formation from Preformed Ions

“…In an early version of the lucky survivor model [9], electrons generated in the MALDI process, especially the photoelectrons from the metal or the metal-matrix interface [22][23][24], were proposed to contribute to the neutralization of positive analyte ions. In this regard, we measured IYs for two samples loaded on an anchor target coated with a hydrophobic organic layer, one on the bare metal surface and the other on the organic layer.…”