Improved Reproducibility and Increased Signal Intensity in Matrix-assisted Laser Desorption/Ionization as a Result of Electrospray Sample Preparation

The properties of several cinnamic acid compounds used as matrices for matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) were investigated as standard dried droplet (DD) and vacuum sublimed preparations. The differences between both preparation methods were analyzed with regard to matrix grain size, internal ion energy, initial velocity, analyte intensity, and analyte incorporation depth. Some of the used cinnamic acid derivatives exhibit clearly reduced grain sizes as sublimed preparations compared with standard DD approaches. In these cases higher effective temperatures could be measured accompanied by increased analyte intensities, which can be explained by stronger volatilization processes caused by a hindered heat dissipation resulting in a raised analyte transfer into the gas phase. For all sublimed compounds, a strong increase of the initial ion velocity compared with DD preparations could be measured. Higher initial ion velocities correlate with a decrease in internal ion energy which might be attributed to the very uniform crystal morphology exhibited by sublimed compounds. For sublimed matrices without reduced grain size, at least slightly higher analyte intensities could be detected at raised laser fluences. Analyte accumulation in the uppermost matrix layers or the detected higher ion stability can be explanations for these results. Ever increasing numbers of samples call for automation in high-throughput workflows. One of the key areas for progress in automated MALDI analyses consists of improved sample preparation techniques. Numerous efforts have been conducted for improving the preparation step, e.g., the generation of thin uniform matrix layers with high shot-to-shot reproducibility using the aerospray technique by combination of matrix and nitrocellulose [4] or for analysis of synthetic polymers [5]. Also, electrospray deposition and vacuum deposition were reported to yield homogeneous sample preparations with increased analyte intensities [6 -9]. Matrix deposition by sublimation for MALDI imaging has been reported to yield more intense signals with less alkali adducts compared with the electrospray technique [10]. Additionally, sublimed matrix spots deposited on extremely hydrophobic surfaces described in [11][12][13][14] offer a variety of beneficial properties compared with standard dried droplet (DD) preparations: The main advantage of sublimed matrix spots compared with standard DD preparations are higher analyte sensitivities [14 -17]. This is likely related to the formation of considerably smaller crystals and narrow size distribution of thin-film sublimed matrices, such as ␣-cyano-4-hydroxycinnamic acid (CHCA), compared with DD preparations with comparatively large particle sizes and broader size distribution [15]. As a consequence of this, the specific matrix surface area as well as the amount of potential binding places increases compared with standard DD precipitates, which will result in increased analyte binding properties. Additionally, th...

Section: Sample Preparation and Measurementmentioning

confidence: 99%

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

Jaskolla

Karas

Roth

et al. 2009

“…The homogeneity over a large surface area of the sample is a major concern regarding reproducibility in MALDI [23]. To prevent unwanted effects caused by inconsistencies of the target preparation, all MALDI targets were routinely prepared in the following way.…”

Section: Target Preparationmentioning

confidence: 99%

Alkali cation attachment to derivatized fullerenes studied by matrix-assisted laser desorption/ionization

Fati

Leeman

Vasil’ev

et al. 2002

The complexation of alkali metal ions with amphiphilic fullerene derivatives has been investigated by matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry. The formation of analyte ions occurs via two competing mechanisms including electron transfer from matrix-derived ions and metal ion attachment. The interplay of these processes has been examined by laser fluence dependent sample activation and by variation of the target composition. The attachment of metal ions has been established as the gentler and thus more efficient route towards the formation of intact analyte ions. Investigations into the metal ion complexation have been conducted to reveal the reactivity order of the alkali metals in these reactions and to elucidate the influence of structural differences of the analytes, as well as to unravel effects caused by the anionic counter ion of the metal. The experimental data have been derived by two complementary approaches. Competing reactants were either studied simultaneously, so that the product distribution would provide direct insight into the reactivity pattern, and/or product distributions were obtained in a large variety of separate experiments and normalized for reliable comparison. It has been found that the extent to which complexation is observed follows the charge density order of the alkali metal ions. The structural features of the fullerene-attached ligands were of profound influence on the attachment of the metal ion, inducing enhanced selectivity for the complexation with less reactive metals. The metal ion attachment is reduced with the use of smaller anionic counter ions. Rationalization of these findings is provided within the framework of the mechanisms of ion formation in

“…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.…”

mentioning

confidence: 99%

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

McCombie

Knochenmuss

2006

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...