New insights into mechanisms of material ejection in MALDI mass spectrometry for a wide range of spot sizes

Marcel Niehaus,Jens Soltwisch

doi:10.1038/s41598-018-25946-z

Marcel Niehaus, Jens Soltwisch

Open Access

https://doi.org/10.1038/s41598-018-25946-z

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Journal: Scientific Reports	Publication Date: May 17, 2018
Citations: 32	License type: open-access

Affiliation: University of Münster

Abstract

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is widely used for the analysis of large biomolecules in numerous applications. The technique utilizes nanosecond-long laser pulses at various spot sizes to eject and ionize large molecules embedded in a highly absorptive chemical matrix. Despite the methods name, ‘molecular desorption’ from the matrix crystal surface is not the sole mechanism discussed for material ejection in MALDI, but additional ablation of larger clusters has been reported. Here we present results on the influence of laser fluence and spot size on the mechanisms of the initial material ejection in MALDI and subsequent plume development. We used a laser-based postionization (MALDI-2) as well as a complementary photoacoustic method to monitor the material ejection step. The photoacoustic data reveal a quasi-thermal sublimation process up to a transition fluence. Above this threshold fluence additional ablation processes are observed. Complementary investigations on plume dynamics by MALDI-2 showed an ejection of predominantly fast particles for desorption conditions while ablation produces considerably slower ejecta. Additionally the presented results revealed a peculiar influence of the spot size on analyte fragmentation as well as plume development and allows for new insights into the unexplained spot size effect reported for MALDI.

Highlights

Matrix-assisted laser desorption/ionization (MALDI) is a major technique for the mass spectrometric (MS) analysis of large, thermally labile biomolecules such as peptides/proteins, phospho- and glycolipids, or oligosaccharides[1]
On the one hand the normalized photoacoustic analysis (PA) data shows that in general the amount of material ejection from a certain area as well as the underlying mechanism is only dependent on fluence and largely independent of the applied spot size
On the other hand postionization data suggests that the optimal fluence region to produce molecules accessible to MALDI-2 drastically changes with spot size

Summary

Introduction

Matrix-assisted laser desorption/ionization (MALDI) is a major technique for the mass spectrometric (MS) analysis of large, thermally labile biomolecules such as peptides/proteins, phospho- and glycolipids, or oligosaccharides[1]. Over the course of 20 some years a multitude of fundamental studies has deepened our understanding of these processes It is generally agreed and experimentally well verified that the sensitivity of a MALDI-MS analysis is critically determined by the physicochemical properties of the analyte/matrix system (such as optical absorptivity, proton affinities, and molar analyte-matrix ratios), on the one hand, and by a proper choice of irradiation parameters (such as laser wavelength, pulse duration, fluence, and focal spot size), on the other[8,9,10,11,12,13,14,15]. Utilizing laser postionization to probe neutrally desorbed molecules, Dreisewerd et al observed a similar steep increase in laser fluence needed for material ejection, when focal spot sizes are decreased to the low ten to sub ten-micrometre diameter range[8]

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