Irradiation effects in MALDI, ablation, ion production, and surface modifications. Part II: 2,5-dihydroxybenzoic acid monocrystals

Abstract

Irradiation effects at low and high laser fluence on 2,5-dihydroxybenzoic acid large crystals were investigated. Contrary to what was observed for matrices as cinnamic acid derivatives, no chemical degradation of matrix is evidenced and continuous ablation as well as ion production resulted of extended irradiation in all the fluence range corresponding to classical matrix-assisted laser desorption /ionization. Ripples are formed on the base of the crater for a limited number of laser shots under moderate fluence. For extended irradiation, conical shape craters are formed with the axis of the crater oriented along the incident direction of the laser beam. A study of the craters showed that ablation through the ablated volume slowly varied with the laser fluence when a strong increase of ion production (matrix and analyte) was recorded. Ablation volume was found to vary non-linearly with the number of laser shots. On a same spot, the ablated volume and the ion production were measured as a function of the laser energy. With an increasing laser energy (or fluence), the ablated volume slowly increases when the ion production strongly increases. This gives evidence of a decoupling between ablation and ionization. is now a powerful method to produce ions of large molecules and its coupling with mass spectrometry (MS) provides a major analytical tool, particularly in proteomic studies and synthetic polymers characterization. To date, many of the methodologies developed for MALDI have been empirically determined. Indeed, the mechanisms involved in the ion production are very complex and cannot be experimentally studied in detail in all the time range of elementary processes. Briefly, these mechanisms involve ablation of molecules and aggregates from a solid state, the expansion of a plume from a very dense state to a very dilute gas. Ionization processes are likely involved in all the steps of the ablation and expansion. In addition, it is still difficult to predict for a small molecule absorbing at the laser wavelength whether it will be an effective or an inefficient matrix for the analysis of a given analyte. Some recent reviews gave an overview of the common ideas on these ionization mechanisms [2, 3] involving specific chemical reaction, ion-molecule reaction [2] or proton transfer in aggregates [3,4]. Generally, the mechanisms in MALDI are studied in two different ways. One deals with the ablation phenomenon mainly by molecular dynamics simulation [5,6] at very short scale (Ͻ Ͻ1 ns) and experimentally on the plume expansion [7] at medium and long-time scale (a few s to a few tens of s). The second deals with the ions produced by MALDI at the time scale of the ion time-of-flight, the role of the laser parameters [8], the target preparations and the nature of analyte ions. This subject occupies most publications in this field.In this article, we will focus on the relationship between the ablated volume and the analyte ion production and on the irradiation effects using large 2,5-dihydroxybenzoic acid single crystals...

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

“…Then a large peak appears, as expected from Figure 1. The peak has a significant tail since the ablation crater is conical rather than flat, so after initial contact with the surface, the edges of the crater also slowly ablate down to the metal [18].…”

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“…Experimental evidence for entrainment, late ion release and analyte embedding in clusters has been available for some time, [56,57,74] and cluster-based models followed soon after. [3,17] Massive clusters of analyte ions with matrix have been suggested, up to 50 000 Da and more, [18] and various pathways for their decay into observable analyte ions suggested. [9,19 -22] Although more extensive simulations are needed to better understand the role of clusters in MALDI, the current results are quite consistent with analyte entrainment in large clusters early in the plume.…”

MALDI produced ions inspected with a linear ion trap-orbitrap hybrid mass analyzer

“…For example, for the conditions used for instrument calibration (see below), ion population may increase faster with additional laser shots than predicted by the pre-scan evaluation, and later the trend may reverse when the sample is continuously consuming at this given x-y location [39,40]. This effect may be sample-, matrix-, preparation-, or laser energy-dependent.…”

“…This effect may be sample-, matrix-, preparation-, or laser energy-dependent. To appropriately take care of the MALDI phenomenon that the ion current per laser shot can potentially first increase and then decrease [40,41], the instrument control software does not allow for more than 80 laser shots onto a given location under AGC in Orbitrap detection. Spoken with ESI and LT terminology, the "maximum ion injection time" is reached if 80 laser shots are requested.…”