Target Plate Material Influence on Fullerene-C₆₀ Laser Desorption/Ionization Efficiency

Abstract

Abstract. Systematic laser desorption/ionization (LDI) experiments of fullerene-C 60 on a wide range of target plate materials were conducted to gain insight into the initial ion formation in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. The positive and negative ion signal intensities of precursor, fragment, and cluster ions were monitored, varying both the laser fluence (0-3.53 Jcm) and the ion extraction delay time (0-950 ns). The resulting species-specific ion signal intensities are an indication for the ionization mechanisms that contribute to LDI and the time frames in which they operate, providing insight in the (MA)LDI primary ionization. An increasing electrical resistivity of the target plate material increases the fullerene-C 60 precursor and fragment anion signal intensity. Inconel 625 and Ti90/Al6/V4, both highly electrically resistive, provide the highest anion signal intensities, exceeding the cation signal intensity by a factor~1.4 for the latter. We present a mechanism based on transient electrical field strength reduction to explain this trend. Fullerene-C 60 cluster anion formation is negligible, which could be due to the high extraction potential. Cluster cations, however, are readily formed, although for high laser fluences, the preferred channel is formation of precursor and fragment cations. Ion signal intensity depends greatly on the choice of substrate material, and careful substrate selection could, therefore, allow for more sensitive (MA)LDI measurements.

Advances in Ionization for Mass Spectrometry

“…Zeegers et al monitored LDI signal intensities of fullerene anions and cations as a function of target plate material and laser fluence. 145 Targets with high electrical resistivity yielded significantly higher anion signal intensities. This provided insight into LDI ionization mechanisms, which was extended to MALDI mechanisms.…”

“…In addition, several research groups published papers related to understanding the mechanism of MALDI and laser desorption/ionization (LDI). Zeegers et al monitored LDI signal intensities of fullerene anions and cations as a function of target plate material and laser fluence . Targets with high electrical resistivity yielded significantly higher anion signal intensities.…”

Competing Deprotonation and Electron Capture Dissociation in MALDI Mass Spectrometry

“…In a static electric field, in addition to electron mobility, increased bias voltages between the sample plate and the aperture also facilitate the emission of free electrons out of the surfaces of substrates . Transfer of emitted photoelectrons to molecules involves several aspects including the adsorption and orientation on substrates as well as the presence of charge-deficient atoms to accept transferred photoelectrons . In particular, the electron affinity (χ) of adsorbed molecules plays a key role in the capture of transferred electrons and the formation of radical anions.…”

Microtrace Analysis of Rare Earth Element Residues in Femtogram Quantities by Laser Desorption and Laser Postionization Mass Spectrometry

“…The residues were prepared by evaporating a 500 nL solution of REEs on the stainless steel target via a hot drier. The metal target was employed for a higher desorption efficiency instead of a quartz glass plate because of its high absorbance of laser irradiance and better thermal conductivity . One issue to consider is that the constituents of the metal target are also determined, but it should be interference-free for REE determination.…”