Optimized laser foci for imaging MALDI-mass spectrometry

• Applicant: Privatdozent Dr. Jens Soltwisch
• Subject Area: Analytical Chemistry
• Term: from 2014 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 259818831

Imaging MALDI mass spectrometry is a fairly recent analytical method with enormous development potential. The major advantage of the technique is that it allows for the simultaneous, label-free and laterally-resolved registration of numerous biomolecules in tissue slices. The lateral resolution is largely defined by the focal diameter of the laser spot on the sample. Resolutions between 20-50 µm, currently achieved as standard, do not provide for a cellular resolution as it is required in many biological and medical research issues. Generally, this boundary is only surpassed for laser spots with sub 10 µm-diameters. However, the realization of a true mass spectrometric resolution <10 µm is hampered by several instrumental and MALDI process-related factors. For example, due to a hitherto not well understood spot size effect miniaturization of the focal laser spot is accompanied by an over-proportional increase of the pulse energy per area necessary for the production of ions. Moreover, with respect to the MS performance characteristics also the laser intensity profile plays has a significant role, especially for small spot sizes. The use of Gaussian or stronger structured laser beam profiles with hot spots, like they are produced by Nd:YAG and N2-lasers employed for MALDI, can lead to an increased analyte fragmentation in the intensity peaks. Only by using homogeneous flat top laser profiles can optimal excitation conditions be achieved over the whole irradiated area. Because of the steepness of the flanks and the thereby well-defined material ejection these profiles are also particular well suited for imaging MALDI-MS. This would be even more so if flat top profiles with square dimensions were utilized.Building on beam shaping techniques that are already implemented successfully in the areas of laser material processing and laser medicine we propose to use flat top beam profiles with diameters between 2 and 200 µm in the focal plane of customary MALDI ion sources. For this purpose, different concepts will be investigated and further developed for imaging MALDI-MS that in their entirety encompass the two cases of commonly used MALDI laser systems, namely the spatially high-coherent Nd:YAG and the spatially low-coherent N2-Laser. As mass spectrometers, a Synapt G2-S (Waters) and an instrument similar to the QStar (AB Sciex) will be employed. In a second, strongly linked part of the research project fundamentals of the MALDI process will be investigated with regard to the influence of the focal spot diameter and its intensity profile. MALDI ion signals as well as the laser-induced material ablation will be recorded as function of these parameters and of the laser fluence, using a mass spectrometer and a photoacoustic detection set-up, respectively. Important hints towards possibilities for improving imaging MALDI-MS are expected from these studies.

• DFG Programme: Research Grants