Infrared Mass Spectrometric Imaging below the Diffraction Limit

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI−MS) is an established technique for the analysis of biological macromolecules. Its relative insensitivity to pollutants makes MALDI−MS very suitable for the direct analysis of biological samples. As such, it has facilitated great advances in the field of biomolecular imaging mass spectrometry. Traditionally, MALDI−MS imaging is performed in a scanning microprobe methodology. − However, in a recent study we have demonstrated an alternative methodology; the so-called microscope mode, where the requirement for a highly focused ionization beam is removed. Spatial details from within the desorption area are conserved during the flight of the ions through the mass analyzer, and a magnified ion image is projected onto a 2D-detector. In this paper, we demonstrate how imaging mass spectrometry benefits from the microscope mode approach. For the first time, high-lateral resolution ion images were recorded using infrared MALDI at 2.94 μm wavelength. The ion optical resolution achieved was well below the theoretical limit of (light-) diffraction for the setup used, which is impossible to achieve in the conventional scanning microprobe approach. Keywords: IR-MALDI • imaging mass spectrometry • microscope mode • time-of-flight • stigmatic ion optics