Infrared mass spectrometric imaging below the diffraction limit

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS)(1) 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.(2-4) However, in a recent study we have demonstrated an alternative methodology; the so-called microscope mode,5 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 mu m 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.