High Mass Resolution MALDI-MS Imaging Part II: Using the "MALDIVision" from PREMIER Biosoft
Introduction
Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MS Imaging) is a powerful tool for the biochemical analyses of surfaces. Previously, this technique has been used to determine the spatial distribution of hundreds of unknown compounds in thinly sliced tissue sections. The mass spectral images are generated by changing the laser irradiation point at regular intervals across the sample surface and collecting a mass spectrum for each point. Time-of-flight mass spectrometers (TOFMS) are widely used as the mass analyzer for MALDI-MS Imaging because they are well matched for the MALDI ionization process. Ultra-high mass resolution achieving isobaric peak separation is important for lipid profiling using MALDI-Imaging [1, 2]. However, the fine structure of the matrix crystals and small irregularities in the tissue surface flatness can cause peak drift in the collected mass spectra that is caused by slight differences in the starting point of the flight path for the ions at each laser irradiation point. As a result, the typical reflectron type TOFMS systems have a difficult time achieving high mass resolution from spot to spot over a thinly sliced biological surface.
Conversely, the JEOL JMS-S3000 “SpiralTOF™”, which has 5-10 times longer flight path than the traditional reflectron type TOF, is able to reduce the effect of this mass drift to achieve high mass resolution and high mass accuracy [3]. The SpiralTOF is a MALDI-TOFMS that uses the unique JEOL spiral ion optical system which increases the ion flight path length to 17m. One consequence of this improvement in resolution is that there are more data points across each mass spectral peak, resulting in an overall increase in file size. This feature, along with the dimensions of the surface, the spatial resolution of each image data point, and the mass range measured, can all result in producing very large data files (10-20GB). Consequently, SpiralTOF imaging data generally requires file compression in order for the typical imaging software to reconstruct the MS images. However, a new software package called MALDIVision from PREMIER Biosoft [4] can easily handle these larger sized data files without the need for file compression. In this work, we used the SpiralTOF to collect the mass spectral data for a mouse brain section and then used MALDIVision to reconstruct the MS images using the uncompressed data files.