Dispersion of the mass accuracy in a MALDI image (TOF TOF analyser)

Abstract
Mass spectrometry (MS) coupled with matrix assisted laser desorption ionization (MALDI) is a powerful technique which allows spatial localization of ions of interest. Indeed, MALDI imaging (MSI) leads to the acquisition of an image of a sample deposited on a conductive slide where each “pixel” contains a mass spectrum. An “average” spectrum results from the compilation of the individual mass spectra. Mass selected (m/z) images can be reconstructed by software.
The analysis of lipids in MSI is a present challenge. Lipids form a very large group of small molecules (<1,000 Da) classified into 8 classes which contain isobaric compounds present in biologically relevant samples. In order to identify each detected lipid, the exact mass with a maximum error of 10 ppm and a full width at middle height (FWHM) resolving power of at least 80,000 (at 800 Da) are usually required for MS analysis.
The RapifleX™ (Bruker Daltonics) is a MALDI-TOF-TOF spectrometer specially designed for MSI. It allows a faster image acquisition than previous MALDI MSI generations. Single spectra usually provide narrow peak width but the mass accuracies mainly depend on the location of the sample on the conductive slide. This results in an apparent loss of mass resolving power and mass accuracy in the averaged spectrum, decreasing so the efficiency of lipid identification. Full image acquisition is then affected and shows a significant dispersion of the resolving power and mass accuracy depending on the topology of the analyzed surface.
The ionization of the molecules of interest requires the presence of small organic molecules able to absorb the wavelength of the laser, called matrix. The mode of deposition (e.g. spray or sublimation), the nature of the matrix, the composition of the solvents, and the temperature greatly affect the shape of the matrix crystals. The size of the crystals also affects the maximum lateral resolution of an image and the homogeneity of matrix deposition assures a steady mass accuracy. The deposition method also influences the efficiency of co-crystallization of the matrix with the sample and determines the ionization efficiency of the ion of interest. The application of matrix is therefore a critical step affecting the quality of the image.
In this work, we used MALDI images to map the dispersion of the mass accuracy according to the dimensions of the crystals. This allows a rapid evaluation of the best operating conditions to reach an optimum mass accuracy and resolving power on the whole image using first authentic lipid standard, then real sample such as dried blood spots.
This work is funded by the Interreg Eurlipids project.