Introduction

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is a powerful tool that enables the simultaneous detection and identification of biomolecules in analytes. MALDI-imaging mass spectrometry (MALDI-IMS) is a two-dimensional MALDI-MS technique used to visualize the spatial distribution of biomolecules without extraction, purification, separation, or labeling of biological samples. This technique can reveal the distribution of hundreds of ion signals in a single measurement and also helps in understanding the cellular profile of the biological system. MALDI-IMS has already revealed the characteristic distribution of several kinds of lipids in various tissues. The versatility of MALDI-IMS has opened a new frontier in several fields, especially in lipidomics.

Fig1. The protocol workflow of MALDI-Imaging lipidomics (Noh SA.; et al, 2019)

Type of instrument for MALDI-IMS

TOF is the most widely-used technology. TOF analyzers allow the separation of ionized accelerated molecules according to their mass-to-charge ratio (m/z). TOF-MS offers suitable performance for MALDI-IMS, namely a good transmission ratio (50–100%), sensitivity, and repetition rate. However, TOF-MS lacks the capability to perform effective tandem MS analyses for identification. This disadvantage of TOF-MS has been addressed with the introduction of hybrid analyzers pairing various technologies with TOF: a quadrupole mass analyzer and TOF, a quadrupole ion trap and TOF, an ion mobility spectrometer and TOF, and tandem TOF mass spectrometers. These combination systems have revolutionized the application of TOF-MS to structural analysis with tandem MS analyses.

Sample preparation for IMS

• The preferred samples for MALDI-IMS consist of fresh-frozen and chemically unmodified tissue. Fresh-frozen tissues can be prepared using powdered dry ice, liquid nitrogen, liquid nitrogen-chilled isopentane, etc. Among these fresh-freezing methods, the tissue section morphology appears to be well maintained when samples are frozen by powdered dry ice. However, the optimal freezing method is different for different samples. It is important to ensure that the tissue section morphology is well maintained;
• Embedding with an optimal cutting temperature (OCT) compound usually allows samples to retain their shape and facilitates the cutting process, but in the preparation of tissue sections for MALDI-IMS measurements, the use of embedding agents such as OCT compound must be avoided because the attachment and penetration of such polymer molecules in tissues cause serious inhibition of biomolecule ionization. We used a precooled semiliquid gel of 2% sodium carboxymethylcellulose as an alternative embedding compound that does not interfere with the detection sensitivity of biomolecules by MS analysis;
• Ionization efficiency is partly dependent on the thickness of the tissue section. In general, 5- to 20-μm-thick sections are prepared for the analysis of low-molecular-weight molecules. The use of thinner tissue sections (2–5 μm in thickness) has been recommended for analysis of high-molecular-weight molecules (range, 3–21 kDa);
• In conventional MALDI-MS analyses, choosing the appropriate matrix is the most important step. Generally, 2,5-dihydroxybenzoic acid (DHB) and 9-aminoacridine (9-AA) are used as suitable matrices for lipids and small metabolites.

Application

MALDI-IMS is currently one of the only methodologies enabling simultaneous visualization of lipids. Visualization of various types of lipids, such as phospholipids, neutral lipids, glycolipids, and fatty acids has been reported.

Reference:

1. Noh SA.; et al. Alterations in Lipid Profile of the Aging Kidney Identified by MALDI Imaging Mass Spectrometry. J Proteome Res. 2019, 5;18(7):2803-2812.