Mass Spectrometry Imaging of Lipids

Mass Spectrometry Imaging of Cholesterol and Oxysterols

“…Both MALDI and DESI are desorption ionisation methods (5). In MALDI-MSI the tissue section to be analysed is coated with matrix, usually a UV absorbing matrix, by spraying the matrix from a pneumatic sprayer or by sublimation of the matrix onto the tissue surface in a vacuum chamber.…”

“…The tissue itself is usually of 5 -20 µm thickness cryo-sectioned using a microtome. The tissue is thaw-mounted onto a microscope slide which often has a conductive surface of indium tin oxide for MALDI-MS (5). In the MALDI process the exact ionisation mechanism is still a subject of debate but can be imagined in simple terms as the excitation of the matrix by the absorption of laser light (usually 337, 349 or 355 nm) leading to vaporisation and ionisation of matrix and surface analytes, and in the positive ion mode the transfer of a cation, often a proton or alkali metal, from the matrix to the analyte.…”

“…It is more likely that the authors were detecting 7-DHC, 8-DHC, their isomer desmosterol or a dehydrated cholesterol autoxidation product or perhaps vitamin D3 artifactually generated by laser light (Figure 2). Interestingly, MALDI-2 has been used to image cholesterol esters in multiple sclerosis brain tissue at the small pixel size of 6 µm (5). Cholesterol esters, if present in mammalian brain are present at very low levels ( 16), but perhaps their presence in multiple sclerosis brain is a consequence of the disease itself.…”

“…At its simplest, a probe (e.g., laser or ion beam) samples the surface of a tissue section pixel by pixel with mass spectra being recorded of the sampled-surface at each pixel. An image of any ion recorded by the MS can then be generated across the sampled surface with ion abundance pixel by pixel illustrated on a colour scale (Figure 1) (5,6). As an ion's mass/charge (m/z), as recorded by MS, is related to its parent compound's molecular mass, and hence to a particular compound, MSI at least to a first approximation, gives a molecular image of a compound in a tissue.…”

“…As an ion's mass/charge (m/z), as recorded by MS, is related to its parent compound's molecular mass, and hence to a particular compound, MSI at least to a first approximation, gives a molecular image of a compound in a tissue. There are many differing sampling probes providing different degrees of spatial resolution, ranging from 100 to 400 nm or smaller for secondary ion mass spectrometry (SIMS) at the high spatial resolution end of the spectrum (7,8); 50 to 100 µm typical for matrix-assisted laser desorption ionisation (MALDI) (4)(5)(6) and desorption electrospray ionisation ionisation (DESI) (9) and 20 µm or less for the more sensitive MALDI2 (10,11); to 300 -1,000 µm for liquid extraction for surface analysis (LESA) at the low spatial resolution end of the spectrum (12,13). However, as spatial resolution is increased the pixel size get smaller and the available tissue to be analysed is reduced meaning that low-abundance metabolites are discriminated against at high spatial resolution, this is less of a problem for highly abundant cholesterol but is for its lower abundance precursors and metabolites.…”