Effect of MALDI matrices on lipid analyses of biological tissues using MALDI‐2 postionization mass spectrometry

Abstract

Matrix‐assisted laser desorption/ionization imaging mass spectrometry (MALDI IMS) allows for highly multiplexed, untargeted detection of many hundreds of analytes from tissue. Recently, laser postionization (MALDI‐2) has been developed for increased ion yield and sensitivity for lipid IMS. However, the dependence of MALDI‐2 performance on the various lipid classes is largely unknown. To understand the effect of the applied matrix on MALDI‐2 analysis of lipids, samples including an equimolar lipid standard mixture, various tissue homogenates, and intact rat kidney tissue sections were analyzed using the following matrices: α‐cyano‐4‐hydroxycinnamic acid, 2′,5′‐dihydroxyacetophenone, 2′,5′‐dihydroxybenzoic acid (DHB), and norharmane (NOR). Lipid signal enhancement of protonated species using MALDI‐2 technology varied based on the matrix used. Although signal improvements were observed for all matrices, the most dramatic effects using MALDI‐2 were observed using NOR and DHB. For lipid standards analyzed by MALDI‐2, NOR provided the broadest coverage, enabling the detection of all 13 protonated standards, including nonpolar lipids, whereas DHB gave less coverage but gave the highest signal increase for those lipids recorded. With respect to tissue homogenates and rat kidney tissue, mass spectra were compared and showed that the number and intensity of neutral lipids tentatively identified with MALDI‐2 using NOR increased significantly (e.g., fivefold intensity increase for triacylglycerol). In the cases of DHB with MALDI‐2, the number of protonated lipids identified from tissue homogenates doubled with 152 on average compared with 76 with MALDI alone. High spatial resolution imaging (~20 μm) of rat kidney tissue showed similar results using DHB with 125 lipids tentatively identified from MALDI‐2 spectra versus just 72 using standard MALDI. From the four matrices tested, NOR provided the greatest increase in sensitivity for neutral lipids (triacylglycerol, diacylglycerol, monoacylglycerol, and cholesterol ester), and DHB provided the highest overall number of lipids detected using MALDI‐2 technology.

MALDI-2 Mass Spectrometry for Synthetic Polymer Analysis

“…The results in the protonated species are in agreement with earlier studies in which the signal of these species is enhanced by the second laser. – The boost in ion yield for protonated species can be explained by a mechanism proposed for MALDI-2 by Soltwisch et al, in which the matrix ( m ) is ionized by resonance-enhanced multiphoton ionization (REMPI), causing the formation of radical matrix cations (Scheme ). ,, These radical ions abstract a proton from another matrix molecule, which, given their acidity (e.g., DHB), they then transfer to neutral analyte molecules (M).…”

A Caged In‐Source Laser‐Cleavable MALDI Matrix with High Vacuum Stability for Extended MALDI‐MS Imaging

“…MALDI‐2 postionization is an emerging technology, in which a secondary laser that irradiates the primary ion plume generated by the first laser, increases ionization efficiency for multiple classes of analytes. In particular, MALDI‐2 postionization has been shown to be able to boost lipid ion intensity in MSI and, consequently, sensitivity [20–22] . For this reason, we sought to investigate if DMNB‐2,5‐DHAP could promote a similar MALDI‐2 boost in lipid ion intensity as 2,5‐DHAP.…”

Quantitative mass spectrometry imaging: therapeutics & biomolecules

“…Negative polarity measurements display mainly phosphatidylethanolamine (PE), phosphatidylinositol (PI), phosphatidylserine (PS), and phosphatidylglycerol (PG) using matrices such as DAN, 9-AA, DHA, and others. 75,90 Recently, Angerer and colleagues, empirically determined that DAN matrix is best suited for high spatial resolution MALDI-MSI and tandem-MS of lipids in negative polarity. 72 DAN matrix predominantly displayed PE lipids, but also exhibited broad lipid coverage in negative polarity over other traditional matrices.…”