Multifunctional Reactive MALDI Matrix Enabling High-Lateral Resolution Dual Polarity MS Imaging and Lipid C═C Position-Resolved MS2 Imaging

Mass Spectrometry Imaging for the Characterization of C═C Localization in Unsaturated Lipid Isomers at the Single-Cell Level

Unsaturated lipids with carbon-carbon double bonds (C═C) have been implicated in the pathogenesis of various diseases. While mass spectrometry imaging (MSI) has been employed to map the distribution of lipid isomers in tissue sections, the identification of lipid C═C positional isomers at the single-cell level using MSI poses a significant challenge. In this study, we developed a novel approach utilizing ToF-SIMS in conjunction with the Paternò-Büchi (P-B) photochemical reaction to characterize the C═C localization in unsaturated lipid isomers at the single-cell level. The P-B reaction was employed to produce adduct products, which were subsequently subjected to collision-induced dissociation by the primary ion beam of ToF-SIMS to generate characteristic ion pairs indicative of the presence of C═C bonds. Utilizing this approach, lipid isomers in brain and skeletal tissues from mice, as well as different cell lines, were visualized at single-cell resolution. Furthermore, distinct variations in the composition of FA 18:1 isomers across different microregions and cell types were revealed. Our P-B ToF-SIMS approach enables the accurate identification and characterization of complex lipid structures with remarkable spatial resolution and can be helpful in understanding the physiological role of these C═C positional isomers.

Multi-modal mass spectrometry imaging of a single tissue section

Mass spectrometry imaging (MSI) was developed to visualize spatial chemical information within tissues, thereby facilitating spatial multi-omic analysis. However, due to the limited spatial information provided by individual modal MSI, correlating various chemical data within tissues remains a significant challenge. In recent years, multimodal MSI has garnered considerable attention due to its ability to visualize the spatial distributions of multiple biomolecules within tissues. Among the strategies employed in this field, multimodal imaging on a single tissue section circumvents multiple issues introduced by integration of images of consecutive tissue sections. In this minireview, we provide an overview of multimodal MSI on a single tissue section, with a particular focus on the use of Matrix-Assisted Laser Desorption/Ionization-MSI for spatial multi-omic investigations that offer a comprehensive and in-depth elucidation of the biological state and activities, aiming to inspire the development of new approaches in this field.

Structural Elucidation and Relative Quantification of Fatty Acid Double Bond Positional Isomers in Biological Tissues Enabled by Gas-Phase Charge Inversion Ion/Ion Reactions

Fatty acids (FAs) contain a vast amount of structural diversity, and differences in fatty acid structure have been associated with various disease states. Accurate identification and characterization of fatty acids is critical to fully understand the biochemical roles these compounds play in disease progression. Conventional tandem mass spectrometry (MS/MS) workflows do not provide sufficient structural information, necessitating alternative dissociation methods. Gas-phase charge inversion ion/ion reactions can be used to alter the ion type subjected to activation to provide improved or complementary structural information. Herein, we have used an ion/ion reaction between fatty acid (FA) anions and magnesium tris-phenanthroline [Mg(Phen)3] dications to promote charge remote fragmentation of carbon-carbon bonds along the fatty acid chain, allowing for localization of carbon-carbon double bond (C=C) positions to successfully differentiate monounsaturated fatty acid isomers. Relative quantification was also performed to obtain the relative abundance of fatty acid isomers in different biological tissues. For example, the relative abundance of FA 18:1 (9) was determined to vary across regions of rat brain, rat kidney, and mouse pancreas, and FA 16:1 (9) was found to have a higher relative abundance in the dermis layer compared to the sebaceous glands in human skin tissue.

Imaging with mass spectrometry: Which ionization technique is best?

The use of mass spectrometry (MS) to acquire molecular images of biological tissues and other substrates has developed into an indispensable analytical tool over the past 25 years. Imaging mass spectrometry technologies are widely used today to study the in situ spatial distributions for a variety of analytes. Early MS images were acquired using secondary ion mass spectrometry and matrix-assisted laser desorption/ionization. Researchers have also designed and developed other ionization techniques in recent years to probe surfaces and generate MS images, including desorption electrospray ionization (DESI), nanoDESI, laser ablation electrospray ionization, and infrared matrix-assisted laser desorption electrospray ionization. Investigators now have a plethora of ionization techniques to select from when performing imaging mass spectrometry experiments. This brief perspective will highlight the utility and relative figures of merit of these techniques within the context of their use in imaging mass spectrometry.

Shotgun Lipidomic Profiling of Sebum Lipids via Photocatalyzed Paternò-Büchi Reaction and Ion Mobility-Mass Spectrometry

Sebum lipids are composed of nonpolar lipids, and they pose challenges for mass spectrometry-based analysis due to low ionization efficiency and the existence of numerous isomers and isobars. To address these challenges, we have developed ethyl 2-oxo-2-(pyridine-3-yacetate as a charge-tagging Paternò-Büchi reagent and Michler's ketone as a highly efficient photocatalyst, achieving ∼90% conversion for C═C derivatization under 440 nm LED irradiation. This derivatization, when coupled with electrospray ionization-tandem mass spectrometry, boosts the detection of sebum lipids and pinpoints C═C location in a chain-specific fashion. Identification and quantitation of isomers are readily achieved for wax esters, a class of underexplored sebum lipids, which have C═C bonds distributed in fatty alcohol and fatty acyl chains. A shotgun analysis workflow has been developed by pairing the offline PB derivatization with cyclic ion mobility spectrometry-mass spectrometry. Besides the dominant n-10 C═C location in unsaturated wax esters, profiling of low abundance isomers, including the rarely reported n-7 and n-13 locations, is greatly enhanced due to separations of C═C diagnostic ions by ion mobility. Over 900 distinct lipid structures from human sebum lipid extract have been profiled at the chain-specific C═C level, including wax esters (500), glycerolipids (393), and cholesterol esters (22), far more exceeding previous reports. Overall, we have developed a fast and comprehensive lipidomic profiling tool for sebum samples, a type of noninvasive biofluids holding potential for the discovery of disease markers in distal organs.

Sample preparation for fatty acid analysis in biological samples with mass spectrometry-based strategies

Fatty acids (FAs) have attracted many interests for their pivotal roles in many biological processes. Imbalance of FAs is related to a variety of diseases, which makes the measurement of them important in biological samples. Over the past two decades, mass spectrometry (MS) has become an indispensable technique for the analysis of FAs owing to its high sensitivity and precision. Due to complex matrix effect of biological samples and inherent poor ionization efficiency of FAs in MS, sample preparation including extraction and chemical derivatization prior to analysis are often employed. Here, we describe an updated overview of FA extraction techniques, as well as representative derivatization methods utilized in different MS platforms including gas chromatography-MS, liquid chromatography-MS, and mass spectrometry imaging based on different chain lengths of FAs. Derivatization strategies for the identification of double bond location in unsaturated FAs are also summarized and highlighted. The advantages, disadvantages, and prospects of these methods are compared and discussed. This review provides the development and valuable information for sample pretreatment approaches and qualitative and quantitative analysis of interested FAs using different MS-based platforms in complex biological matrices. Finally, the challenges of FA analysis are summarized and the future perspectives are prospected.

MS3 Imaging Enables the Simultaneous Analysis of Phospholipid C═C and sn-Position Isomers in Tissues

Mass spectrometry (MS) imaging of lipids in tissues with high structure specificity is challenging in the effective fragmentation of position-selective structures and the sensitive detection of multiple lipid isomers. Herein, we develop an MS3 imaging method for the simultaneous analysis of phospholipid C═C and sn-position isomers by on-tissue photochemical derivatization, nanospray desorption electrospray ionization (nano-DESI), and a dual-linear ion trap MS system. A novel laser-based sensing probe is developed for the real-time adjustment of the probe-to-surface distance for nano-DESI. This method is validated in mouse brain and kidney sections, showing its capability of sensitive resolving and imaging of the fatty acyl chain composition, the sn-position, and the C═C location of phospholipids in an MS3 scan. MS3 imaging of phospholipids has shown the capability of differentiation of cancerous, fibrosis, and adjacent normal regions in liver cancer tissues.

Advances in mass spectrometry imaging for spatial cancer metabolomics

Mass spectrometry (MS) has become a central technique in cancer research. The ability to analyze various types of biomolecules in complex biological matrices makes it well suited for understanding biochemical alterations associated with disease progression. Different biological samples, including serum, urine, saliva, and tissues have been successfully analyzed using mass spectrometry. In particular, spatial metabolomics using MS imaging (MSI) allows the direct visualization of metabolite distributions in tissues, thus enabling in-depth understanding of cancer-associated biochemical changes within specific structures. In recent years, MSI studies have been increasingly used to uncover metabolic reprogramming associated with cancer development, enabling the discovery of key biomarkers with potential for cancer diagnostics. In this review, we aim to cover the basic principles of MSI experiments for the nonspecialists, including fundamentals, the sample preparation process, the evolution of the mass spectrometry techniques used, and data analysis strategies. We also review MSI advances associated with cancer research in the last 5 years, including spatial lipidomics and glycomics, the adoption of three-dimensional and multimodal imaging MSI approaches, and the implementation of artificial intelligence/machine learning in MSI-based cancer studies. The adoption of MSI in clinical research and for single-cell metabolomics is also discussed. Spatially resolved studies on other small molecule metabolites such as amino acids, polyamines, and nucleotides/nucleosides will not be discussed in the context.

Next-generation pathology practices with mass spectrometry imaging

Mass spectrometry imaging (MSI) is a powerful technique that reveals the spatial distribution of various molecules in biological samples, and it is widely used in pathology-related research. In this review, we summarize common MSI techniques, including matrix-assisted laser desorption/ionization and desorption electrospray ionization MSI, and their applications in pathological research, including disease diagnosis, microbiology, and drug discovery. We also describe the improvements of MSI, focusing on the accumulation of imaging data sets, expansion of chemical coverage, and identification of biological significant molecules, that have prompted the evolution of MSI to meet the requirements of pathology practices. Overall, this review details the applications and improvements of MSI techniques, demonstrating the potential of integrating MSI techniques into next-generation pathology practices.

Development of an Efficient On-Tissue Epoxidation Reaction Mediated by Urea Hydrogen Peroxide for MALDI MS/MS Imaging of Lipid C═C Location Isomers

Unsaturated lipids containing different numbers and locations of C═C bonds are significantly associated with a variety of cellular and metabolic functions. Although matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) has been used to visualize the spatial distribution patterns of various lipids in biological tissues, in situ identification, discrimination, and visualization of lipid C═C location isomers remain challenging. Herein, an efficient and fast on-tissue chemical derivatization (OTCD) approach was developed to pinpoint the locations of C═C bonds in complex lipids in situ via methyltrioxorhenium (MTO)-catalyzed epoxidation of C═C with a urea hydrogen peroxide (UHP)/hexafluoroisopropanol (HFIP) system. The efficiency of OTCD could reach 100% via one-step spray deposition of the solution mixture of MTO/UHP/HFIP at room temperature. The developed OTCD method provided rich structural information on lipid C═C location isomers, and their accurate spatial distribution patterns were resolved in mouse brain tissues. Tissue-specific distributions and changes of lipid C═C location isomers in the liver sections of obese ob/ob and diabetic db/db mice were further investigated, and their correlation in two animal models was revealed. The simplicity and high efficiency of the OTCD method developed for MALDI tandem MSI of lipid C═C location isomers possess great potential for functional spatial lipidomics.

Visible-Light Paternò-Büchi Reaction for Lipidomic Profiling at Detailed Structure Levels

The Paternò-Büchi (PB) derivatization of carbon-carbon double bond (C═C) has been increasingly employed with tandem mass spectrometry to analyze unsaturated lipids. It enables the discovery of altered or uncanonical lipid desaturation metabolism, which would be otherwise undetected by conventional methods. Although highly useful, the reported PB reactions only provide moderate yield (∼30%). Herein, we aim to determine the key factors that affect the PB reactions and develop a system with improved capabilities for lipidomic analysis. An Ir(III) photocatalyst is chosen as the triplet energy donor for the PB reagent under 405 nm light irradiation, while phenylglyoxalate and its charge-tagging version, pyridylglyoxalate, are developed as the most efficient PB reagents. The above visible-light PB reaction system provides higher PB conversions than all previously reported PB reactions. Around 90% conversion can be achieved at high concentrations (>0.5 mM) for different classes of lipids but drops as the lipid concentration decreases. The visible-light PB reaction has then been integrated with shotgun and liquid chromatography-based workflows. The limits of detection for locating C═C in standard lipids of glycerophospholipids (GPLs) and triacylglycerides (TGs) are in the sub-nM to nM range. More than 600 distinct GPLs and TGs have been profiled at the C═C location level or the sn-position level from the total lipid extract of bovine liver, demonstrating that the developed method is capable of large-scale lipidomic analysis.

Structural Elucidation and Relative Quantification of Sodium- and Potassium-Cationized Phosphatidylcholine Regioisomers Directly from Tissue Using Electron Induced Dissociation

Comprehensive structural characterization of phosphatidylcholines (PCs) is essential to understanding their biological functions and roles in metabolism. Electron induced dissociation (EID) of protonated PCs directly generated from biological tissues has previously been shown to provide in-depth structural information on the lipid headgroup, regiosiomerism of fatty acyl tails and double bond positions. Although phosphatidylcholine ions formed via alkali metal cationization (i.e., [M + Na]+ and [M + K]+) are commonly generated during matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry experiments, the gas-phase ion chemistry behavior of EID on sodium- and potassium-cationized phosphatidylcholine ion types has not been studied for ions generated directly from tissue. Herein, we demonstrate EID on [M + Na]+ and [M + K]+ ion types in a MALDI imaging mass spectrometry workflow for lipid structural characterization. Briefly, near-complete structural information can be obtained upon EID of sodium- and potassium-cationized PCs, including diagnostic fragmentation of the lipid headgroup as well as identification of fatty acyl chain positions and double bond position. EID of cationized lipids generates sn-specific glycerol backbone cleavages as well as a favorable combined loss of sn-2 fatty acid with choline over sn-1, allowing for facile differentiation and relative quantification of PC regioisomers. Moreover, relative quantification of sn-positional isomers from biological tissue reveals that the relative percentages of sodium- and potassium-cationized sn-positional isomers varies significantly in different regions of rat brain tissue.

sn-1 Specificity of Lysophosphatidylcholine Acyltransferase-1 Revealed by a Mass Spectrometry-Based Assay

Lysophosphatidylcholine acyltransferase-1 (LPCAT1) plays a critical role in the remodeling of phosphatidylcholines (PCs) in cellular lipidome. However, evidence is scarce regarding its sn-selectivity, viz. the preference of assembling acyl-Coenzyme A (CoA) at the C1 or C2-hydroxyl on a glycerol backbone because of difficulty to quantify the thus-formed PC sn-isomers. We have established a multiplexed assay to measure both sn- and acyl-chain selectivity of LPCAT1 toward a mixture of acyl-CoAs by integrating isomer-resolving tandem mass spectrometry. Our findings reveal that LPCAT1 shows exclusive sn-1 specificity regardless of the identity of acyl-CoAs. We further confirm that elevated PC 18 : 1/16:0 relative to its sn-isomer results from an increased expression of LPCAT1 in human hepatocellular carcinoma (HCC) tissue as compared to normal liver tissue. MS imaging via desorption electrospray ionization of PC 18 : 1/16:0 thus enables visualization of HCC margins in human liver tissue at a molecular level.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2019-2020

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2020. Also included are papers that describe methods appropriate to analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. The review is basically divided into three sections: (1) general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, quantification and the use of arrays. (2) Applications to various structural types such as oligo- and polysaccharides, glycoproteins, glycolipids, glycosides and biopharmaceuticals, and (3) other areas such as medicine, industrial processes and glycan synthesis where MALDI is extensively used. Much of the material relating to applications is presented in tabular form. The reported work shows increasing use of incorporation of new techniques such as ion mobility and the enormous impact that MALDI imaging is having. MALDI, although invented nearly 40 years ago is still an ideal technique for carbohydrate analysis and advancements in the technique and range of applications show little sign of diminishing.

Molecular Networking and On-Tissue Chemical Derivatization for Enhanced Identification and Visualization of Steroid Glycosides by MALDI Mass Spectrometry Imaging

Spatial metabolomics describes the spatially resolved analysis of interconnected pathways, biochemical reactions, and transport processes of small molecules in the spatial context of tissues and cells. However, a broad range of metabolite classes (e.g., steroids) show low intrinsic ionization efficiencies in mass spectrometry imaging (MSI) experiments, thus restricting the spatial characterization of metabolic networks. Additionally, decomposing complex metabolite networks into chemical compound classes and molecular annotations remains a major bottleneck due to the absence of repository-scaled databases. Here, we describe a multimodal mass-spectrometry-based method combining computational metabolome mining tools and high-resolution on-tissue chemical derivatization (OTCD) MSI for the spatially resolved analysis of metabolic networks at the low micrometer scale. Applied to plant toxin sequestration in Danaus plexippus as a model system, we first utilized liquid chromatography (LC)-MS-based molecular networking in combination with artificial intelligence (AI)-driven chemical characterization to facilitate the structural elucidation and molecular identification of 32 different steroidal glycosides for the host-plant Asclepias curassavica. These comprehensive metabolite annotations guided the subsequent matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) analysis of cardiac-glycoside sequestration in D. plexippus. We developed a spatial-context-preserving OTCD protocol, which improved cardiac glycoside ion yields by at least 1 order of magnitude compared to results with untreated samples. To illustrate the potential of this method, we visualized previously inaccessible (sub)cellular distributions (2 and 5 μm pixel size) of steroidal glycosides in D. plexippus, thereby providing a novel insight into the sequestration of toxic metabolites and guiding future metabolomics research of other complex sample systems.

Novel Aza-Paternò-Büchi Reaction Allows Pinpointing Carbon-Carbon Double Bonds in Unsaturated Lipids by Higher Collisional Dissociation

The evaluation of double bond positions in fatty acyl chains has always been of great concern given their significance in the chemical and biochemical role of lipids. Despite being the foremost technique for lipidomics, it is difficult in practice to obtain identification beyond the fatty acyl level by the sole high-resolution mass spectrometry. Paternò–Büchi reactions of fatty acids (FAs) with ketones have been successfully proposed for pinpointing double bonds in FAs in combination with the collision-induced fragmentation technique. In the present paper, an aza-Paternò–Büchi (aPB) reaction of lipids with 6-azauracil (6-AU) was proposed for the first time for the determination of carbon–carbon double bonds in fatty acyl chains using higher collisional dissociation in the negative ion mode. The method was optimized using free FA and phospholipid analytical standards and compared to the standard Paternò–Büchi reaction with acetone. The introduction of the 6-AU moiety allowed enhancing the ionization efficiency of the FA precursor and diagnostic product ions, thanks to the presence of ionizable sites on the derivatizing agent. Moreover, the aPB derivatization allowed the obtention of deprotonated ions of phosphatidylcholines, thanks to an intramolecular methyl transfer from the phosphocholine polar heads during ionization. The workflow was finally applied for pinpointing carbon–carbon double bonds in 77 polar lipids from an yeast (Saccharomyces cerevisiae) extract.

Spatially revealed perfluorooctane sulfonate-induced nephrotoxicity in mouse kidney using atmospheric pressure MALDI mass spectrometry imaging

Perfluorooctane sulfonate (PFOS), an emerging environmental persistent pollutant, has attracted extensive attention due to its potential nephrotoxicity. However, little is known about the spatial variations of lipid metabolism associated with PFOS exposure. In this study, atmospheric pressure matrix-assisted laser desorption/ionization mass spectrometry imaging (AP-MALDI MSI) was used to reveal the spatial distributions of PFOS and its adverse effect on lipid metabolism directly in mouse kidney sections. We have observed that PFOS accumulated in the renal pelvis and outer cortex regions, with some found in the medulla and inner cortex regions. Hematoxylin and eosin (H&E) staining results also demonstrated that the accumulation of PFOS caused damage to the mouse kidney, which was consistent with AP-MALDI MSI results. Furthermore, a total of 42 lipids were shown to be significantly different in the spatial distribution patterns and variations between control and PFOS exposure mice groups, including the significant down-regulation of lyso-glycerophospholipids (Lyso-GPs), phosphatidic acids (PA), phosphatidylcholines (PC), phosphatidylethanolamines (PE), phosphatidylserines (PS) sphingomyelins (SM) and sulfatides (ST) in renal medulla or cortex region of mouse kidney sections, and remarkable up-regulation of cholesterol and phosphatidylinositols (PI) in the cortex regions of mouse kidney sections. The AP-MALDI MSI provides a new tool to explore spatial distributions and variations of the endogenous metabolites for the risk assessment of environmental pollutants.

Quaterization Derivatization with Bis(Pyridine) Iodine Tetrafluoroboride: High-Sensitivity Mass Spectrometric Analysis of Unsaturated Fatty Acids in Human Thyroid Tissues

Accurate quantification of disease-related unsaturated fatty acids (UFAs) in biomedical samples plays an important role in clinical diagnosis. Here, we reported a quaterization derivatization-stable isotope labeling strategy for accurate quantitative analysis of UFAs by high-performance liquid chromatography-mass spectrometry. [d₀]/[d₁₀]-Bis(pyridine) iodine tetrafluoroboride ([d₀]/[d₁₀]-IPy₂BF₄) was employed as the carbon-carbon double bond derivatization reagent with high efficiency and high specificity, to introduce a charge tag on UFAs and avoid the interference of saturated fatty acids. After labeling, the detection sensitivity was significantly enhanced by up to three orders of magnitude compared to intact UFAs. The standard curves showed good linearity (R² > 0.999) over a wide concentration range. This strategy was successfully applied to determine the content of 12 UFAs in human thyroid carcinoma and para-carcinoma tissues. A significant difference was found in the content of several UFAs between these two kinds of tissues (p < 0.05). These results indicated that the proposed strategy may be valuable for the discovery of abnormal UFA content in early clinical diagnosis.

On-tissue pyrene-1-boronic acid labeling assisted MALDI imaging of catecholamines in porcine adrenal gland

In this study, an on-tissue chemical labeling - matrix assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI) method was developed for visualization of the distribution of three catecholamine (CA) compounds (dopamine, epinephrine and norepinephrine) in porcine adrenal gland. Commercially available pyrene-1-boronic acid (PBA) was employed as an effective in situ derivatizing reagent dissolved in acetonitrile containing 0.1% pyridine for the chemical labeling and the matrix coating. Without extra matrix coating, the tissue section was directly analyzed by MALDI-MS. The detection specificity and sensitivity were greatly improved with the on-tissue PBA labeling and successful imaging of the three CAs in porcine adrenal gland was achieved. Compared with previously reported methods for MALDI-MSI of the CAs, the analytical strategy proposed in the study provided a robust, easy-to-use and low-cost on-tissue chemical derivatization method that facilitated simultaneous molecular imaging of the three compounds.

What are we imaging? Software tools and experimental strategies for annotation and identification of small molecules in mass spectrometry imaging

Mass spectrometry imaging (MSI) has become a widespread analytical technique to perform nonlabeled spatial molecular identification. The Achilles' heel of MSI is the annotation and identification of molecular species due to intrinsic limitations of the technique (lack of chromatographic separation and the difficulty to apply tandem MS). Successful strategies to perform annotation and identification combine extra analytical steps, like using orthogonal analytical techniques to identify compounds; with algorithms that integrate the spectral and spatial information. In this review, we discuss different experimental strategies and bioinformatics tools to annotate and identify compounds in MSI experiments. We target strategies and tools for small molecule applications, such as lipidomics and metabolomics. First, we explain how sample preparation and the acquisition process influences annotation and identification, from sample preservation to the use of orthogonal techniques. Then, we review twelve software tools for annotation and identification in MSI. Finally, we offer perspectives on two current needs of the MSI community: the adaptation of guidelines for communicating confidence levels in identifications; and the creation of a standard format to store and exchange annotations and identifications in MSI.

Deep-lipidotyping by mass spectrometry: recent technical advances and applications

In-depth structural characterization of lipids is an essential component of lipidomics. There has been a rapid expansion of mass spectrometry methods that are capable of resolving lipid isomers at various structural levels over the past decade. These developments finally make deep-lipidotyping possible, which provides new means to study lipid metabolism and discover new lipid biomarkers. In this review, we discuss recent advancements in tandem mass spectrometry (MS/MS) methods for identification of complex lipids beyond the species (known headgroup information) and molecular species (known chain composition) levels. These include identification at the levels of carbon-carbon double bond (C=C) location and sn-position as well as characterization of acyl chain modifications. We also discuss the integration of isomer-resolving MS/MS methods with different lipid analysis workflows and their applications in lipidomics. The results showcase the distinct capabilities of deep-lipidotyping in untangling the metabolism of individual isomers and sensitive phenotyping by using relative fractional quantitation of the isomers.

Changes in the lipid profile of hamster liver after Schistosoma mansoni infection, characterized by mass spectrometry imaging and LC-MS/MS analysis

Schistosomiasis, caused by the human parasite Schistosoma mansoni, is one of the WHO-listed neglected tropical diseases (NTDs), and it has severe impact on morbidity and mortality, especially in Africa. Not only the adult worms but also their eggs are responsible for health problems. Up to 50% of the eggs produced by the female worms are not excreted with the feces but are trapped in the host tissue, such as the liver, where they provoke immune responses and a change in the lipid profile. We built up a database with 372 infection markers found in livers of S. mansoni-infected hamsters, using LC-MS/MS for identification, followed by statistical analysis. Most of them belong to the lipid classes of phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), and triglycerides (TGs). We assigned some of these markers to specific anatomical structures by applying high-resolution MALDI MSI to cryosections of hamster liver and generating ion images based on the marker list from the LC-MS/MS experiments. Furthermore, enrichment and depletion of several markers were visualized.

Mass Spectrometry Imaging Techniques Enabling Visualization of Lipid Isomers in Biological Tissues

This Feature focuses on a review of recent developments in mass spectrometry imaging (MSI) of lipid isomers in biological tissues. The tandem MS techniques utilizing online and offline chemical derivatization procedures, ion activation techniques such as ozone-induced dissociation (OzID), ultraviolet photodissociation (UVPD), or electron-induced dissociation (EID), and other techniques such as coupling of ion mobility with MSI are discussed. The importance of resolving lipid isomers in diseases is highlighted.

Reactive Matrices for Analytical Matrix-Assisted Laser Desorption/Ionization (MALDI) Mass Spectrometry

In recent years, a special focus is placed on the usage of reactive matrices for analytical matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS). Since 2003, when the term "reactive matrices" was suggested and the dignity of compounds, possessing dualistic properties as matrices and derivatization agents was demonstrated, corresponding approach has found application in various fields and, in particular, in bioanalysis (metabolomics, lipidomics, etc.). The main advantage of this methodology is that it reduces sample treatment time, simplifies the procedure of sample handling, improves the sensitivity of analysis, enhances the molecular identification and profiling. Within the framework of this review, the main attention is paid to "true" reactive matrices that interact with analyte molecules through an exchange or addition reactions. A special section discusses practical application of reactive matrices in the determination of the distribution of targeted and non-targeted organic substances on the surface of biological tissue sections by MALDI-MS imaging. In this critical review, a controversial proposal is made to consider protonating and deprotonating matrices as reactive, because they can undergo a chemical reaction such as proton transfer that occurs in both target solution and MALDI plume. In this respect, special attention is paid to "proton sponge" matrices that have found a wide application in the analysis of various acidic compounds by MALDI-MS in the negative mode. Historical data on the formation of ions and the fate of matrices in MALDI are considered at the beginning of this article.

Multi-wavelength visible-light induced [2+2] cycloaddition for identification of lipid isomers in biological samples

Ultraviolet (UV) light-catalyzed Paternò-Büchi (PB) reaction has been developed as an efficient lipid C=C double bond (DB) derivatization strategy, which can accurately assign the position of C=C bond in unsaturated lipids when coupled with tandem mass spectrometry (MS/MS). Inspired by this, here we proposed a novel visible-light induced [2+2] cycloaddition reaction combined with ESI-MS/MS and MALDI-MS/MS to identify lipid C=C position isomers. Benz[g]isoquinoline-5,10-dione (BIQD) and 6,9-difluorobenzo[g]isoquinoline-5,10-dione (DF-BIQD) were developed as a new type of [2+2] cycloaddition reagent, which can not only react with C=C bond under 254 nm UV light irradiation, but also quickly combine with lipid C=C bond under the irradiation of 405 nm visible-light and > 400 nm compact fluorescent lamp visible-light. High cycloaddition reaction conversion efficiency can be achieved by irradiating under compact fluorescent lamp light for 2 min. Moreover, we discovered that 437 nm, 489 nm, 545 nm, 581 nm, and 613 nm monochromatic light appearing in compact fluorescent lamp can individually induce the [2 + 2] cycloaddition reaction between DF-BIQD and unsaturated lipids. Using this method, we found that the expressions of lipid DB-positional isomers in rat heart, brain, lung, spleen, thymus, kidney, liver and plasma vary greatly. The relative content of FA-18:1 (Δ9) in rat heart is only 1.49 times that of FA-18:1 (Δ11), while the relative content of FA-18:1 (Δ9) in rat plasma is 5.20 times that of FA-18:1 (Δ11). The above results offer new insight into the development of photocatalytic reagent for visible-light induced [2+2] cycloaddition and structural lipidomic studies.

Unsaturated lipid isomeric imaging based on the Paternò-Büchi reaction in the solid phase in ambient conditions

In recent years, the development of unsaturated lipid isomeric imaging based on the Paternò-Büchi (PB) reaction has improved significantly. However, research on this imaging method in ambient conditions needs to expand. In this paper, a method of PB reaction in the solid phase in ambient conditions is developed, which is combined with air-flow-assisted desorption electrospray ionisation mass spectrometry (AFADESI-MS) to achieve in situ imaging of lipids at an isomeric level. Experiments showed that the efficiency of the PB reaction was much higher when spraying the reagent solution than when sprinkling the reactant powder directly, and it was not lower than that in the liquid phase. This method can simplify the reaction conditions in the imaging process and can be applied to tissue section samples with only 10 min of pre-processing. The study successfully demonstrated the spatial distribution of unsaturated lipid isomers, and the isomeric ratio corresponded to the lesion areas in mouse brain cancer tissues. Due to its simple operation and performance in ambient conditions, this method may be useful for future studies on lipid isomers in tissues.

Unravel the Local Complexity of Biological Environments by MALDI Mass Spectrometry Imaging

Classic metabolomic methods have proven to be very useful to study functional biology and variation in the chemical composition of different tissues. However, they do not provide any information in terms of spatial localization within fine structures. Matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI MSI) does and reaches at best a spatial resolution of 0.25 μm depending on the laser setup, making it a very powerful tool to analyze the local complexity of biological samples at the cellular level. Here, we intend to give an overview of the diversity of the molecules and localizations analyzed using this method as well as to update on the latest adaptations made to circumvent the complexity of samples. MALDI MSI has been widely used in medical sciences and is now developing in research areas as diverse as entomology, microbiology, plant biology, and plant–microbe interactions, the rhizobia symbiosis being the most exhaustively described so far. Those are the fields of interest on which we will focus to demonstrate MALDI MSI strengths in characterizing the spatial distributions of metabolites, lipids, and peptides in relation to biological questions.

TLC-based MS Imaging Analysis of Glycosphingolipids and Glycerin Fatty Acid Esters after 1,2-Dichloroethane Washing

Matrix-assisted laser desorption/ionization-based mass spectrometry imaging (MSI) of separated lipids on thin-layer chromatography (TLC) plates or followed by blotted hydrophilic polyvinylidene fluoride (PVDF) membranes has become a powerful tool in lipidomic analyses. However, background peaks in MS spectra often cover lipid peaks in a low amount/ionization effect; consequently, only low intensities/resolutions MSI are obtained. To address the aforementioned problem, we attempted 1,2-dichloroethane pre-washing of TLC plates before development and found that backgrounds could successfully be removed from the TLC plate or PVDF membrane.

Enabling High Structural Specificity to Lipidomics by Coupling Photochemical Derivatization with Tandem Mass Spectrometry

Lipids have pivotal roles in many biological processes, including energy storage, signal transduction, and plasma membrane formation. A disruption of lipid homeostasis is found to be associated with a range of diseases, such as cardiovascular diseases, diabetes, and cancer. Fundamental lipid biology and disease diagnostics can benefit from monitoring lipid changes in cells, tissues, organs, or the whole biological system. Therefore, it is important to develop lipid analysis tools to achieve comprehensive lipid characterization and quantitation. Over the past two decades, mass spectrometry (MS) has become the method of choice for qualitative and quantitative analyses of lipids, owing to its high sensitivity, multiplexed analysis, and soft ionization features. With the rapid development and adoption of ultrahigh-resolution MS, isobaric lipids can now be routinely resolved. By contrast, the structural characterization and quantitation of isomeric lipids remain an analytical challenge. Although some lipid C═C location or sn-isomers can be resolved by chromatography, ion mobility, or selective ionization approaches, a detailed structural characterization on the lipidome-wide level needs to be achieved.Over the past six years, we have successfully combined the Paternò-Büchi (PB) reaction, which is a UV-promoted photocycloaddition reaction specific to the C═C, with tandem MS (MS/MS) to locate the C═C in lipids and quantify lipid C═C location isomers. The PB reactions have analytical advantages such as a simple experimental setup, rapid lipid C═C derivatization, and highly specific C═C cleavage during PB-MS/MS to produce abundant diagnostic ions. More importantly, without a need of isomer separation or a comparison to authentic standards, PB-MS/MS can be directly applied to identify and quantify a mixture of lipid C═C location isomers, often coexisting with molar ratios sensitive to the biological state of the system. The PB-MS/MS method is compatible with conventional shotgun lipidomics employing a nanoelectrospray ionization or a large-sale lipid structural analysis via liquid chromatography (LC) coupled to any mass spectrometer with tandem MS capability. The PB-MS/MS method is highly versatile, as a variety of PB reagents can be tailored to a broad range of applications. Besides UV-promoted PB reactions, visible-light PB reactions have also been developed to offer more flexibility for a lipid analysis. By using selected PB reagents, the sn-positions of fatty acyls can be resolved together with C═C locations in phospholipids. This method has been used in lipidomic analyses of tissue, blood, and plasma from animal models and clinical samples, demonstrating the potential of using lipid C═C or sn-location isomer ratios for phenotyping and disease diagnostics. Lipid isomer-resolving MS imagings of tissues and single-cell lipid analysis have also been demonstrated by a proper implementation of PB-MS/MS.

Advanced tandem mass spectrometry in metabolomics and lipidomics-methods and applications

Metabolomics and lipidomics are new drivers of the omics era as molecular signatures and selected analytes allow phenotypic characterization and serve as biomarkers, respectively. The growing capabilities of untargeted and targeted workflows, which primarily rely on mass spectrometric platforms, enable extensive charting or identification of bioactive metabolites and lipids. Structural annotation of these compounds is key in order to link specific molecular entities to defined biochemical functions or phenotypes. Tandem mass spectrometry (MS), first and foremost collision-induced dissociation (CID), is the method of choice to unveil structural details of metabolites and lipids. But CID fragment ions are often not sufficient to fully characterize analytes. Therefore, recent years have seen a surge in alternative tandem MS methodologies that aim to offer full structural characterization of metabolites and lipids. In this article, principles, capabilities, drawbacks, and first applications of these "advanced tandem mass spectrometry" strategies will be critically reviewed. This includes tandem MS methods that are based on electrons, photons, and ion/molecule, as well as ion/ion reactions, combining tandem MS with concepts from optical spectroscopy and making use of derivatization strategies. In the final sections of this review, the first applications of these methodologies in combination with liquid chromatography or mass spectrometry imaging are highlighted and future perspectives for research in metabolomics and lipidomics are discussed.

A liquid chromatography-mass spectrometry workflow for in-depth quantitation of fatty acid double bond location isomers

Tracing compositional changes of fatty acids (FAs) is frequently used as a means of monitoring metabolic alterations in perturbed biological states. Given that more than half of FAs in the mammalian lipidome are unsaturated, quantitation of FAs at a carbon-carbon double bond (C=C) location level is necessary. The use of 2-acetylpiridine (2-acpy) as the charge-tagging PB reagent led to a limit of identification in the subnanomolar range for mono- and polyunsaturated as well as conjugated FAs. Conjugated free FAs of low abundance such as FA 18:2 (n-7, n-9) and FA 18:2 (n-6, n-8) were quantified at concentrations of 0.61 ± 0.05 and 0.05 ± 0.01 mg per 100 g in yak milk powder, respectively. This workflow also enabled deep profiling of eight saturated and 37 unsaturated total FAs across a span of four orders of magnitude in concentration, including ten groups of C=C location isomers in pooled human plasma. A pilot survey on total FAs in plasma from patients with type 2 diabetes revealed that the relative compositions of FA 16:1 (n-10) and FA 18:1 (n-10) were significantly elevated compared with that of normal controls. In this work, we have developed a workflow for global quantitation of FAs, including C=C location isomers, via charge-tagging Paternò-Büchi (PB) derivatization and liquid chromatography-tandem mass spectrometry (LC-MS/MS).

Stereoselective Synthesis of Novel Sphingoid Bases Utilized for Exploring the Secrets of Sphinx

Sphingolipids are ubiquitous in eukaryotic plasma membranes and play major roles in human and animal physiology and disease. This class of lipids is usually defined as being derivatives of sphingosine, a long-chain 1,3-dihydroxy-2-amino alcohol. Various pathological conditions such as diabetes or neuropathy have been associated with changes in the sphingolipidome and an increased biosynthesis of structurally altered non-canonical sphingolipid derivatives. These unusual or non-canonical sphingolipids hold great promise as potential diagnostic markers. However, due to their low concentrations and the unavailability of suitable standards, the research to explore the secret of this class of 'Sphinx' lipids is ultimately hampered. Therefore, the development of efficient and facile syntheses of standard compounds is a key endeavor. Here, we present various chemical approaches for stereoselective synthesis and in-depth chemical characterization of a set of novel sphingoid bases which were recently utilized as valuable tools to explore the metabolism and biophysical properties of sphingolipids, but also to develop efficient analytical methods for their detection and quantification.

Deep Structural Annotation of Glycerolipids by the Charge-Tagging Paterno-Büchi Reaction and Supercritical Fluid Chromatography-Ion Mobility Mass Spectrometry

Glycerolipids (GLs) are essential for cellular lipid homeostasis, while dysregulation in GL metabolism is often associated with the onset or progression of human-related metabolic diseases. The profile of GLs is thus frequently used as a molecular readout for disease phenotyping. Although mass spectrometry (MS) is the method of choice for GL profiling, the current MS methods are unable to differentiate two major types of structural isomers due to the fact that fatty acyls can be linked to different positions on the glycerol backbone (sn-positions) and the site(s) of unsaturation in acyl chains. Herein, by utilizing charge-tagging Paterno-Büchi (PB) derivatization of carbon-carbon double bond (C═C), supercritical fluid chromatography (SFC), and mobility aligned tandem mass spectrometry (MS/MS), a workflow has been developed for the sensitive and structurally informative analysis of GLs. SFC allows fast separation (within 25 min) of sn-isomers of diacylglycerols (DGs) and separation of triacylglycerols (TGs) of different chain lengths and degrees of unsaturation. Time-aligned parallel fragmentation enables multiple-stage MS/MS of the PB-derivatized lipids in a high-throughput fashion and allows pinpointing C═C location to a specific fatty acyl chain. This workflow reveals the presence of more than 500 molecular structures of neutral lipids from pooled human plasma. A comparison of human plasma samples between type 2 diabetes (N = 7) and control (N = 7) shows significant changes in isomer compositions (C18:1 Δ9 vs Δ11) from nine groups of TG and DG. These findings suggest that the developed workflow can be potentially applied to lipid marker discovery for disease monitoring or diagnosis.

Quantification and molecular imaging of fatty acid isomers from complex biological samples by mass spectrometry

Elucidating the isomeric structure of free fatty acids (FAs) in biological samples is essential to comprehend their biological functions in various physiological and pathological processes. Herein, we report a novel approach of using peracetic acid (PAA) induced epoxidation coupled with mass spectrometry (MS) for localization of the C[double bond, length as m-dash]C bond in unsaturated FAs, which enables both quantification and spatial visualization of FA isomers from biological samples. Abundant diagnostic fragment ions indicative of the C[double bond, length as m-dash]C positions were produced upon fragmentation of the FA epoxides derived from either in-solution or on-tissue PAA epoxidation of free FAs. The performance of the proposed approach was evaluated by analysis of FAs in human cell lines as well as mapping the FA isomers from cancer tissue samples with MALDI-TOF/TOF-MS. Merits of the newly developed method include high sensitivity, simplicity, high reaction efficiency, and capability of spatial characterization of FA isomers in tissue samples.

MALDI Matrices for the Analysis of Low Molecular Weight Compounds: Rational Design, Challenges and Perspectives

The analysis of low molecular weight (LMW) compounds is of great interest to detect small pharmaceutical drugs rapidly and sensitively, or to trace and understand metabolic pathways. Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) plays a central role in the analysis of high molecular weight (bio)molecules. However, its application for LMW compounds is restricted by spectral interferences in the low m/z region, which are produced by conventional organic matrices. Several strategies regarding sample preparation have been investigated to overcome this problem. A different rationale is centred on developing new matrices which not only meet the fundamental requirements of good absorption and high ionization efficiency, but are also vacuum stable and "MALDI silent", i. e., do not give matrix-related signals in the LMW area. This review gives an overview on the rational design strategies used to develop matrix systems for the analysis of LMW compounds, focusing on (i) the modification of well-known matrices, (ii) the search for high molecular weight matrices, (iii) the development of binary, hybrid and nanomaterial-based matrices, (iv) the advance of reactive matrices and (v) the progress made regarding matrices for negative or dual polarity mode.

Rapid Imaging of Unsaturated Lipids at an Isomeric Level Achieved by Controllable Oxidation

Lipid imaging plays an important role in the research of some diseases, such as cancers. Unsaturated lipids are often present as isomers that can have different functions; however, traditional tandem mass spectrometry imaging (MSI) cannot differentiate between different isomers, which presents difficulties for the pathological study of lipids. Herein, we propose a method for the MSI of the C═C double-bond isomers of unsaturated lipids based on oxidative reactions coupled with air flow-assisted desorption electrospray ionization, which can conveniently achieve rapid MSI of unsaturated lipids at an isomeric level. Using this method, tissue sections can be scanned directly with MSI after only 10 min of accelerated oxidation. This method was used for the imaging of mouse lung cancer tissues, revealing a distributional difference in the unsaturated lipid isomers of normal and pathological regions. Through the MSI of unsaturated lipids at an isomeric level in tissues infected with cancer cells, the regions where the isomers were enriched were exhibited, indicating that these regions were the most concentrated regions of cancer cells. This method provides a convenient platform for studying the functional effects of the isomers of unsaturated lipids in pathological tissues.