On-Tissue Derivatization via Electrospray Deposition for Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging of Endogenous Fatty Acids in Rat Brain Tissues

High-resolution imaging mass spectrometry combined with transcriptomic analysis identified a link between fatty acid composition of phosphatidylinositols and the immune checkpoint pathway at the primary tumour site of breast cancer

BACKGROUND

The fatty acid (FA) composition of phosphatidylinositols (PIs) is tightly regulated in mammalian tissue since its disruption impairs normal cellular functions. We previously found its significant alteration in breast cancer by using matrix-assisted laser desorption and ionisation imaging mass spectrometry (MALDI-IMS).

METHODS

We visualised the histological distribution of PIs containing different FAs in 65 primary breast cancer tissues using MALDI-IMS and investigated its association with clinicopathological features and gene expression profiles.

RESULTS

Normal ductal cells (n = 7) predominantly accumulated a PI containing polyunsaturated FA (PI-PUFA), PI(18:0/20:4). PI(18:0/20:4) was replaced by PIs containing monounsaturated FA (PIs-MUFA) in all non-invasive cancer cells (n = 12). While 54% of invasive cancer cells (n = 27) also accumulated PIs-MUFA, 46% of invasive cancer cells (n = 23) accumulated the PIs-PUFA, PI(18:0/20:3) and PI(18:0/20:4). The accumulation of PI(18:0/20:3) was associated with higher incidence of lymph node metastasis and activation of the PD-1-related immune checkpoint pathway. Fatty acid-binding protein 7 was identified as a putative molecule controlling PI composition.

CONCLUSIONS

MALDI-IMS identified PI composition associated with invasion and nodal metastasis of breast cancer. The accumulation of PI(18:0/20:3) could affect the PD-1-related immune checkpoint pathway, although its precise mechanism should be further validated.

Enhancement of On-tissue Chemical Derivatization by Laser-Assisted Tissue Transfer for MALDI MS Imaging

The development of on-tissue chemical derivatization methods for matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) of small endogenous metabolites in tissues has attracted great attention for their advantages in improving detection sensitivity and ionization efficiency of poorly ionized and low abundant metabolites. Herein, a laser-assisted tissue transfer (LATT) technique was developed to enhance on-tissue derivatization of small molecules. Using a focused blue laser, a thin-layer tissue film (∼1 μm) was transferred to an acceptor slide from a 6 μm dry tissue section preliminarily coated with derivatization and matrix reagents. The acceptor slide with its ablated constituents was then imaged by MALDI MS. On-tissue chemical derivatization with amino-specific derivatization reagent 4-hydroxy-3-methoxycinnamaldehyde (CA) was carried out on LATT system. 20-235 folds increase in signal intensity for CA derivatized metabolites such as amino acids, neurotransmitters, and dipeptides were observed from rat brain tissues in comparison with conventional incubation-based derivatization. This technique was further extended to derivatize steroids with Girard reagent T (GirT). The remarkable derivatization efficiency can mainly be attributed to the minimization of ion suppression effects due to the reduced thickness of tissue section and endogenous components. Additionally, shorter derivatization time with no obvious metabolite delocalization was achieved using LATT method. These results demonstrate the advantages of LATT in the enhancement of on-tissue derivatization for the more specific and sensitive imaging of small metabolites in tissues with MALDI MS.

Analytical challenges of shotgun lipidomics at different resolution of measurements

The essence of shotgun lipidomics is to maintain consistency of the chemical environment of lipid samples during mass spectrometry acquisition. This strategy is suitable for large-scale quantitative analysis. This strategy also allows sufficient time to collect data to improve the signal-to-noise ratio. The initial approach of shotgun lipidomics was the electrospray ionization (ESI)-based direct infusion mass spectrometry strategy. With development of mass spectrometry for small molecules, shotgun lipidomics methods have been extended to matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) and ambient mass spectrometry, including MS imaging methods. Furthermore, the object of analysis has extended from organ and body fluid levels to tissue and cell levels with technological developments. In this article, we summarize the status and technical challenges of shotgun lipidomics at different resolution of measurements from the mass spectrometry perspective.

Mass spectrometry imaging to detect lipid biomarkers and disease signatures in cancer

BACKGROUND

Current methods to identify, classify, and predict tumor behavior mostly rely on histology, immunohistochemistry, and molecular determinants. However, better predictive markers are required for tumor diagnosis and evaluation. Due, in part, to recent technological advancements, metabolomics and lipid biomarkers have become a promising area in cancer research. Therefore, there is a necessity for novel and complementary techniques to identify and visualize these molecular markers within tumors and surrounding tissue.

RECENT FINDINGS

Since its introduction, mass spectrometry imaging (MSI) has proven to be a powerful tool for mapping analytes in biological tissues. By adding the label-free specificity of mass spectrometry to the detailed spatial information of traditional histology, hundreds of lipids can be imaged simultaneously within a tumor. MSI provides highly detailed lipid maps for comparing intra-tumor, tumor margin, and healthy regions to identify biomarkers, patterns of disease, and potential therapeutic targets. In this manuscript, recent advancement in sample preparation and MSI technologies are discussed with special emphasis on cancer lipid research to identify tumor biomarkers.

CONCLUSION

MSI offers a unique approach for biomolecular characterization of tumor tissues and provides valuable complementary information to histology for lipid biomarker discovery and tumor classification in clinical and research cancer applications.

Emerging role of clinical mass spectrometry in pathology

Mass spectrometry-based assays have been increasingly implemented in various disciplines in clinical diagnostic laboratories for their combined advantages in multiplexing capacity and high analytical specificity and sensitivity. It is now routinely used in areas including reference methods development, therapeutic drug monitoring, toxicology, endocrinology, paediatrics, immunology and microbiology to identify and quantify biomolecules in a variety of biological specimens. As new ionisation methods, instrumentation and techniques are continuously being improved and developed, novel mass spectrometry-based clinical applications will emerge for areas such as proteomics, metabolomics, haematology and anatomical pathology. This review will summarise the general principles of mass spectrometry and specifically highlight current and future clinical applications in anatomical pathology.

Toward Mass Spectrometry Imaging in the Metabolomics Scale: Increasing Metabolic Coverage Through Multiple On-Tissue Chemical Modifications

Exploring the metabolic differences directly on tissues is essential for the comprehensive understanding of how multicellular organisms function. Mass spectrometry imaging (MSI) is an attractive technique toward this goal; however, MSI in metabolomics scale has been hindered by multiple limitations. This is most notable for single cell level high-spatial resolution imaging because of the limited number of molecules in small sampling size and the low ionization yields of many metabolites. Several on-tissue chemical derivatization approaches have been reported to increase MSI signals of targeted compounds, especially in matrix-assisted laser desorption/ionization (MALDI)-MSI. Herein, we adopt a combination of chemical derivatization reactions, to selectively enhance metabolite signals of a specific functional group for each consecutive tissue section. Three well-known on-tissue derivatization methods were used as a proof of concept experiment: coniferyl aldehyde for primary amines, Girard's reagent T for carbonyl groups, and 2-picolylamine for carboxylic acids. This strategy was applied to the cross-sections of leaves and roots from two different maize genotypes (B73 and Mo17), and enabled the detection of over six hundred new unique metabolite features compared to without modification. Statistical analysis indicated quantitative variation between metabolites in the tissue sections, while MS images revealed differences in localization of these metabolites. Combined, this untargeted approach facilitated the visualization of various classes of compounds, demonstrating the potential for untargeted MSI in the metabolomics scale.

Electrochemistry at the Synapse

Electrochemical measurements of neurotransmitters provide insight into the dynamics of neurotransmission. In this review, we describe the development of electrochemical measurements of neurotransmitters and how they started with extrasynaptic measurements but now are pushing toward synaptic measurements. Traditionally, biosensors or fast-scan cyclic voltammetry have monitored extrasynaptic levels of neurotransmitters, such as dopamine, serotonin, adenosine, glutamate, and acetylcholine. Amperometry and electrochemical cytometry techniques have revealed mechanisms of exocytosis, suggesting partial release. Advances in nanoelectrodes now allow spatially resolved, electrochemical measurements in a synapse, which is only 20-100 nm wide. Synaptic measurements of dopamine and acetylcholine have been made. In this article, electrochemical measurements are also compared to optical imaging and mass spectrometry measurements, and while these other techniques provide enhanced spatial or chemical information, electrochemistry is best at monitoring real-time neurotransmission. Future challenges include combining electrochemistry with these other techniques in order to facilitate multisite and multianalyte monitoring.

Recent advances in single-cell analysis by mass spectrometry

Cells are the most basic structural units that play vital roles in the functioning of living organisms. Analysis of the chemical composition and content of a single cell plays a vital role in ensuring precise investigations of cellular metabolism, and is a crucial aspect of lipidomic and proteomic studies. In addition, structural knowledge provides a better understanding of cell behavior as well as the cellular and subcellular mechanisms. However, single-cell analysis can be very challenging due to the very small size of each cell as well as the large variety and extremely low concentrations of substances found in individual cells. On account of its high sensitivity and selectivity, mass spectrometry holds great promise as an effective technique for single-cell analysis. Numerous mass spectrometric techniques have been developed to elucidate the molecular profiles at the cellular level, including electrospray ionization mass spectrometry (ESI-MS), secondary ion mass spectrometry (SIMS), laser-based mass spectrometry and inductively coupled plasma mass spectrometry (ICP-MS). In this review, the recent advances in single-cell analysis by mass spectrometry are summarized. The strategies of different ionization modes to achieve single-cell analysis are classified and discussed in detail.

Visualization and Identification of Neurotransmitters in Crustacean Brain via Multifaceted Mass Spectrometric Approaches

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) has emerged as a label-free analytical tool for fast biomolecule profiling on tissue sections. Among various functional molecules, mapping neurotransmitters and related metabolites is of tremendous significance, as these compounds are critical to signaling in the central nervous system. Here, we demonstrated the use of both derivatization and reaction-free approaches that greatly reduced signal complexity and thus enabled complementary signaling molecule visualization on crab brain sections via MALDI-LTQ-Orbitrap XL platform. Pyrylium salt served as a primary amine derivatization reagent and produced prominent signal enhancement of multiple neurotransmitters, including dopamine, serotonin, γ-aminobutyric acid, and histamine that were not detected in underivatized tissues. Molecules with other functional groups, such as acetylcholine and phosphocholine, were directly imaged after matrix application. The identities of discovered neurotransmitters were verified by standards using LC-MS/MS. This study broadens our understanding of metabolic signaling in the crustacean nervous system and highlights potential of multifaceted MS techniques for unambiguous neurotransmitter characterization.

Derivatization Strategy for Simultaneous Molecular Imaging of Phospholipids and Low-Abundance Free Fatty Acids in Thyroid Cancer Tissue Sections

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI) has been applied in many fields for detecting and imaging a variety of metabolites. In cancer research, this fast-growing imaging method also helps to elucidate the connection between the changes of metabolites in the microenvironment and the proliferation and survival of cancer cells. Free fatty acids (FFAs) are a vital building block of phospholipids (PLs) that can serve as a second cellular messenger and provide nutrients in the cancer microenvironment. The metabolism process of FFAs and PLs is highly relevant to the initiation and progression of different cancers. To better understand the metabolism process in cancer tissues, simultaneously detecting and imaging FFAs and PLs is essential. Despite the crucial developments that have been performed in the field of lipids imaging, FFAs and PLs have rarely been detected and imaged simultaneously in positive ion mode with good detection sensitivity. In this work, an on-tissue derivatization method was used to add a permanently quaternary amine onto FFAs; then, the FFAs and PLs were simultaneously imaged in positive ion mode. The derivatized FFAs are suitable for detection in positive ion mode. In comparison with the traditional matrix and the previous derivatization method, our derivatization reagent has a higher sensitivity for imaging FFAs. In addition, for simultaneous imaging analysis of FFAs and PLs, the number of imaged FFAs and PLs is greater than that with the previous on-tissue derivatization method. This high-sensitivity on-tissue derivatization method was applied to detect and image PLs and fatty acids in thyroid cancer tissues. In the MSI experiment, FFA derivatives and PLs were imaged while molecular localization and tissue integrity were maintained. Meanwhile, the correlation between PLs and FFAs was also studied, and the results showed that the correlations between saturated FFAs of C16:0 and C18:0 and PLs are better than the correlations of unsaturated FFAs with PLs.

Organic washes of tissue sections for comprehensive analysis of small molecule metabolites by MALDI MS imaging of rat brain following status epilepticus

INTRODUCTION

In-situ detection and in particular comprehensive analysis of small molecule metabolites (SMMs, m/z < 500) using matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI MSI) remain a challenge, mainly due to ion suppression effects from more abundant molecules in tissue section like lipids.

OBJECTIVE

A strategy based on organic washes to remove most ionization-suppressing lipids from tissue section was firstly explored for improved analysis of SMMs by MALDI MSI.

METHODS

The tissue sections after rinse with different organic solvents were analyzed by MALDI MSI, and the results were compared for the optimized washing conditions.

RESULTS

The rinse with chloroform for 15 s at - 20 °C significantly removed most glycerophospholipids and glycerolipids from tissue section. Consequentially, ATP-related energy metabolites, amino acids and derivatives, glucose derivatives, glycolysis pathway metabolites and other SMMs were able to be well-visualized with enhanced ion intensity and good reproducibility. The organic washes-based MALDI MSI was applied to the metabolic pathway analysis in rat brain following status epilepticus (SE) model, which was, as far as we know, the first report about in-situ detection of a broad range of metabolites in the model of SE by MALDI MSI technique. The alterations of cyclic adenosine monophosphate (cyclic AMP), inosine, glutamine, glutathione, taurine and spermine during SE were observed.

CONCLUSION

A simple organic washing protocol enables comprehensive analysis of tissue SMMs in MALDI MSI by removing ionization-suppressing lipids. The application in the SE model indicates that MALDI MSI analysis potentially provides new insight for understanding the disease mechanism.

Optimal Preparation of Formalin Fixed Samples for Peptide Based Matrix Assisted Laser Desorption/Ionization Mass Spectrometry Imaging Workflows

The use of matrix-assisted laser desorption/ionization, mass spectrometry imaging (MALDI MSI) has rapidly expanded, since this technique analyzes a host of biomolecules from drugs and lipids to N-glycans. Although various sample preparation techniques exist, detecting peptides from formaldehyde preserved tissues remains one of the most difficult challenges for this type of mass spectrometric analysis. For this reason, we have created and optimized a robust methodology that preserves the spatial information contained within the sample, while eliciting the greatest number of ionizable peptides. We have also aimed to achieve this in a cost effective and simple way, thereby eliminating potential bias or preparation error, which can occur when using automated instrumentation. The end result is a reproducible and inexpensive protocol.

1,6-Diphenyl-1,3,5-hexatriene (DPH) as a Novel Matrix for MALDI MS Imaging of Fatty Acids, Phospholipids, and Sulfatides in Brain Tissues

1,6-Diphenyl-1,3,5-hexatriene (DPH) is a commonly used fluorescence probe for studying cell membrane-lipids due to its affinity toward the acyl chains in the phospholipid bilayers. In this work, we investigated its use in matrix-assisted laser desorption/ionization (MALDI) as a new matrix for mass spectrometry imaging (MSI) of mouse and rat brain tissue. DPH exhibits very minimal matrix-induced background signals for the analysis of small molecules (below m/z of 1000). In the negative ion mode, DPH permits the highly sensitive detection of small fatty acids (m/z 200-350) as well as a variety of large lipids up to m/z of 1000, including lyso-phospholipid, phosphatidic acid (PA), phosphoethanolamine (PE), phosphatidylserine (PS), phosphatidylglycerol (PG), phosphatidylinositol (PI), and sulfatides (ST). The analytes were mostly detected as the deprotonated ion [M - H]^-. Our results also demonstrate that sublimated DPH is stable for at least 24 h under the vacuum of our MALDI mass spectrometer. The ability to apply DPH via sublimation coupled with its low volatility allows us to perform tissue imaging of the above analytes at high spatial resolution. The degree of lipid fragmentation was determined experimentally at varying laser intensities. The results illustrated that the use of relatively low laser energy is important to minimize the artificially generated fatty acid signals. On the other hand, the lipid fragmentation obtained at higher laser energies provided tandem MS information useful for lipid structure elucidation.

Oxidative Phospholipidomics in health and disease: Achievements, challenges and hopes

Phospholipid peroxidation products are recognized as important bioactive lipid mediators playing an active role as modulators in signalling events in inflammation, immunity and infection. The biochemical responses are determined by the oxidation structural features present in oxPL modulating biophysical and biological properties in model membranes and lipoproteins. In spite of the extensive work conducted with model systems over the last 20 years, the study of oxPL in biological systems has virtually stagnated. In fact, very little is known concerning the predominant oxPL in fluids and tissues, their basal levels, and any variations introduced with age, gender and ethnicity in health and disease. In consequence, knowledge on oxPL has not yet translated into clinical diagnostic, in the early and timely diagnosis of "silent" diseases such as atherosclerosis and cardiovascular diseases, or as prognosis tools in disease stratification and particularly useful in the context of multimorbidities. Their use as therapeutic solutions or the development of innovative functional biomaterials remains to be explored. This review summarizes the achievements made in the identification of oxPL revealing an enormous structural diversity. A brief overview of the challenges associated with the analysis of such diverse array of products is given and a critical evaluation on key aspects in the analysis pipeline that need to be addressed. Once these issues are addressed, Oxidative Phospholipidomics will hopefully lead to major breakthrough discoveries in biochemistry, pharmaceutical, and clinical areas for the upcoming 20 years. This article is part of Special Issue entitled 4-Hydroxynonenal and Related Lipid Oxidation Products.

Fischer Indole Synthesis in the Gas Phase, the Solution Phase, and at the Electrospray Droplet Interface

Previous reports have shown that reactions occurring in the microdroplets formed during electrospray ionization can, under the right conditions, exhibit significantly greater rates than the corresponding bulk solution-phase reactions. The observed acceleration under electrospray ionization could result from a solution-phase, a gas-phase, or an interfacial reaction. This study shows that a gas-phase ion/molecule (or ion/ion) reaction is not responsible for the observed rate enhancement in the particular case of the Fischer indole synthesis. The results show that the accelerated reaction proceeds in the microdroplets, and evidence is provided that an interfacial process is involved. Graphical Abstract <!-- [INSERT GRAPHICAL ABSTRACT TEXT HERE] -->.

Improving the analyte ion signal in matrix-assisted laser desorption/ionization imaging mass spectrometry via electrospray deposition by enhancing incorporation of the analyte in the matrix

RATIONALE

Matrix-assisted laser desorption/ionization (MALDI) is widely used as the ionization method in high-resolution chemical imaging studies that seek to visualize the distribution of analytes within sectioned biological tissues. This work extends the use of electrospray deposition (ESD) to apply matrix with an additional solvent spray to incorporate and homogenize analyte within the matrix overlayer.

METHODS

Analytes and matrix are sequentially and independently applied by ESD to create a sample from which spectra are collected, mimicking a MALDI imaging mass spectrometry (IMS) experiment. Subsequently, an incorporation spray consisting of methanol is applied by ESD to the sample and another set of spectra are collected. The spectra prior to and after the incorporation spray are compared to evaluate the improvement in the analyte signal.

RESULTS

Prior to the incorporation spray, samples prepared using α-cyano-4-hydroxycinnamic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB) as the matrix showed low signal while the sample using sinapinic acid (SA) initially exhibited good signal. Following the incorporation spray, the sample using SA did not show an increase in signal; the sample using DHB showed moderate gain factors of 2-5 (full ablation spectra) and 12-336 (raster spectra), while CHCA samples saw large increases in signal, with gain factors of 14-172 (full ablation spectra) and 148-1139 (raster spectra).

CONCLUSIONS

The use of an incorporation spray to apply solvent by ESD to a matrix layer already deposited by ESD provides an increase in signal by both promoting incorporation of the analyte within and homogenizing the distribution of the incorporated analyte throughout the matrix layer. Copyright © 2017 John Wiley & Sons, Ltd.

Dopamine-modified TiO2 monolith-assisted LDI MS imaging for simultaneous localization of small metabolites and lipids in mouse brain tissue with enhanced detection selectivity and sensitivity

Localization of metabolites using multiplexed mass spectrometry imaging (MSI) provides important chemical information for biological research. In contrast to matrix-assisted laser desorption/ionization (MALDI), TiO2-assisted laser desorption/ionization (LDI) for MSI improves detection of low molecular mass metabolites (<500 Da) by reducing matrix background. However, the low UV absorption of TiO2 nanoparticles and their ester hydrolysis catalytic activity hinder the detection of phospholipids and many low-abundance molecules. To address these challenges, we evaluated and optimized the material morphology and composition of TiO2. Dopamine (DA) was found to be an efficient ligand for TiO2, resulting in increased UV light absorption, higher surface pH, and formation of monolithic TiO2-DA structures. The sub-micron scale and higher surface pH of the TiO2 particle sizes led to improved detection of phospholipid signals. Compared to unmodified TiO2 sub-micron particles, the DA-modified TiO2 monolith led to 10- to 30-fold increases in the signal-to-noise ratios of a number of compound peaks. The TiO2-DA monolith-assisted LDI MSI approach has higher selectivity and sensitivity for Lewis basic compounds, such as fatty acids, cholesterols, ceramides, diacylglycerols, and phosphatidylethanolamine, when analyzed in positive mode, than traditional MALDI MS. Using this new method, over 100 molecules, including amino acids, alkaloids, free fatty acids, peptides, and lipids, were localized in mouse brain sections. By comparing the presence and localization of those molecules in young and old mouse brains, the approach demonstrated good performance in the determination of aging-related neurochemical changes in the brain.

Matrix-assisted laser desorption/ionization mass spectrometric imaging for the rapid segmental analysis of methamphetamine in a single hair using umbelliferone as a matrix

Segmental hair analysis offers a longer period for retrospective drug detection than blood or urine. Hair is a keratinous fiber and is strongly hydrophobic. The embedding of drugs in hydrophobic hair at low concentrations makes it difficult for extraction and detection with matrix-assisted laser desorption/ionization (MALDI) coupled with mass spectrometric imaging (MSI). In this study, a single scalp hair was longitudinally cut with a cryostat section to a length of 4 mm and fixed onto a stainless steel MALDI plate. Umbelliferone was used as a new hydrophobic matrix to enrich and assist the ionization efficiency of methamphetamine in the hair sample. MALDI-Fourier transform ion cyclotron resonance (FTICR)-MS profiling and imaging were performed for direct detection and mapping of methamphetamine on the longitudinal sections of the single hair sample in positive ion mode. Using MALDI-MSI, the distribution of methamphetamine was observed throughout five longitudinally sectioned hair samples from a drug abuser. The changes of methamphetamine were also semi-quantified by comparing the ratios of methamphetamine/internal standard (I.S). This method improves the detection sensitivity of target drugs embedded in a hair matrix for imaging with mass spectrometry. The method could provide a detection level of methamphetamine down to a nanogram per milligram incorporated into hair. The results were also compared with the conventional high performance liquid chromatography -tandem mass spectrometry (HPLC-MS/MS) method. Changes in the imaging results over time by the MSI method showed good semi-quantitative correlation to the results from the HPLC-MS/MS method. This study provides a powerful tool for drug abuse control and forensic medicine analysis in a narrow time frame, and a reduction in the sample amount required.

Mass Spectrometry Imaging of Complex Microbial Communities

In the two decades since mass spectrometry imaging (MSI) was first applied to visualize the distribution of peptides across biological tissues and cells, the technique has become increasingly effective and reliable. MSI excels at providing complementary information to existing methods for molecular analysis—such as genomics, transcriptomics, and metabolomics—and stands apart from other chemical imaging modalities through its capability to generate information that is simultaneously multiplexed and chemically specific. Today a diverse family of MSI approaches are applied throughout the scientific community to study the distribution of proteins, peptides, and small-molecule metabolites across many biological models.

The inherent strengths of MSI make the technique valuable for studying microbial systems. Many microbes reside in surface-attached multicellular and multispecies communities, such as biofilms and motile colonies, where they work together to harness surrounding nutrients, fend off hostile organisms, and shield one another from adverse environmental conditions. These processes, as well as many others essential for microbial survival, are mediated through the production and utilization of a diverse assortment of chemicals. Although bacterial cells are generally only a few microns in diameter, the ecologies they influence can encompass entire ecosystems, and the chemical changes that they bring about can occur over time scales ranging from milliseconds to decades. Because of their incredible complexity, our understanding of and influence over microbial systems requires detailed scientific evaluations that yield both chemical and spatial information. MSI is well-positioned to fulfill these requirements. With small adaptations to existing methods, the technique can be applied to study a wide variety of chemical interactions, including those that occur inside single-species microbial communities, between cohabitating microbes, and between microbes and their hosts.

In recognition of this potential for scientific advancement, researchers have adapted MSI methodologies for the specific needs of the microbiology research community. As a result, workflows exist for imaging microbial systems with many of the common MSI ionization methods. Despite this progress, there is substantial room for improvements in instrumentation, sample preparation, and data interpretation. This Account provides a brief overview of the state of technology in microbial MSI, illuminates selected applications that demonstrate the potential of the technique, and highlights a series of development challenges that are needed to move the field forward. In the coming years, as microbial MSI becomes easier to use and more universally applicable, the technique will evolve into a fundamental tool widely applied throughout many divisions of science, medicine, and industry.

Recent advancements in matrix-assisted laser desorption/ionization mass spectrometry imaging

Matrix assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) is a robust tool for spatially resolved analysis of biomolecules in situ. Recent advances in high ionization-efficiency MALDI matrices, new matrix deposition procedures, and the development of high spatial-resolution and high sensitivity MS instruments continue to drive new applications of MALDI-MSI, along with other MSI techniques, which allow us to visualize and determine the regio-specific and temporal changes in proteins, peptides, lipids, drug molecules, and metabolites within the tissues, cells and microorganisms. These provide researchers with a new route to the discovery of potential biomarkers of human disease and elucidation of the underlying biology of metabolic regulation, thus bringing our understanding of human health to a new level.

Molecular labels for analysis of amines and diols by spray based ionization-mass spectrometry

A family of imidazolium and pyridinium salts was synthesized for the purpose of labeling amines and 1,2-diols for highly sensitivite analysis by mass spectrometry. The chosen mass labels are shown to serve as effective reporters when bound to particles functionalized with amines or 1,2-diols and the binding is reversible. The straightforward synthetic route allows analogous internal standards to be generated quickly. Solvents amenable to electrospray ionization facilitate the rapid liberation of imidazolium and pyridinium mass labels from particles by hydrolysis in aqueous acid, while the acetal bond remains stable in anhydrous or buffered aqueous solution. Detection of the labels is demonstrated at 1 nM and quantitation of mass labels released from particles is also demonstrated.