A survey of useful salt additives in matrix-assisted laser desorption/ionization mass spectrometry and tandem mass spectrometry of lipids: introducing nitrates for improved analysis

Enhancement of Lipid Signals in Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry with Ammonium Fluoride as a Matrix Additive

Lipids are essential macromolecules that play a crucial role in numerous biological events. Lipids are structurally diverse which allows them to fulfill multiple functional roles. Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) is a powerful tool to understand the spatial localization of lipids within biological systems. Herein, we report the use of ammonium fluoride (NH4F) as a comatrix additive to enhance lipid detection in biological samples, with a signal increase of up to 200%. Emphasis was placed on anionic lipid enhancement with negative polarity measurements, with some preliminary work on cationic lipids detailed. We observed lipid signal enhancement of [M-H]- ions with the addition of NH4F additive attributed to a proton transfer reaction in several different lipid classes. Overall, our study demonstrates that the use of the NH4F comatrix additive substantially improves sensitivity for lipid detection in a MALDI system and is capable of being applied to a variety of different applications.

Comparison of Small Biomolecule Ionization and Fragmentation in Pseudomonas aeruginosa Using Common MALDI Matrices

Different bacterial cell surface associated biomolecules can be analyzed by matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry and coupled with collision induced dissociation (CID) for identification. Pseudomonas aeruginosa is an opportunistic, Gram-negative bacterium that causes acute or chronic biofilm infections. Cells of P. aeruginosa communicate through a system of signaling biomolecules known as quorum sensing (QS). The QS system can result in the production of biosurfactant rhamnolipids known to associate and alter the cellular membrane. MALDI-TOF utilizes a variety of matrices that can interact differently with biomolecules for selective ionization. We examined six common matrices to determine the optimal matrix specific to different molecule classes in P. aeruginosa associated with cell surfaces. Three major molecule classes (quinolones, rhamnolipids, and phospholipids) were observed to ionize selectively with the different matrices tested. Sodiated and protonated adducts differed between matrices utilized in our study. Isobaric ions were identified as different molecule classes depending on the matrix used. We highlight the role of matrix selection in MALDI-TOF identification of molecules within a complex biological mixture.

There is detectable variation in the lipidomic profile between stable and progressive patients with idiopathic pulmonary fibrosis (IPF)

Background

Idiopathic pulmonary fibrosis (IPF) is a chronic interstitial lung disease characterized by fibrosis and progressive loss of lung function. The pathophysiological pathways involved in IPF are not well understood. Abnormal lipid metabolism has been described in various other chronic lung diseases including asthma and chronic obstructive pulmonary disease (COPD). However, its potential role in IPF pathogenesis remains unclear.

Methods

In this study, we used ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS) to characterize lipid changes in plasma derived from IPF patients with stable and progressive disease. We further applied a data-independent acquisition (DIA) technique called SONAR, to improve the specificity of lipid identification.

Results

Statistical modelling showed variable discrimination between the stable and progressive subjects, revealing differences in the detection of triglycerides (TG) and phosphatidylcholines (PC) between progressors and stable IPF groups, which was further confirmed by mass spectrometry imaging (MSI) in IPF tissue.

Conclusion

This is the first study to characterise lipid metabolism between stable and progressive IPF, with results suggesting disparities in the circulating lipidome with disease progression.

Imaging of Neurotransmitters and Small Molecules in Brain Tissues Using Laser Desorption/Ionization Mass Spectrometry Assisted with Zinc Oxide Nanoparticles

Inorganic nanostructured materials such as silicon, carbon, metals, and metal oxides have been explored as matrices of low-background signals to assist the laser desorption/ionization (LDI) mass spectrometric (MS) analysis of small molecules, but their applications for imaging of small molecules in biological tissues remain limited in the literature. Titanium dioxide is one of the known nanoparticles (NP) that can effectively assist LDI MS imaging of low molecular weight molecules (LMWM). TiO₂ NP is commercially available as dispersions, which can be applied using a chemical solution sprayer. However, aggregation of NP can occur in the dispersions, and the aggregated NP can slowly clog the sprayer nozzle. In this work, the use of zinc oxide (ZnO) NP for LDI MS imaging is investigated as a superior alternative due to its dissolution in acidic pH. ZnO NP was found to deliver similar or better results in the imaging of LMWM in comparison to TiO₂ NP. The regular acid washes were effective in minimizing clogging and maintaining high reproducibility. High-quality images of mouse sagittal and rat coronal tissue sections were obtained. Ions were detected predominately as Na⁺ or K⁺ adducts in the positive ion mode. The number of LMWM detected with ZnO NP was similar to that obtained with TiO₂ NP, and only a small degree of specificity was observed.

Lithium Hydroxide Hydrolysis Combined with MALDI TOF Mass Spectrometry for Rapid Sphingolipid Detection

Sphingolipids have diverse structural and bioactive functions that play important roles in many key biological processes. Factors such as low relative abundance, varied structures, and a dynamic concentration range provide a difficult analytical challenge for sphingolipid detection. To further improve mass-spectrometry-based sphingolipid analysis, lithium adduct consolidation was implemented to decrease spectral complexity and combine signal intensities, leading to increased specificity and sensitivity. We report the use of lithium hydroxide as a base in a routine hydrolysis procedure in order to effectively remove common ionization suppressants (such as glycolipids and glycerophospholipids) and introduce a source of lithium into the sample. In conjunction, an optimized MALDI matrix system, featuring 2',4',6'-trihydroxyacetophenone (THAP) is used to facilitate lithium adduct consolidation during the MALDI process. The result is a robust and high-throughput sphingolipid detection scheme, particularly of low-abundance ceramides. Application of our developed workflow includes the detection of differentially expressed liver sphingolipid profiles from a high-fat-induced obesity mouse model. We also demonstrate the method's effectiveness in detecting various sphingolipids in brain and plasma matrices. These results were corroborated with data from UHPLC HR MS/MS and MALDI FT-ICR, verifying the efficacy of the method application. Overall, we demonstrate a high-throughput workflow for sphingolipid analysis in various biological matrices by the use of MALDI TOF and lithium adduct consolidation.

Complementary neuropeptide detection in crustacean brain by mass spectrometry imaging using formalin and alternative aqueous tissue washes

Neuropeptides are low abundance signaling molecules that modulate almost every physiological process, and dysregulation of neuropeptides is implicated in disease pathology. Mass spectrometry (MS) imaging is becoming increasingly useful for studying neuropeptides as new sample preparation methods for improving neuropeptide detection are developed. In particular, proper tissue washes prior to MS imaging have shown to be quick and effective strategies for increasing the number of detectable neuropeptides. Treating tissues with solvents could result in either gain or loss of detection of analytes, and characterization of these wash effects is important for studies targeting sub-classes of neuropeptides. In this communication, we apply aqueous tissue washes that contain sodium phosphate salts, including 10% neutral buffered formalin (NBF), on crustacean brain tissues. Our optimized method resulted in complementary identification of neuropeptides between washed and unwashed tissues, indicating that our wash protocol may be used to increase total neuropeptide identifications. Finally, we show that identical neuropeptides were detected between tissues treated with 10% NBF and an aqueous 1% w/v sodium phosphate solution (composition of 10% NBF without formaldehyde), suggesting that utilizing a salt solution wash affects neuropeptide detection and formaldehyde does not affect neuropeptide detection when our wash protocol is performed.

Lipidomic analysis of lactic acid bacteria strains by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Analysis by MALDI-TOF mass spectrometry and gas chromatography-mass spectrometry was used to characterize the lipid profile of 3 lactic acid bacteria strains. By gas chromatography coupled with mass spectrometry, 23 fatty acids were identified. Dominant acids were palmitic (C16:0), oleic (C18:1), and α-linoleic acid (C18:3n-3) for Lactobacillus paracasei; for Lactococcus lactis they were palmitic (C16:0), gondoic (C20:1), myristoleic (C14:1), and eicosadienoic acid (C20:2), respectively; and in the case of Lactobacillus curvatus were C18:1, C18:2n-6, and C16:0, respectively. The effect of the medium on fatty acid composition was also determined. In addition, the fatty acid profile was also compared using MALDI MS analysis. The MALDI-TOF MS was used for qualitative analysis and identification of bacterial lipids. Phosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol, phosphatidylcholine, triacylglycerols, and ceramides were the most abundant species in lactic acid bacteria. One hundred different combinations of fatty acids in polar and nonpolar lipids have been identified, including 11 phospholipids (18 phosphatidylglycerol, 16 phosphatidylethanolamine, 10 phosphatidylinositol, 8 phosphatidylcholine, 4 lyso-phosphatidylethanolamine, 3 lyso-phosphatidylcholine, 3 phosphatidylserine, 1 lyso-phosphatidic acid, 1 lyso-phosphatidylglycerol, 1 lyso-phoshatidylinositol, and 1 phosphatidic acid), 23 triacylglycerols, 9 ceramides, and 2 sphingomyelin. The most abundant fatty acids identified were C16:0, C16:1, C18:0, and C18:3. Obtained lipid profiles allowed to distinguish the tested bacterial strains.

MALDI MS Analysis to Investigate the Lipid Composition of Sperm

Background:

The sperm plasma membrane meets the requirements of sperm transit through the female genital tract and subsequent fertilization. Commonly, the (phospho)lipid composition of sperm is characterized by tremendous amounts of highly unsaturated fatty acyl residues such as docosahexaenoic and docosapentaenoic acid. While human sperm contain almost exclusively diacyl lipids, many animal sperm additionally contain significant amounts of ether lipids such as alkylacyl- and alkenyl-acyl lipids (plasmalogens).

Hypothesis/Objective:

It is suggested that deviations from the typical lipid composition are indicative of pathological changes. Therefore, simple methods to elucidate the sperm lipid composition are essential.

Method:

Matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS) is a fast and simple method. Since the selection of the most suitable matrix is a crucial step in MALDI MS, this topic will be highlighted. It will also be shown that MALDI MS can be easily combined with thin-layer chromatography to overcome ion suppression effects.

Results:

The lipid composition of sperm from different species can be elucidated by MALDI MS. However, different matrix compounds have to be used to record positive and negative ion mass spectra. Since some sperm (glyco)lipids are characterized by the presence of sulfate residues which suppress the detection of less acidic lipids in the negative ion mode, previous separation is often necessary. It will be also emphasized that plasmalogens can be easily identified by either enzymatic digestion or treatment with acids.

Conclusion:

MALDI MS is a reliable method to obtain sperm lipid fingerprints in a simple and convenient way.

Direct liquid extraction surface analysis mass spectrometry of cell wall lipids from mycobacteria: Salt additives for decreased spectral complexity

RATIONALE

Lipids are important mycobacterium cell wall constituents; changes are linked with drug resistance. Liquid extraction surface analysis (LESA) enables direct sampling in a highly sensitive manner. Here we describe protocols for the analysis of lipids from bacterial colonies. Lipids form various adducts, complicating spectra. Salt additives were investigated to circumvent this problem.

METHODS

Chloroform:methanol mixtures were studied for lipid extraction and analysis by LESA-MS. The inclusion of (ESI-compatible) acetate salts of sodium, potassium or lithium in the extraction solvent was investigated.

RESULTS

We report the detection of bacterial cell wall lipids from mycobacterial species using LESA for the first time. Sampling protocols were optimised for the use of volatile extraction solvents. The inclusion of acetate salt additives in the sampling solvent significantly reduces spectral complexity in comparison with no additives being used.

CONCLUSIONS

LESA offers a sensitive technique for bacterial lipid phenotyping. The inclusion of an acetate salt in the sampling solvent drives adduct formation towards a specific adduct type and thus significantly reduces spectral complexity.

Recent Developments of Useful MALDI Matrices for the Mass Spectrometric Characterization of Lipids

Matrix-assisted laser desorption/ionization (MALDI) is one of the most successful “soft” ionization methods in the field of mass spectrometry and enables the analysis of a broad range of molecules, including lipids. Although the details of the ionization process are still unknown, the importance of the matrix is commonly accepted. Both, the development of and the search for useful matrices was, and still is, an empirical process, since properties like vacuum stability, high absorption at the laser wavelength, etc. have to be fulfilled by a compound to become a useful matrix. This review provides a survey of successfully used MALDI matrices for the lipid analyses of complex biological samples. The advantages and drawbacks of the established organic matrix molecules (cinnamic or benzoic acid derivatives), liquid crystalline matrices, and mixtures of common matrices will be discussed. Furthermore, we will deal with nanocrystalline matrices, which are most suitable to analyze small molecules, such as free fatty acids. It will be shown that the analysis of mixtures and the quantitative analysis of small molecules can be easily performed if the matrix is carefully selected. Finally, some basic principles of how useful matrix compounds can be “designed” de novo will be introduced.

Toward Higher Sensitivity in Quantitative MALDI Imaging Mass Spectrometry of CNS Drugs Using a Nonpolar Matrix

Tissue-specific ion suppression is an unavoidable matrix effect in MALDI mass spectrometry imaging (MALDI-MSI), the negative impact of which on precision and accuracy in quantitative MALDI-MSI can be reduced to some extent by applying isotope internal standards for normalization and matrix-matched calibration routines. The detection sensitivity still suffers, however, often resulting in significant loss of signal for the investigated analytes. An MSI application considerably affected by this phenomenon is the quantitative spatial analysis of central nervous system (CNS) drugs. Most of these drugs are low molecular weight, lipophilic compounds, which exhibit inefficient desorption and ionization during MALDI using conventional polar acidic matrices (CHCA, DHB). Here, we present the application of the (2-[(2 E)-3-(4- tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile) matrix for high sensitivity imaging of CNS drugs in mouse brain sections. Since DCTB is usually described as an electron-transfer matrix, we provide a rationale (i.e., computational calculations of gas-phase proton affinity and ionization energy) for an additional proton-transfer ionization mechanism with this matrix. Furthermore, we compare the extent of signal suppression for five different CNS drugs when employing DCTB versus CHCA matrices. The results showed that the signal suppression was not only several times lower with DCTB than with CHCA but also depended on the specific tissue investigated. Finally, we present the application of DCTB and ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry to quantitative MALDI imaging of the anesthetic drug xylazine in mouse brain sections based on a linear matrix-matched calibration curve. DCTB afforded up to 100-fold signal intensity improvement over CHCA when comparing representative single MSI pixels and >440-fold improvement for the averaged mass spectrum of the adjacent tissue sections.

Carbon Nanotubes and Graphene Promote Pyrolysis of Free-Base Phthalocyanine

Unsubstituted phthalocyanines (including free-base H₂Pc and many of its metal complexes) are among the most stable organic compounds. They can sublime without decomposition under reduced pressure and temperatures of up to 550 °C. This property was previously employed to design a novel approach to noncovalent functionalization of pristine single-walled carbon nanotubes (SWNTs) with 3d metal(II) phthalocyanine complexes. However, when we attempted to use the same sublimation protocol to prepare a SWNTs-H₂Pc hybrid, an unexpected side effect of partial H₂Pc pyrolysis was detected, phthalonitrile being a main decomposition product, under the conditions when H₂Pc is supposed to be totally stable. By using density functional theory calculations, we offer an explanation for the thermal behavior of H₂Pc based on its covalent attachment to the pentagonal-ring topological defects, which are very common in all graphene-derived carbon nanomaterials and capable of reacting with amines via nucleophilic addition process.

Exploring Ion Suppression in Mass Spectrometry Imaging of a Heterogeneous Tissue

In this study we have explored several aspects of regional analyte suppression in mass spectrometry imaging (MSI) of a heterogeneous sample, transverse cryosections of mouse brain. Olanzapine was homogeneously coated across the section prior to desorption electrospray ionization (DESI) and matrix-assisted laser desorption ionization (MALDI) mass spectrometry imaging. We employed the concept of a tissue extinction coefficient (TEC) to assess suppression of an analyte on tissue relative to its intensity in an off tissue region. We expanded the use of TEC, by first segmenting anatomical regions using graph-cuts clustering and calculating a TEC for each cluster. The single ion image of the olanzapine [M + H]⁺ ion was seen to vary considerably across the image, with anatomical features such as the white matter and hippocampus visible. While trends in regional ion suppression were conserved across MSI modalities, significant changes in the magnitude of relative regional suppression effects between techniques were seen. Notably the intensity of olanzapine was less suppressed in DESI than for MALDI. In MALDI MSI, significant differences in the concentration dependence of regional TECs were seen, with the TEC of white matter clusters exhibiting a notably stronger correlation with concentration than for clusters associated with gray matter regions. We further employed cluster-specific TECs as regional normalization factors. In comparison to published pixel-by-pixel normalization methods, regional TEC normalization exhibited superior reduction ion suppression artifacts. We also considered the usefulness of a segmentation-based approach to compare spectral information obtained from complementary modalities.

Addition of CsCl reduces ion suppression effects in the matrix-assisted laser desorption/ionization mass spectra of triacylglycerol/phosphatidylcholine mixtures and adipose tissue extracts

RATIONALE

Ion suppression is a known disadvantage in mixture analysis. Matrix-assisted laser desorption/ionization (MALDI) mass spectra of crude adipose tissue extracts are dominated by triacylglycerol (TAG) signals while less abundant phospholipids such as phosphatidylcholines (PC) and particularly phosphatidylethanolamines (PE) are suppressed. It is suggested that addition of an excess of cesium (Cs) ions helps to overcome this problem.

METHODS

Selected lipid mixtures of known compositions and organic adipose tissue extracts were investigated by positive ion MALDI-time-of-flight mass spectrometry (TOF MS). 2,5-Dihydroxybenzoic acid (DHB) in methanol was used as the matrix. In selected cases the methanolic DHB solution was saturated by the addition of different solid alkali chlorides (such as NaCl, KCl, RbCl and CsCl). Studies on the solubilities of these salts in methanol and the interaction with DHB (by ¹³ C NMR) were also performed.

RESULTS

Saturation of the DHB matrix with solid CsCl leads to tremendous intensity differences, i.e. the intensities of the TAG signals (which otherwise dominate the mass spectra) are significantly reduced. In contrast, the intensity of small signals of phospholipids increases considerably. Decrease in the TAG signal intensity is particularly caused by the considerable size of the Cs⁺ ion which prevents successful analyte ionization.

CONCLUSIONS

The addition of CsCl improves the detectability of otherwise invisible or weak phospholipid ions. This is a simple approach to detect small amounts of phospholipids in the presence of an excess of TAG. No laborious and time-consuming separation of the total lipid extract into the individual lipid classes is required. Copyright © 2016 John Wiley & Sons, Ltd.

Classification of Bacillus and Brevibacillus species using rapid analysis of lipids by mass spectrometry

Bacillus are aerobic spore-forming bacteria that are known to lead to specific diseases, such as anthrax and food poisoning. This study focuses on the characterization of these bacteria by the detection of lipids extracted from 33 well-characterized strains from the Bacillus and Brevibacillus genera, with the aim to discriminate between the different species. For the purpose of analysing the lipids extracted from these bacterial samples, two rapid physicochemical techniques were used: matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF-MS) and liquid chromatography in conjunction with mass spectrometry (LC-MS). The findings of this investigation confirmed that MALDI-TOF-MS could be used to identify different bacterial lipids and, in combination with appropriate chemometrics, allowed for the discrimination between these different bacterial species, which was supported by LC-MS. The average correct classification rates for the seven species of bacteria were 62.23 and 77.03 % based on MALDI-TOF-MS and LC-MS data, respectively. The Procrustes distance for the two datasets was 0.0699, indicating that the results from the two techniques were very similar. In addition, we also compared these bacterial lipid MALDI-TOF-MS profiles to protein profiles also collected by MALDI-TOF-MS on the same bacteria (Procrustes distance, 0.1006). The level of discrimination between lipids and proteins was equivalent, and this further indicated the potential of MALDI-TOF-MS analysis as a rapid, robust and reliable method for the classification of bacteria based on different bacterial chemical components. Graphical abstract MALDI-MS has been successfully developed for the characterization of bacteria at the subspecies level using lipids and benchmarked against HPLC.

Large-Scale Metabolite Analysis of Standards and Human Serum by Laser Desorption Ionization Mass Spectrometry from Silicon Nanopost Arrays

The unique challenges presented by metabolomics have driven the development of new mass spectrometry (MS)-based techniques for small molecule analysis. We have previously demonstrated silicon nanopost arrays (NAPA) to be an effective substrate for laser desorption ionization (LDI) of small molecules for MS. However, the utility of NAPA-LDI-MS for a wide range of metabolite classes has not been investigated. Here we apply NAPA-LDI-MS to the large-scale acquisition of high-resolution mass spectra and tandem mass spectra from a collection of metabolite standards covering a range of compound classes including amino acids, nucleotides, carbohydrates, xenobiotics, lipids, and other classes. In untargeted analysis of metabolite standard mixtures, detection was achieved for 374 compounds and useful MS/MS spectra were obtained for 287 compounds, without individual optimization of ionization or fragmentation conditions. Metabolite detection was evaluated in the context of 31 metabolic pathways, and NAPA-LDI-MS was found to provide detection for 63% of investigated pathway metabolites. Individual, targeted analysis of the 20 common amino acids provided detection of 100% of the investigated compounds, demonstrating that improved coverage is possible through optimization and targeting of individual analytes or analyte classes. In direct analysis of aqueous and organic extracts from human serum samples, spectral features were assigned to a total of 108 small metabolites and lipids. Glucose and amino acids were quantitated within their physiological concentration ranges. The broad coverage demonstrated by this large-scale screening experiment opens the door for use of NAPA-LDI-MS in numerous metabolite analysis applications.

Multigrid MALDI mass spectrometry imaging (mMALDI MSI)

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) is an important technique for the spatially resolved molecular analysis of tissue sections. The selection of matrices influences the resulting mass spectra to a high degree. For extensive and simultaneous analysis, the application of different matrices to one tissue sample is desirable. To date, only a single matrix could be applied to a tissue section per experiment. However, repetitive removal of the matrix makes this approach time-consuming and damaging to tissue samples. To overcome these drawbacks, we developed a multigrid MALDI MSI technique (mMALDI MSI) that relies on automated inkjet printing to place differing matrices onto predefined dot grids. We used a cooled printhead to prevent cavitation of low viscosity solvents in the printhead nozzle. Improved spatial resolution of the dot grids was achieved by using a triple-pulse procedure that reduced droplet volume. The matrices can either be applied directly to the thaw-mounted tissue sample or by precoating the slide followed by mounting of the tissue sample. During the MALDI imaging process, we were able to precisely target different matrix point grids with the laser to simultaneously produce distinct mass spectra. Unlike the standard method, the prespotting approach optimizes the spectra quality, avoids analyte delocalization, and enables subsequent hematoxylin and eosin (H&E) staining. Graphical Abstract Scheme of the pre-spotted multigrid MALDI MSI workflow.

Profiling and tandem mass spectrometry analysis of aminoacylated phospholipids in Bacillus subtilis

Cationic modulation of the dominantly negative electrostatic structure of phospholipids plays an important role in bacterial response to changes in the environment. In addition to zwitterionic phosphatidylethanolamine, Gram-positive bacteria are also abundant in positively charged lysyl-phosphatidylglycerol. Increased amounts of both types of lipids render Gram-positive bacterial cells more resistant to cationic antibiotic peptides such as defensins.  Lysyl and alanyl-phosphatidylglycerol as well as alanyl-cardiolipin have also been studied by mass spectroscopy. Phospholipids modified by other amino acids have been discovered by chemical analysis of the lipid lysate but have yet to be studied by mass spectroscopy. We exploited the high sensitivity of modern mass spectroscopy in searching for substructures in complex mixtures to establish a sensitive and thorough screen for aminoacylated phospholipids. The search for deprotonated aminoacyl anions in lipid extracted from Bacillus subtilis strain 168 yielded strong evidence as well as relative abundance of aminoacyl-phosphatidylglycerols, which serves as a crude measure of the specificity of aminoacyl-phosphatidylglycerol synthase MprF. No aminoacyl-cardiolipin was found. More importantly, the second most abundant species in this category is D-alanyl-phosphatidylglycerol, suggesting a possible role in the D-alanylation pathway of wall- and lipo-teichoic acids.

2,5-dihydroxybenzoic acid salts for matrix-assisted laser desorption/ionization time-of-flight mass spectrometric lipid analysis: simplified spectra interpretation and insights into gas-phase fragmentation

RATIONALE

In the last decades the interest in lipids as important components of membranes has considerably increased. Nowadays, lipids are often routinely analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). In this regard, many relevant aspects are so far unknown, e.g., gas-phase stabilities, adduct formation and fragmentation. To fill this gap, MALDI matrix salts are presented which allow for simplified lipid analysis and elucidation of the underlying gas-phase fragmentation mechanisms.

METHODS

MALDI-TOF MS was used due to its beneficial properties for lipid investigations, e.g., high sensitivity, simple sample preparations, and a high tolerance to contaminants. The lipid hydrolysis, ionization and fragmentation properties of synthesized near neutral Na(+) and NH4 (+) salts of the commonly used MALDI matrix 2,5-dihydroxybenzoic acid were compared to that of DHB free acid itself as well as to base addition to DHB during dried-droplet sample preparation.

RESULTS

Many lipid classes such as sterols, triacylglycerols, phosphatidylcholines and -ethanolamines undergo initial protonation with subsequent prompt partial up to quantitative fragmentation when analyzed with classical acidic matrices by MALDI-TOF MS. Neutral matrix salts can prevent initial analyte fragmentation by suppression of analyte protonation. Additionally, intramolecular gas-phase fragmentation reactions can be inhibited due to analyte stabilization by cation chelation. Base addition during sample preparation leads not only to in situ generation of matrix salts but also to analyte hydrolysis.

CONCLUSIONS

Neutral DHB salts avoid separation of lipid species into several ionization states when used as matrices in MALDI-TOF MS. This allows for simplified lipid spectra interpretation. Due to the high cationization efficiency of DHB matrix salts, certain lipid classes become detectable which cannot be analyzed easily using standard acidic DHB.

Formal lithium fixation improves direct analysis of lipids in tissue by mass spectrometry

Mass spectrometry imaging is a powerful method for imaging and in situ characterization of lipids in thin tissue sections. Structural elucidation of lipids is often achieved via collision induced dissociation, and lithium-lipid adducts have been widely reported as providing the most structurally informative fragment ions. We present a method for the incorporation of lithium salts into tissue imaging experiments via fixation of samples in formal lithium solutions. The method is suitable for preparation of single tissue sections, or as an immersion fixation method for whole tissue blocks or organs prior to sectioning. We compare lithium adduct detection and MALDI-MSI of murine brain from analysis of tissues prepared in different ways. Tissues prepared in formal solutions containing lithium or sodium salts before coating in matrix via air-spray deposition are compared with fresh samples coated in lithium-doped matrix preparations by either dry-coating or air-spray deposition. Sample preparation via fixation in formal lithium is shown to yield the highest quality images of lithium adducts, resulting in acquisition of more informative product ion spectra in MALDI MS/MS profiling and imaging experiments. Finally, the compatibility of formal lithium solutions with standard histological staining protocols (hemotoxylin and eosin, Van Giessen and Oil Red O) is demonstrated in a study of human liver tissue.

Fluorometric beam profiling of UV MALDI lasers

The photon distribution (beam profile) of the laser as projected onto the sample is an important variable in matrix assisted laser desorption ionization mass spectrometry (MALDI-MS). Measurement of the beam profile is, therefore, an important factor within MALDI-MS. In this study a simple, low-cost fluorometric laser beam profiling technique is presented and applied in conjunction with MALDI-MS experiments. A comparison of the beam profile information afforded by a commercial system and the fluorometric method is carried out to determine the variation of beam profile for an Nd:YVO4 laser operated between 1 and 25 kHz. The beam profile information can be used, in conjunction with corresponding ion yields, to inform MALDI-MS experiments. The fluorometric beam profiling technique is used to obtain information about the beam dimensions as incident upon the MALDI-MS sample plate in-source. These values are compared with equivalent information obtained from ablation of thin film α-cyano-4-hydroxycinnamic acid (CHCA). In this study, area estimation by ablation provided a value 1.6 times smaller than that obtained by the fluorometric method, demonstrating the need for caution when measuring beam profile and, therefore, fluence, in MALDI-MS. ᅟ

para-Nitroaniline is a promising matrix for MALDI-MS imaging on intermediate pressure MS systems

para-Nitroaniline (PNA) is presented as a promising matrix for matrix-assisted laser desorption ionization mass spectrometry imaging (MALDI-MSI) on an intermediate-pressure ion source (~1 Torr) QqTOF instrument using an Nd:YVO4 laser operated at 5 kHz. An imaging study was carried out to determine the utility of PNA at this pressure by analyzing 14 tissue sections. We demonstrate acquisition of high-quality imaging data over a 6-h period in the ion source. In this study, comparisons were made between PNA and α-cyano-4-hydroxycinnamic acid (CHCA) in positive ion mode to demonstrate the utility of PNA in these circumstances. PNA performed as well as or better than CHCA in terms of lipid ion intensities, resulting in lower levels of ion fragmentation and in lower incidences of analyte migration at the edges of the tissue sections when using airspray matrix deposition.

Memory efficient principal component analysis for the dimensionality reduction of large mass spectrometry imaging data sets

A memory efficient algorithm for the computation of principal component analysis (PCA) of large mass spectrometry imaging data sets is presented. Mass spectrometry imaging (MSI) enables two- and three-dimensional overviews of hundreds of unlabeled molecular species in complex samples such as intact tissue. PCA, in combination with data binning or other reduction algorithms, has been widely used in the unsupervised processing of MSI data and as a dimentionality reduction method prior to clustering and spatial segmentation. Standard implementations of PCA require the data to be stored in random access memory. This imposes an upper limit on the amount of data that can be processed, necessitating a compromise between the number of pixels and the number of peaks to include. With increasing interest in multivariate analysis of large 3D multislice data sets and ongoing improvements in instrumentation, the ability to retain all pixels and many more peaks is increasingly important. We present a new method which has no limitation on the number of pixels and allows an increased number of peaks to be retained. The new technique was validated against the MATLAB (The MathWorks Inc., Natick, Massachusetts) implementation of PCA (princomp) and then used to reduce, without discarding peaks or pixels, multiple serial sections acquired from a single mouse brain which was too large to be analyzed with princomp. Then, k-means clustering was performed on the reduced data set. We further demonstrate with simulated data of 83 slices, comprising 20,535 pixels per slice and equaling 44 GB of data, that the new method can be used in combination with existing tools to process an entire organ. MATLAB code implementing the memory efficient PCA algorithm is provided.