Reducing fragmentation observed in the matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis of triacylglycerols in vegetable oils

Sodium doping and trapped ion mobility spectrometry improve lipid detection for novel MALDI-MSI analysis of oats

Oat (Avena sativa L.) is an important cereal grain with a unique nutritional profile including a high proportion of lipids. Understanding lipid composition and distribution in oats is valuable for plant, food and nutritional research, and can be achieved using MALDI mass spectrometry imaging (MALDI-MSI). However, this approach presents several challenges for sample preparation (hardness of grains) and analysis (isobaric and isomeric properties of lipids). Here, oat sections were successfully mounted onto gelatin-coated indium tin oxide slides with minimal tearing. Poor detection of triacylglycerols was resolved by applying sodium chloride during mounting, increasing signal intensity. In combination with trapped ion mobility spectrometry (TIMS), lipid identification significantly improved, and we report the separation of several isobaric and isomeric lipids with visualisation of their "true" spatial distributions. This study describes a novel MALDI-TIMS-MSI analytical technique for oat lipids, which may be used to improve the discovery of biomarkers for grain quality.

Combined Use of MALDI-TOF Mass Spectrometry and 31P NMR Spectroscopy for the Analysis of (Phospho)Lipids

Lipids are important and abundant constituents of all biological tissues and body fluids. In particular, phospholipids (PLs) constitute a major part of the cellular membrane and play a role in signal transduction, and some selected PLs are increasingly considered as potential disease markers. Unfortunately, methods of lipid analysis are less established in comparison to techniques of protein analysis. Mass spectrometry (MS) is an increasingly used technique to analyze lipids, especially in combination with electrospray ionization MS, which is the most commonly used ionization technique in lipidomics. Matrix-assisted laser desorption/ionization coupled to time-of-flight MS (MALDI-TOF MS) has itself proven to represent a useful tool in the field of lipid analysis. ³¹P nuclear magnetic resonance (NMR) spectroscopy, another powerful method for PL analysis, represents a direct quantitative method and does not suffer from suppression effects.This paper gives an overview of methodological aspects of MALDI-TOF MS and ³¹P NMR in lipid research and summarizes the specific advantages and drawbacks of both methods. In particular, suppression effects in MS will be highlighted, and possible ways to overcome this problem, e.g., the use of different matrices and separation of the relevant lipid mixture prior to analysis, will be discussed.

A Comparative Study of Milk Fat Extracted from the Milk of Different Goat Breeds in China: Fatty Acids, Triacylglycerols and Thermal and Spectroscopic Characterization

Goat milk (GM) is an excellent alternative to cow milk and has recently been used in commercial infant formula preparation due to its superior fat composition. Here, the fatty acid (FA) composition, triacylglycerol (TAG) molecular species, thermal behavior and infrared spectra of extracted milk fat from the milk of the two main breeds of dairy goat bred in China (Guanzhong GM (GZG) and Xinong Saanen GM (XSG)) are investigated. Gas chromatography, Fourier-transform infrared spectroscopy, differential scanning calorimetry and ultra-performance convergence chromatography with quadrupole time-of-flight mass spectrometry are applied. The obtained results evidence significant fat compositional differences based on the breed that produced the considered GM. The major FAs in both GM fats were capric (C10:0), myristic (C14:0), palmitic (C16:0), stearic (C18:0) and oleic (C18:1 n-9c). GZG presented a higher content of medium-chain saturated FAs, while XSG had higher unsaturated FAs with higher ratios of L/Ln and n-6/n-3. A total of 339 and 359 TAGs were detected and quantified in GZG and XSG, and the major TAGs were those of m/z 740.6712 (14.10 ± 0.27%) and m/z 684.6094 (10.94 ± 0.02%), respectively. Milk TAGs of GZG and XSG showed 24–54 and 26–54 total acyl carbon numbers with a 0–4 and 0–5 double bond number at 68 and 72 various retention times, respectively. Thermal analysis showed that all GM fat samples melted below normal body temperature. Infrared spectra revealed higher absorption values of GZG milk fat. This study provides valuable information to the dairy industry sector about GM fat produced in China, assessing the appropriateness of Chinese GM fat to be applied in Chinese infant formula.

A new update of MALDI-TOF mass spectrometry in lipid research

Matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS) is an indispensable tool in modern lipid research since it is fast, sensitive, tolerates sample impurities and provides spectra without major analyte fragmentation. We will discuss some methodological aspects, the related ion-forming processes and the MALDI MS characteristics of the different lipid classes (with the focus on glycerophospholipids) and the progress, which was achieved during the last ten years. Particular attention will be given to quantitative aspects of MALDI MS since this is widely considered as the most serious drawback of the method. Although the detailed role of the matrix is not yet completely understood, it will be explicitly shown that the careful choice of the matrix is crucial (besides the careful evaluation of the positive and negative ion mass spectra) in order to be able to detect all lipid classes of interest. Two developments will be highlighted: spatially resolved Imaging MS is nowadays well established and the distribution of lipids in tissues merits increasing interest because lipids are readily detectable and represent ubiquitous compounds. It will also be shown that a combination of MALDI MS with thin-layer chromatography (TLC) enables a fast spatially resolved screening of an entire TLC plate which makes the method competitive with LC/MS.

Mass spectrometry-based lipid analysis and imaging

Mass spectrometry imaging (MSI) is a powerful tool for in situ mapping of analytes across a sample. With growing interest in lipid biochemistry, the ability to perform such mapping without antibodies has opened many opportunities for MSI and lipid analysis. Herein, we discuss the basics of MSI with particular emphasis on MALDI mass spectrometry and lipid analysis. A discussion of critical advancements as well as protocol details are provided to the reader. In addition, strategies for improving the detection of lipids, as well as applications in biomedical research, are presented.

2-Cyano-3-(2-thienyl)acrylic Acid as a New MALDI Matrix for the Analysis of a Broad Spectrum of Analytes

A low-cost synthetic 2-cyano-3-(2-thienyl)acrylic acid (CTA) is developed as a new MALDI matrix for the analysis of various classes of compounds such as lipids (e.g., fatty acids), peptides, proteins, saccharides, natural products (i.e., iridoids), PEGs, and organometallics in the positive-ion mode. The difficulty in the analysis of high molecular mass PEGs was overcome by using CTA as matrix even at low concentrations. Both high molecular mass proteins and peptides were successfully analyzed using CTA. The mass spectra of all of the studied analytes with CTA showed high signal-to-noise (S/N) ratios and spectral resolutions when compared to other conventional matrices such as SA, DHB, DT, and HCCA. However, in the case of peptide analysis with CTA, the resulting mass spectra are found to be similar to that of the well-established HCCA matrix. On the basis of the physicochemical properties of the analytes, the CTA works as a proton/cation or electron-transfer matrix. It proves that the CTA can be used as a common matrix for the analysis of majority classes of analytes instead of using a specific matrix for the particular class of analytes. Further, the CTA provides an advantage in the analysis of unknown samples as it rules out ambiguity in the selection of particular matrix and it may also offer a complete profile of the tissue surface in the MALDI-imaging experiments.

Matrix-assisted ionization Fourier transform mass spectrometry for the analysis of lipids

RATIONALE

Assessing the utility of vacuum matrix-assisted ionization (MAI) for the direct and rapid analysis of lipids in complex samples with emphasis on bacterial taxonomy.

METHODS

Matrix-assisted ionization Fourier transform mass spectrometry (MAI-FTMS) was used to characterize polar and non-polar lipids in mixtures.

RESULTS

For non-polar lipid triacylglycerols (TAGs), MAI-FTMS produced lipid-specific ions for eight different edible oils and allowed these oils to be identified based on their MAI-FTMS profiles. For polar lipids from bacteria, MAI-FTMS of crude lipid extracts allowed taxonomic identification of eight blind-coded samples based on taxonomy-specific phospholipid profiles. MAI produced results comparable and complementary to benchmark MALDI and ESI methods currently used for characterization of polar and non-polar lipids in the same mixtures.

CONCLUSIONS

The newly developed MAI technique is a rapid, simple and complementary method for the characterization of polar and non-polar lipids in complex mixtures.

Short-term effects of dietary bovine milk on fatty acid composition of human milk: A preliminary multi-analytical study

The fatty acid (FA) composition of human milk (HM) from N = 9 Italian healthy donors following a free diet exhibited FA-dependent ranges of variability, as assessed by GC-FID. The possible short-term changes in the FA profile were monitored in the milk of lactating mothers (three) collected at five time points over a 6 h period, following an oral load (200 mL) of bovine milk. An array of techniques was exploited, including UHPLC-ESI-MS/MS of intact lipids and MALDI-TOF MS before and after chemical hydrogenation or bromination, in addition to MALDI-TOF MS analysis of FA after saponification, to monitor short-chain and odd-chain FA in HM as markers of bovine milk fat. A single administration of bovine milk did not appreciably modify the lipid pattern, suggesting that the maternal diet could induce not detectable short-term changes on the lipid composition of HM. Diet-induced increase of butyric acid was also excluded by ¹³C NMR. The functions that HM FA exert in infant physiology appear finely regulated through maternal metabolism.

Discrimination of bovine milk from non-dairy milk by lipids fingerprinting using routine matrix-assisted laser desorption ionization mass spectrometry

An important sustainable development goal for any country is to ensure food security by producing a sufficient and safe food supply. This is the case for bovine milk where addition of non-dairy milks such as vegetables (e.g., soya or coconut) has become a common source of adulteration and fraud. Conventionally, gas chromatography techniques are used to detect key lipids (e.g., triacylglycerols) has an effective read-out of assessing milks origins and to detect foreign milks in bovine milks. However, such approach requires several sample preparation steps and a dedicated laboratory environment, precluding a high throughput process. To cope with this need, here, we aimed to develop a novel and simple method without organic solvent extractions for the detection of bovine and non-dairy milks based on lipids fingerprint by routine MALDI-TOF mass spectrometry (MS). The optimized method relies on the simple dilution of milks in water followed by MALDI-TOF MS analyses in the positive linear ion mode and using a matrix consisting of a 9:1 mixture of 2,5-dihydroxybenzoic acid and 2-hydroxy-5-methoxybenzoic acid (super-DHB) solubilized at 10 mg/mL in 70% ethanol. This sensitive, inexpensive, and rapid method has potential for use in food authenticity applications.

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.

Benefits of NaCl addition for time-of-flight secondary ion mass spectrometry analysis including the discrimination of diacylglyceride and triacylglyceride ions

RATIONALE

Diacylglycerides (DAGs) and triacylglycerides (TAGs) are two important lipid classes present in all mammalian cells that share similar chemical structures but differ in biological function in cells and tissues. Differentiation of these two species during time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis is therefore important, but has been difficult due to the formation of DAG-like ions during the ionization process of TAGs.

METHODS

We investigated the use of salt adduct formation as a quick and simple method to determine the origin of the DAG-like ions in ToF-SIMS spectra. NaCl was added to lipid standards of a DAG and a TAG and differences in fragmentation patterns were identified. The salt was then applied to prepared tissue samples by spraying with a saturated solution of NaCl in methanol and samples were analysed with ToF-SIMS using a 40 keV (CO₂ )_6k ⁺ primary ion beam.

RESULTS

A 40 Da peak shift was observed in the DAG spectrum that was not observed in the TAG spectrum ([M + H - H₂ O]⁺ to [M + Na]⁺ ) while the isobaric [M - RCOO]⁺ peak did not shift allowing differentiation between the two species. Spraying NaCl on to tissue sections indicated that the DAG-like ions originated from TAGs.

CONCLUSIONS

With the method described in this paper, simple addition of salt by spraying on the sample leads to better interpretation of complex mass spectra from biological tissue samples, discriminating DAG and TAG fragment peaks.

Phosphonate coating of SiO2 nanoparticles abrogates inflammatory effects and local changes of the lipid composition in the rat lung: a complementary bioimaging study

BACKGROUND

The well-known inflammatory and fibrogenic changes of the lung upon crystalline silica are accompanied by early changes of the phospholipid composition (PLC) as detected in broncho-alveolar lavage fluid (BALF). Amorphous silica nanoparticles (NPs) evoke transient lung inflammation, but their effect on PLC is unknown. Here, we compared effects of unmodified and phosphonated amorphous silica NP and describe, for the first time, local changes of the PLC with innovative bioimaging tools.

METHODS

Unmodified (SiO₂-n), 3-(trihydroxysilyl) propyl methylphosphonate coated SiO₂-n (SiO₂-p) as well as a fluorescent surrogate of SiO₂-n (SiO₂-FITC) nanoparticles were used in this study. In vitro toxicity was tested with NR8383 alveolar macrophages. Rats were intratracheally instilled with SiO₂-n, SiO₂-p, or SiO₂-FITC, and effects on lungs were analyzed after 3 days. BALF from the right lung was analyzed for inflammatory markers. Cryo-sections of the left lung were subjected to fluorescence microscopy and PLC analyses by matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MS), Fourier transform infrared microspectroscopy (FT-IR), and tandem mass spectrometry (MS/MS) experiments.

RESULTS

Compared to SiO₂-p, SiO₂-n NPs were more cytotoxic to macrophages in vitro and more inflammatory in the rat lung, as reflected by increased concentration of neutrophils and protein in BALF. Fluorescence microscopy revealed a typical patchy distribution of SiO₂-FITC located within the lung parenchyma and alveolar macrophages. Superimposable to this particle distribution, SiO₂-FITC elicited local increases of phosphatidylglycerol (PG) and phosphatidylinositol (PI), whereas phoshatidylserine (PS) and signals from triacylgyceride (TAG) were decreased in the same areas. No such changes were found in lungs treated with SiO₂-p or particle-free instillation fluid.

CONCLUSIONS

Phosphonate coating mitigates effects of silica NP in the lung and abolishes their locally induced changes in PLC pattern. Bioimaging methods based on MALDI-MS may become a useful tool to investigate the mode of action of NPs in tissues.

Combined Use of MALDI-TOF Mass Spectrometry and 31P NMR Spectroscopy for Analysis of Phospholipids

Lipids are important and abundant constituents of all biological tissues and body fluids. In particular, phospholipids (PL) constitute a major part of the cellular membrane, play a role in signal transduction, and some selected PL are increasingly considered as potential disease markers. However, methods of lipid analysis are less established in comparison to techniques of protein analysis. Mass spectrometry (MS) is an increasingly used technique to analyze lipids, especially in combination with electrospray ionization (ESI) MS which is the so far best established ionization method. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS has itself proven to be also useful in the field of lipid analysis. ³¹P nuclear magnetic resonance (NMR) spectroscopy is another powerful method of PL analysis, represents a direct quantitative method, and does not suffer from suppression effects.This chapter gives an overview of methodological aspects of MALDI-TOF MS and ³¹P NMR in lipid research and summarizes the specific advantages and drawbacks of both methods. In particular, suppression effects in MS will be highlighted and possible ways to overcome this problem (use of different matrices, separation of the relevant lipid mixture prior to analysis) will be discussed.

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.

Functionalization of silicon nanowire arrays by silver nanoparticles for the laser desorption ionization mass spectrometry analysis of vegetable oils

In this work, novel hybrid nanostructured surfaces, consisting of dense arrays of silicon nanowires (SiNWs) functionalized by Ag nanoparticles (AgNP/SiNWs), were used for the laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) analysis of some typical unsaturated food components (e.g. squalene, oleic acid) to assess their MS performance. The synthesis of the novel platforms is an easy, cost-effective process based on the maskless wet-etching preparation at room temperature of SiNWs followed by their decoration with AgNPs, produced by pulsed laser deposition. No particular surface pretreatment or addition of organic matrixes/ionizers was necessary. Moreover, oil extracts (e.g. extra virgin olive oil, peanut oil) could be investigated on AgNP/SiNWs surfaces, revealing their different MS profiles. It was shown that such substrates operate at reduced laser energy, typically generating intense silver cluster ions and analyte adducts. A comparison with bare SiNWs was also performed, indicating the importance of AgNP density on NW surface. In this case, desorption/ionization on silicon was invoked as probable LDI mechanism. Finally, the influence of SiNW length and surface composition on MS results was assessed. The combination of typical properties of SiNWs (hydrophobicity, antireflectivity) with ionization ability of metal NPs can be a valid methodology for the further development of nanostructured surfaces in LDI-TOF MS applications. Copyright © 2016 John Wiley & Sons, Ltd.

Sodium-Doped Gold-Assisted Laser Desorption Ionization for Enhanced Imaging Mass Spectrometry of Triacylglycerols from Thin Tissue Sections

The deposition of sodium salts followed by a sputtered layer of gold has been demonstrated to be a power combination for the analysis of triacylglycerols (TAGs) from tissue sections by laser desorption ionization (LDI) imaging mass spectrometry (IMS). Various sodium salts were tested for their capability to ionize TAGs and their ability to produce fast drying, small crystals (≤3 μm). The spray deposition of a sodium acetate and carbonate buffer mixture at pH 10.3 on which a 28 ± 3 nm sputtered layer of gold (Au-CBS) is subsequently deposited was found to provide the most effective combination for TAG analysis by high imaging resolution IMS. Under these conditions, a 30-fold increase in TAG signal intensity was observed when compared to matrix-assisted LDI (MALDI) methods using 2,5-dihydrobenzoic acid as matrix. Furthermore, Au-CBS led to an increase in the number of detected TAG species from ∼7 with DHB to more than 25 with the novel method, while few phospholipid signals were observed. These results were derived from the IMS investigation of fresh frozen mouse liver and rabbit adrenal gland tissue sections with a range of higher spatial resolutions between 35 and 10 μm. Au-CBS-LDI MS presents a highly sensitive and specific alternative to MALDI MS for imaging of TAGs from tissue sections. This novel approach has the potential to provide new biological insights on the role of TAGs in both health and disease.

Rapid screening of mixed edible oils and gutter oils by matrix-assisted laser desorption/ionization mass spectrometry

Authentication of edible oils is a long-term issue in food safety, and becomes particularly important with the emergence and wide spread of gutter oils in recent years. Due to the very high analytical demand and diversity of gutter oils, a high throughput analytical method and a versatile strategy for authentication of mixed edible oils and gutter oils are highly desirable. In this study, an improved matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) method has been developed for direct analysis of edible oils. This method involved on-target sample loading, automatic data acquisition and simple data processing. MALDI-MS spectra with high quality and high reproducibility have been obtained using this method, and a preliminary spectral database of edible oils has been set up. The authenticity of an edible oil sample can be determined by comparing its MALDI-MS spectrum and principal component analysis (PCA) results with those of its labeled oil in the database. This method is simple and the whole process only takes several minutes for analysis of one oil sample. We demonstrated that the method was sensitive to change in oil compositions and can be used for measuring compositions of mixed oils. The capability of the method for determining mislabeling enables it for rapid screening of gutter oils since fraudulent mislabeling is a common feature of gutter oils.

The use of hydrazine-based derivatization reagents for improved sensitivity and detection of carbonyl containing compounds using MALDI-MSI

Hydrazine-based derivatization reagents have been used to detect the presence of the carbonyl containing glucocorticoid fluticasone proprionate in rat lung tissue by MALDI-MSI. Such reagents also act as a matrix for analysis by MALDI-MS and have been termed "reactive matrices". Cryosections of rat lung tissue (12 μm), spotted with a range of concentrations of fluticasone proprionate, were derivatized in situ with 2,4-dinitrophenylhydrazine (DNPH) and 4-dimethylamino-6-(4-methoxy-1-naphthyl)-1,3,5-triazine-2-hydrazine (DMNTH) by the use of an acoustic reagent spotter. It has been demonstrated that DMNTH gave superior results compared to DNPH and that analysis of samples immediately after application of DMNTH resulted in the detection of the protonated hydrazone derivative ([MD + H](+)) of fluticasone propionate at a concentration of 500 ng/μL. It has been further shown that a prolonged reaction time (~48 h) improves the detection limit of the protonated hydrazone derivative to 50 ng/μL and that improvements in sensitivity and limits of detection are obtained when a conventional MALDI matrix CHCA is employed in conjunction with the DNPH/DMNTH reactive matrix.

Triacylglyceride composition and fatty acyl saturation profile of a psychrophilic and psychrotolerant fungal species grown at different temperatures

Pseudogymnoascus destructans is a psychrophilic fungus that infects cutaneous tissues in cave dwelling bats, and it is the causal agent for white nose syndrome (WNS) in North American (NA) bat populations. Geomyces pannorum is a related psychrotolerant keratinolytic species that is rarely a pathogen of mammals. In this study, we grew P. destructans and G. pannorum in static liquid cultures at favourable and suboptimal temperatures to: 1) determine if triacylglyceride profiles are species-specific, and 2) determine if there are differences in fatty acyl (FA) saturation levels with respect to temperature. Total lipids isolated from both fungal spp. were separated by thin-layer chromatography and determined to be primarily sterols (∼15 %), free fatty acids (FFAs) (∼45 %), and triacylglycerides (TAGs) (∼50 %), with minor amounts of mono-/diacylglycerides and sterol esters. TAG compositions were profiled by matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF). Total fatty acid methyl esters (FAMEs) and acyl lipid unsaturation levels were determined by gas chromatography-mass spectrometry (GC-MS). Pseudogymnoascus destructans produced higher proportions of unsaturated 18C fatty acids and TAGs than G. pannorum. Pseudogymnoascus destructans and G. pannorum produced up to a two-fold increase in 18:3 fatty acids at 5 °C than at higher temperatures. TAG proportion for P. destructans at upper and lower temperature growth limits was greater than 50 % of total dried mycelia mass. These results indicate fungal spp. alter acyl lipid unsaturation as a strategy to adapt to cold temperatures. Differences between their glycerolipid profiles also provide evidence for a different metabolic strategy to support psychrophilic growth, which may influence P. destructans' pathogenicity to bats.

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.

Laser Desorption/Ionization Mass Spectrometry (LDI-MS) of Lipids with Iron Oxide Nanoparticle-Coated Targets

Iron oxide nanoparticle (NP)-coated target plates were employed for the direct detection and analysis of low molecular weight lipids by laser desorption/ionization (LDI) mass spectrometry (MS). We have demonstrated that the use of the iron oxide NP-coated target provides a simple, direct, and rapid detection method for lipid standards and epidermal surface lipids without any cumbersome sample pretreatment as well as mass spectra that are free of background matrix peaks. Lipid standards (1-stearoyl-sn-glycero-3-phosphocholine, 1,2-dioleoyl-sn-glycerol, 1-palmitoyl-2-oleoyl-3-linoleoyl-rac-glycerol, 1,2-distearoyl-sn-glycero-3-phosphocholine) were detected as either protonated or cationated species. Clean MS/MS spectra for each lipid were also successfully obtained. Pre-MS surface cleaning of the target plates with UV-ozone treatment successfully removed organic contaminants that would interfere with the mass spectra especially in the low molecular weight region. Preliminary application of the presented target plate to the detection of endogenous lipids in latent fingerprints showed promising results and for potential use in the visualization and chemical composition determination of latent fingerprints by nanoparticle assistance.

Imaging of lipids in rat heart by MALDI-MS with silver nanoparticles

Lipids are a major component of heart tissue and perform several important functions such as energy storage, signaling, and as building blocks of biological membranes. The heart lipidome is quite diverse consisting of glycerophospholipids such as phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), phosphatidylinositols (PIs), phosphatidylglycerols (PGs), cardiolipins (CLs), and glycerolipids, mainly triacylglycerols (TAGs). In this study, mass spectrometry imaging (MSI) enabled by matrix implantation of ionized silver nanoparticles (AgNP) was used to map several classes of lipids in heart tissue. The use of AgNP matrix implantation was motivated by our previous work showing that implantation doses of only 10(14)/cm(2) of 2 nm gold nanoparticulates into the first 10 nm of the near surface of the tissue enabled detection of most brain lipids (including neutral lipid species such as cerebrosides) more efficiently than traditional organic MALDI matrices. Herein, a similar implantation of 500 eV AgNP(-) across the entire heart tissue section results in a quick, reproducible, solvent-free, uniform matrix concentration of 6 nm AgNP residing near the tissue surface. MALDI-MSI analysis of either positive or negative ions produce high-quality images of several heart lipid species. In negative ion mode, 24 lipid species [16 PEs, 4 PIs, 1 PG, 1 CL, 2 sphingomyelins (SMs)] were imaged. Positive ion images were also obtained from 29 lipid species (10 PCs, 5 PEs, 5 SMs, 9 TAGs) with the TAG species being heavily concentrated in vascular regions of the heart.

Profiling the triacylglyceride contents in bat integumentary lipids by preparative thin layer chromatography and MALDI-TOF mass spectrometry

The mammalian integument includes sebaceous glands that secrete an oily material onto the skin surface. Sebum production is part of the innate immune system that is protective against pathogenic microbes. Abnormal sebum production and chemical composition are also a clinical symptom of specific skin diseases. Sebum contains a complex mixture of lipids, including triacylglycerides, which is species-specific. The broad chemical properties exhibited by diverse lipid classes hinder the specific determination of sebum composition. Analytical techniques for lipids typically require chemical derivatizations that are labor-intensive and increase sample preparation costs. This paper describes how to extract lipids from mammalian integument, separate broad lipid classes by thin-layer chromatography, and profile the triacylglyceride contents using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. This robust method enables a direct determination of the triacylglyceride profiles among species and individuals, and it can be readily applied to any taxonomic group of mammals.

Multiplex mass spectrometry imaging for latent fingerprints

We have previously developed in-parallel data acquisition of orbitrap mass spectrometry (MS) and ion trap MS and/or MS/MS scans for matrix-assisted laser desorption/ionization MS imaging (MSI) to obtain rich chemical information in less data acquisition time. In the present study, we demonstrate a novel application of this multiplex MSI methodology for latent fingerprints. In a single imaging experiment, we could obtain chemical images of various endogenous and exogenous compounds, along with simultaneous MS/MS images of a few selected compounds. This work confirms the usefulness of multiplex MSI to explore chemical markers when the sample specimen is very limited.

Triacylglyceride (TAG) profiles of integumentary lipids isolated from three bat species determined by matrix-assisted laser desorption–ionization time-of-flight mass spectrometry (MALDI–TOF MS)

Lipids secreted from sebaceous glands in bat integument may play a role in determining host pathogenicity by the fungus Geomyces destructans in white-nose syndrome (WNS). To investigate this, we have determined the triacylglycerol (TAG) profiles for three bat species: eastern red bats ( Lasiurus borealis (Müller, 1776)), evening bats ( Nycticeius humeralis (Rafinesque, 1818)), and big brown bats ( Eptesicus fuscus (Beauvois, 1796)). Neutral lipids extracted from the hair and wing tissue were fractionated by preparative thin-layer chromatography (TLC) into four major lipid bands corresponding to cholesterol, free fatty acids (FFAs), TAGs, and sterol–wax esters. Densitometry showed higher proportions of TAGs in hair than from wing tissue. TAG bands were recovered and analyzed by matrix-assisted laser desorption–ionization time-of-flight mass spectrometry (MALDI–TOF MS). Mass spectra showed sodiated TAG species with variable fatty acyl (FA) moieties range from m/z 715.6–911.8. High intensity ion peaks were consistent with 16:0 and 18:1 as dominant FA moieties, and these were identified as palmitic and oleic acids, respectively, by liquid chromatography electrospray ionization mass spectrometry. We determined significant differences in TAG profiles between three bat species by MALDI–TOF MS, providing the first description of integumentary lipids in bats. In this study, we performed the first TAG profiling of bats, which suggest such profiles may be species-specific in bats.

Oxidative changes of lipids monitored by MALDI MS

Oxidation processes of lipids are of paramount interest from many viewpoints. For instance, oxidation processes are highly important under in vivo conditions because molecules with regulatory functions are generated by oxidation of lipids or free fatty acids. Additionally, many inflammatory diseases are accompanied by lipid oxidation and, therefore, oxidation products are also useful disease (bio)markers. Thus, there is also considerable interest in methods of (oxidized) lipid analysis. Nowadays, soft ionization mass spectrometric (MS) methods are regularly used to study oxidative lipid modifications due to their high sensitivities and the extreme mass resolution. Although electrospray ionization (ESI) MS is so far most popular, applications of matrix-assisted laser desorption and ionization (MALDI) MS are increasing. This review aims to summarize the so far available data on MALDI analyses of oxidized lipids. In addition to model systems, special attention will be paid to the monitoring of oxidized lipids under in vivo conditions, particularly the oxidation of (human) lipoproteins. It is not the aim of this review to praise MALDI as the "best" method but to provide a critical survey of the advantages and drawbacks of this method.

Thin-layer chromatography combined with MALDI-TOF-MS and 31P-NMR to study possible selective bindings of phospholipids to silica gel

High-performance thin-layer chromatography (HPTLC) is a highly established separation method in the field of lipid and (particularly) phospholipid (PL) research. HPTLC is not only used to identify certain lipids in a mixture but also to isolate lipids (preparative TLC). To do this, the lipids are separated and subsequently re-eluted from the silica gel. Unfortunately, it is not yet known whether all PLs are eluted to the same extent or whether some lipids bind selectively to the silica gel. It is also not known whether differences in the fatty acyl compositions affect the affinities to the stationary phase. We have tried to clarify these questions by using a readily available extract from hen egg yolk as a selected example of a lipid mixture. After separation, the complete lanes or selected spots were eluted from the silica gel and investigated by a combination of MALDI-TOF MS and (31)P NMR spectroscopy. The data obtained were compared with the composition of the total extract (without HPTLC). Although there were significant, solvent-dependent losses in the amount of each lipid, the relative composition of the mixture remained constant; there were also only very slight changes in the fatty acyl compositions of the individual PL classes. Therefore, lipid isolation by TLC may be used without any risk of major sample alterations.

An update of MALDI-TOF mass spectrometry in lipid research

Although matrix-assisted laser desorption and ionization (MALDI) mass spectrometry (MS)--often but not exclusively coupled with a time-of-flight (TOF) mass analyzer--is primarily established in the protein field, there is increasing evidence that MALDI MS is also very useful in lipid research: MALDI MS is fast, sensitive, tolerates sample impurities to a relatively high extent and provides very simple mass spectra without major fragmentation of the analyte. Additionally, MALDI MS devices originally purchased for "proteomics" can be used also for lipids without the need of major system alterations. After a short introduction into the method and the related ion-forming process, the MALDI mass spectrometric characteristics of the individual lipid (ranging from completely apolar hydrocarbons to complex glycolipids with the focus on glycerophospholipids) classes will be discussed and the progress achieved in the last years emphasized. Special attention will be paid to quantitative aspects of MALDI MS because this is normally considered to be the "weak" point of the method, particularly if complex lipid mixtures are to be analyzed. Although the detailed role of the matrix is not yet completely clear, it will be also explicitly shown that the careful choice of the matrix is crucial in order to be able to detect all compounds of interest. Two rather recent developments will be highlighted: "Imaging" MS is nowadays widely established and significant interest is paid in this context to the analysis of lipids because lipids ionize particularly well and are, thus, more sensitively detectable in tissue slices than other biomolecules such as proteins. It will also be shown that MALDI MS can be very easily combined with thin-layer chromatography (TLC) allowing the spatially-resolved screening of the entire TLC plate and the detection of lipids with a higher sensitivity than common staining protocols.

Instantaneous characterization of vegetable oils via TAG and FFA profiles by easy ambient sonic-spray ionization mass spectrometry

A fast and reliable method is presented for the analysis of vegetable oils. Easy ambient sonic-spray ionization mass spectrometry (EASI-MS) is shown to efficiently desorb and ionize the main oil constituents from an inert surface under ambient conditions and to provide comprehensive triacylglyceride (TAG) and free fatty acid (FFA) profiles detected mainly as either [TAG + Na](+) or [FFA-H](-) ions. EASI(+/-)-MS analysis is simple, easily implemented, requires just a tiny droplet of the oil and is performed without any pre-separation or chemical manipulation. It also causes no fragmentation of TAG ions hence diacylglyceride (DAG) and monoacylglyceride (MAG) profiles and contents can also be measured. The EASI(+/-)-MS profiles of TAG and FFA permit authentication and quality control and can be used, for instance, to access levels of adulteration, acidity, oxidation or hydrolysis of vegetable oils in general.

A rapid separation technique for overcoming suppression of triacylglycerols by phosphatidylcholine using MALDI-TOF MS

Phospholipids and triacylglycerols (TAGs) are important classes of lipids in biological systems. Rapid methods have been developed for their characterization in crude samples, including MALDI time-of-flight MS. For mixtures, MALDI often selectively shows only some components. For example, phosphatidylcholine (PC) suppresses detection of other lipids. Most rapid MS methods detect either TAGs or phospholipids but not both. Herein, we demonstrate a simple approach to rapidly screen mixtures containing multiple lipid classes. To validate this approach, reference lipids [PC, tripalmitin (PPP), and phosphatidyl-ethanolamine (PE)] and real samples (beef, egg yolk) were used. In a binary mixture with a strong suppressor (PC), PPP was greatly suppressed. After a simple separation, suppression was virtually eliminated. A mixture of nominally nonsuppressing lipids (PE and PPP) was not adversely affected by separation. Ground beef and egg yolk were used to demonstrate detection of known lipid compositions where other methods have missed one or more lipids or lipid classes. Separation was performed using solid phase extraction with a PrepSep florisil column. A 10 min separation allows rapid screening for lipids and changes in lipids. It is sufficient to clearly detect all lipids and overcome suppression effects in complex lipid mixtures.

Amazonian vegetable oils and fats: fast typification and quality control via triacylglycerol (TAG) profiles from dry matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry fingerprinting

Amazonian oils and fats display unique triacylglycerol (TAG) profiles and, because of their economic importance as renewable raw materials and use by the cosmetic and food industries, are often subject to adulteration and forgery. Representative samples of these oils (andiroba, Brazil nut, buriti, and passion fruit) and fats (cupuaçu, murumuru, and ucuúba) were characterized without pre-separation or derivatization via dry (solvent-free) matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Characteristic profiles of TAG were obtained for each oil and fat. Dry MALDI-TOF MS provides typification and direct and detailed information, via TAG profiles, of their variable combinations of fatty acids. A database from spectra could be developed and may be used for their fast and reliable typification, application screening, and quality control.

The new matrix 4-chloro-alpha-cyanocinnamic acid allows the detection of phosphatidylethanolamine chloramines by MALDI-TOF mass spectrometry

Phosphatidylethanolamines (PEs) are abundant lipid constituents of the cellular membrane. The amino group of PEs exhibits high reactivity with hypochlorous acid that is generated under inflammatory conditions in vivo. The analysis of the resulting PE mono- and dichloramines is of significant interest since these species represent important mediators of lipid peroxidation. We have shown in a previous communication that mass spectrometric detection of PE chloramines is only possible with ESI MS, whereas MALDI-TOF MS fails to detect these products if standard matrices are used. In this work we demonstrate that the detection of PE chloramines is also possible by MALDI-TOF MS if 4-chloro-alpha-cyanocinnamic acid is used as matrix. The underlying processes leading to ionization of these species will be discussed in detail. Both, experimental and theoretical studies taking into account possible intramolecular rearrangements were performed to clarify these aspects.

Capabilities and drawbacks of phospholipid analysis by MALDI-TOF mass spectrometry

The important roles of lipids particularly certain phospholipids in signal transduction processes and as important disease markers are becoming increasingly evident. Unfortunately, however, sensitive methods of lipid analysis are established to a much lesser extent than, e.g., methods of protein analysis. Mass spectrometry (MS) is an increasingly used technique of lipid analysis and electrospray ionization (ESI) MS is the so far most established ionization method. Although matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) was so far primarily used for protein analysis, however, this method has itself proven to be very useful in the field of lipid analysis, too. This chapter gives an overview of methodological aspects of MALDI-TOF MS in lipid research and summarizes the specific advantages and drawbacks of this soft-ionization method. In particular, suppression effects of some lipid classes, especially those with quaternary ammonia groups such as phosphatidylcholine, will be highlighted and possible ways to overcome this problem (use of different matrices, separation of the relevant lipid mixture prior to analysis) will be discussed on the example of an organic liver extract.

Lipid and phospholipid profiling of biological samples using MALDI Fourier transform mass spectrometry

Here we describe a study of the feasibility of lipid and phospholipid (PL) profiling using matrix assisted laser desorption/ionization (MALDI) Fourier transform mass spectrometry (FTMS) for two different applications. In this work PL profiles of different mammalian tissues as well as those of whole cell organisms were examined. In particular, comparative analysis of lipid and PL profiles of tissues from mice fed different diets was done and, in another application, MALDI FTMS was used to analyze PL profiles of genetically modified Saccharomyces cerevisiae. Computational sorting of the observed ions was done in order to group the lipid and PL ions from complex MALDI spectra. The PL profiles of liver tissues from mice fed different diets showed a cross correlation coefficient of 0.2580, indicating significant dissimilarity, and revealed more than 30 significantly different peaks at the 99.9% confidence level. Histogram plots derived from the spectra of wild type and genetically modified yeast resulted in a cross correlation coefficient 0.8941 showing greater similarity, but still revealing a number of significantly different peaks. Based on these results, it appears possible to use MALDI FTMS to identify PLs as potential biomarkers for metabolic processes in whole cells and tissues.