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

Effects of carotenoid pigmentation in salmon on antibiotic extraction recovery, matrix effects and accuracy of quantification by ultrahigh performance liquid chromatography coupled to tandem mass spectrometry

Carotenoid pigmentation in salmon may interfere with the accuracy of antibiotic analysis with ultra-high pressure liquid chromatography coupled to tandem spectrometry (UPLC-MS/MS) by causing matrix effects or affecting the recovery of compounds during extraction. In the present study, we used both pigmented and non-pigmented salmon to understand the role pigments play on antibiotic analysis, and tested whether clean-up of the extract with dispersive solid phase extraction (dSPE) or hydrophilic-lipophilic balance (HLB) SPE clean-up reduces matrix effects. Thirty antibiotics and their respective class-specific surrogate standards were measured in Sockeye (pigmented), King (pigmented) and Ivory King (non-pigmented) salmon extracted using the QUEChERS method, or a modified QUEChERS method involving dSPE or HLB SPE clean-up (for Sockeye salmon only). Significant matrix effects and lower percent recoveries of spiked antibiotics were observed in pigmented versus non-pigmented salmon extracted with the QUEChERS method. Dispersive SPE clean-up did not improve extraction recoveries or matrix effects. However, SPE clean-up with HLB columns improved matrix effects for several antibiotics but reduced the percent recovery to < 30%. Across all types of salmon analyzed, the accuracy of quantitation was minimally impacted, likely due to similar behavior of the surrogate standards tagged to each antibiotic class during extraction. Our results demonstrate that carotenoids in salmon are associated with significant matrix effects and low extraction recoveries, but do not impact accuracy.

Phosphorus deprivation affects composition and spatial distribution of membrane lipids in legume nodules

In legumes, symbiotic nitrogen (N) fixation (SNF) occurs in specialized organs called nodules after successful interactions between legume hosts and rhizobia. In a nodule, N-fixing rhizobia are surrounded by symbiosome membranes, through which the exchange of nutrients and ammonium occurs between bacteria and the host legume. Phosphorus (P) is an essential macronutrient, and N2-fixing legumes have a higher requirement for P than legumes grown on mineral N. As in the previous studies, in P deficiency, barrel medic (Medicago truncatula) plants had impaired SNF activity, reduced growth, and accumulated less phosphate in leaves, roots, and nodules compared with the plants grown in P sufficient conditions. Membrane lipids in M. truncatula tissues were assessed using electrospray ionization-mass spectrometry. Galactolipids were found to increase in P deficiency, with declines in phospholipids (PL), especially in leaves. Lower PL losses were found in roots and nodules. Subsequently, matrix-assisted laser desorption/ionization-mass spectrometry imaging was used to spatially map the distribution of the positively charged phosphatidylcholine (PC) species in nodules in both P-replete and P-deficient conditions. Our results reveal heterogeneous distribution of several PC species in nodules, with homogeneous distribution of other PC classes. In P poor conditions, some PC species distributions were observed to change. The results suggest that specific PC species may be differentially important in diverse nodule zones and cell types, and that membrane lipid remodeling during P stress is not uniform across the nodule.

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.

In Situ Separation and Analysis of Lipids by Paper Spray Ionization Mass Spectrometry

Paper spray ionization (PSI) is an extractive ambient ionization technique for mass spectrometry (MS), whereby a triangular paper tip serves as the sampling base and the electrospray tip. During PSI, analytes are extracted and transported to the edge of the paper tip by the applied spraying solvent. Analytes can be purified from a sample matrix and separated from each other by this transportation process. In this study, we investigated and utilized the analyte transportation process of PSI for the in situ separation and analysis of lipid mixtures. We found that differential transport of phosphatidylcholine (PC) and triacylglycerol (TAG), the two most abundant lipid classes in animals, occurred during PSI. We also found that the order in which these lipids moved strongly depended on how the spraying solvent was applied to the paper base. The more polar PC moved faster than the less polar TAG during PSI, when a polar solvent was slowly fed into a paper tip, whereas TAG was transported faster than PC when excess solvent was applied to the tip at once. In addition, we achieved a complete separation and detection of PC and TAG by slowly supplying a nonpolar solvent to a PSI tip.

Mass spectrometry imaging of triglycerides in biological tissues by laser desorption ionization from silicon nanopost arrays

Mass spectrometry imaging (MSI) is used increasingly to simultaneously detect a broad range of biomolecules while mapping their spatial distributions within biological tissue sections. Matrix-assisted laser desorption ionization (MALDI) is recognized as the method-of-choice for MSI applications due in part to its broad molecular coverage. In spite of the remarkable advantages offered by MALDI, imaging of neutral lipids, such as triglycerides (TGs), from tissue has remained a significant challenge due to ion suppression of TGs by phospholipids, e.g. phosphatidylcholines (PCs). To help overcome this limitation, silicon nanopost array (NAPA) substrates were introduced to selectively ionize TGs from biological tissue sections. This matrix-free laser desorption ionization (LDI) platform was previously shown to provide enhanced ionization of certain lipid classes, such as hexosylceramides (HexCers) and phosphatidylethanolamines (PEs) from mouse brain tissue. In this work, we present NAPA as an MSI platform offering enhanced ionization efficiency for TGs from biological tissues relative to MALDI, allowing it to serve as a complement to MALDI-MSI. Analysis of a standard lipid mixture containing PC(18:1/18:1) and TG(16:0/16:0/16:0) by LDI from NAPA provided an ~49 and ~227-fold higher signal for TG(16:0/16:0/16:0) relative to MALDI, when analyzed without and with the addition of a sodium acetate, respectively. In contrast, MALDI provided an ~757 and ~295-fold higher signal for PC(18:1/18:1) compared with NAPA, without and with additional Na⁺ . Averaged signal intensities for TGs from MSI of mouse lung and human skin tissues exhibited an ~105 and ~49-fold increase, respectively, with LDI from NAPA compared with MALDI. With respect to PCs, MALDI provided an ~2 and ~19-fold increase in signal intensity for mouse lung and human skin tissues, respectively, when compared with NAPA. The complementary coverage obtained by the two platforms demonstrates the utility of using both techniques to maximize the information obtained from lipid MS or MSI experiments.

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.

Multimodal imaging of biological tissues using combined MALDI and NAPA-LDI mass spectrometry for enhanced molecular coverage

Sequential imaging of a tissue section by MALDI and NAPA-LDI mass spectrometry provides enhanced molecular coverage.

Characterisation of triacylglycerols from bovine milk fat fractions with MALDI-TOF-MS fragmentation

In this study, we used MALDI-TOF-MS to profile and characterise the triacylglycerol (TAG) species of anhydrous bovine milk fat (AMF) and its low melting (olein) and high melting (stearin) fractions obtained by dry fractionation. Silver-ion solid phase extraction (Ag⁺-SPE) cartridges were utilised to separate the TAGs according to saturation degree. Then, several TAG species were selected and fragmented via post-source decay (PSD) fragmentation. MALDI-TOF-MS TAG profiles and fragmentation patterns were compared to the TAG and fatty acid (FA) compositions obtained by gas chromatography-flame ionization detector (GC-FID). We found that, olein was rich in medium length chain TAG species like CN38:0 and CN40:1, whereas stearin was rich in saturated long chain TAG species from CN42:0 to CN52:0. Separation of the TAGs based on saturation degree allowed successful selection of the TAG parent-ion for fragmentation by eliminating the interferences of TAG species that have the same carbon number but vary in number of double bonds. The TAG fragmentation patterns indicated significant differences between AMF, olein and stearin as a result of the dry fractionation process. Compared to AMF, olein yielded in higher fragments of short-chain saturated and middle-chain unsaturated FAs. Whereas, stearin yielded in saturated and monounsaturated long chain FA fragments. Fragmentation of unsaturated long chain TAGs showed that the TAGs in olein contained more C18:1 and C18:2 than that of AMF and stearin. Stearin on the other hand, contained higher amount of TAG species containing C16:0. These results were in line with the FA compositions obtained from GC-FID and suggest that Ag⁺-SPE cartridges coupled with MALDI-TOF-MS offer an informative and practical approach to characterise fats and oils with complex TAG composition.

Graphene oxide aggregate-assisted LDI-MS for the direct analysis of triacylglycerol in complex biological samples

Knowledge of blood triacylglycerol (TAG) species is essential to clarify the physiological functions of individual TAG molecules and also to develop potential biomarkers for related diseases. Commonly, lipid samples prepared by organic liquid-liquid extraction contain complex components, thus cannot be directly characterized by mass spectrometry (MS) and often require an additional purification step. Here, we described a laser desorption ionization - mass spectrometry (LDI-MS) method that utilized aggregated graphene oxide (AGO) as both lipid extractant and MS matrix (AGOLDI-MS), to characterize and quantify plasma TAG species without the use of harmful solvent or complex separation step. We first designed and synthesized the AGO material with a multi-layered sheet structure, which could efficiently break up the structure of lipoproteins, and extract plasma TAGs as solid-phase extraction material. Furthermore, in AGOLDI-MS procedure, the AGO could directly act as matrix and selectively produce the MS signals of TAGs without the interferences of phospholipids, which was hardly achieved by using the routine LDI-MS method based on liquid-liquid extraction and small molecular matrix. We confirmed the suitability of AGOLDI-MS as characterization and quantitative tool for TAG species through studying the analysis performances in TAG standards and real plasma samples. To establish potential utility of our method, we characterized 42 human plasmas from healthy and hyperlipemic donators, indicating that the AGOLDI-MS could not only generate comparable quantitative results of total TAGs to current clinical technology, but also monitor the changes of TAG species between different sample groups. This approach could further characterize the compositions of the fatty acid moieties in even low abundant TAGs by the assistance of tandem MS-MS. This concise, specific, and high-throughput approach will facilitate the rapid and precise characterizations of plasma TAGs, and make the MS approach for TAGs more adaptable for clinical uses.

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.

Organic–inorganic binary mixture matrix for comprehensive laser-desorption ionization mass spectrometric analysis and imaging of medium-size molecules including phospholipids, glycerolipids, and oligosaccharides

A binary matrix of DHB and Fe₃O₄ NPs provides broad coverage for medium size metabolites.

A sample preparation method for recovering suppressed analyte ions in MALDI TOF MS

In matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS), analyte signals can be substantially suppressed by other compounds in the sample. In this technical note, we describe a modified thin-layer sample preparation method that significantly reduces the analyte suppression effect (ASE). In our method, analytes are deposited on top of the surface of matrix preloaded on the MALDI plate. To prevent embedding of analyte into the matrix crystals, the sample solution were prepared without matrix and efforts were taken not to re-dissolve the preloaded matrix. The results with model mixtures of peptides, synthetic polymers and lipids show that detection of analyte ions, which were completely suppressed using the conventional dried-droplet method, could be effectively recovered by using our method. Our findings suggest that the incorporation of analytes in the matrix crystals has an important contributory effect on ASE. By reducing ASE, our method should be useful for the direct MALDI MS analysis of multicomponent mixtures.

Improved fatty acid analysis of conjugated linoleic acid rich egg yolk triacylglycerols and phospholipid species

Reports from chicken conjugated linoleic acid (CLA) feeding trials are limited to yolk total fatty acid composition, which consistently described increased saturated fatty acids and decreased monounsaturated fatty acids. However, information on CLA triacylglycerol (TAG) and phospholipid (PL) species is limited. This study determined the fatty acid composition of total lipids in CLA-rich egg yolk produced with CLA-rich soy oil, relative to control yolks using gas chromatography with flame ionization detection (GC-FID), determined TAG and PL fatty acid compositions by thin-layer chromatography-GC-FID (TLC-GC-FID), identified intact PL and TAG species by TLC-matrix-assisted laser desorption/ionization mass spectrometry (TLC-MALDI-MS), and determined the composition of TAG and PL species in CLA and control yolks by direct flow infusion electrospray ionization MS (DFI ESI-MS). In total, 2 lyso-phosphatidyl choline (LPC) species, 1 sphingomyelin species, 17 phosphatidyl choline species, 19 TAG species, and 9 phosphatidyl ethanolamine species were identified. Fifty percent of CLA was found in TAG, occurring predominantly in C52:5 and C52:4 TAG species. CLA-rich yolks contained significantly more LPC than did control eggs. Comprehensive lipid profiling may provide insight on relationships between lipid composition and the functional properties of CLA-rich eggs.

Direct analysis of triacylglycerols from crude lipid mixtures by gold nanoparticle-assisted laser desorption/ionization mass spectrometry

Triacylglycerols (TAGs), essential energy storage lipids, are easily detected by conventional MALDI MS when occurring on their own. However, their signals are easily overwhelmed by other lipids, mainly phosphatidylcholines (PCs) and, therefore, require purification. In order to profile TAGs from crude lipid mixtures without prefractionation, we investigated alternative matrixes that can suppress phospholipid ion signals and enhance cationization of TAGs. We found that an aqueous solution of citrate-capped gold nanoparticles (AuNPs) with a diameter of 12 nm is a superior matrix for the laser desorption/ionization mass spectrometry (LDI MS) of TAGs in crude lipid mixtures. The AuNP matrix effectively suppressed other lipid signals such as phospholipids and also provided 100 times lower detection limit for TAGs than 2,5-dihydroxybenzoic acid (DHB), the best conventional MALDI matrix for TAGs. The AuNP-assisted LDI MS enabled us to obtain detailed TAG profiles including minor species directly from crude beef lipid extracts without phospholipid interference. In addition, we could detect TAGs at a trace level from a total brain lipid extract.

Lipidomics in tissues, cells and subcellular compartments

Mass spectrometry (MS) advances in recent years have revolutionized the biochemical analysis of lipids in plants, and made possible new theories about the structural diversity and functional complexity of lipids in plant cells. Approaches have been developed to profile the lipidome of plants with increasing chemical and spatial resolution. Here we highlight a variety of methods for lipidomics analysis at the tissue, cellular and subcellular levels. These procedures allow the simultaneous identification and quantification of hundreds of lipids species in tissue extracts by direct-infusion MS, localization of lipids in tissues and cells by laser desorption/ionization MS, and even profiling of lipids in individual subcellular compartments by direct-organelle MS. Applications of these approaches to achieve improved understanding of plant lipid metabolism, compartmentation and function are discussed.

Spatial mapping of lipids at cellular resolution in embryos of cotton

Advances in mass spectrometry (MS) have made comprehensive lipidomics analysis of complex tissues relatively commonplace. These compositional analyses, although able to resolve hundreds of molecular species of lipids in single extracts, lose the original cellular context from which these lipids are derived. Recently, high-resolution MS of individual lipid droplets from seed tissues indicated organelle-to-organelle variation in lipid composition, suggesting that heterogeneity of lipid distributions at the cellular level may be prevalent. Here, we employed matrix-assisted laser desorption/ionization-MS imaging (MALDI-MSI) approaches to visualize lipid species directly in seed tissues of upland cotton (Gossypium hirsutum). MS imaging of cryosections of mature cotton embryos revealed a distinct, heterogeneous distribution of molecular species of triacylglycerols and phosphatidylcholines, the major storage and membrane lipid classes in cotton embryos. Other lipids were imaged, including phosphatidylethanolamines, phosphatidic acids, sterols, and gossypol, indicating the broad range of metabolites and applications for this chemical visualization approach. We conclude that comprehensive lipidomics images generated by MALDI-MSI report accurate, relative amounts of lipid species in plant tissues and reveal previously unseen differences in spatial distributions providing for a new level of understanding in cellular biochemistry.

Triacylglycerol profiling of marine microalgae by mass spectrometry

We present a method for the determination of triacylglycerol (TAG) profiles of oleaginous saltwater microalgae relevant for the production of biofuels, bioactive lipids, and high-value lipid-based chemical precursors. We describe a technique to remove chlorophyll using quick, simple solid phase extraction (SPE) and directly compare the intact TAG composition of four microalgae species (Phaeodactylum tricornutum, Nannochloropsis salina, Nannochloropsis oculata, and Tetraselmis suecica) using MALDI time-of-flight (TOF) mass spectrometry (MS), ESI linear ion trap-orbitrap (LTQ Orbitrap) MS, and ¹H NMR spectroscopy. Direct MS analysis is particularly effective to compare the polyunsaturated fatty acid (PUFA) composition for triacylglycerols because oxidation can often degrade samples upon derivatization. Using these methods, we observed that T. suecica contains significant PUFA levels with respect to other microalgae. This method is applicable for high-throughput MS screening of microalgae TAG profiles and may aid in the commercial development of biofuels.

Mass-spectrometry based oxidative lipidomics and lipid imaging: applications in traumatic brain injury

Lipids, particularly phospholipids, are fundamental to CNS tissue architecture and function. Endogenous polyunsaturated fatty acid chains of phospholipids possess cis-double bonds each separated by one methylene group. These phospholipids are very susceptible to free-radical attack and oxidative modifications. A combination of analytical methods including different versions of chromatography and mass spectrometry allows detailed information to be obtained on the content and distribution of lipids and their oxidation products thus constituting the newly emerging field of oxidative lipidomics. It is becoming evident that specific oxidative modifications of lipids are critical to a number of cellular functions, disease states and responses to oxidative stresses. Oxidative lipidomics is beginning to provide new mechanistic insights into traumatic brain injury which may have significant translational potential for development of therapies in acute CNS insults. In particular, selective oxidation of a mitochondria-specific phospholipid, cardiolipin, has been associated with the initiation and progression of apoptosis in injured neurons thus indicating new drug discovery targets. Furthermore, imaging mass-spectrometry represents an exciting new opportunity for correlating maps of lipid profiles and their oxidation products with structure and neuropathology. This review is focused on these most recent advancements in the field of lipidomics and oxidative lipidomics based on the applications of mass spectrometry and imaging mass spectrometry as they relate to studies of phospholipids in traumatic brain injury.