Analysis of triacylglycerols with non-aqueous reversed-phase liquid chromatography and positive ion electrospray tandem mass spectrometry

Triacylglycerol regioisomers containing palmitic acid analyzed by ultra-performance supercritical fluid chromatography and quadrupole time-of-flight mass spectrometry: Comparison of standard curve calibration and calculation equation

Triacylglycerol (TAG) regioisomers containing palmitic acid (16:0) was identified using ultra-performance supercritical fluid chromatography and quadrupole time-of-flight mass spectrometry (UPSFC-Q-TOF-MS) and quantified using calibration curve method and calculation equation method. There were negative linear correlation between [R_A-A]⁺/[R_A-A]⁺+[R_A-B]⁺ and content of sn-A-B-A (%) for AAB/ABA type TAGs, [R_{sn-1 FA-sn-3 FA}]⁺/[R_B-C]⁺+[R_A-C]⁺+[R_A-B]⁺ and content of fatty acid (FA) at sn-2 position (%) for BAC/ABC/ACB type TAGs. The difference between calculation equation and standard curve method was acceptable. The TAG regioisomers in human milk, mammalian milk, lard and fish oil were identified and quantified using the developed methods. This study provided a reliable and facile method for analysis of the TAG regioisomers, which was capable of the selection of oil materials for infant formula production.

Ammonia Concentration in the Eluent Influences Fragmentation Pattern of Triacylglycerols in Mass Spectrometry Analysis

Correct assessment of the fatty acyl at the glycerol sn-2 position in triacylglycerol (TAG) analysis by liquid chromatography and mass spectrometry (LC-MS) is challenging. Ammonium hydroxide (NH₄OH) is the preferred choice for the solvent additive for the formation of the ammonium adduct ([M + NH₄]⁺). In this study, the influence of different NH₄OH concentrations in the eluents on TAG adduct formation and fragmentation under LC-MS analysis was assessed. Increasing NH₄OH concentrations delayed the chromatographic elution time according to a power function. The [M + NH₄]⁺ and [M + ACN + NH₄]⁺ adducts (where ACN means acetonitrile) were formed at all ammonium concentrations assayed. [M + ACN + NH₄]⁺ predominated above 18.26 mM [NH₄OH], and the intensity of [M + NH₄]⁺ dropped. TAG fragmentation for fatty acyl release in the MS^E was reduced with increasing [M + ACN + NH₄]⁺ adduct, which suggests that ACN stabilizes the adduct in a way that inhibits the rupture of the ester bonds in TAGs. A linear equation (H_sn-I = a × H_[M+NH4]+, where sn-I refers to the sn position of the glycerol (I = 1, 2, or 3) and H is the peak height) was deduced to quantify the dehydroxydiacylglycerol fragment intensity in relation to [M + NH₄]⁺ intensity in the full scan. This equation had a slope mean value of 0.369 ± 0.058 for the sn-1 and sn-3 positions, and of 0.188 ± 0.007 for the sn-2 position.

The impact of the complexing agent on the sensitivity of collision-induced dissociation spectra to fatty acid position for a set of XYZ-type triglycerides

RATIONALE

The development of an automated platform for the positional analysis of triglycerides (TAGs) based on electrospray ionization tandem mass spectrometry (ESI-MS/MS) continues to be pursued. This work evaluates the positional sensitivities of the collision-induced dissociation (CID) spectra of a representative set of XYZ triglycerides using sodium, lithium, and ammonium salts as complexing agents.

METHODS

A set of triglycerides were synthesized and analyzed via ESI-MS/MS using an ion trap mass spectrometer. Using three different complexing agents, the product ion spectra of the corresponding precursor ions for twelve XYZ TAGs were collected, where X, Y, and Z represent C_16:0 , C_18:1(c-9) , C_{18:2(cc-9,12)} , and C_{20:4(cccc-5,8,11,14)} fatty acid chains. These data were then used to prepare ternary plots for four positional isomer systems to evaluate the positional sensitivity differences among the three different complexing agents.

RESULTS

The positional sensitivities for each of the four positional isomer systems were robust for the sodium and lithium adducts. The CID data for the sodium and lithium TAGs demonstrated an unfavorable loss of the fatty acid in the center position and showed a higher sensitivity to fatty acid position, when compared with the CID data for ammonium adducts, especially for the arachidonic acid containing triglycerides.

CONCLUSIONS

The data shows that that the relative abundances of the DAG product ions for the XYZ-type TAGs when using sodium and lithium complexing agent adducts are sensitive to fatty acid position and are consistent for the diverse array of TAGs studied in this work. This suggests that using sodium or lithium as the complexing agent may be advantageous for the development of an automated platform for the positional analysis of complex TAG mixtures based on ESI-MS/MS.

Liquid chromatography and mass spectrometry for the analysis of acylglycerols in art and archeology

Lipid characterization in art and archeology, together with the study of lipid degradation processes, is an important research area in heritage science. Lipid-based materials have been used as food since ancient times, but also employed as illuminants and as ingredients in cosmetic, ritual, and pharmaceutical preparations. Both animal and plant lipids have also been processed to produce materials used in art and crafts, such as paint binders, varnishes, waterproofing agents, and coatings. Identifying the origin of the lipid materials is challenging when they are found in association with artistic historical objects. This is due to the inherent complex composition of lipids, their widespread occurrence, and the chemical alterations induced by ageing. The most common approach for lipid characterization in heritage objects entails profiling fatty acids by gas chromatography/mass spectrometry after saponification or transesterification. New developments in high-performance liquid chromatography coupled with mass spectrometry (HPLC-MS) for the characterization of acylglycerols, together with more efficient sample treatments, have fostered the introduction of liquid chromatography for characterizing the lipid profile in heritage objects. This review reports the latest developments and applications of HPLC-MS for the characterization of lipid materials in the field of heritage science. We describe the various approaches for sample pretreatment and highlight the advantages and limitations of HPLC-MS in the analysis of paint and archeological samples. © 2020 John Wiley & Sons Ltd.

Structural elucidation of triacylglycerol using online acetone Paternò-Büchi reaction coupled with reversed-phase liquid chromatography mass spectrometry

Triacylglycerol (TG) is a class of lipids that is responsible for energy storage and cell metabolism in biological systems; it is found in relatively high abundances in biological fluids such as human plasma. Due to structural complexity, analyzing TGs using shotgun lipidomic approaches is challenging because of the presence of multiple fatty acyl compositional isomers. In this work, reversed-phase liquid chromatography (RPLC) was used for separation of TG species due to the capability of separating lipids based on fatty acyl chain lengths and degrees of unsaturation. RPLC alone does not provide structurally informative information for the location of carbon-carbon double-bonds (C[double bond, length as m-dash]Cs) without using synthesized standards that correspond to each species analyzed. The Paternò-Büchi (PB) reaction was employed online to confidently characterize the location of C[double bond, length as m-dash]Cs within lipid species via photo-initiated modification of the alkene group with acetone, which was later subjected to electrospray ionization (ESI) and tandem mass spectrometry (MS/MS) to form signature fragmentation peaks. This online RPLC-PB-MS/MS system was able to distinguish fatty acyl level and C[double bond, length as m-dash]C level isomeric species. The systems allowed for the identification of 46 TG molecular species in human plasma with confident C[double bond, length as m-dash]C location assignment in fatty acyls at a limit of identification of 50 nM.

Antidiabetic components from the hexane extract of red kidney beans (Phaseolus vulgaris L.): isolation and structure determination

Red kidney beans (Phaseolus vulgaris L.) contain bioactive compounds that are known to exhibit antidiabetic effects via inhibition of α-glucosidase. However, information on the nonpolar components that exhibit antidiabetic activity is limited. Here, we report the isolation and structure determination of components with α-glucosidase inhibitory activity, which were obtained from the hexane extract of red kidney beans. Triacylglycerols (TAGs) were identified as the major components exhibiting inhibitory activity against α-glucosidase. The chemical structure of TAGs was determined by a combination of GC-MS and UPLC-MS/MS. The primary TAGs identified were LnLnLn (trilinolenin) and LnLLn (1,3-dilinolenoyl-2-linoleoyl glycerol). The major fatty acids present in these TAGs were α-linolenic acid (ω-3) and linoleic acid (ω-6). These TAGs were also found to inhibit the α-glucosidase activity in a similar fashion as acarbose. These results suggest that TAGs have potency as antidiabetics and support the potential suitability of red kidney beans for diabetes treatment.

Characterization of pharmaceutic structured triacylglycerols by high-performance liquid chromatography/tandem high-resolution mass spectrometry and its application to structured fat emulsion injection

RATIONALE

Structured triacylglycerols (STAGs) are a complex mixture of triacylglycerols which are esterified by long-chain fatty acids and medium-chain fatty acids with the same glycerol molecular backbone. As a kind of lipid pharmaceutic excipients, STAGs are used in the pharmaceutical industry as major components of structured fat emulsion injections and play an important role in pharmaceutic energy material because they improve nutritional status with faster elimination in a safe way. The composition and proportion of triacylglycerols in STAGs are closely related to the stability of pharmaceutical preparations and curative effects in the clinic. Therefore, it is necessary to characterize pharmaceutic STAGs using a rapid and accurate method.

METHODS

An analytical method for rapid and accurate determination of triacylglycerols in pharmaceutic STAGs was developed using high-performance liquid chromatography/tandem high-resolution mass spectrometry (HPLC/HRMS). Triacylglycerol components could be well separated on a Waters Xterra MS C8 (2.1 × 100 mm, 3.5 μm) column. Four-dimensional HPLC/HRMS data (high-resolution m/z, MS² data, retention time and isotopic intensity distribution) were used to identify triacylglycerols using Lipid Data Analyzer (LDA) software and the LIPID MAPS database. Then, these identified triacylglycerol components were relatively quantified by their corresponding normalized peak areas using representative standard curves of structurally similar standard substances.

RESULTS

Forty-seven kinds of triacylglycerol components in pharmaceutic STAGs and structured fat emulsion injection were identified and relatively quantified by this method. It has been shown that their retention times are in good correlation with the number of carbon atoms and carbon-carbon double bonds. The main components in pharmaceutic STAGs and structured fat emulsion injection were triacylglycerols containing both medium-chain fatty acids and long-chain fatty acids, while the other components, including triacylglycerols containing three medium-chain fatty acids and triacylglycerols containing three long-chain fatty acids, were relatively low.

CONCLUSIONS

This study has provided a rapid and accurate approach for the identification and quality control of pharmaceutic STAGs and structured fat emulsion injection and this approach can be extended to other lipid pharmaceutic excipients and used as an effective and reasonable control to guarantee the quality of pharmaceutical preparations.

Development of a qualitative/quantitative strategy for comprehensive determination of polar lipids by LC-MS/MS in human plasma

Polar lipids, especially glycerophospholipids, constitute the main components of cell membranes and are precursors of signaling molecules in many cellular and physiological processes. For this reason, the development of methods with high capability for detection of polar lipids in biological samples is required. In this research, the objective was to develop a method for comprehensive qualitative/quantitative determination of polar lipids in plasma by a combination of acquisition methods with a triple quadrupole mass analyzer. The strategy was optimized in two steps: (a) a first step for detection of lipids by monitoring selective fragmentation patterns representative of each lipid family and (b) a second step for confirmation of lipid species by detection and identification of product ions associated with the conjugated fatty acids. The acquisition list was divided into two multiple reaction monitoring (MRM) methods to ensure the detection of all transitions with suited instrumental sensitivity according to chromatographic retention time and relative abundance in plasma. The combination of the two MRM methods allowed the detection of 398 polar lipids in plasma in 64 min. Precision, estimated as within-day variability, was below 6.8% for all determined lipid families, while between-day variability was below 24.0%. This strategy has been applied to a cohort formed by 384 individuals in order to obtain a qualitative and quantitative distribution of polar lipids in human plasma. The most concentrated lipid families in relative terms were lysophospholipids, plasmalogens, and phosphatydilcholines, with mean relative concentration of 58.0, 17.1, and 8.3%, respectively. Then, sphingomyelins and phosphatidylethanolamines reported a relative concentration of 2.0%, followed by phosphatidylserines, with 1.1%. Graphical abstract.

An untargeted lipidomic approach for qualitative determination of latent fingermark glycerides using UPLC-IMS-QToF-MSE

More detailed fundamental information is required about latent fingermark composition in order to better understand fingermark properties and their impact on detection efficiency, and the physical and chemical changes that occur with time following deposition. The composition of the glyceride fraction of latent fingermark lipids in particular is relatively under-investigated due in part to their high structural variability and the limitations of the analytical methods most frequently utilised to investigate fingermark composition. Here, we present an ultra performance liquid chromatography-ion mobility spectroscopy-quadrupole time-of-flight mass spectrometry (UPLC-IMS-QToF-MS^E) method to characterise glycerides in charged latent fingermarks using data-independent acquisition. Di- and triglycerides were identified in fingermark samples from a population of 10 donors, through a combination of in silico fragmentation and monitoring for fatty acid neutral losses. 23 diglycerides and 85 families of triglycerides were identified, with significant diversity in chain length and unsaturation. 21 of the most abundant triglyceride families were found to be common to most or all donors, presenting potential targets for further studies to monitor chemical and physical changes in latent fingermarks over time. Differences in relative peak intensities may be indicative of inter- and intra-donor variability. While this study represents a promising step to obtaining more in-depth information about fingermark composition, it also highlights the complex nature of these traces.

The Impact of the Complexing Cation on the Sensitivity of the Collisional-Induced Dissociation Spectra to Fatty Acid Position for a Set of YXY/YYX-type Triglycerides

RATIONAL

The development of an automated platform for the positional analysis of triglycerides based on electrospray tandem mass spectrometry continues to be pursued. This work compares the positional sensitivities of the collisional-induced dissociation spectra for a representative set of YXY/YYX triglycerides using ammonium, silver, sodium and lithium as complexing agents.

METHODS

A set of triglycerides were synthesized and analyzed by electrospray tandem mass spectrometry using an ion trap mass spectrometer. Using different salt additives, the product ion spectra of the corresponding parent ions for twelve systems of the form YXY/YYX, where Y and X represent C16:0 , C18:1(c-9), C18:2(cc-9,12) and C20:4(cccc-5,8,11,14) , were collected. The data was used to prepare two-point calibration plots for each of the twelve positional isomer systems using each of the four complexing agents.

RESULTS

The positional sensitivities for all twelve positional isomer systems were robust for both the sodium and lithium TAG adducts. The CID data for both the sodium and lithium TAG adducts are much less sensitive to the degree of unsaturation and double bond position of the fatty acids constituents than the CID data for the ammonium adducts.

CONCLUSION

Using sodium or lithium TAG adducts may be advantageous for the development of an accurate predictive model for performing positional analysis of complex TAG mixtures based on electrospray tandem mass spectrometry. Ammonium adducts are likely complicated by the ability of the ammonium ion to provide extra stability to some parent ions through hydrogen bond-like interactions.

A simple and economical strategy for obtaining calibration plots for relative quantification of positional isomers of YYX/YXY triglycerides using high-performance liquid chromatography/tandem mass spectrometry

RATIONALE

Positional analysis of intact triglycerides could provide greater insights into the link between fatty acid position and lipotoxic diseases. However, this methodology has been impeded by lack of commercial availability of positionally pure triglycerides. This work reports on a strategy for defining calibration plots for YXY/YYX triglyceride systems based on the product ion intensities in the collision-induced dissociation spectra of ammoniated precursor ions.

METHODS

A set of triglycerides were synthesized and analyzed by electrospray ionization tandem mass spectrometry using an ion trap mass spectrometer. The product ion spectra of the ammoniated precursor ions were collected for 42 triglyceride systems of the form YXY/YYX, where Y represents C_16:0 , C_18:1(c-9) and C_{20:4(cccc-5,8,11,14)} . Three-point calibration plots were prepared by plotting the relative abundance of the YY⁺ product ion vs. the relative abundance of the YYX positional isomer.

RESULTS

The calibration plots were shown to give relative abundances of positional isomers accurate to within ±0.02 for most systems. Using an ion trap, under a controlled set of collision parameters, the slopes of the calibration plots can be used to compare the sensitivities of the product ion intensities to fatty acid position for various triglyceride systems. The average slopes of the calibration plots for the C_16:0 , C_18:1(c-9) and C_{20:4(cccc-5,8,11,14)} systems were 0.29 ± 0.05, 0.21 ± 0.05 and 0.045 ± 0.005, respectively.

CONCLUSIONS

While the presence of multiple unsaturated fatty acids tends to slightly decrease the slopes of the calibration plots, the data suggest that the sensitivities are sufficient for performing positional analysis of most triglyceride systems. However, the presence of unsaturated fatty acids that contain double bonds close to the carbonyl group, such as arachidonic acid, tends to dramatically decrease positional sensitivity.

Phytochemicals in whole grain wheat and their health-promoting effects

Accumulated evidence in epidemiological studies has consistently shown that consumption of whole grains (WGs) is inversely associated with risk of major chronic diseases such as certain types of cancer, type 2 diabetes, and cardiovascular diseases. Dietary fiber (DF) has been reported to be responsible for the health effects of WG consumption. Evidence from in vitro and in vivo studies is emerging that, in addition to DF and minerals, the unique phytochemicals in WGs may in part contribute to these health-promoting effects. WGs are rich sources of various phytochemicals. However, phytochemical contents and profiles in WG wheat are not systematically summarized yet, and the rapid rate of discovery of wheat phytochemicals necessitates an update on the current state of this field. Furthermore, the biological roles of phytochemicals in protective effects of WGs are also relatively underestimated compared to DFs. This manuscript summarized current research literature regarding phytochemicals that have been identified and characterized from wheat grains and wheat bran, and their corresponding contributions to the major health benefits of WG wheat consumption.

Rapid Quantification of Low-Viscosity Acetyl-Triacylglycerols Using Electrospray Ionization Mass Spectrometry

Acetyl-triacylglycerols (acetyl-TAG) possess an sn-3 acetate group, which confers useful chemical and physical properties to these unusual triacylglycerols (TAG). Current methods for quantification of acetyl-TAG are time consuming and do not provide any information on the molecular species profile. Electrospray ionization mass spectrometry (ESI-MS)-based methods can overcome these drawbacks. However, the ESI-MS signal intensity for TAG depends on the aliphatic chain length and unsaturation index of the molecule. Therefore response factors for different molecular species need to be determined before any quantification. The effects of the chain length and the number of double-bonds of the sn-1/2 acyl groups on the signal intensity for the neutral loss of short chain length sn-3 groups were quantified using a series of synthesized sn-3 specific structured TAG. The signal intensity for the neutral loss of the sn-3 acyl group was found to negatively correlated with the aliphatic chain length and unsaturation index of the sn-1/2 acyl groups. The signal intensity of the neutral loss of the sn-3 acyl group was also negatively correlated with the size of that chain. Further, the position of the group undergoing neutral loss was also important, with the signal from an sn-2 acyl group much lower than that from one located at sn-3. Response factors obtained from these analyses were used to develop a method for the absolute quantification of acetyl-TAG. The increased sensitivity of this ESI-MS-based approach allowed successful quantification of acetyl-TAG in various biological settings, including the products of in vitro enzyme activity assays.

Comparison of Molecular Species Distribution of DHA-Containing Triacylglycerols in Milk and Different Infant Formulas by Liquid Chromatography-Mass Spectrometry

Long-chain polyunsaturated fatty acids (LC-PUFA) are an important nutritional lipid and have potential in being able to promote human health. Docosahexaenoic acid (DHA, C22:6ω3) is often added in infant formulas to meet the nutritional requirement of formula-fed infants. A comprehensive survey on DHA-containing triacylglycerol (DHA-TAG) molecular species has been conducted for seven infant formulas (IFs) sourced from Australia, Europe, and the USA as well as bovine milk and human milk. Using LC-triple quadrupole MS and LC-LTQ-orbitrap MS we were able to identify and quantify 56 DHA-TAG species in these samples; the fatty acid structure of these species was assigned using their MS(2) spectra. The species composition of DHA-TAG was found to be different between bovine milk, human milk, and IFs and also between different brands of IFs. Bovine milk and human milk contain DHA-TAG of smaller molecular size (728-952 Da), whereas five out of the seven IF samples contain species of broader mass range (from 728 to 1035 Da). Our study indicates that two types of DHA were used in the seven IF products surveyed and that there is very large difference in molecular species distribution in different IF products that may influence the fine nutritional profile and biological functions of IF products.

Generation and esterification of electrophilic fatty acid nitroalkenes in triacylglycerides

Electrophilic fatty acid nitroalkenes (NO(2)-FA) are products of nitric oxide and nitrite-mediated unsaturated fatty acid nitration. These electrophilic products induce pleiotropic signaling actions that modulate metabolic and inflammatory responses in cell and animal models. The metabolism of NO(2)-FA includes reduction of the vinyl nitro moiety by prostaglandin reductase-1, mitochondrial β-oxidation, and Michael addition with low molecular weight nucleophilic amino acids. Complex lipid reactions of fatty acid nitroalkenes are not well defined. Herein we report the detection and characterization of NO(2)-FA-containing triacylglycerides (NO(2)-FA-TAG) via mass spectrometry-based methods. In this regard, unsaturated fatty acids of dietary triacylglycerides are targets for nitration reactions during gastric acidification, where NO(2)-FA-TAG can be detected in rat plasma after oral administration of nitro-oleic acid (NO(2)-OA). Furthermore, the characterization and profiling of these species, including the generation of beta oxidation and dehydrogenation products, could be detected in NO(2)-OA-supplemented adipocytes. These data revealed that NO(2)-FA-TAG, formed by either the direct nitration of esterified unsaturated fatty acids or the incorporation of nitrated free fatty acids into triacylglycerides, contribute to the systemic distribution of these reactive electrophilic mediators and may serve as a depot for subsequent mobilization by lipases to in turn impact adipocyte homeostasis and tissue signaling events.

Characterization of Shrimp Oil from Pandalus borealis by High Performance Liquid Chromatography and High Resolution Mass Spectrometry

Northern shrimp (Pandalus borealis) oil, which is rich in omega-3 fatty acids, was recovered from the cooking water of shrimp processing facilities. The oil contains significant amounts of omega-3 fatty acids in triglyceride form, along with substantial long-chain monounsaturated fatty acids (MUFAs). It also features natural isomeric forms of astaxanthin, a nutritional carotenoid, which gives the oil a brilliant red color. As part of our efforts in developing value added products from waste streams of the seafood processing industry, we present in this paper a comprehensive characterization of the triacylglycerols (TAGs) and astaxanthin esters that predominate in the shrimp oil by using HPLC-HRMS and MS/MS, as well as ¹³C-NMR. This approach, in combination with FAME analysis, offers direct characterization of fatty acid molecules in their intact forms, including the distribution of regioisomers in TAGs. The information is important for the standardization and quality control, as well as for differentiation of composition features of shrimp oil, which could be sold as an ingredient in health supplements and functional foods.

Triacylglycerol profile in cocoa liquors using MALDI-TOF and LC-ESI tandem mass spectrometry

Triacylglycerols are responsible for chocolate's peculiar melting behavior: the type and position of fatty acids on the glycerol molecule strongly affect the melting range of cocoa butter. For this reason, the characterization of triglyceride composition in cocoa products is particularly important. In this work, triacylglycerols extracted from cocoa liquor samples were analyzed by matrix-assisted laser desorption/ionization time-of-flight (TOF) and electrospray ionization tandem mass spectrometry (MS/MS) coupled to liquid chromatography. Extracted samples were initially analyzed by direct injection in MS to obtain information on triglyceride molecular weights; relevant MS parameters were optimized, and the possible formation of the adducts [M + Na](+) and [M + NH(4)](+) was studied. Tandem mass experiments (both with triple quadrupole and TOF/TOF) were performed to study the fragmentation pathways (in particular, the loss of palmitic, stearic and oleic acid) and identify the triacylglycerols in cocoa liquors. Some signals of the spectra obtained with both MS techniques could indicate the presence of diacylglycerols in the cocoa extract, but different experimental evidences demonstrated that they were generated by the in-source fragmentation of triglycerides. A nonaqueous reversed-phase chromatographic separation was also developed and used to support the identification of the analytes; nine triacylglycerols were recognized in the cocoa liquor extracts. The three different batches of Ecuador cocoa liquor did not show significant differences in the triacylglycerol profile.

Quantitative profiling of major neutral lipid classes in human meibum by direct infusion electrospray ionization mass spectrometry

PURPOSE

The purpose of this investigation was to better understand lipid composition in human meibum.

METHODS

Intact lipids in meibum samples were detected by direct infusion electrospray ionization mass spectrometry (ESI-MS) analysis in positive detection mode using sodium iodide (NaI) as an additive. The peak intensities of all major types of lipid species, that is, wax esters (WEs), cholesteryl esters (CEs), and diesters (DEs) were corrected for peak overlapping and isotopic distribution; an additional ionization efficiency correction was performed for WEs and CEs, which was simplified by the observation that the corresponding ionization efficiency was primarily dependent on the specific lipid class and saturation degree of the lipids while independent of the carbon chain length. A set of WE and CE standards was spiked in meibum samples for ionization efficiency determination and absolute quantitation.

RESULTS

The absolute amount (μmol/mg) for each of 51 WEs and 31 CEs in meibum samples was determined. The summed masses for 51 WEs and 31 CEs accounted for 48 ± 4% and 40 ± 2%, respectively, of the total meibum lipids. The mass percentages of saturated and unsaturated species were determined to be 75 ± 2% and 25 ± 1% for CEs and 14 ± 1% and 86 ± 1% for WEs. The profiles for two types of DEs were also obtained, which include 42 α,ω Type II DEs, and 21 ω Type I-St DEs.

CONCLUSIONS

Major neutral lipid classes in meibum samples were quantitatively profiled by ESI-MS analysis with NaI additive.

Use of [13C18] oleic acid and mass isotopomer distribution analysis to study synthesis of plasma triglycerides in vivo: analytical and experimental considerations

We have previously reported on a liquid chromatography-mass spectrometry method to determine the disposition of [(13)C18]-oleic acid following intravenous and oral administration in vivo. This approach has enabled us to study a variety of aspects of lipid metabolism including a quantitative assessment of triglyceride synthesis. Here we present a more rigorous evaluation of the constraints imposed upon the analytical method in order to generate accurate data using this stable-isotope tracer approach along with more detail on relevant analytical figures of merit including limits of quantitation, precision, and accuracy. The use of mass isotopomer distribution analysis (MIDA) to quantify plasma triglyceride synthesis is specifically highlighted, and a re-evaluation of the underlying mathematics has enabled us to present a simplified series of equations. The derivation of this MIDA model and the significance of all underlying assumptions are explored in detail, and examples are given of how it can successfully be applied to detect differences in plasma triglyceride synthesis in lean and high-fat diet fed mouse models. More work is necessary to evaluate the applicability of this approach to triglyceride stores with slower rates of turnover such as in adipose or muscle tissue; however, the present report provides investigators with the tools necessary to conduct such studies.

Mapping the regioisomeric distribution of fatty acids in triacylglycerols by hybrid mass spectrometry

This study describes the use of hybrid mass spectrometry for the mapping, identification, and semi-quantitation of triacylglycerol regioisomers in fats and oils. The identification was performed based on the accurate mass and fragmentation pattern obtained by data-dependent fragmentation. Quantitation was based on the high-resolution ion chromatograms, and relative proportion of sn-1(3)/sn-2 regioisomers was calculated based on generalized fragmentation models and the relative intensities observed in the product ion spectra. The key performance features of the developed method are inter-batch mass accuracy < 1 ppm (n = 10); lower limit of detection (triggering threshold) 0.1 μg/ml (equivalent to 0.2 weight % in oil); lower limit of quantitation 0.2 μg/ml (equivalent to 0.4 weight % in oil); peak area precision 6.5% at 2 μg/ml concentration and 15% at 0.2 μM concentration; inter-batch precision of fragment intensities < 1% (n = 10) independent of the investigated concentration; and averaged accuracy using the generic calibration 3.8% in the 1-10 μg/ml range and varies between 1-23% depending on analytes. Inter-esterified fat, beef tallow, pork lard, and butter fat samples were used to show how well regioisomeric distribution of palmitic acid can be captured by this method.

Determination of triacylglycerol regioisomers using electrospray ionization-quadrupole ion trap mass spectrometry with a kinetic method

The kinetic method was applied to differentiate and quantify mixtures of regioisomeric triacylglycerols (TAGs) by generating and mass selecting alkali ion bound metal dimeric clusters with a TAG chosen as reference (ref) and examining their competitive dissociations in a quadrupole ion trap mass spectrometer. This methodology readily distinguished pairs of regioisomers (AAB/ABA) such as LLO/LOL, OOP/OPO and SSP/SPS and consequently distinguished sn-1/sn-3, sn-2 substituents on the glycerol backbone. The dimeric complex ions [ref, Li, TAG((AAB and/or ABA))](+) generated by electrospray ionization mass spectrometry were subjected to collision induced dissociation causing competitive loss of either the neutral TAG reference (ref) leading to [Li(AAB and/or ABA)](+) or the neutral TAG molecule (TAG((AAB and/or ABA))) leading to [ref, Li](+). The ratio of the two competitive dissociation rates, defined by the product ion branching ratio (R(iso)), was related via the kinetic method to the regioisomeric composition of the investigated TAG mixture. In this work, a linear correlation was established between composition of the mixture of each TAG regioisomer and the logarithm of the branching ratio for competitive fragmentation. Depending on the availability of at least one TAG regioisomer as standard, the kinetic method and the standard additions method led to the quantitative analysis of natural TAG mixtures.

Triacylglycerol profiling of microalgae strains for biofuel feedstock by liquid chromatography-high-resolution mass spectrometry

Biofuels from photosynthetic microalgae are quickly gaining interest as a viable carbon-neutral energy source. Typically, characterization of algal feedstock involves breaking down triacylglycerols (TAG) and other intact lipids, followed by derivatization of the fatty acids to fatty acid methyl esters prior to analysis by gas chromatography (GC). However, knowledge of the intact lipid profile could offer significant advantages for discovery stage biofuel research such as the selection of an algal strain or the optimization of growth and extraction conditions. Herein, lipid extracts from microalgae were directly analyzed by ultra-high pressure liquid chromatography–mass spectrometry (UHPLC-MS) using a benchtop Orbitrap mass spectrometer. Phospholipids, glycolipids, and TAGs were analyzed in the same chromatographic run, using a combination of accurate mass and diagnostic fragment ions for identification. Using this approach, greater than 100 unique TAGs were identified over the six algal strains studied and TAG profiles were obtained to assess their potential for biofuel applications. Under the growth conditions employed, Botryococcus braunii and Scenedesmus obliquus yielded the most comprehensive TAG profile with a high abundance of TAGs containing oleic acid.

Figure

Analytical strategies for discriminating archeological fatty substances from animal origin

Mass spectrometry (MS) is an essential tool in the field of biomolecular archeology to characterize amorphous organic residues preserved in ancient ceramic vessels. Animal fats of various nature and origin, namely subcutaneous fats of cattle, sheep, goats, pigs, horses, and also of dairy products, are those most commonly identified in organic residues in archeological pottery. Fats and oils of marine origin have also been revealed. Since the first applications of MS coupled with gas chromatography (GC) in archeology at the end of 1980s, several developments have occurred, including isotopic determinations by GC coupled to isotope ratio MS and identification of triacylglycerols (TAGs) structure by soft ionization techniques (ESI and APCI). The combination of these methods provides invaluable insights into the strategies of exploitation of animal products in prehistory. In this review, I focus on the analytical strategies based upon MS that allow elucidation of the structure of biomolecular constituents and determination of their isotopic values to identify the nature of animal fat components preserved in highly complex and degraded archeological matrices.

Quantitative analysis of positional isomers of triacylglycerols via electrospray ionization tandem mass spectrometry of sodiated adducts

Herein we report a reversed-phase high-performance liquid chromatography tandem mass spectrometry (RP-HPLC/MS/MS) method for the analysis of positional isomers of triacylglycerols (TAGs) in vegetable oils. The fragmentation behavior of [M + X](+) ions (X = NH(4), Li, Na or Ag) was studied on a quadrupole-time-of-flight (Q-TOF) mass spectrometer under low-energy collision-induced dissociation (CID) conditions. Mass spectra that were dependent on the X(+) ion and the nature and position of the acyl substituents were observed for four pairs of 'AAB/ABA'-type TAGs, namely PPO/POP, OOP/OPO, LLO/LOL and OOL/OLO (where P is 16:0, palmitic acid; O is 18:1, oleic acid; and L is 18:2, linoleic acid). For the majority of [M + X](+) adducts, the loss of the fatty acid in the outer positions (sn-1 or sn-3) was favored over the loss in the central position (sn-2), which enabled the determination of the fractional abundance of the isomers. Ratios of the intensity of fragment ions at various AAB/ABA compositions produced linear calibration curves with positive slopes, comparable to those obtained traditionally by ESI-MS/MS of [M + NH(4)](+) adducts. The only exceptions were the [M + Ag](+) adducts of the PPO/POP system, which produced calibration curves with negative slopes. Sodium adducts provided the most consistent level of isomeric discrimination for the TAGs studied and also offered the most convenience in that they required no additive to the mobile phase. Therefore, calibration curve data derived from [M + Na](+) adducts were applied to the quantification of TAG regioisomers in sunflower and olive oils. The regiospecific analysis showed that palmitic acid was typically located at positions sn-1 or sn-3, whereas unsaturated fatty acids, oleic and linoleic acids were mostly found at the sn-2 position.

Regioisomer characterization of triacylglycerols by non-aqueous reversed-phase liquid chromatography/electrospray ionization mass spectrometry using silver nitrate as a postcolumn reagent

Triacylglycerols (TAGs) provide a challenge for mass spectrometry (MS) analysis because of their complexity. In particular, for dietary, nutritional and metabolic purposes, the positional placement of fatty acids on the glycerol backbone of TAGs is a crucial aspect. To solve this problem, we have investigated the TAGs' fragmentation patterns using an ion trap mass spectrometer. A series of pure regioisomeric pairs of TAGs (POP/PPO, POO/OPO and OSO/SOO) were cationized by Ag(+) after their separation by non-aqueous reversed-phase liquid chromatography (NARP-LC) before MS to improve MS sensitivity. Electrospray ionization-MS (ESI-MS) conditions were optimized in order to produce characteristic [M + Ag + AgNO(3)](+) ions from each TAG, which were then fragmented to produce MS/MS spectra and then fragmented further to produce up to MS(5) spectra. The observation of ions produced by LC-MS(5) of on-line Ag(+)-cationized TAG provided unambiguous information on the fatty acid distribution on the glycerol backbone. These strategies of MS to MS(5) experiments were applied to identify components and to determine the regiospecificity of TAG within a complex mixture of lipids in natural oils.

Regioisomer compositions of vaccenic and oleic acid containing triacylglycerols in sea buckthorn (Hippophae rhamnoides) pulp oils: influence of origin and weather conditions

Triacylglycerols (TAGs) 16:1(n-7)/16:1(n-7)/18:1(n-7) (Po/Po/V) and 16:1(n-7)/16:1(n-7)/18:1(n-9) (Po/Po/O) in pulp/peel oils of various sea buckthorn (Hippophae rhamnoides) subspecies and varieties were separated by reversed-phase high-performance liquid chromatography. The regioisomerism of the TAGs was determined by tandem mass spectrometry using ammonia supplemented in the nebulizer gas to produce ammonium adducts. The regioisomer compositions of Po/Po/V (8-24% of PoVPo) and Po/Po/O (43-61% of PoOPo) both differed from the random distribution of fatty acids (33.3% of ABA) in all 32 sea buckthorn samples investigated. The regioisomer compositions were different between cultivated ssp. rhamnoides varieties, wild ssp. rhamnoides, and wild ssp. sinensis. Differences were also found in the regioisomerism of both Po/Po/V and Po/Po/O between the two cultivated ssp. rhamnoides varieties, Tytti and Terhi. In addition, growth location and harvesting years showed clear impacts on the regioisomer compositions of Po/Po/V and Po/Po/O. Higher temperatures showed positive correlations with the proportion of PoOPo in ssp. rhamnoides and wild ssp. sinensis and with the proportion of sn-PoPoV+sn-VPoPo in wild ssp. sinensis. However, higher temperatures, higher temperature sums, and radiation sums increased the accumulation of PoVPo in wild ssp. rhamnoides.

Quantification of triacylglycerol regioisomers by ultra-high-performance liquid chromatography and ammonia negative ion atmospheric pressure chemical ionization tandem mass spectrometry

The regioisomer composition of triacylglycerols (TAGs) in various vegetable oils was determined with a new liquid chromatography/tandem mass spectrometry (LC/MS/MS method). A direct inlet ammonia negative ion chemical ionization (NICI) MS/MS method was improved by adapting it to LC negative ion (NI) atmospheric pressure chemical ionization (APCI) MS/MS system using ammonia as nebulizer gas. The method is based on the preferential formation of [M-H-RCOOH-100](-) ions during collision-induced dissociation by loss of sn-1/3 fatty acids from [M-H](-) ions. Calibration curves were created from nine reference TAGs: Ala/L/L, Gla/L/L, L/L/O, L/O/O, P/O/O, P/P/O, Po/Po/V, Po/Po/O, and C/O/O. The calibration curves were used to quantify the regioisomer compositions of selected TAGs in rapeseed oil, sunflower seed oil, palm oil, black currant seed oil, and sea buckthorn pulp oil. The method discriminates the different regioisomers and the results obtained by this method were in good agreement with previous results. This proves that this new method can be used for the determination of regiospecific distribution of fatty acids in TAGs.

Contribution of two approaches using electrospray ionization with multi-stage mass spectrometry for the characterization of linseed oil

A detailed characterization of triacylglycerols (TAGs) present in linseed oil samples from a local producer was performed using electrospray ionization and two mass spectrometric approaches; direct infusion multi-stage mass spectrometry (MS(n)) experiments and liquid chromatography/tandem mass spectrometry (LC/MS/MS) using non-aqueous reversed-phase chromatographic conditions. The combination of both approaches permitted the identification of 26 TAGs. Comparison of the two analytical approaches showed that discrimination of regioisomers was achieved from MS3 data while other isobaric species were separated and identified by LC/MS/MS analysis. The results we obtained were also compared with those previously reported. The TAG composition of the studied linseed oil is qualitatively identical to that of linseed oils from various sources in Europe, Canada, Argentina or India. However, a few differences were observed with regard to the proportions of some TAGs; these can be explained by variations in the culture conditions, climate, and variety of the seeds.

Effect of latitude and weather conditions on the regioisomer compositions of alpha- and gamma-linolenoyldilinoleoylglycerol in currant seed oils

The regioisomer compositions of triacylglycerols (TAG) 18:3(n-3)/18:2(n-6)/18:2(n-6) (Ala/L/L) and 18:3(n-6)/18:2(n-6)/18:2(n-6) (Gla/L/L) in seed oils of black and green currant ( Ribes nigrum L.) and red and white currant ( Ribes rubrum L.) varieties were determined by silver-ion high-performance liquid chromatography and mass spectrometry. The aim was to investigate whether latitude and weather conditions affect the regioisomer compositions of Ala/L/L and Gla/L/L and whether the regioisomerism differs between species and varieties. In R. rubrum the proportion of the symmetric regioisomer LAlaL among Ala/L/L was higher (14.1%) than in R. nigrum (12.1%) (p < 0.001). Generally in currants, the proportion of LAlaL was lower (p < 0.001) in northern Finland (12.1%) than in southern Finland (13.5%), where temperature and radiation sums were higher. In R. rubrum varieties grown in the south, the proportion of LGlaL among Gla/L/L was significantly higher in the years 2005 and 2007 (30.7-32.0%) than in 2006 (24.2-25.4%), when temperature and radiation sums were higher and the amount of precipitation was lower.

Global analysis of retina lipids by complementary precursor ion and neutral loss mode tandem mass spectrometry

Despite an increasing recognition of the causative and diagnostic role of lipids in the onset and progression of retinal disease, information on the global lipid profile of the normal retina is quite limited. Here, a "shotgun" tandem mass spectrometry approach involving the use of multiple lipid class-specific precursor ion and neutral loss scan mode experiments has been employed to analyze lipid extracts from normal rat retina, obtained with minimal sample handling prior to analysis. Redundant information for the identification and characterization of molecular species in each lipid class was obtained by complementary analysis of their protonated or deprotonated precursor ions, or by analysis of their various ionic adducts (e.g., Na+, NH4+, Cl-, CH3OCO2-). Notably, "alternative" precursor ion or neutral loss scan mode MS/MS experiments are introduced that were used to identify rat retina lipid molecular species that were not detected using "conventional" scan types typically employed in large-scale lipid-profiling experiments. This chapter outlines the principles and advantages of utilizing complementary/redundant identification of lipid species as a strategy to overcome inherent challenges and limitations of shotgun lipid analysis, and provides examples of the application of this strategy in the analysis of the retina lipidome.

Detection of the abundance of diacylglycerol and triacylglycerol molecular species in cells using neutral loss mass spectrometry

Triacylglycerols (TAGs) are neutral lipids present in all mammalian cells as energy reserves, and diacylglycerols (DAGs) are present as intermediates in phospholipid biosynthesis and as signaling molecules. The molecular species of TAGs and DAGs present in mammalian cells are quite complex, and previous investigations revealed multiple isobaric species having molecular weights at virtually every even mass between 600 and 900 Da, making it difficult to assess changes of individual molecular species after cell activation. A method has been developed, using tandem MS and neutral loss scanning, to quantitatively analyze changes in those glyceryl ester molecular species containing identical fatty acyl groups. This was carried out by neutral loss scanning of 18 common fatty acyl groups where the neutral loss corresponded to the free carboxylic acid plus NH(3). Deuterium-labeled internal standards were used to normalize the signal for each nominal [M+NH(4)](+) ion undergoing this neutral loss reaction. This method was applied in studies of TAGs in RAW 264.7 cells treated with the toll-like receptor 4 ligand Kdo(2)-lipid A. A 50:1-TAG containing 18:1 was found to increase significantly over a 24-h time course after Kdo(2)-lipid A exposure, whereas an isobaric 50:1-TAG containing 16:1 did not change relative to controls.

Atmospheric pressure covalent adduct chemical ionization tandem mass spectrometry for double bond localization in monoene- and diene-containing triacylglycerols

We report a method to elucidate the structure of triacyl-glycerols (TAGs) containing monoene or diene fatty acyl groups by atmospheric pressure covalent adduct chemical ionization (APCACI) tandem mass spectrometry using acetonitrile as an adduct formation reagent. TAGs were synthesized with the structures ABB and BAB, where A is palmitate (C16:0) and B is an isomeric C18 monoene unsaturated at position 9, 11, or 13 or an isomeric diene unsaturated at positions 9 and 11, 10 and 12, or 9 and 12. In addition to the species at m/z 54 observed in previous CI studies of fatty acid methyl esters, we also found that ions at m/z 42, 81, and 95 undergo covalent reaction with TAGs containing double bonds to yield ions at m/z 40, 54, 81, and 95 units greater than that of the parent TAG: [M + 40]+, [M + 54]+, [M + 81]+, and [M + 95]+ ions. When collisionally dissociated, these ions fragment to produce two or three diagnostic ions that locate the double bonds in the TAG. In addition, ions [RCH=C=O + 40]+ and [RCH=C=O + 54]+ formed from collisional dissociation are of strong abundance in MS/MS spectra, and collisional activation of these ions produces two intense confirmatory diagnostic ions in the MS3 spectra. Fragment ions reflecting neutral loss of an sn-1-acyl group from [M + 40]+ and [M + 54]+ are more abundant than those reflecting neutral loss of an sn-2-acyl group, analogous to previous reports for protonated TAGs. The position of each acyl group on the glycerol backbone is thus determined by the relative abundances of these ions. Under the conditions in our instrument, the [M + 40]+ adduct is at the highest signal and also yields all information about the double bond position and TAG stereochemistry. With the exception of geometries about the double bonds, racemic TAG isomers containing two monoenes or dienes and a saturate can be fully characterized by APCACI-MS/MS/MS.

Isolation of esterified fatty acids bound to serum albumin purified from human plasma and characterised by MALDI mass spectrometry

Human serum albumin (HSA) is the most abundant protein in plasma. It is known to transport drugs as well as endogenous ligands, like free fatty acids (FFA). A mass spectrometry based method was applied to analyze the albumin bound lipid ligands. HSA was isolated from a human plasma pool by cold ethanol fractionation and ion exchange chromatography. HSA was defatted using a solvent extraction method to release the copurified lipids bound to the protein. The extracts were then analyzed by matrix-assisted laser desorption ionisation (MALDI) mass spectrometry (MS). Using this method, phospholipids and acylglycerols were detected. The phospholipids were identified to be lyso-phosphatidylcholine (lyso-PC) with distribution of different fatty acids (palmitic, stearic, oleic, and linoleic acids). An abundant species in the HSA lipid extract was found to be a diacylglycerol, composed of two linoleic and/or oleic acid chains. The identified motifs reflect structures that are known to be present in plasma. The binding of lysophospholipids has already been described but it is the first ever-reported evidence of native diacylglycerol ligands bound to HSA. Besides the native ligands from plasma a triacylglycerol was detected that has been added during the albumin preparation steps.

Quantification of triacylglycerol regioisomers in oils and fat using different mass spectrometric and liquid chromatographic methods

The regioisomers (sn-ABA/sn-AAB) of four triacylglycerols (TAGs), 18:2/18:2/18:1 (LLO), 18:2/18:1/18:1 (LOO), 16:0/18:1/18:1 (POO), and 16:0/16:0/18:1 (PPO), were quantified in lard, rapeseed oil, and sunflower seed oil by three different mass spectrometric methods using liquid chromatography (LC) and two different mass spectrometers. The ionization methods used were positive ion atmospheric pressure chemical ionization (APCI), positive ion electrospray ionization (ESI), and negative ion chemical ionization (NICI) with ammonia as the reagent gas. The LC/APCI-MS results with two different instrumentation types, LC/ESI-MS/MS and direct inlet ammonia NICI-MS/MS, were compared. The LC/APCI-MS method is based on the preferential formation of diacylglycerol (DAG) fragment ions during ionization by loss of sn-1/3 fatty acids from [M+H]+ ions. Similar formation of the DAG ions from [M+NH4]+ ions by collision-induced dissociation (CID) in the LC/ESI-MS/MS method and the [M-H--RCOOH-100]- ions from [M-H]- ions by CID in the direct inlet ammonia NICI-MS/MS method is observed. These methods were found to be useful and reliable in determining the regioisomeric structure of TAGs. No statistically significant differences were found between the results obtained with these methods. For LLO, LOO, and POO the proportions of sn-ABA isomer calculated from the results from all four methods were in rapeseed oil 7.7 +/- 6.5, 57.9 +/- 3.3, and 4.5 +/- 6.1%, respectively, and in sunflower seed oil 12.2 +/- 6.9, 34.0 +/- 5.2, and 1.4 +/- 2.8%, respectively. The proportions of ABA of POO and PPO in lard were 95.3 +/- 3.2 and 4.9 +/- 5.6%, respectively. This study also proved that the LC/APCI-MS/MS method examined is not applicable in the quantification of TAG regioisomers because the formation of DAG ions is not clearly dependent on the positional distribution of the fatty acids.