Mass lipid library

The identification of the lipids is a very challenging task. The lack of comprehensive mass spectral libraries often limits the identification of compounds in LC-MS and shotgun methods. Some spectral libraries are available, such as the Human Metabolome Database (http /www.hmdb.ca), the METLIN Metabolite Database (http /metlin.scripps.edu) (24), and the MassBank (http /www. massbank.jp) (25). However, construction of universal spectral databases for API-MS is challenging due to the poor reproducibility and high interinstru-ment variability of fragmentation patterns. 

The identification of individual classes of fatty acids has relied on the use of gas chromatography (GC), equipped with a flame ionization detector. Lipids are sapoiufled after extraction and the fatty acids converted to methyl esters. The fatty acid methyl esters (FAME) are separated using GC. The use of standards ahowed for the identification of individual species of fatty acids based on retention time. The method is quite sensitive and permits the quantification of fatty acid species. The mode of detection has been enhanced with the use of mass spectrometry (MS), which allows for the detection and quantification of unknowns, thus increasing the utility of these methods. The Lipid Library section on the gas chromatography of lipids http //www.lipidlibrary.co.uk/GCJipid/01 intro/index. htm) provides a comprehensive description of these methods. 

The interpretation of mass spectra of common wax components are described elsewhere (Hamilton, 1995b Evershed, 1992b Christie, 1994). The mass spectra of fatty acids, alcohols, wax esters and other lipids can be found in an open access website (The AOCS Lipid Library, 2011). Briefly, the identification is performed on the basis of the characteristic fragment and molecular ions. For example, the mass spectra of saturated fatty add methyl... 

The ACXZS Lipid Library. (2011). Mass spectrometry of fatty acid derivatives. In The Lipid Library, 2011-08-24, Available from ... 

Figure 3.46 Spectrum of the DMOX derivative of 9,12,15-octadecatrieenoate (CIS 3, co3). Masses indicated by are the diagnostic ions with 12 Da mass distance for locating double bonds (Christie - The Lipid Library, AOCS, 2014).

A number of cationic lipids have been prepared using solid-phase methods [147— 159]. Along with the well-known advantages that solid-phase chemistry provide (e.g. mass action, simple purification, compatibility with microwave synthesis [ 160, 161]), the main reason to use this approach is that it facilitates parallel synthesis of libraries of compounds, allowing potential structure activity relationships to be rapidly determined by the systematic modification of the cationic lipid structure per domain. 

Table II compares the composition by wei t of Croteau s and Fogerson s neutral lipid extract and our SF extract. The SF extracts were richer in fatty acids as compared to the neutral lipid extract. The SF extracts contained trace amounts of docosanoic acid and eicosanoic acid which were not identified in the neutral lipid extract. Since the paper published by Croteau and Fogerson has no chromatograms, it is not possible to Imow whether all the peaks in their GC-FID trace were identified. In addition, the analytes in the extract were identified by comparing retention times of a standard with the unknown. This method does not permit the unambiguous identification of an analyte because two different analytes can have similar retention time. In our study, the SF extracts were analyzed by GC-MS which is a superior technique compared to GC-FID because the peaks are identified by comparison of the mass spectrum of the peak component to the mass spectrum of a standard from the Wiley library.

LipidBlast is a library containing tandem mass spectra in the product-ion mode created in silico, validated to a great extent, and maintained by the Fiehn laboratory at University of California-Davis. LipidBlast contains a total of 212,516 tandem mass spectra for 119,200 different lipids in 26 lipid classes [7]. This library is freely available for commercial and noncommercial use at http //fiehnlab.ucdavis.edu/projects/ LipidBlast/. 

Demonstrated the applications of the library for high-throughput lipid identification [7]. The group analyzed lipid extracts of the human plasma using a low-resolution mass spectrometer. Using LipidBlast, they structurally annotated a total of 264 lipids. The data set was cross-checked with manual peak annotations and data available from Lipid MAPS. Using accurate mass LC-MS/MS, they annotated a total of 523 lipid molecular species. A similar number of plasma lipids were obtained in comparison to those previously published [8, 9]. 

The developers concluded that LipidBlast could be successfully applied to analyze MS/MS data from over 40 different mass spectrometer types and used with other available search engines and scoring algorithms, which represents a paradigm shift in lipidomics because it is not feasible to chemically synthesize all metabolites or natural products as authentic standards for library generation or quantification purposes. Moreover, the current array of MS/MS mass spectra for plant, animal, viral, and bacterial lipids in LipidBlast could be readily extended to many other important lipid classes. 

Although elution time from a LC separation provides valuable information for a particular species, definitive identification of the lipid species has to be performed based on its product-ion mass spectrum. For those researchers who are using LC-MS for identification and quantification of lipid species, familiar with the fragmentation patterns of lipid classes, therefore deriving the product-ion mass spectra of individual lipid species, is very important although a few databases and/or libraries (see Chapter 5) can be used to aid the identification. Resolving complex biological lipids into a... 

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