Structural Characterization for Lipid Identification

Characterization of lipid structures is a major and essential part of lipidomic research. This is largely due to the identification and even quantification of individual lipid species present in biological samples that ultimately depend on the information learned from the extensive characterization of representative authentic lipid species of individual class. Accordingly, characterization of common ions of lipid species from the majority of the cellular lipid classes is summarized in this part. 

For clarification, a few points related to characterization of lipid species should be made clear. 

Lipidomics Comprehensive Mass Spectrometry of Lipids, First Edition. Xianlin Han. 2016 John Wiley Sons, Inc. Published 2016 by John Wiley Sons, Inc. 

It is always good to characterize a wide variety of representative molecular species of a lipid class based on the degree of unsaturation, fatty acyl chain length, different regioisomers, etc. However, due to the cost for extensive characterization, only a minimal set of species is usually employed for the purpose and a fragmentation pattern is derived from the limited characterization for general utilization. 

Since the ESI ion source is compatible with nearly all of mass analyzers in which tandem MS can be performed, a variety of types of mass spectrometers are employed for characterization of lipids. Some examples of the instruments that are used to perform tandem MS include tandem sectors, QqQ, ion-trap, ion-cyclotron resonance, TOF/TOF, and hybrid instruments such as Q-TOF. The majority of tandem MS analyses presented in this part are conducted by using the QqQ-type instrument since majority of the early studies on characterization of lipid species were conducted with this type of instrument, and the characterized fragmentation pattern from this type of instrument can well represent those obtained from other types of instruments even including MALDI-MS [4, 5]. 

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