Fragmentation Patterns of Sphingolipids

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

In this chapter, similar to the last one, only the fragmentation patterns of individual lipid classes of sphingolipids under some common experimental conditions are overviewed. Any advanced readers should always look for the details from the original studies on the topic and/or smdy several invaluable review articles [1-3]. 

Cer species can readily form protonated ions ([M-fH] ) under acidic conditions in the positive-ion mode. These ions are very labile and readily lose a water molecule to become the [M-l-H-18] ion in the ion source [4-6]. The fragmentation pattern of protonated Cer species after CID contains the following  

The m/z 282 and 284 ions arise from the neutral loss of a FA ketene from the predominant [M-i-H-18] of Cer and dihydroceramide species, respectively. The fragment ions at m/z 264 and 266 could result from the further loss of a water molecule from the m/z 282 and 284 ions, respectively, or alternatively are produced from the neutral loss of a FA ketene from [M-i-H-36]+. More complicated Cer species, especially those found in skin, containing di- and trihydroxy sphingoid bases in combination with hydroxy and nonhydroxy FA amides were also characterized by product-ion ESI-MS analyses of protonated species [7-11]. 

Cer species can also be ionized as lithium adducts (i.e., [M+Li] ) in modest sensitivity in the positive-ion mode if lithium ions are present in the matrix [4], The fragmentation pattern of lithiated Cer species ([M-l-Li]+) has been well characterized [4], which is possibly used to identify the FA amide substituent and the sphingoid bases of the species [4], Fragment ions specific to each Cer subclass are also observed [4], Structural information can also be obtained from characterization of the sodium/potassium adducts of Cer species [5-7], 

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The following are a few examples to demonstrate how an elucidated fragmentation pattern can be used for identification of individual species of a lipid class present in biological samples by MDMS-SL. These examples well represent different lipid classes (e.g glycerophospholipids, sphingolipid, and glycerolipids), different mass levels (from low abundance to abundance), different polarities (Ifom polar to non-polar), etc. 

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