Lipid coupled with MALDI

ESI enables the production of molecular ions directly from samples in solution. It can be used for small as well as large biopolymers up to about 200,000 Da including peptides, proteins, carbohydrates, DNA fragments and lipids. Unlike MALDI, ESI is a continuous ionisation method and suitable for coupling with liquid separation methods like HPLC (chapter 2) or CE (chapter 3.3). 

Unlike conventional ionization techniques (e.g., electron or chemical ionization), soft ionization techniques (e.g., ESI and MALDI) (see above) yield minimal in-source fragmentation under appropriate experimental conditions. In-source fragmentation can provide structural information when used properly, but generally leads to complication of lipid analysis since frequently multiple lipid species enter the ion source simultaneously even when coupled with chromatographic separation (i.e., an LC-MS lipidomics approach) (see Chapter 3). Therefore, the absence of in-source fragmentation becomes a big advantage of these ionization techniques for lipid analysis in lipidomics. 

Electrospray ionization and atmospheric pressure chemical ionization are popular as ionization techniques, for qualitative and quantitative LC—MS analysis of lipids [63—65]. Based on flieir ionization mechanisms, ESI is more suitable for ionization of polar and ionic compounds and is capable of ionizing both small and large biomolecules. APCI can ionize less polar and neutral compounds more efficiently than ESI. Consequently, APCI—MS coupled to LC is the most used tool for TAG identification, because of the full compatibility with common NARP LC conditions, easy ionization of nonpolar TAGs, and the attainment of both protonated molecules [M + H]+ and fragment ions [M - - H — RzCOOH]. On the other hand, ESI is usually employed for the more-polar phospholipids. However, ESI or matrix-assisted laser desorption—ionization (MALDI) have been used for TAGs, as well [66,67]. 

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