Table 1.
Summary of commonly observed in-source fragmentation and strategies for recognition and avoidance of these generated artifacts in lipidomics analysis.
| MS-based lipidomics approach | Lipid class | In-source fragmentation | Strategies for recognition and avoidancea |
|---|---|---|---|
|
| |||
| LC-MS and shotgun lipidomicsb | PS - PA | NLS of serine - generation of PA | 1) A less harsh ionization condition; 2) an exogenous standard of PS included; 3) the lower concentration of PA species compared with PS species; and 4) the structural characteristics of endogenous PA and PS species. |
| PC - DMPE/PE | NLS of CH3X from [PC+X]- - generation of DMPE species (mis-annotation as isomeric PE) | 1) A less harsh ionization condition; 2) inclusion of an exogenous standard of PC; 3) very low concentration of DMPE species; 4) the structural characteristics of endogenous DMPE species; 5) further confirmation through product-ion ESI-MS analysis; and 6) chemical derivatization. | |
| Other lysophospholipid - lysoPA | NLS of choline/serine headgroup from lysoPC and lysoPS -generation of lysoPA species | 1) Inclusion of exogenous standards of lysoPC and lysoPS; 2) a less harsh ionization condition; and 3) the novel prefractionation method. | |
| Free cholesterol/CE - cholestadiene ion | NLS of water/fatty acid from free cholesterol/CE, respectively - generation of cholestadiene ion | 1) A less harsh ionization condition employed; 2) chemical derivatization; and 3) using more advanced commercial mass instruments. | |
| Glycosphingolipid - ceramide | NLS of sugar residue - generation of ceramide | The aforementioned methods for lysoPA and cholesterol. | |
| MALDI-MSI | TAG-DAG | NLS of fatty acid/fatty acid salt from TAG - generation of DAG or DAG analogs | 1) Reduction of the laser power; 2) the much lower levels of endogenous DAGs compared with that of TAG species; 3) the more complicated profile of ion source generated DAG in an identical tissue spot; 4) using new matrices; and 5) novel approaches of pretreatment of samples, such as on-tissue digestion and on-tissue derivatization |
| Glycerophospholipids - PA | NLS of trimethylamine/ vinylamine/serine from sodiated glycerophospholipids (e.g., PC, PE and PS) - generation of PA/ isomeric species of PA | 1) Reduction of the laser power; 2) the unique chemical structures of endogenous PA species; 3) much lower concentrations of PA species compared with PC species; 4) tissue washing procedures; and 5) determination of PA species in the negative-ion mode. | |
| SM - C1P | NLS of trimethylamine headgroup from sodiated SM species - generation of isomeric species of C1P | 1) Reduction of the laser power; 2) much lower concentrations of C1P species compared with SM counterparts; 3) tissue washing procedures; and 4) determination of C1P species in the negative-ion mode. | |
a
The original references from which these strategies were employed are cited in the text. See text for the abbreviations.
b
All these artifacts discussed in LC-MS and shotgun lipidomics also exist in MALDI-MSI analysis with more severity.