LIPID MAPS Lipid Classification and Shorthand Notation
Summary¶
The LIPID MAPS standard organizes lipids into a chemical and biosynthetic hierarchy while separately encoding how much structural detail an experiment resolved. Its core rule is epistemic: an annotation must stop at the highest level justified by the measurement rather than naming a fully defined isomer from mass alone [1,2].
Classification¶
The top-level categories are fatty acyls (FA), glycerolipids (GL), glycerophospholipids (GP), sphingolipids (SP), sterol lipids (ST), prenol lipids (PR), saccharolipids (SL), and polyketides (PK). Categories are divided into classes and subclasses, with a fourth level available where structural diversity requires it [1].
The 2009 revision expanded coverage beyond mammalian lipids, replaced source-based sterol groupings with core-structure classes, incorporated natural-product hierarchies for isoprenoids and polyketides, and adopted familiar glycerophospholipid abbreviations such as PC, PE, PS, PI, and PA [1]. The hierarchy continues to evolve as new lipid classes are curated [3].
Annotation Hierarchy¶
| Level | What is established | Example |
|---|---|---|
| Species | Class plus summed carbons, double-bond equivalents, and optional oxygen count | PC 38:3;O2 |
| Molecular species | Constituent acyl/alkyl residues, but not necessarily their positions | TG 16:0_18:1_18:1 |
| sn-position | Residues assigned to glycerol positions | TG 16:0/18:1/18:1 |
| Double-bond/DBE position | Positions of double bonds or equivalent unsaturation | FA 18:2(9,12) |
| Structure defined | Functional-group identities known without all positions/stereochemistry | FA 18:2;OH |
| Full structure | Positions and geometries known, but not every stereochemical feature | FA 18:2(9Z,11E);13OH |
| Complete structure | All defining functional groups and stereochemistry resolved | 13R-HODE |
The separator carries information: an underscore separates known chains with unknown order, while a slash records assigned positions. Semicolons introduce oxygen counts or functional groups; double-bond positions and geometry are added only when demonstrated [2].
Minimum Evidence¶
At minimum, an MS-derived identification should report intact m/z, ion/adduct, chromatographic retention time when used, and diagnostic fragment m/z values. Double-bond positions, functional-group locations, and stereochemistry require additional techniques such as diagnostic fragmentation, derivatization, OzID, photochemical reactions, or chiral separation, depending on the claimed feature [2].
Database matching by precursor m/z alone is therefore compatible with species-level candidates, not complete structures. The standard explicitly warns that returning chemically complete molecules from such a search over-interprets the experiment [2,3].
Identifier Caveat¶
Historically, the positions within an LM identifier reflected the lipid category, class, subclass, and optional fourth level. Because LIPID MAPS now preserves identifiers when records are reclassified, current and legacy LM identifiers should not be decoded as immutable statements of classification [1,3].
Ongoing Updates¶
The 2026 resource update added stable identifiers for species- and molecular-species-level shorthand, expanded bile-acid conjugate notation, and changed the taurine abbreviation from ;T to ;Tau to remove ambiguity [4]. Users should therefore consult the maintained online shorthand guide when producing new data.
Citations¶
[1] Fahy et al. 2009. Update of the LIPID MAPS comprehensive classification system for lipids. Supports: Eight-category hierarchy, fourth-level classification, nomenclature changes, and historical LM identifier layout. Location: Classification Updates; Nomenclature Updates; Updates by Lipid Category.
[2] Liebisch et al. 2020. Update on LIPID MAPS classification, nomenclature, and shorthand notation for MS-derived lipid structures. Supports: Annotation levels, delimiters, minimum evidence, functional-group rules, and warning against structural over-annotation. Location: Experimental Prerequisites; Hierarchical Concept; General Rules; Discussion and Conclusions.
[3] Conroy et al. 2024. LIPID MAPS: update to databases and tools for the lipidomics community. Supports: Evolving hierarchy, persistent identifier policy, and species-level bulk-search behavior. Location: Updates to LMSD; Bulk Searches.
[4] Cockayne et al. 2026. LIPID MAPS: Powering discovery in lipidomics. Supports: Shorthand DB and 2026 bile-acid and taurine-notation updates. Location: Main text, page 2.