Lipidomics

Lipidomics, a field focused on large-scale analysis of cellular lipids, has advanced rapidly since its emergence in the early 2000s. Lipidomics enables the identification and quantification of various lipid species. In contrast, metabolomics focuses on the high-throughput analysis of metabolites—substrates, intermediates, and products of cellular metabolism.

About lipids

Lipid classes encompass a wide range of molecules, including free fatty acids, triacylglycerols, diacylglycerols, monoacylglycerols, phosphatidylinositols (PIs), phosphatidylethanolamines (PEs), phosphatidylcholines (PCs), phosphatidic acids (PAs), phosphatidylglycerols (PGs), lysophosphatidylcholines (lysoPCs), lysophosphatidylserines (lysoPSs), lysophosphatidylglycerols (lysoPGs), lysophosphatidylinositols (lysoPIs), as well as sphingolipid biosynthesis intermediates, sphinganines, ceramides, sphingomyelins, sulfatides, cerebrosides, and gangliosides. The LIPID MAPS® Structure Database (LMSD) currently holds 48,548 unique lipid structures (https://www.lipidmaps.org/).

Untargeted lipidomics

We use high-resolution mass spectrometry (HRMS) coupled with ultra-high-performance liquid chromatography (UHPLC) to analyze the lipidome in both positive and negative ionization (polarity switching). Deuterated internal lipid standards belonging to 14 different lipid classes are used for data normalization, providing accurate relative quantification. Up to 7,000 features in typical biological samples are relatively quantified. QC samples are injected multiple times throughout the analysis to monitor instrument stability (platform variability, retention time shift, and mass shift) and sample stability to ensure high-quality datasets. QC data is evaluated for each data set with multivariate statistics. Identification of lipids is performed in silico and with the aid of a library of standard substances of >900 in stock.

Platform strengths

Incorporating deuterated lipid standards allows for accurate, relative quantification for sample comparisons. MS/MS provides high-confidence lipid identification.

Lipid identification

Tier 3 is provided as optional (upon request) by using accurately measured masses (errors ≤2 ppm) to query lipidomic databases for preliminary assignments with putative identities. Tier 1 and Tier 2 are provided to identify and structurally confirm statistics relevant to lipid biomarkers by combining metabolome/lipidome database searching, LC-MS, and LC-MS/MS spectral matching and interpretation with an in-house library of >900 authentic compounds. Upon request, we can perform additional LC-MS and LC-MS/MS runs and acquire new standards (if available) to confirm the identity of novel metabolites not currently in our library.

Example data

Serum: In 120 samples, 3,462 lipid features were found across all samples (allowing 10% missing values) with 2,936 putative assigned IDs.