Untargeted lipidomics and targeted lipidomics are two approaches used in lipidomics, the study of lipids and their functions in biological systems.
Analytical strategies for mass spectrometry-based lipidomics (Xu et al., 2020)
Untargeted lipidomics is a comprehensive approach that aims to identify and quantify all lipid species present in a sample, without prior knowledge of the specific lipid species of interest, and regardless of their known or unknown presence. This method provides a comprehensive overview of the lipid profile in a sample but may be limited by sensitivity, specificity, and the ability to accurately identify and quantify all lipid species. Untargeted lipidomics is commonly used in areas such as disease diagnosis, drug development, and fundamental biology research.
Targeted lipidomics is a hypothesis-driven approach that focuses on the analysis of specific lipids or lipid classes of interest. This approach provides quantification of the targeted lipids with high precision and accuracy, and is commonly used in areas such as biomarker discovery, pharmacokinetics and pharmacodynamics studies, and nutritional research. However, it provides a limited view of the lipid profile, and may miss other important lipids present in the sample.
The choice between untargeted lipidomics and targeted lipidomics depends on the research question and the specific goals of the analysis.
In general, untargeted lipidomics is best suited for exploratory studies, while targeted lipidomics is best suited for more focused and specific studies. Typically, untargeted lipidomics screening for differential lipids is followed by target validation using targeted lipidomics. The validation of untargeted lipidomics findings using targeted lipidomics provides increased confidence in the results and allows for more robust conclusions to be drawn from the lipidomics data.
The primary instruments used for untargeted lipidomics and targeted lipidomics are mass spectrometers (MS) and liquid chromatography (LC) systems.
Untargeted lipidomics often employs liquid chromatography-mass spectrometry (LC-MS) techniques, such as shotgun lipidomics or liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) to identify and quantify all lipid species present in a sample.
Targeted lipidomics primarily employs liquid chromatography-tandem mass spectrometry (LC-MS/MS) techniques. In this approach, the lipid species of interest are pre-selected and specific mass transitions are monitored using multiple reaction monitoring (MRM) or selected reaction monitoring (SRM) to accurately quantify the targeted lipids.
Examples of mass spectrometers used in lipidomics include quadrupole time-of-flight (Q-TOF), Orbitrap, and linear ion trap (LIT) mass spectrometers. Examples of liquid chromatography systems include reversed-phase LC, normal-phase LC, and hydrophilic interaction LC (HILIC).
The lipidomics workflow, including all essential steps from sample to biological outcome (Züllig et al., 2020)
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