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)
Different Purposes of Experiments in Untargeted/Targeted Lipidomics
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.
Techniques and Instruments Used in Untargeted/Targeted Lipidomics
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.
- Shotgun lipidomics is a specific type of LC-MS that is focused on identifying and quantifying all lipid species in a sample. Shotgun lipidomics uses high-resolution mass spectrometry (HR-MS) to identify and quantify all lipid species present in a sample, without prior knowledge of the specific lipid species of interest. In this approach, lipids are extracted from a sample, separated by liquid chromatography, and then analyzed by HR-MS. The high-resolution mass spectrometry allows for the identification of many lipid species simultaneously.
- LC-MS/MS is a comprehensive and untargeted approach that uses liquid chromatography in combination with tandem mass spectrometry (MS/MS) to identify and quantify all lipid species in a sample. In this approach, lipids are extracted from a sample, separated by liquid chromatography, and then analyzed by tandem mass spectrometry. LC-MS/MS can be used in a “data-dependent” or “data-independent” manner. In a data-dependent manner, the mass spectrometer automatically switches between different analysis modes, such as full scan and product ion scan, to acquire information about the lipids in a sample. In a data-independent manner, the mass spectrometer collects information about all lipid species present in a sample without prior selection, providing a comprehensive overview of the lipid profile.
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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References
- Xu, Tianrun, et al. “Recent advances in analytical strategies for mass spectrometry-based lipidomics.” Analytica Chimica Acta 1137 (2020): 156-169.
- Züllig, Thomas, Martin Trötzmüller, and Harald C. Köfeler. “Lipidomics from sample preparation to data analysis: a primer.” Analytical and bioanalytical chemistry 412 (2020): 2191-2209.
