Features of Shotgun Lipidomics

These principles of shotgun lipidomics can only be achieved in conjunction with the major feature of direct infusion, that is, ESI-MS analysis of lipids is conducted at a constant concentration of the solution. This feature in shotgun lipidomics provides many advantages for lipid analysis, particularly for the quantification of individual lipid species. Some of these advantages are as follows. First, constant interactions between lipid species are maintained under a constant concentration condition therefore, contribution of individual lipid species to the ion current in an ESI source is constant, thereby leading to a constant ratio of ion peak intensities between lipid species of a class. Such a constant ratio can be achieved under different experimental conditions (see Chapter 4), on different MS instruments, and in different laboratories. Second, also due to the constant interactions between lipid species under the condition, ion suppression between each other within a lipid class or between lipid classes is constant. Third, lipid aggregation, which is a big concern for lipid quantification, can be well controlled and minimized. 

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It should be recognized that loop injection (delivering sample solution with an LC system, but without an LC column) is also used in lipidomics and carries some features of direct infusion [4], Since this sample delivery method is unable to continuously maintain a constant lipid concentration as solvent is pushed through the sample loop, this method is excluded in the category of shotgun lipidomics. In contrast, direct infusion of individual fractions collected after LC separation (including those from a solid-phase extraction (SPE) column) falls under shotgun lipidomics due to its maintenance of a constant concentration condition. 

Based on these unique features, at least three different approaches of shotgun lipidomics are developed and well documented in the literature, including tandem MS-based shotgun lipidomics, high mass accuracy-based shotgun lipidomics, and multidimensional MS-based shotgun lipidomics. 

The third factor is to set up the MS (or MS/MS) parameters to identify and quantify the eluted individual lipid species as many as possible. The particular features in LC-MS analysis are that the lipid concentrations in eluents are constantly changing, and identification and quantification of lipid species have to be done in a very limited time frame. These features are in contrast to shotgun lipidomics therefore, totally different settings and methodologies from those in shotgun lipidomics have to be employed. 

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