Reversed-Phase LC-MS-Based Approaches

The history and many applications of reversed-phase LC for lipid analysis could be found in a review paper [56]. Here only a few examples with LC-ESI/MS analysis of lipid species are given. 

At the earliest stage of LC-ESI/MS analysis of lipids, Kim et al. employed a C18 column (150x2.1 mm, 3-pm particle size) with a mobile phase consisting of the 0.5% ammonium hydroxide in a water-methanol-hexane mixture employing a linear gradient from 12 88 0 to 0 88 12 [69], They detected ions that were shown as the protonated or sodiated form in the positive-ion mode. They determined PI, PS, PC, PE, and SM with an SIM approach. 

Analysis of individual lipid species of a class in the Lipid MAPS protocols was largely developed based on reversed-phase LC-MS. For example, in the protocol for eicosanoid analysis, a C18 column (250 X 2.1 mm) was employed and a specific gradient from mobile phase A (water-acetonitrile-formic acid, 63 37 0.02, v/v/v) to B (acetonitrile-isopropanol, 50 50, v/v) was applied [71]. Individual eicosanoid species were detected by using the MRM approach and quantified in comparison to the relevant internal standard. 

Sommer et al. [72] used reversed-phase LC-MS or LC-MS/MS with a C18 capillary column to fully characterize the individual lipid species including fatty acid compositions after fractionation of different lipid classes utilizing normal-phase LC-MS on an offline setting. Similar approaches using 2D LC-MS online or offline were also used by Byrdwell for a total lipid analysis [73] as well as for others [74—77]. 

Overall, LC-MS analysis with a reversed-phase column is very powerful and common for the analysis of individual species after fractionation of an interest class, whereas a UPLC system is popularly used for global lipid analysis. Regarding the MS detection, the former is more associated with an MRM approach while the SIM approach is the likely choice in the latter case. 

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