g., selected reaction monitoring (SRM) or multiple reaction monitoring (MRM)) is employed. Among the OSI-744 mw methods based on the three requirements, the first method or its variants has been used broadly in practice [19,20]. The second one is impractical for quantification of numerous species
in a lipidomic approach while studies with one or limited species have been widely reported . The third one makes it possible to use one standard (or one standard curve) to quantify individual lipid species in a class but is mostly used for a rough quantitation with less accuracy compared to the former two methods [22-24]. To perform quantitative analysis of lipids by LC-MS, the
limit Inhibitors,research,lifescience,medical of detection, the standard curves and their linear dynamic ranges are generally pre-determined Inhibitors,research,lifescience,medical before sample analysis. In practice, at least one internal standard for each lipid class is generally included in the sample to normalize the differential ionization efficiencies from different lipid classes that possess differential head groups [25,26]. Inhibitors,research,lifescience,medical Accordingly, each of the ion peaks of individual species is first normalized to the internally added control species from the same class prior to comparison with the appropriate standard curve(s) for quantification. This approach reduces the variability of quantification by diminishing the effects of the variations of chromatographic separation conditions and/or ESI-MS conditions that can dramatically alter the detected
absolute Inhibitors,research,lifescience,medical ion counts of a particular species but much less affect the relative ion counts of the species obtained by normalizing to the ion counts of the internal standard detected under identical conditions if co-eluted or nearly identical conditions if eluted at different times. Two major LC-MS techniques for quantitative analysis of lipids include selected ion extraction Inhibitors,research,lifescience,medical (SIE) and SRM. The SIE approach utilizes a survey scan for quantification while the SRM (or MRM) approach performs tandem MS and monitors a particular pair (or pairs) of precursor/product ions at a specified elution time for quantification. The SIE approach is usually used for “global” lipid analysis where mass spectra are acquired continuously whatever during a chromatographic separation. The particular ions of interest are extracted from the acquired data array and the reconstituted peak of each extracted ion can be quantified with comparisons to either the reconstituted ion peak of an internal standard or a standard curve of the particular ion established under identical experimental conditions. The advantage of this approach is its simple instrumentation because no tandem MS is required but the specificity of the extracted ion to the targeted species is always a concern.