In contrast to SIE, SRM is generally more specific than the SIE approach if the monitored precursor-product transition is specific to the targeted precursor eluted at a specified elution time while co-eluents have no interfering transitions. However, this approach requires previous knowledge of the transition from a targeted precursor ion to its specific

fragment ion and the numbers of transitions that can be monitored during column elution (“on the fly”) are limited. An instrument possessing a high duty cycle capability is therefore crucial to employ this approach for quantification of multiple species. In comparison to SIE (i.e., LC-MS) approach, #selleck kinase inhibitor keyword# SRM (i.e., LC-MS/MS) approach has not only higher specificity but also higher sensitivity [20]. The former is due to the specific monitoring of a pair of transitions while the latter is due to the marked noise reduction through filtering with tandem MS. These LC-MS techniques are theoretically suitable for many stationary phases (normal-phase, reversed-phase, ion exchange, hydrophilic interaction, etc.) Inhibitors,research,lifescience,medical as long as the elution conditions are effectively coupled with the mass spectrometer. In practice, LC-MS

has been employed for many applications in lipid identification and quantification. For example, Hermansson and colleagues separated over 100 lipid Inhibitors,research,lifescience,medical species employing a diol-modified silica column and identified and quantified these species

through two-dimensional maps of elution time and masses of the ions [27]. Sommer, Byrdwell, and others have employed dual LC coupled with MS (e.g., fractionation of lipid classes by normal-phase LC-MS followed by reversed-phase LC-MS or LC-MS/MS) to analyze lipid species in different classes Inhibitors,research,lifescience,medical [28,29]. Masukawa and colleagues have employed normal-phase LC-MS with a non-linear gradient to quantify over 182 ceramide species in human stratum corneum Inhibitors,research,lifescience,medical [30]. Merrill and colleagues have employed normal-phase and reversed-phase LC-MS to identify and quantify lipid species in sphingolipidomes [5]. Many researchers have broadly employed reversed-phase LC in conjunction with negative ion ESI-MS/MS to identify and quantify eicosanoids from biological samples [21,31]. Recently, Bohlinger, etc. have developed a charge-switch methodology about employing derivatization to markedly increase the sensitivity of eicosanoid analysis by coupling HPLC with positive-ion ESI-MS/MS [32]. Many researchers have employed ultra-performance LC (UPLC) to replace the sequential separation with normal- and reversed-phase HPLC and succeeded in analysis of different lipid classes including phospholipids, sphingolipids, and triacylglycerols [23,33-35]. It should be recognized that discovery and quantification of low and very low abundance lipid species is one of the major advantages of the LC-MS compared to direct infusion-based MS.