chromatography/mass spectrometry. Lipid molecular species were profiled using a dual-column nanocapillary LC system equipped with 75 mm665 cm columns, each packed with 3 mm Jupiter particles (Phenomenex, Torrance, CA). The mobile phases were (A) 10 mM ammonium acetate in 50:50 water/methanol (v/v) and (B) 10 mM ammonium acetate in 50:50 methanol/acetonitrile (v/v). The LC system was equilibrated at 10,000 psi with mobile phase A prior to injecting 0.4 mL of sample. Exponential gradient elution was initiated 50 min after sample loading with an initial column flow of ,300 nL/min. After 90 min of gradient separation, the mobile phase mixer was purged with 3 mL of mobile phase B, followed by a 5 min column wash. Finally, the mobile phase mixer was purged with 10 mL of mobile phase A, which represented the end of one separation cycle. While gradient elution was performed on one column, the other column was equilibrated with mobile phase A. The LC system was coupled to a hybrid linear ion-trap-Orbitrap mass spectrometer (ThermoFisher, San Jose, CA), and the capillary temperature and electrospray voltage were 200uC and +2.2 kV, respectively. The Orbitrap was used as the mass analyzer during MS survey scans over the m/z range 200-2000 with a duty cycle of ,1.2 s. Data-dependant MS/MS was performed in the LTQ for the top 5 ions, with a normalized collision energy of ,35%. Dynamic exclusion in the LTQ during data-dependant MS/MS experiments was enabled as follows: repeat count of 2, repeat duration of 30 s, exclusion list size of 250, and exclusion duration of 60 s.