All gas-phase charge inversion data were collected on a Sciex QTRAP 4000 hybrid triple quadrupole/linear ion trap mass spectrometer (SCIEX, Concord, ON, Canada) with modifications analogous to those previously described. Alternately, pulsed nano-electrospray ionization (nESI) allows for sequential injection of lipid anions and charge inversion reagent bis-terpyridine magnesium dications, [MgTerpy2] 2+ . Mass spectra were produced via the analysis of ions using mass-selective axial ejection (MSAE). 61 Scheme 1 illustrates the generation of [FA -H + MgTerpy] + complex cations. Briefly, singly deprotonated FA anions are transformed via gas-phase ion/ion reaction with bis-terpyridine magnesium dications. First, [FA -H]¯ were generated via direct negative nESI of either (1) a methanolic solution of fatty acid standard or (2) from the liberation fatty acyl carboxylate anions from a phospholipid precursor anion utilizing collisional activation of lipid anions in q2 via single frequency resonance excitation which results in the cleavage of sn-1 and sn-2 ester bonds and production of singly deprotonated FA anions. Following the generation of [FA -H]¯ anions, reagent dications were mass-selected during transit through Q1 and transferred to q2. Together in q2, reagent [MgTerpy2] 2+ dications and [FA -H]¯ anions were permitted to react, yielding charge-inverted [FA -H + MgTerpy] 2+ complex cations. Next, monoisotopic isolation and ion trap CID of charge-inverted FA complex cations permit unambiguous isomeric anteiso branched-and straight-chain lipid discrimination via exploitation of reproducible spectral patterns. Mass spectra were recorded using an Agilent 6495C triple quadrupole mass spectrometer (Santa Clara, CA) equipped with a Jet Stream ESI source. The instrument was operated in positive ion mode.