Collision Cross-Section Profiling of Lipids

Traveling wave ion mobility mass spectrometers
(Synapt G2 HDMS, Waters, Corporation, Manchester, UK), located in
different laboratories, were used to derive CCS values for various
lipid classes. A mass range of m/z 50–1500, with the mass spectrometer operating in both positive
and negative electrospray ionization, was used. A summary of the MS
settings appears in Table S2 (Supporting Information). Direct injection at 5 μL/min was used. CCS values, obtained
in nitrogen, were experimentally determined using previously published
CCS values for singly charged polyalanine oligomers as the TWIM calibrant
species in both ESI⁺ and ESI^– mode^{26 (link),27 (link)} (Table S3 in the Supporting Information). Poly-dl-alanine was prepared in 50:50 (v/v) water/acetonitrile
at a concentration of 10 mg/L. Calibration was performed using singly
charged oligomers from n = 3 to n = 14. The calibration encompassed a mass range extending from 231
to 1012 Da and a CCS range extending from 151 Ų to
306 Ų, in ESI⁺, and from 150 Ų to 308 Ų, in ESI^– (Table
S3 in the Supporting Information).^{26 (link)} CCS values were derived according to previously
reported procedures.^{26 (link),28 (link)} The ion-mobility resolution was
∼40 Ω/ΔΩ(fwhm). The ion-mobility peak, or
arrival-time distribution (ATD), may represent a combination of structurally
similar isomers that remain unresolved. The CCS values reported were
determined at the apex of the ATD.^{29 (link)}