Trifluoracetic acid

Acetonitrile (ACN), acetic acid, formic acid (FA), trifluoracetic acid (TFA), ethanol, ammonium bicarbonate, and chloroform were purchased from Fisher Scientific (Pittsburgh, PA, USA). 2,5-dihydroxyacetphenone (DHA), hematoxylin stain, aluminum potassium sulphate, glycerol, and mass spectrometry sequence-grade trypsin from porcine pancreas were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA).

The lipid reference standard mixture was purchased from Avanti Polar Lipids (Differential Ion Mobility System System Suitability Lipidomix, Alabaster, AL). Trifluoracetic acid (TFA), acetonitrile (ACN), and 1.0 M NaOH were purchased from Fisher Scientific (Hampton, NH). acetonitrile/aqueous sodium trifluoroacetic acid solution (1:1 ACN/0.1% TFA, pH 3.5) was prepared according to Moini et al.^{20 (link)} Pierce negative ion calibration solution (Calmix) was purchased from Thermo Scientific (Rockford, IL). Lipids were extracted from HAP1 and yeast cells using a CHCl3/MeOH/H2O (6:3:2, v/v/v) solvent system, an aliquot of the organic layer was dried under vacuum, and the dried lipid material resuspended in ACN/IPA/H2O (65:30:5, v/v/v). For mouse cecum and plasma samples, lipids were extracted using an MTBE/MeOH/H2O (10:3:2.5, v/v/v) solvent system, an aliquot of the organic layer was dried under vacuum, and the dry lipid material resuspend in MeOH/toluene (9:1). Further sample preparation details are provided in Supplemental Methods.