Tartaric acid

Chemical standards of the compounds in this study were purchased from Sigma-Aldrich (St. Louis, MO, USA), TCI America (Portland, OR, USA), EKC Inc. (Rosemont, IL, USA), Alfa Aesar (Ward Hill, MA, USA), and EMD Chemical Inc. (Gibbstown, NJ, USA), and their purities were >90% in all cases. The details are shown in a supplementary table. Milli-Q quality water was obtained from a Milli-Q purification system (Millipore, Boston, MA, USA). Methanol (HPLC grade) was from EM Science (Gibbstown, NJ, USA). Acetonitrile (HPLC grade) was bought from Omnisolv (McLean, VA, USA). Dichloromethane (HPLC grade) from Burdick & Jackson (Muskegon, MI, USA) was freshly distilled before use. Tartaric acid was from Mallinckrodt Inc. (Paris, France). Anhydrous sodium sulfate and sodium chloride (99.9%, ACS certified) were supplied by Mallinckrodt Baker. The LiChrolut-EN cartridge with a two-gram absorbent bed was packed in the laboratory.
Standard stock solutions were prepared in methanol individually except that fatty acids were in acetonitrile. Internal standard solution (IS) of 3,4-dimethylphenol and 4-octanol, was prepared at a concentration of 50 ppm in methanol, separately. All the solutions were stored in dark bottles at −20 °C.