Materials
SPO was purchased from Qinghai Kangpu Co., Ltd. (Xining, Qinghai). Standards of 37 fatty acid methyl esters were purchased from Sigma-Aldrich Chemical Co., Ltd. (Shanghai, China). Hexane, methanol, ethanol, acetone and isopropanol were provided by Sinopharm Chemical Regent (Shanghai, China). All of the other reagents were analytically pure and were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).