The absolute quantification of ρ-coumaric acid, isoferulic acid, caffeic acid, sinapic acid, campesterol, and β-sitosterol was accomplished using calibration curves of each standard compound. Nine concentration solutions were prepared to achieve a concentration of 0.625 to 200 μg/mL by dissolving precisely weighed amounts of ρ-coumaric acid, isoferulic acid, caffeic acid, campesterol, and β-sitosterol in ethanol and sinapic acid in methanol. Two hundred microliters of each standard solution was transferred into GC vials and dried with nitrogen gas for 5 min. For derivatization, 30 μL of 20,000 μg/mL methoxylamine hydrochloride in pyridine, 50 μL of BSTFA (N,O-bis (trimethylsilyl) trifluoroacetamide; Alfa Aesar, Ward Hill, MA, USA) containing 1% trimethylchlorosilane, and 10 μL of 3000 μg/mL myristic acid-d27 (Tokyo Chemical Industry Co., Ltd.) in pyridine used as an internal standard (IS), were added to each standard compound of various concentrations. The samples were incubated in a 65°C water bath for 60 min, and the derivatized standard solutions of each compound were injected into the GC-MS in triplicate. The standard calibration curves were achieved by plotting concentration against intensity ratio between compounds and the internal standard. The means of the slopes (S) and standard deviation of the intercepts (σ) were calculated. The limit of detection (LOD) and the limit of quantification (LOQ) were calculated by following equations.
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