the dried residues were derivatized with methoxylamine hydrochloride and propionic anhydride sequentially adapted from a previously described method (12 (link)). 25 μL of methoxylamine hydrochloride (2% (w/v) in pyridine) was added to the dried residues and incubated for 60 minutes at 70°C. After centrifugation for 2 minutes at 12,000 g, 50 μL propionic anhydride was added and incubated for 30 minutes at 60°C prior to another round of drying under nitrogen. The dried residues were resuspended with 55ul pure ethyl acetate and transferred to GC vials for analysis. GC/MS analysis was conducted using the Agilent 7890B GC system and Agilent 5977A Mass Spectrometer. Specifically, 1 μL of the derivatized sample was injected into the GC column. GC temperature gradient started at 90°C, increased to 260°C at the speed of 9°C per minute and further increased to 290 °C at the speed of 30°C per minute. Then temperature was held at 290°C for 5 minutes with a run time of 24.9 minutes. Ionization was conducted by EI at 70 eV with Helium flow at 1 mL/min. Temperatures of the source, the MS quad, the interface, and the inlet were maintained at 230°C, 150°C, 280°C, and 250°C, respectively. Mass spectra were recorded in mass scan mode from m/z 50 to 700.