GC-MS Analysis. Sample extracts were analyzed using a Trace 1310 gas chromatography coupled to a Thermo ISQ-LT mass spectrometer (ThermoFisher, Waltham, MA). Samples (1 µL) was injected onto a DB-WAXUI column (Agilent, 30 m x 0.25 mm x 0.25 µm) at a 5:1 split ratio. Helium carrier gas flow was held at 1.2 mL/min. The injector temperature was set at 220°C. The oven temperature program started at 100°C, ramped to 140°C at a rate of 10°C/min, then to 160°C at a rate of 5°C/min, to 250°C at a rate of 30°C/min with a final hold of 3 min. Temperatures of transfer line and ion source were both held at 250°C. Single Ion Monitoring was used at a rate of 10 scans/sec under electron impact mode for scanning the following m/z 44, 62, 64, 74, 76, 89, 94. Mass 62 and 64 were used as quantification ions for EC and EC-d5, respectively.