Sample Extraction
A volume of 100 µL D4-choline (150 µmol/L in water) was added to a 100 µL sample volume as an internal standard, as well as 5 µL dibutylhydroxytoluene (BHT) solution (20 mg/mL in ethanol) as an antioxidant. Afterwards, 0.6 mL water, 2.4 mL methanol and 0.8 mL chloroform were added and the mixture stirred to achieve a ternary mixture kept at 4 °C for 1 h. To separate hydrophilic from lipophilic compounds, 1.6 mL water and 2.4 mL chloroform were added, and the sample was vigorously stirred and centrifuged at 3000× 