The PGs were converted to bis(3,5-dinitrophenylurethanes), i.e., bis-DNPU derivatives, as described by Itabashi and Kuksis , with a minor modification. The products (~ 1 mg), all four isomers, were dissolved in 1 mL of dry toluene, 5 mg of 3,5-dinitrophenyl isocyanate, and 20 μL of dry pyridine was added, and the solution was stirred gently at 30°C. After 12 h, the solvent was evaporated to dryness, and LC-MS analyzed the resulting bis-DNPU derivatives on the chiral column; see below. The flow rate, the column temperature, and the injection volume were 0.5 mLmin -1 , 24 °C, and 10 µL, respectively. Other conditions were the same as those used for HILIC (see above).