HPLC-MS Measurements. HPLC-MS measurements were
performed using the HPLC method used for HPLC-SPE. ESI mass
spectra were recorded in both positive- and negative-ion modes
with a drying temperature of 350 °C, nebulizer pressure of 50 psi,
drying gas flow of 10 L/min, spray voltage of 3500 V, and trap
drive of 50 (arbitrary units).