The gradient started at 10% B and was linearly increased to 100% B over the course of 5 min followed by a hold of 3 min at 100% B. Equilibrium was achieved by holding at
10% B for 8 min. The flow rate was 0.6 mL/min and each injection was 10 lL. The liquid chromatography system was connected to an Agilent 6540 Quadrupole Time-of-Flight Mass Spectrometer with a Dual Agilent Jetstream ESI source in positive ion mode; data were collected at an acquisition rate of 2 spectra/s in in the extended dynamic range mode (2 GHz).