The LC system was coupled on-line to a TSQ Quantum Access triple quadrupole mass spectrometer
(TQMS) (Thermo Fisher Scientific) equipped with an electrospray ionisation (ESI) source and operated in the positive ion mode. The analytical column was a Luna C8 (150 2 mm ID,
5 lm particle size), coupled with a C8 Security Guard cartridge system (4 2 mm ID, 5 lm particle size), both from Phenomenex(Torrance, CA, USA).