The conditions of the ESI source operating in the negative po-larity mode were set as follows: 200 C drying gas temperature, 20 l/min drying gas flow, 40 psi nebulizer pressure, 325 C sheath gas temperature,10 l/min sheath gas flow, 3000 V capillary voltage and 1000 V nozzle voltage. Dwell time 10 ms was set to each MS/MS transition acquired in multiple reaction monitoring (MRM) mode.