• C, drying gas flow 12 L/min, sheath gas temperature 350 • C, sheath gas flow 11 L/min, and a nebulizer pressure of 35 psi. The capillary and nozzle voltages were kept at 3500 V and 500 V, respectively. A reference solution, containing purine and hexakis(1H,1H,3Htetrafluoropropoxy)phosphazine (HP-0921), was continuously supplied to the second sprayer of the ESI source using an isocratic pump (G7110B, Agilent Technologies, Germany).