Washing optimization Elution optimization ESI APCI %RE %Ion suppression %RE %Ion suppression with 1mL water with 1mL methanol 83 8.9 85 4.2 with 1mL water with 1mL acetonitrile 72 18 75 15.2 with 1mL water twice with 1mL methanol twice 86 3.4 89 2.6 with 1mL 2% methanol in water twice with 1mL methanol twice 69 16 73 13 with 1mL 2% methanol in water twice with 1mL2% ammoniated  methanol twice 63 17.6 57 19

Table 4: Effect of extraction condition of SPE on ion suppression and method efficiency.