Solid phase extractionTo 1 mL spiked plasma samples, 10 L IS solution was added,and 500 L of this mixture was applied to a 100 mg HyperSep C8cartridge (which was conditioned prior to sample loading by 3 suc-cessive washes of HPLC grade methanol, followed by 3 successivewashes of ultrapure water). Serum components were eluted by cen-trifugation at 1500 rpm for 1 min. This first eluent was discarded,and the simultaneous elution of piperacillin, tazobactam, and ISwas achieved by washing the cartridge with 0.5 mL of a 60:40 (v/v)water: methanol mobile phase with centrifugation at 1500 rpm for