Jan 15, 2012 · Subsequently 250 μL internal standard was added. The vials were vortexed for 1 min and centrifuged at 10,900 rpm for 3 min. 200 μL of the clear upper layer of the supernatant was transferred into a vial and 250 μL formic acid 2% was added and vortexed for 5 s. 5 μL was injected into the LC–MS/MS system. 2.4. Get Price.