ICP-QMS/QMS was applied to whole blood samples to determine the quantity of trace As. It was found that Fe also causes polyatomic interference with As in conventional ICP-QMS, in addition to Ca and Cl, and ICP-QMS/QMS can remove these interferences in mass-shift mode using O2as reaction gas. The ICP-QMS/QMS technique allows the determination of As at sub-ng mL−1levels.