An apparatus (100) is described for determining the mass of ions, the apparatus configured to hold a plasma (101 ) having a plasma potential. The apparatus (100) comprises an electrode (102) having a surface extending in a surface plane and an insulator (104) interfacing with the electrode (102). An electric potential provider (106) is configured to provide an electric potential different than the potential of the plasma to the electrode (102), thereby forming a curved (110) potential distribution in the plasma surrounding the electrode (102). A magnetic field source (114, 116) is configured to provide a magnetic field (B) across at least part of the curved potential distribution in the plasma surrounding the electrode (102). An ion impact detector (108) is configured to detect impacts of ions arriving at the electrode (102), the detecting comprising detecting of locations of the impacts, and a processing unit (108) configured to interpret the detected impact locations in terms of the mass of the impacting ions.