The Cameca Nanosims 50 has been designed to provide for parallel detection along a focal plane over a large mass range but spacing between adjacent mass lines makes it necessary to use miniature electron multipliers (EMs). A new version of a miniature EM, designed to reduce aging effects, has been tested. Means to characterize aging effects are presented. Data are interpreted by simulations with a simple model. Consequences of aging on isotopic measurements in a parallel detection context are examined in the special case of oxygen. Measurements of the most abundant isotope with a Faraday cup (FC) and weak isotopes with EMs are reported. Advantages of FC are emphasized: no dead time correction and no quasi-simultaneous arrivals to account for in a situation where high collection efficiencies (8% for O- for instance) leads to large corrections. (C) 2002 Elsevier Science B.V. All rights reserved.