The Cation Mass Spectrometer (CMS) should lead to high MCschi+ useful yields by allowing to reach an optimum value of the stationary Cs surface concentration, which is known to be a very critical parameter in MCchi+ analysis due to its strong influence on the probability of secondary Cs+ ionisation. For the present work, variations of the Cs concentration were obtained by modifying the impact angle of the primary Cs+ beam and by bombarding the sample simultaneously with Cs+ and Ga+ beams. Results show that these techniques allow to cover a large and continuous range of Cs surface concentrations and to reach an optimisation of the MCschi+ useful yields (enhancements up to a factor 100 compared to Cameca 4f and 6f instruments) mainly due to work function conditions resulting in high Cs+ ionisation probabilities. (C) 2002 Elsevier Science B.V. All rights reserved.