The Cation Mass Spectrometer (CMS) is a SIMS prototype developed in our laboratory in order to perform quantitative analysis with optimal sensitivity and high depth and lateral resolution in the MCsx+ and M-modes. For this aim, a patented neutral Cs evaporator for varying the Cs surface concentration over the whole range has been developed. The sputtering and the Cs introduction have been decoupled successfully by applying simultaneously Xy+ bombardment and neutral Cs deposition. X stands for any element except Cs. Currently the CMS is equipped with a Cs+ and a Ga+ gun. These guns are used for studying the useful yield variations of negative secondary ions (Si-, Al-, Ni-, In-, P-) with respect to the Cs surface concentration. Furthermore, the observed variations of the useful yield were linked to work function shifts. Qualitative agreement with the predictions of the electron tunnelling model are obtained. Several applications give further evidence of successful optimization of the useful yield for elements with high electron affinity. (c) 2006 Elsevier B.V. All rights reserved.