Negative silicon ions sputtered from a set of olivines under Cs bombardment show useful yield UY variations depending on sample compositions. Those matrix effects are correlated with atomic concentrations of implanted cesium close to surface, which in turn depend on sputtering yields. It is shown that instrumental mass fractionations IMF on isotopes are depending on UY. A linear relationship between IMF and the inverse of Af, the Cs atomic fraction, is experimentally established for Si as well as a linear dependence of UY on Af. A model linking the two sets of experimental data is proposed. Measurement artefacts are reviewed. (C) 2011 Elsevier B.V. All rights reserved.