MEASUREMENT OF LIGHT STABLE-ISOTOPE RATIOS BY SIMS - MATRIX EFFECTS FOR OXYGEN, CARBON, AND SULFUR ISOTOPES IN MINERALS

Mass bias occurring during analysis of the light stable isotopes of ox ygen, carbon, and sulfur in geological materials by secondary ionizati on mass spectrometry has been investigated. The effects of instrumenta l parameters (primary ion beam, secondary ion energy, and polarity) we re evaluated by measuring sulfur isotope ratios in conductive sulfide minerals. The role of analyte chemical composition (matrix effect) on mass bias was investigated in sulfides (sulfur), silicates and oxides (oxygen), and carbonates (oxygen and carbon). For oxygen and carbon, v arious correlations between mass bias and matrix parameters have been identified. The application of several empirical models for prediction of oxygen isotopic mass bias indicates that for silicates, depending on mineral composition, bias can be predicted with an accuracy that is typically within two times that of the precision. However, extension of these models to other matrices has proved problematic, indicating t hat additional factors are important. (Int J Mass Spectrom 178 (1998) 81-112) (C) 1998 Elsevier Science B.V.