Paper presented at the EGS 23rd General Assembly 20-24 April, 1998, Nice, France.
Using the high-resolution mass spectrometer CELIAS/MTOF on board SOHO we have measured the solar wind isotopic abundance ratios of Ne, Mg, and Si in different solar wind regimes with bulk velocities ranging from 350 km/s to 650 km/s. Data indicate a systematic depletion of the heavier isotopes in the slow solar wind compared to their abundances in the fast solar wind from coronal holes. These variations in the solar wind isotopic composition represent a pure mass-dependent effect because the different isotopes of an element pass the inner corona with the same charge state distribution. The influence of particle mass on the acceleration of minor solar wind ions will be discussed in the context of theoretical models and recent optical observations with other SOHO instruments.