Investigation of instrumental mass discrimination (higher transmission efficiency for the heavier of two isotopes) affecting the measurement of isotope ratios with multi-collector ICP-mass spectrometry (MC-ICP-MS). Obtaining insight into the origin of mass discrimination via computer simulation and experimental research. Specific attention for the isotopic analysis of mercury - assessment of the potential existence of the occurrence of mass-idenpendent fractionation (next to the well-known mass-dependent fractionation) during MC-ICP-MS measurement and selection of the most suited method for correction for these approaches.