New generation multicollector ICP mass spectrometers are now being widely used for precise isotope ratio measurements. Different instruments show marked differences in abundance sensitivity, which, if corrected using the conventional thermal ionization mass spectrometry method of half-mass zeroes, result in large inaccuracies in isotope ratios. It is shown that a tail correction based on measurements of mono-isotopic ion beams is essential to obtain isotope ratio accuracy on instruments with poorer abundance sensitivity. This effect can largely explain published artefacts in MC-ICP-MS Pb isotope determination (M. Rehkämper and K. Mezger, J. Anal. At. Spectrom., 2000, 15, 1451).
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