This manuscript was published in Analytical Chemistry 2009, 81, 6774-6778 [DOI: 10.1021/ac9008435] by the American Chemical Society Mass Bias Fractionation Laws for Multi-Collector ICP-MS: assumptions and their experimental verification Juris Meija*, Lu Yang, Ralph Sturgeon and Zoltán Mester Institute for National Measurement Standards, National Research Council Canada, 1200 Montreal Rd., Ottawa ON K1A 0R6, Canada *E-mail: juris.meija@nrc.ca ABSTRACT Although mass bias fractionation correction is among the most studied theoretical aspects of analytical mass spectrometry, several assumptions imbedded in the conventional correction models remain largely untested. Experimental evidence is given herein highlighting parts-per-thousand deviations from the conventional mass bias correction models which can occur with use of multi-collector inductively coupled plasma mass spectrometry. Furthermore, current mass spectrometric approaches are not capable of elucidating the fractionation exponent.