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 multicollector inductively coupled plasma mass spectrometry. Furthermore, current mass spectrometric approaches are not capable of elucidating the fractionation exponent.