Correction of instrumentally produced mass fractionation during isotopic analysis of Fe by thermal ionization mass spectrometry

High-precision (similar to 0.015%/mass) isotope ratio measurements of Fe may be obtained by using magnetic-sector thermal ionization mass spectrometry (TIMS), where rigorous correction of instrumentally produced mass fractionation can be made. Such corrections are best done by using a double-spike approach, which was first introduced several decades ago. However, previous derivations do not lend themselves to the high-precision isotope analysis that modern TIMS instruments are capable of because of various assumptions of mass fractionation laws or constant atomic weights. Moreover, some of these previous approaches took iterative approaches to the calculation, and none presented detailed error propagations. Here we present a completely general derivation to the double-spike approach that may be used for any appropriate isotope system and is applicable to the mass fractionation laws that are known to occur in TIMS. In addition, we present an assessment of error propagation as a function of algorithm and spike isotope composition. This approach has produced the highest precision Fe isotope ratio measurements yet reported, on the order of +/-0.2 to 0.3 per mil for the Fe-54/Fe-56 ratio, that correct for instrumentally produced mass fractionation and yet retain natural, mass-dependent isotopic variations in samples. (Int J Mass Spectrom 193 (1999) 87-99) (C) 1999 Elsevier Science B.V.