High-precision measurement of W isotopes in Fe-Ni alloy and the effects from the nuclear field shift

We present an analytical protocol for high-precision measurements of Wisotopes by multi-collector ICPMS in metal alloy samples, with a particular emphasis on the least abundant W isotope (W-180). The external reproducibility, based on replicate analysis of an NIST Fe-Ni steel (SRM 129c), for epsilon W-180 (6/4) is +/- 0.49. This is an improvement of at least a factor of approximate to 2.4 compared to previous studies using MC-ICPMS. External precisions for other isotope ratios are comparable to previous studies of metallic samples. In addition, we observed resolvable deviations relative to the measurement reference standard (NIST SRM 3163) in isotope ratios that contain W-183. Similar effects have been reported in several previous studies of W isotopes. Our new data demonstrate that these effects are induced during chemical purification of W in the laboratory and that the isotopic variations are consistent with nuclear field shift effects. Such effects have been reported for various other elements but not yet for W. Possible bias introduced by this effect has implications for the use of W isotopes in early solar system chronology.