High precision Fe isotope measurements with high mass resolution MC-ICPMS

This study presents a new technique for high precision measurement of iron isotope ratios using high mass resolution MC-ICPMS on the ThermoFinnigan Neptune. A mass resolving power of about 8000-9000 was used to resolve the mass interferences of isobaric polyatontic ions (e.g., (ArO+)-Ar-40-O-16 on Fe-56(+) , Ar-40(16) OH+ on Fe-57(+) and (ArN+)-Ar-40-N-14 on Fe-54(+)) from the Fe isotopes and to produce "flat top" peak shapes with a plateau width of about 100ppm in mass (Deltam/m). The abundance sensitivity was determined to 50ppm contribution of the (ArO+)-Ar-40-O-16 peak tail on the Fe-56(+) peak. This enables Fe isotope measurements with a Fe-isotope/interference ratio of up to 1, without significant contribution from the tail of the interference peak. The performance of the technique was tested by determining the reproducibility and accuracy of synthetic Fe samples and spiked standards against the IRMM014 standard. Various interface setups, including wet and dry plasma techniques, and a wide range of concentrations (from 50 ppb to 10 ppm) have been used to demonstrate the flexibility, robustness and sensitivity of the technique. An external precision of ca. 0.10parts per thousand for delta(56)Fe and ca. 0.15parts per thousand for delta(57)Fe (2 S.D.) was routinely achieved. Measured and calculated delta values between spiked standards and the IRMM014 standard agree within uncertainties. In the investigated concentration range, precision and accuracy are independent of both the sample introduction system and the sample concentration. The new technique presented in this study is a robust and simple method to perform high precision Fe isotope measurements, and it allows to measure much lower sample concentrations compared to previous techniques. (C) 2003 Elsevier Science B.V. All rights reserved.