Germanium stable isotope measurements by double-spike MC-ICPMS

Abstract

We present analytical procedures for the measurement of mass-dependent Ge isotope compositions using a ⁷⁰Ge–⁷³Ge double spike and multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Two different mass spectrometers (ThermoScientific Neptune Plus and Neoma) and two different sample introduction systems (Teledyne Cetac Technologies HGX-200 hydride generator and, for the first time in Ge isotope analyses, a Cetac Technologies Aridus II desolvator) were used. A series of analytical tests demonstrate that our analytical procedure efficiently separates Ge from the sample matrix and provides accurate and precise Ge concentration and isotope data for both instruments and sample introduction methods. The external reproducibility (2 s.d.) of the entire analytical procedure is ±0.09‰ for δ^74/70Ge (the permil deviation from the National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 3120a). Using the new methods, we obtained Ge isotope data for an Alfa Aesar Ge standard solution, the NIST SRM metals 129c and 261, two terrestrial basalts (BHVO-2 and BCR-2), and eight iron meteorites. We find excellent agreement between the data obtained using the hydride generator and using the Aridus II desolvator. Due to the overall easier use of the latter, the desolvator may be the sample introduction system of choice for most Ge isotope applications. Overall, the samples of this study show δ^74/70Ge variations up to ∼2‰, and in agreement with literature data, indicate that mass-dependent Ge isotope variations may be used to study a wide range of geochemical and cosmochemical processes.