COSMO: Double spike optimization for sample-limited analyses of isotopically anomalous materials

Abstract

The double spike (DS) technique is the gold standard by which high-precision and high-accuracy mass-dependent isotope fractionations are quantified, and has played a critical role in the recent development of numerous non-traditional stable isotope systems. The democratization of the technique is in great part due to the availability of the so-called 'DS toolbox', a software suite that allows for the straightforward identification of optimal DS compositions. As new applications for DS measurements arise, some additional considerations must be taken into account in deciding on an optimal double spike. In particular, sample-limited investigations of combined mass-dependent and independent isotope effects (e.g., in Early Solar System materials) present an additional challenge in determining optimal spikes. Here, we describe the cosmo software package, which specifically addresses this upcoming need in cosmochemistry/isotope geochemistry to optimize DS measurements for small samples with mass-independent anomalies (e.g., nucleosynthetic anomalies, radiogenic ingrowth). These measurements are subject to additional errors from a complementary unspiked measurement, which is necessary to properly quantify mass-dependent isotope effects during the DS inversion. The software package addresses this additional complication by offering users the ability to (i) specify additional parameters relevant to practical sample-limited analyses (e.g., instrumental transmission efficiency, number of cycles of analyses), (ii) optimize how a sample is split between unspiked and spiked measurements, and (iii) identify the internal normalization scheme that leads to the lowest uncertainty on the mass-dependent fractionation factor, α, and/or the isotope anomalies, ε. These additional functionalities were designed to operate within the DS toolbox framework and expands its applicability to a wider array of samples (i.e., extraterrestrial samples) and measurement scenarios to push the limits of new and improved instrumentation.

Additional details