Testing the Ce Limit of Mass Bias Correction Using Nd/Nd as Normalizing Ratio in Radiogenic Neodymium Isotope Analysis by MC-ICP-MS.

Rationale: Neodymium isotopes are a powerful geochemical tool that has widely been used in terrestrial and extraterrestrial studies. Modern multicollector inductively coupled plasma mass spectrometers (MC-ICP-MS) allow fast, accurate, and precise analysis of the radiogenic Nd isotope ratio Nd/Nd. These analyses comprise relatively high instrumental mass bias that is typically corrected for using the stable Nd/Nd of 0.7219 and an exponential law. The instrument is usually tuned to optimize the operating conditions for isotope analysis, but this tuning is a trade-off primarily between signal intensity, stability, and accuracy. Alternative, more effective approaches for mass bias correction have been proposed that use Nd/Nd as normalizing ratio. However, one drawback of using this ratio is that the efficient removal of Ce from Nd is required to minimize the effect of isobaric interference of Ce on Nd.

Methods: Here, we analyzed international Nd and rock reference materials using a Thermo Scientific Neptune Plus MC-ICP-MS to evaluate the sensitivity of Nd/Nd-based mass bias correction to varying Ce/Nd and in comparison with the commonly used Nd/Nd-based correction.

Results: Our results show that the corrected Nd/Nd of Ce-doped JNdi-1 and Ce-containing USGS BCR-2, NOD-A-1, and NOD-P-1 reference materials are insensitive to Ce/Nd of up ~1.

Conclusions: The correction of instrumental mass bias with Nd/Nd as a normalizing ratio yields, as previously suggested, improved trueness and precision of Nd/Nd data in comparison with Nd/Nd-based corrections, even under high Ce/Nd of to ~1. This allows improved optimization of signal intensity during instrument tuning, which is particularly useful for natural samples with low Nd content.