Argon versus helium as carrier gas for LA-ICP-MS impurity mapping on ice cores

Abstract

Impurity records in polar ice cores have provided invaluable insights into atmospheric aerosol concentrations of the past environment. The investigation of the oldest, deepest and highly thinned ice core layers is one of the most pressing tasks in today’s state-of-the-art ice core research. This calls for impurity analysis at high spatial resolution, which has to take into account post-depositional processes through the interaction of impurities with the ice matrix. To this end, mapping the impurity distribution in ice with laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) has great potential. Here we explore to what extent the use of Ar as a carrier gas has merits in ice core impurity mapping with LA-ICP-MS. This is motivated by the fact that a) the use of Ar is more economical over He, especially when used in conjunction with large volume sample cells, and b) an increase in sensitivity has previously been reported when adding ”wet” Ar to He as a carrier gas. We show that, albeit not fully matching the single-pulse-performance of He, it is possible to achieve mapping at up to 500 Hz with Ar in a system that is originally designed for He. In contrast to what we find on NIST glass standards and a sample of decorative murrina glass, maps obtained on ice core samples show higher intensities in Ar than in He. In an extreme case example, we show how Ar may permit to obtain signals in a deep interglacial ice sample from Antarctica with very low impurity concentrations, which was not possible when using He with the same LA-ICP-MS system.