Gold-coated silver capsule for elemental analyzer-isotope ratio mass spectrometer: Robust against pretreatment of rock material for organic carbon and δ¹³C analyses

Abstract

For organic C content and stable isotope analysis of a sediment sample, an essential pretreatment procedure is acidification to remove carbonates in the sample. Although strong acids are required for removal of recalcitrant carbonates (e.g., siderite, magnesite, and dolomite), the long duration of acidification for frequently results in destruction of the Ag foil capsule in which the sample is placed. Another troublesome issue with the acid treatment is that capillary action of acids through wrinkles in the capsule wall leads to the sample spilling out. In this study, we successfully develop a new capsule preparation method by combining two improvements of 1) smoothing the capsule and creating the “bowl” shape that mitigates the acid spill and 2) plasma ablation to coat an Ag capsule with Au nanoparticles that prevents the destruction of the capsule. Through electron microscopy and energy dispersive X-ray analysis, we confirm that the plasma ablation removes ingrained C compounds on the capsule surface, contributing to a reduction in the C blank, and that the coating of Au nanoparticles protects the Ag surface from acid attack. None of 24 Au capsules and only one of 24 Ag capsules failed after the pretreatment procedure, which took 6 days with 6 M hydrochloric acid at 60°C and achieved more than 99% carbonate removal. We emphasize that the method developed here (smoothing and Au coating) is most helpful when dealing with recalcitrant carbonates that require more extensive acidification taking days versus overnight for calcite.