Combination of focused ion beam (FIB) and microtome by ultrathin slice preparation for transmission electron microscopy (TEM) observation

There are growing demands for integrated study of isotopes, trace elements and crystallography of micron-sized grains observed on polished sections or scattered on surface of sample holders. However, transmission electron microscopy (TEM) observation is largely restricted by the challenges associated with the preparation of ultrathin sections from given micron-sized grains after chemical and isotopic analyses. Here, we report a new method of combining focused ion beam (FIB) and microtome techniques. This method is demonstrated by the successful preparation of several ultrathin slices from a presolar graphite grain. The presolar graphite grain was a spherule with a diameter of 5 μm. It was found by nanoscale secondary ion mass spectrometry (NanoSIMS) mapping of a carbonaceous acid residue from the Qingzhen enstatite chondrite (EH3), deposited on a gold foil mount. The spherule showed isotope anomalies in C and Si, with 12C/13C = 99 ± 2 and δ29Si/28Si = 172 ± 36‰, and likely originated in an asymptotic branch giant (AGB) star. After NanoSIMS analysis, this spherule was transported and fixed on a pre-cut top surface of an epoxy stub, using FIB. The fixed spherule was then embedded entirely in resin, and cut into ~ 70 nm slices with a microtome. TEM observation of these ultrathin sections revealed a turbostratic structure free of any subgrains. The morphology, Raman spectrum, C and Si isotopic compositions, internal texture and crystallography of this graphite spherule suggest that it condensed from a carbon-rich outflow of an AGB star.