Ultramicrotomy Sample Preparation of Bennu Particles at NASA Johnson Space Center

OSIRIS-REx and Hayabusa2 are sample return missions that collected carbon-rich material from asteroid Bennu and Ryugu, both containing OH-bearing surface materials in the form of clays [1]. Roughly 70 percent of the OSIRIS-REx sample collection [2] and 10 percent of the JAXA Hayabusa2 sample collection that was shared with NASA [3] are stored separately in a new curation ISO 5 cleanroom laboratory at NASA Johnson Space Center (JSC). Located between the two lab suites is an ISO 7 carbonaceous asteroid ultramicrotomy sample preparation laboratory that has the capability to fulfil requests for ultramicrotomed sections of Bennu and Ryugu particles from the scientific community.

Ultramicrotomy is a technique used to produce 70-100 nm thick cross sections of particulate materials. The resulting thin sections are available for a wide array of analytical techniques including non-destructive spectroscopies (e.g. Fourier-transform infrared spectroscopy), transmission electron microscopy (TEM), and NanoSIMS isotopic analyses, often in a coordinated fashion. For this study, we used ultramicrotomy to prepare particles from Bennu samples OREX-800068-0 and OREX-501018-0 for TEM analysis. This abstract will discuss the JSC curation-related technique development used to achieve high quality sections.

For these samples, the process began by mixing fresh embed 812 epoxy resin to cure particles from each sample onto epoxy bullets. A borosilicate glass needle was used to apply a small bead of epoxy onto the face of the bullet. The same epoxy-coated needle was then used to gently lift and transfer a roughly 10-15μm sized particle into the bead of epoxy. Ultramicrotomy has the advantage over focused ion beam (FIB) analysis in that up to 100 sections can be produced from a 10μm particle while far fewer are produced by FIB. To prevent cross-contamination, a fresh needle was used for each sample. The thin epoxy coating on the needle creates a sticky surface for the particle to adhere to, making it unlikely to become dislodged by static and allowing for a successful transfer to the bullet for embedding. During the embedding process for each sample, buoyancy would cause the particle to float to the surface and drift towards the edge of the bullet, suggesting a low particle density. A glass needle was used to gently manipulate the particle until it stabilized in the resin. The embedded particles were then transferred to an apparatus to ensure a level surface and then cured in a 70°C oven for roughly 88 hours. A cured sample was then transferred to a Leica EM UC6 ultramicrotome for trimming. A diamond chisel with an incline angle of 45° was first used to trim the surface of the potted butt into a trapezoidal shape then, used to remove excess epoxy from the face of the trapezoid until the particle was about 2μm from the surface. Next, a diamond knife with an incline angle of 45° and reservoir filled with deionized water was mounted onto the ultramicrotome for cutting electron-transparent slices of the particle. Sample thickness can be estimated by observing the interference color of the section: silver to yellow colors is indicative of a 70-100 nm section thickness. Sections within the silver to pale gold range are suitable for normal work and typically thinner sections are required when high resolution is needed [4]. Each trapezoidal slice adheres to the previous slice, thereby producing a ribbon of particle cross-sections. Particle plucking will often occur when microtoming samples that are greater than 10-15μm in size and containing hard minerals. This is where fragments from the main particle start to break apart and dislodge from the cured epoxy, creating holes in the trapezoidal slices. Once this occurs, little can be done to improve cutting conditions. This was observed in the particle from OREX-800068-0 after about 10-15 slices were produced and the knife had reached a larger surface area of the particle. After the first initial slices of the samples were taken, a single hair manipulation tool was used to detach the sections from the knife and separate the ribbon of particle cross-sections. A Perfect Loop tool was used to carefully transfer the sections from the reservoir onto a carbon coated TEM grid for analysis.

This collaboration has provided curation with valuable ultramicrotomy experience in sectioning carbonaceous asteroid sample while also benefiting the science team in furthering their analysis of Bennu samples. The experience and developed techniques from this collaboration will be carried over to forthcoming processor trainings and will be utilized in the carbonaceous asteroid ultramicrotomy sample preparation laboratory for future collaborations and requests. Sulfur embedding is another capability that curation will make available for future requests. This has the advantage of excluding contributions from carbon-based epoxies, making these sections especially useful for studies of indigenous organic matter in these primitive samples.

References: [1] Lauretta, D. S. et al. (2024). Meteoritics and Planetary Science, DOI: 10.1111/maps.14227.[2] Wilcox, Kevin. (2023, October 30). OSIRIS REx Sample Wows Scientists. [3] Lindsey, Lauren. (2024, November 3). Hayabusa2. [4] Reid, N. (1975). Ultramicrotomy (Practical Methods in Electron Microscopy Series,: Vol 3, Pt 2), Elsevier Press, 353 pp.