A preparation sequence for multi‐analysis of µm‐sized extraterrestrial and geological samples

Abstract

With the recent and ongoing sample return missions and/or the developments of nano-to microscale 3-D and 2-D analytical techniques, it is necessary to develop sample preparation and analysis protocols that allow combination of different nanometer-to micrometer-scale resolution techniques and both maximize scientific outcome and minimize sample loss and contamination. Here, we present novel sample preparation and analytical procedures to extract a maximum of submicrometer structural, mineralogical, chemical, molecular, and isotopic information from micrometric heterogeneous samples. The sample protocol goes from a nondestructive infrared (IR) tomography of~10 to~70 µm-sized single grains, which provides the distribution and qualitative abundances of both mineral and organic phases, followed by its cutting in several slices at selected sites of interest for 2-D mineralogical analysis (e.g., transmission electron microscopy), molecular organic and mineral analysis (e.g., Raman and/or IR microspectroscopy), and isotopic/chemical analysis (e.g., NanoSIMS). We also discuss here the importance of the focused ion beam microscopy in the protocol, the problems of sample loss and contamination, and at last the possibility of combining successive different analyses in various orders on the same micrometric sample. Special care was notably taken to establish a protocol allowing correlated NanoSIMS/TEM/IR analyses with NanoSIMS performed first. Finally, we emphasize the interest of 3-D and 2-D IR analyses in studying the organics-minerals relationship in combination with more classical isotopic and mineralogical grain characterizations.

Multiscale correlated analysis of the Aguas Zarcas CM chondrite

“…The FIB sections were later analyzed by energy‐dispersive X‐ray spectroscopy (EDS) in a scanning electron microscope (SEM) to retrieve the structure and elemental composition. These analyses at smaller scales are well adapted for studying the carbonaceous component and its mineral host, down to the micrometer scale (Aléon‐Toppani et al, 2021).…”

“…The FIB sections were later analyzed by energydispersive X-ray spectroscopy (EDS) in a scanning electron microscope (SEM) to retrieve the structure and elemental composition. These analyses at smaller scales are well adapted for studying the carbonaceous component and its mineral host, down to the micrometer scale (Al eon-Toppani et al, 2021). Finally, another micrometric fragment AZ_4 was analyzed using a more invasive and classical preparation: fragment AZ_4 was crushed inside a diamond compression cell (as in Merouane et al [2012] and in Dionnet et al [2018]), forming a flattened sample with a thickness of ~11 μm and a surface area below 200 9 200 μm.…”

“…This fragment was naturally detached from the bulk (neither selection nor preparation). It was welded onto the tip of a tungsten needle by platinum ion beam-induced deposition (as in Al eon-Toppani et al, 2021). Measurements in reflection at different angles were first acquired on the small fragment AZ_3 to obtain data on the fragment's surface composition.…”

“…Thanks to this 3D characterization, we were able to visualize the compositional heterogeneity inside the fragment and subsequently select specific areas to study at higher resolution. We then extracted three FIB sections (3 μm thickness) from the AZ_3 fragment by using FIB preparation, and performed FT-IR and Raman microspectroscopy on them (see Al eon-Toppani et al [2021] for the description of the full protocol on a 3D fragment to FIB sections).…”

“…The IR-CT 3D measurements have the advantage of being totally nondestructive and noninvasive; however, they are time-consuming (typically 1/2 days per sample) and because of complex scattering issues, the quality of the spectral data in 3D is not as good as equivalent measurements collected in 2D on a flat surface. To overcome this problem, we applied one more step in the sample preparation (see also Al eon-Toppani et al [2021] for further technical information). The AZ_3 fragment was serially sectioned using FIB technologies to generate a series of three FIB sections (see Fig.…”