PREPARATION OF GEOLOGICAL AND GEOMICROBIOLOGICAL SPECIMENS FOR TEM EXAMINATION BY EMBEDDING IN LIQUID SULFUR

The technique of embedding TEM specimens in polymer resins for subsequent ultra-thin sectioning is well established For geological materials this protocol avoids specimen damage introduced by ion beam thinning. However, traditional carbon-based embedding resins can interfere with high-resolution carbon analyses. Since specimens not amenable to ion milling must be embedded in some medium and ultramicrotomed for TEM analysis, a carbon-free substitute for traditional embedding media is required for accurate carbon analyses. An alternative technique which was first reported by Bradley (Bradley, Keller et al. 1993) is embedding specimens in sulfur. In this technique the solidified sulfur block is ultramicrotomed in as with traditional resins. Unfortunately a detailed description of the protocol has not been published. We present our protocol in an attempt to disseminate this useful and difficult technique to the geology community.

We find, as do others (Joswiak, D.J. 2003), that the greatest difficulty in utilizing this technique is solidifying the sulfur block without cracking. To properly solidify the block, (a) the liquid droplet must be extremely small, (b) the embedded material must occupy a relatively small fraction of this volume, (c) the droplet must be melted to just above it’s 119°C melting point, and (d) solidification must proceed slowly. Another difficulty in preparing these samples is handling the solidified block. To minimize handling and contamination problems, we solidify the block directly onto a rod designed to fit into the ultramicrotome.

For microbial specimens, the technique yields good results for certain experiments. The technique is most useful, however, for carbon-bearing mineral specimens. We present TEM images of several different specimens prepared with these methods, and compare TEM carbon analyses with specimens prepared in traditional resins.

References:

Bradley, J. P., L. Keller, et al. (1993). Carbon analyses of IDPs sectioned in sulfur and supported on beryllium films. 24th lunar and planetary science conference. D. Blanchard and D. Black, Lunar and Planetary Science Conference. 24: 173-174.

Joswiak, D.J. (2003) Personal communication.