Storage for SEM Stubs
Most specimens coated for scanning electron microscopy (SEM) studies may be stored without removing the coating. An exception, pyritized specimens, is discussed under Comments.
Department of Geology
University of Iowa
Iowa City, IA 52242 USA
Tel (319) 335-1822
Fax (319) 335-1821
Photograph: Stephen Shank
The method for storing coated macroscopic, sturdy specimens is the same as for uncoated specimens. Remove the specimen from the SEM specimen mount (SEM stub) using the correct solvent for the mounting adhesive. Then place the specimen in the collection in the same storage container (specimen tray) as any other specimen.
Note the coating composition as well as the date coated, and retain a copy of the SEM photograph as part of the permanent specimen record. Gold and gold-palladium alloy coatings will not peel or flake off. With time, the coating may deteriorate slightly but the specimen may be recoated for SEM study.
Fragile microscopic specimens may require special handling (Fig. 1). Specimens attached directly to the SEM stub or mounted on flexible filters before attachment to the stubs are almost impossible to remove without damaging the specimens. They should be permanently stored on the SEM stubs in SEM mount holders and placed in rigid polystyrene boxes designed to accept the holders.
Figure 1. SEM stubs in mount storage holders in polystyrene box.
If the specimens are extremely sensitive, the boxes should be placed in vacuum desiccators. The preferred method for mounting delicate microscopic specimens is to attach each specimen individually to a glass cover slip, then mount the cover slip to the SEM stub. After SEM examination, the cover slip may be removed and inverted allowing the specimen to be permanently mounted on a glass microscope slide. In many cases, the coating layer will not interfere with subsequent light microscopy.
Materials Tools Supplies
- Polystyrene box, clear with hinged lid
- Universal SEM mount storage holder
Pyritized specimens should not be stored with the coating on. The coating (gold, gold alloy, carbon, etc.) may cause an electro-chemical reaction (galvanic corrosion) that may seriously damage the specimen. The coating should be removed from pyritized specimens before storage.
It may become necessary to remove the gold (or gold alloy) coating from specimens. Aqua-regia may be used to remove the coating from acid-resistant microfossils such as palynomorphs and radiolarians. However, acid-sensitive specimens (e.g., shell, bone or teeth) will be destroyed by aqua-regia. A sodium cyanide solution (10% NaCN in water) may be used to remove the gold coating from most organic and inorganic specimens (Sela and Boyde, 1977). A major drawback is the toxicity of the cyanide solution. Precautions must be taken to perform the procedure in a fume hood, store the solution (labeled “POISON”) in a fume hood, dispose of the used solution in accordance with federal and local regulations for the disposal of hazardous waste materials.