Carbon Evaporation

Samples for electron microscopy are placed on a metal support grid which needs to be first covered with a low absorption material. This is usually a thin film of carbon which has been evaporated onto the metal grid from a graphite rod. The specimen is then deposited on top of this carbon, stained, and blotted dry.

The principle of carbon evaporation is very straightforward. A sheet of freshly cleaved mica to be carbon coated is placed within an evacuated chamber that has a small graphite rod connected to a high voltage circuit. A well-cleaved mica surface should have an atomically flat surface, perfect for high resolution work. When the chamber with the mica sheet (or sheets) has been evacuated to a sufficiently high vacuum, current is applied across the graphite rod until it incandesces. At this point, carbon is evaporated off the rod and deposited on the surface. After sufficient carbon is deposited, the current is turned off and the coated mica retrieved.

Grids to be carbon coated are submerged onto filter paper in a small water tank. The coated mica is then slowly and carefully lowered into the water, in such a way that the carbon film slides off and floats at the surface. When the water is removed from the tank, the carbon follows the receding water level to cover the grids. The grids are now ready for use after drying.


Preparation Techniques: Negative Staining