Critical point drying: Application of the physics of the PVT surface to electron microscopy

In electron microscopy of biological materials, the samples must be specially prepared before they can be placed in the high vacuum of an electron microscope. If the samples are prepared in water and if the last stages of the preparation involve evaporation of water from a liquid–vapor interface, surface tension at the interface can generate very high pressures in micrometer‐size structures that will distort the structures to the point where much information about their original shapes and configurations is lost. In order to avoid this damaging distortion, the samples are immersed in a fluid such as liquid CO₂ in a pressure vessel and the fluid is heated to a temperature higher than its critical temperature where it becomes a gas and can then be removed without the damaging effects of surface tension. The physical details and the advantages and disadvantages of alternative ways of carrying out the process are reviewed. Optimum conditions for carrying out the process are identified, and it is noted that some of the essentials of the process are frequently misunderstood. This ’’practical application’’ of the physics of the critical point can be used to advantage to help students learn about the PVT behavior of real substances and it is of special interest to students in biological sciences.