Why cool samples? With increased use of Low Vacuum or Variable Pressure microscopes, many microscopists now appreciate the need to control water evaporation from wet samples. By cooling a wet specimen, water evaporation may be slowed or depending on chamber pressure stopped altogether. Saturated vapour pressure of water decreases considerably with temperature.

At room temperature water will very quickly evaporate causing considerable changes to the specimen structure. At 300Pa, specimen temperature needs to be less than -9.5°C and at 85Pa specimen temperature needs to be less than -25°C to stop water evaporation. Therefore by cooling a specimen to at least -25°C, chamber pressure may be 85Pa with no water evaporation from the specimen.

Advantages of this procedure are clear to see, by cooling a specimen in low vacuum, changes in specimen structure due to water evaporation can be minimised and the ability to operate at higher vacuum provides a better signal to noise ratio and clearer images.