Outgassing Elements in Electron Microscopes

Image Credit: Thermo Fisher Scientific Phenom-World BV

When electron microscopes function in high vacuum levels, all wet samples loaded into the imaging chamber will instantly begin to outgas.

Particular samples contain microstructures that will combat the phase change, offering positive results with no key concerns. A common example is a fresh leaf. A sample with no firm structure can be imaged if critical point drying or force drying is utilized to prepare it.

Force Drying

To establish if the sample will withstand the vacuum, the use of a further instrument, such as a sputter coater or a desiccator, is suggested. Eventual variations in the sample should be instantly apparent.

Critical Point Drying

Also called supercritical drying, this method causes the evaporation of liquids in the sample, which maintains a low temperature.

The pressure level controls the evaporation, which is brought underneath the vapor tension of the liquid in the sample. Throughout this process, the liquids will produce fractures within the sample, creating changes in the structure.

SEM images of a cucumber’s intact structure. Image Credit: Thermo Fisher Scientific Phenom-World BV

SEM image of a tomato’s peel and interior structure. Image Credit: Thermo Fisher Scientific Phenom-World BV

Cooling

Freezing the sample is an alternative to drying methods and will keep the sample structure fully intact.

If the phase change is efficient enough, the liquids in the sample will not produce crystals and the structure will be well-preserved. It is essential to know that the phase change is brief and long-term exposure to a high vacuum will enhance the rate of evaporation.

Low Vacuum

If the moisture content of the sample is not particularly high, utilizing a small amount of sample at a decreased vacuum level can be sufficient to take images. The total quality of the image will be lower, but the sample can be imaged in its initial state.

Small Amount of Sample

Utilizing a small quantity of sample is sometimes enough to include the influences of evaporation and vacuum. The sample can be gathered with a toothpick and a veil of it can be placed on the stub. This method is especially successful with emulsions and gels.

SEM images of (left) spreadable hazelnuts and (right) cacao cream. Image Credit: Thermo Fisher Scientific Phenom-World BV

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific Phenom-World BV.

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