The correct preparation is vital to getting good quality information from your sample. A poorly mounted or incorrectly processed sample can lead to viewing artefacts. This section is designed to help you choose what processes may be needed to prepare your sample. Text and video help guide you through processes.
Click the linked items below to read information or watch videos about each process.
Although the information we present is fundamental, it is recommended that you consult recent published research papers in your area of research to check on current techniques being used.
Always wear gloves when handling samples and sample holders to keep the sample free from contamination from the user's hands.
Read the appropriate pages of this module that cover the occupational health and safety issues surrounding the handling of samples for electronmicroscopy.
Note on non-conductive samples
For non-conductive samples that need coating to make them conductive, consider whether compositional information is required (e.g. BSE imaging, X-ray spectroscopy), and use a carbon coating if this is the case. Gold or platinum metal coatings will interfere with the compositional signal from the sample.
Powders and colloids
Powdered samples can be particularly challenging and it is important to know from the outset what type of imaging is required. Often a degree of trial-and-error is required. For topographical SE imaging, powders can be dusted onto or pressed onto conducting adhesive carbon tape. For qualitative BSE imaging powders can also be dusted onto or pressed onto a conductive adhesive tab; but topography can interfere with electron collection, providing inaccurate information so for the most reliable data from compositional BSE imaging (or for microanalysis), smooth surfaces are needed. To achieve this, the powder should be hot-mounted in resin or cold-mounted (embedded) in epoxy resin, polymerised and then polished to a smooth mirror surface. Non-conductive samples should be coated to reduce charge-build-up. For BSE imaging carbon coating should be chosen instead of metal coating because a metal coat would detrimentally affect the imaging of average atomic number.
Colloids and magnetic materials require special care. Colloids are very fine particles suspended in another medium. Colloids may need to be dispersed in an ultrasonic bath and then a few drops of suspension are allowed to dry on a glass slide. Magnetic materials can be investigated, but care is needed so that the powder is not attracted to the objective lens pole piece where it could disrupt the electron optics. Usually it is best to use a small quanity of powder and press it firmly onto conductive tape.
Alternative preparation methods less often used for wet particulate samples are to 1) freeze-dry material before mounting or 2) freeze samples in liquid (e.g. on a block cooled by liquid nitrogen) and mount the frozen block of material in a clamp-system, sublimate off any frozen water covering the top of the sample, coat and observe while frozen (using CryoSEM).
Dry materials that can be viewed as a single block, rather than as fragments or powder, are referred to as bulk samples. Examples include metal, ceramic, glass, rock, teeth or bone, various polymers and small dry organisms such as insects. Preparation involves attaching the sample to the viewing mount (SEM stub: pin-type or slug-type). This can be achieved most easily by using a double-sided adhesive tab, preferably one impregnated with carbon to enhance conductivity.
Some bulk samples do not have a flat surface and so do not contact much of the adhesive and break free during handling of the mount. If this is the case then a two-part liquid adhesive may be preferable instead of the adhesive tab. But samples can wick-up fluids, especially if porous so use a liquid adhesive that is dry, or almost dry to the touch of a gloved finger, when the sample is applied.
Conductivity must be considered. If the sample is not conductive then even if coated with metal (e.g. using sputter coating) or carbon (e.g. using evaporative coating) the sample sides or any overhang may be poorly coated or remain uncoated. For this reason, conductive paint is usually applied to a region on the side of non-conductive bulk samples. This paint can be carbon-based or silver-based. Both are liquid and may wick-up or flow over onto the sample surface. Apply with care, using a test piece first, and if wicking-up occurs then use very small amounts of the fluid, waiting for it to dry before applying more, or wait until the conductive paint is paste-like (but not dry) before applying.
Size should be kept to within the diameter of the mounting block (stub) and ideally bulk samples should be no more than 5 mm squared. Tall or wide samples may cause a danger to the viewing equipment. Such samples should be discussed with laboratory staff before viewing and must be inserted into the electron microscope using extra care to ensure that they do not come into contact with machine parts.