A-Z: Terms and Techniques
This glossary provides definitions for electron microscopy techniques, terms and acronyms - as well as links to further information on our site. The terms are listed alphabetically, and are quick-linked to their definitions so you can easily find the information you are looking for.
A metric unit of length measurement equal to 1x10-10metre (0.0000000001m).
Argon (Ar) is widely used as the process gas during sputter coating. The use of argon is essential for clean, contamination-free coating of scanning electron microscopy (SEM) specimens.
The thermal evaporation of carbon (C) is widely used for preparing specimens for electron microscopy (EM). A carbon source - either in the form of woven fibre or graphite rod - is mounted in a vacuum system between two high-current electrical terminals. When the carbon source is heated to its evaporation temperature, a fine stream of carbon is deposited onto specimens.
The main applications of carbon coating in EM are making scanning electron microscopy (SEM) specimens conductive for subsequent examination by X-ray microanalysis, and being used as specimen support films on transmission electron microscopy (TEM) grids.
Also see: Carbon fibre and Carbon rods
Further information: Sputter Coaters and SEM/TEM Carbon Coaters
Carbon fibre, normally in the form of a woven cord, can be used to thermally evaporate thin layers of carbon onto a substrate. The main application in electron microscopy (EM) is the production of thin, electrically-conducting coatings on scanning electron microscopy (SEM) specimens. Carbon fibre can be used for transmission electron microscopy (TEM) applications, but carbon rod is normally preferred due to superior control of the evaporation process.
Also see: Carbon coating and Carbon rods
Further information: Carbon Attachments and Consumables
Carbon (C) in the form of shaped graphite rods can be used to thermally evaporate thin layers of carbon onto a substrate. Common applications in electron microscopy (EM) include the production of carbon-coated transmission electron microscopy (TEM) grid support films and the making of TEM surface replicas. Carbon rod evaporation is also used to produce electrically-conducting surface coatings on scanning electron microscopy (SEM) specimens.
Also see: Carbon coating and Carbon fibre
Further information: Carbon Attachments and Consumables
Critical point dryer (CPD)
This is an instrument used for critical point drying. For scanning electron microscopy (SEM) specimen preparation, small self-contained bench-top systems are normally used. A critical point dryer is essentially a temperature-controlled pressure vessel. The process chamber has facilities for introducing liquid carbon dioxide, and draining and venting liquids and gases. Specimens are held in a holder (‘boat’) during transfer and processing.
Quorum Technologies introduced the first commercial critical point dryer (the E3000) for SEM specimen preparation in 1971, and now produce a range of these instruments.
Also see: Critical point drying
Further information: Critical Point Dryers
Critical point drying
This is a process used to remove liquid from scanning electron microscopy (SEM) specimens in a precise and controlled way. Air drying would result in unacceptably high levels of structural damage, caused by the drying boundary as it passes through specimens. Critical point drying avoids these surface tension effects because the specimens pass from liquid phase to gas phase without crossing the liquid-gas boundary.
Critical point drying is primarily used to dry biological specimens prior to SEM examination, but is also increasingly used to dry micro-electromechanical systems (MEMs), which tend to be broken apart or distorted by strong surface tension forces in the drying front.
Also see: Critical point dryer (CPD)
Further information: Critical point drying technical brief (PDF) on Critical Point Drying Techniques and Advantages
Chromium sputter coating
Sputter coating with chromium (Cr) is widely used for depositing fine grain, high-resolution films onto field emission scanning electron microscopy (FE-SEM) specimens. Chromium oxidises on contact with air, which can present specimen storage problems. For this reason, iridium (Ir) sputter coating is increasingly preferred by many workers.
Also see: Iridium sputter coating
Further information: Q150T Turbo-Pumped Sputter Coater/Carbon Coater
An FIB/SEM is a combined focused ion beam (FIB) microscope and scanning electron microscope (SEM). This instrument, widely known as a DualBeamtm microscope, uses the milling action of a gallium ion beam to reveal internal surfaces and an electron beam to record successive image slices and give 3D morphology. Either secondary electrons (for maximum surface detail) or back-scattered electrons (which have atomic number contrast) can be used to form images.
The addition of a suitable cryo preparation system, such as the PP3010T, allows the observation of biological material, beam-sensitive specimens and many other ‘wet’ materials, such as foods, polymers, oils, foams and greases.
Further information: Cryo-SEM Preparation Systems and Cryo-SEM Preparation Techniques and Advantages
Cryo fixation is the rapid freezing of a water-based specimen to prevent or reduce growth of ice crystals within that specimen. This preserves the specimen in a snapshot of its solution state with the minimal of artefacts. An entire field called cryo-electron microscopy has branched from this technique. With the development of cryo-electron microscopy, it is now possible to observe virtually any biological specimen close to its native state.
See: Cryo fixation
Cryo-scanning electron microscopy (cryo-SEM)
Cryo-SEM is a method for preparing liquid, semi-liquid or beam-sensitive specimens for examination in a scanning electron microscope (SEM). The technique has huge benefits compared to alternative methods, such as critical point drying and freeze drying, because all of the specimen’s water is retained.
Further information: Cryo-SEM Preparation Systems