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Frequently Asked Questions

 

What is an electron microscope (EM)?

The electron microscope (EM) uses a beam of electrons to form an image of a specimen. An EM is capable of much higher magnifications and has a greater resolving power than a light microscope, allowing it to visualise much smaller objects in finer detail.

There are two basic types of electron microscopes: the transmission electron microscope (TEM) produces high resolution two dimensional images, while a scanning electron microscope (SEM) scans surfaces of specimens to produce three-dimensional images.

EMs operate under vacuum conditions and are generally large pieces of equipment. They mostly stand alone in a small, specially-designed room and require trained personnel to operate them.

What is electron microscopy (EM) specimen preparation?

In most cases, materials to be viewed under a scanning electron microscope (SEM) or transmission electron microscope (TEM) require processing to produce a suitable specimen. The technique, or techniques, required varies depending on the specimen and the analysis required. This is where our range of different Products are used,

What is transmission electron microscopy (TEM)?

Transmission electron microscopy (TEM) is a technique whereby a beam of electrons is partially transmitted through an ultra thin specimen, interacting with the specimen as it passes through.

An image is formed from the interaction of the electrons transmitted through the specimen. This image is magnified and focused onto an imaging device, such as a fluorescent screen, a layer of photographic film or, increasingly, it is detected by a sensor such as a CCD camera.

TEMs are capable of imaging at a significantly higher resolution than light microscopes, owing to the small wavelength of electrons, in comparison to light. This enables the examination of fine specimen detail - even as small as a single column of atoms, which is tens of thousands times smaller than the smallest resolvable object in a light microscope. The highest resolution achieved on an aberration-corrected TEM is in the region of 0.5 Ångströms.

The TEM is a major analytical tool in a wide range of scientific fields, in both physical and biological sciences.

What is transmission electron microscopy (TEM) specimen preparation?

A vast range of processing methods and technologies are available for both materials and biological specimens. These processes and associated instrumentation include chemical fixation, embedding and ultra-thin sectioning of biological material, and the thinning of metal specimens prior to TEM examination. The resultant specimens are typically held during observation on TEM support grids.

Our own range of vacuum evaporators are designed to deposit thin carbon support films onto TEM grids, to improve mechanical strength and allow larger areas of specimens to be observed without being hidden behind the grid bars. Vacuum evaporators are also used to create metal and carbon surface replicas of materials that cannot be observed directly - see Product Information.

What is scanning electron microscopy (SEM)?

A scanning electron microscope (SEM) produces surface images by scanning a specimen with a beam of high energy electrons in a raster pattern. The electrons interact with the atoms that make up the specimen, producing signals that contain information about the specimen's surface, topography, composition and other properties.

Depending on the instrument, resolution will vary between less than 1nm and 20nm. In addition to its high resolving capability, the SEM also has a great depth of field, giving the characteristic three-dimensional appearance that is useful for understanding the surface structure of a specimen.

Many SEMs also have a facility to analyse the X-rays given off by the specimen as a result of its electron bombardment. As each element in the periodic table produces its own X-ray spectrum, this can be used to identify the elemental composition and measure the abundance of elements in the specimen.

What is scanning electron microscopy (SEM) specimen preparation?

Preparation instruments such as critical point dryers and freeze dryers are designed to controllably remove water from biological and similar liquid-based specimens. This partially overcomes the adverse effects of air drying (namely, severe collapse of cellular structure as the drying front passes through the specimen).

Prior to drying, most biological specimens require chemical stabilization in order to preserve them and allow them to better withstand subsequent drying and sputter coating processes.

Many modern SEMs can operate without exposing the specimen to high vacuum. Often referred to as environmental/low-vacuum/high-pressure/variable-pressure SEMs, these instruments use higher pressures to minimise out-gassing from volatile specimens. However, variable pressure techniques are generally limited to certain specimen types and have a number of disadvantages in terms of specimen stability and the information that can be obtained.

For water-based, liquid or semi-liquid and beam-sensitive materials, cryo-SEM overcomes many of the problems associated with drying protocols and variable pressure techniques. For more information see the PP3010T.

For most protocols, SEM specimens need to be electrically conductive. For this reason, non-conductive and semi-conductive SEM specimens require the deposition of a thin surface layer of metal. This also increases the amount of secondary electrons that can be detected from the surface of the specimen in the SEM, and therefore increases the signal to noise ratio.

Metal layers need to have a fine grain and be evenly distributed across the specimen surface. We offer a wide choice of sputter coaters, carbon coaters and vacuum evaporators designed to suit most specimen types - see Product Information.

What is cryo-SEM?

Cryo preparation techniques for scanning electron microscopy (cryo-SEM) are now considered essential for the successful observation of many wet or ‘beam sensitive’ specimens.

Cryo-SEM of biological material removes the need for conventional preparation methods, such as chemical fixation and critical point drying, and allows observation of the specimen in its ‘natural’ hydrated state.

A major advantage of cryo-SEM is the capability to cold-fracture specimens to reveal internal microstructure and gain important information from materials with different dispersion phases, eg oils, polymers, fats and food stuffs.

Cryo systems, such as the PP3010T, include facilities to rapidly freeze and transfer specimens. The cryo preparation chamber is directly mounted to the SEM or FIB/SEM and includes tools for cryo fracturing, controlled sublimation and specimen coating.

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