Electron microscopy

20131212084049An Introduction to Electron Microscopy

An electron microscope uses a beam of electrons to create an image. This is unlike traditional light microscopy which uses visible light and a system of lenses. Electron Microscopes are capable of much higher resolving power and magnification. However they are usually much larger, more complex, and require a great deal of technical expertise compared to light microscopes to use.

Around 1870, a German physicist known as Ernst Abbe’s created a formula which proposed the resolution of a lens was limited by the wave nature of light, and this limit was approximately half a micrometre. This meant that a light microscope could distinguish objects on a broad cellular level, but would struggle to differentiate intracellular structures and other smaller particles. For example, a red blood cell has a width approximately 6-8 micrometres. This presented a significant obstacle, and it was until the end of the first third of the 20th century that several generations of brilliant scientists (such as Ernst Ruska and Maximillion Knoll) theorized and demonstrated that a new form of microscopy was possible using electrons instead of light.

Electron microscopes use an electromagnetic lens to control the path of an electron beam. This magnetic lens is similar to the optical lens in light microscopy. Faster electrons will have a shorter wavelength, and the resolution of an electron microscope is directly proportional to the wavelength of the irradiation which will form the image. Therefore reducing the wavelength will increase the resolution, and today objects can be magnified many millions of times their original size. The two main types of electron microscopy are Transmission Electron Microscopy and Scanning Electron Microscopy. Transmission Electron Microscopy was the original form. It involves using a cathode to emit a high voltage electron beam towards a sample.

The electromagnetic lenses are composed of tubes with coils wrapped around them. When an electric current is passed through the coils, an electromagnetic field and this can be used to direct electrons to the sample in the same way a lens would in light microscopy. The sample in Transmission Electron Microscopy must be thinly sliced and the technique can only produce a black and white image.

Another type of electron microscopy is Scanning Electron Microscopy. In SEM electrons are displaced from the sample and sent to a detector, which can produce three-dimensional information about the texture and morphology.