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Electron microscope is an instrument that replaces beams and optical lenses with electron beams and electron lenses according to the principle of electron optics, so that the fine structure of substances can be imaged at very high magnification. In recent years, the research and manufacture of electron microscopy has made great development: on the one hand, the resolution of electron microscopy has been continuously improved, the point resolution of transmission electron microscopy has reached 0.2-0.3nm, the lattice resolution has reached about 0.1nm, through electron microscopy, people have been able to directly observe atomic images; on the other hand, in addition to transmission electron microscopy, a variety of electron microscopy has been developed, such as scanning electron microscopy, analytical electron microscopy, etc. Although the resolving ability of electron microscopes is far better than that of optical microscopes, electron microscopes are difficult to observe living organisms because they need to work under vacuum conditions, and the irradiation of electron beams can also cause irradiation damage to biological samples. Let’s talk to you

electron microscope

The relevant content includes: the principle, structure, disadvantages, application fields, and the difference between light microscopy and the application in agriculture.

Principles of electron microscopy

At present, electron microscopy technology has become an important means to study the fine structure of the body. Commonly used are transmission electron microscopy and scanning electron microscopy. The following describes how each of the two electron microscopes works:

1. Transmission electron microscopy

Transmission electron microscopy, or transmission electron microscope, commonly known as electron microscopy or electron microscopy (EM), is the most widely used type of electron microscope.

1. Working principle: under vacuum conditions, after the electron beam is accelerated by high pressure, scattered electrons and transmitted electrons are formed when penetrating the sample, and they are imaged on the fluorescent screen under the action of electromagnetic lenses. When the electron beam is projected to the sample, the corresponding electron emission can occur with the density of the tissue composition, such as when the electron beam is projected to a massive structure, the electrons are scattered more, so the electrons projected on the fluorescent screen are few and dark, and the electron picture is black.

2. Main advantages: high resolution, can be used to observe the ultrastructure inside tissues and cells and the whole picture of microorganisms and biological macromolecules.

2. Scanning electron microscopy

Scanning electron microscopy is a scanning electron microscope, which is mainly used to observe the surface morphology of the sample, the structure of the cutting surface, and the structure of the inner surface of the lumen.

1. Working principle: scanning electron microscopy is the use of secondary electron signal imaging to observe the surface morphology of the sample. A very fine electron beam is used to scan on the surface of the sample, and the secondary electrons are excited to release on the surface of the sample, and the generated secondary electrons are collected with a special detector to form an electrical signal and transported to the picture tube, where the object is displayed on the fluorescent screen. Stereoscopic conformation of the surface (cells, tissues) can be photographed.

2. Main advantages: long depth of field, strong three-dimensional sense of image obtained, can be used to observe various morphological characteristics of biological samples.

Electron microscope structure

The electron microscope consists of three parts: electron optical system, vacuum system and power supply system, and the following three parts are introduced respectively:

1. Electron-optical system

1. The electron optical system mainly includes electron guns, electron lenses, sample holders, fluorescent screens and camera structures, which are usually assembled into a cylinder from top to bottom.

2. The electron gun is a component composed of tungsten wire hot cathode, gate and cathode. It can emit and form an electron beam with a uniform speed, so the stability of the accelerating voltage is required to be no less than one ten-thousandth.

3. The electron lens is the most important component in the electron microscope tube, which uses a spatial electric field or magnetic field symmetrical to the axis of the lens barrel to bend the electron trajectory to the axis to form focusing, and its role is similar to the role of the glass convex lens to focus the beam, so it is called electron lens. Modern electron microscopes mostly use electromagnetic lenses, in which a very stable DC excitation current is passed through a strong magnetic field generated by a coil with a pole shoe to focus the electrons.

Second, the vacuum system

1. In order to ensure that the truth only interacts with the sample in the entire channel, and does not collide with air molecules, therefore, the entire electronic channel from the electron gun to the photographic bottom plate box must be placed in the vacuum system, and the general vacuum degree is 10-4~10-7 mm Hg.

3. Power supply system

Transmission electron microscopy requires two parts of power supply: one is to supply the high-voltage part of the electron gun, and the other is to supply the low-voltage and steady current part of the electromagnetic lens. The stability of the power supply is an extremely important indicator of the performance of the electron microscope. Therefore, the main requirement for the power supply system is to generate a high and stable accelerating voltage and the excitation current of each lens. In addition to the above-mentioned power supply parts, modern instruments also have automatic operation program control systems and computer systems for data processing.

Disadvantages of electron microscopy

1. In the electron microscope, the sample must be observed in a vacuum, so it is impossible to observe the live sample, with the advancement of technology, environmental scanning electron microscopy will gradually realize the direct observation of the live sample;

2. When processing the sample, the structure of the sample may not be produced, which aggravates the difficulty of analyzing the image later;

3. Due to the strong scattering ability of electrons, secondary diffraction is prone to occur;

4. Because it is a two-dimensional plane projection image of a three-dimensional object, sometimes the image is not unique;

5. Since transmission electron microscopy can only observe very thin samples, it is possible that the structure of the surface of the substance is different from the structure inside the material;

6. Ultra-thin samples (below 100 nm), the sample preparation process is complex and difficult, and the sample preparation is damaged;

7. The electron beam may destroy the sample by collision and heating;

8. In addition, the purchase and maintenance price of electron microscopes is relatively high.

Application areas of electron microscopy

1. Industrial aspects:

1. Industrial inspection, such as circuit boards, precision machinery, etc.;

2. Printing inspection, SMT welding inspection;

3. Textile inspection;

4. IC surface inspection;

2. Beauty:

1. Skin examination;

2. Hair root inspection;

3. Infrared physiotherapy (specific products);

3. Biological applications:

1. Microbial observation;

2. Observation of animal sections;

3. Observation of plant diseases and insects;

IV. Others:

1. Expander to assist people with intellectual disabilities to read;

2. Gemstone identification;

3. Identification of antiques, calligraphy and painting, jade cultural relics, etc.;

4. Some other video image analysis fields.

The difference between electron microscopy and light microscopy

1. Different lighting sources:

The illumination source used in the electron microscope is the electron flow emitted by the electron gun, and the illumination source of the light microscope is visible light (daylight or light), because the wavelength of the electron flow is much higher than the wavelength of the light wave, so the magnification and resolution of the electron microscope is significantly higher than the light mirror.

2. Different lenses:

The objective lens that magnifies in an electron microscope is an electromagnetic lens (a toroidal electromagnetic coil that generates a magnetic field in the center), while the objective lens of a light mirror is an optical lens made of glass milling. There are three groups of electromagnetic lenses in electron microscopy, which are equivalent in function to condensers, objective lenses, and eyepieces in light mirrors.

3. The imaging principle is different:

In electron microscopy, the electron beam acting on the sample to be tested is magnified by an electromagnetic lens and hit the fluorescent screen to form an image or act on the photosensitive film for imaging. The mechanism of the difference in electron concentration is that when the electron beam acts on the sample to be tested, the incident electron collides with the atom of the substance to produce scattering, and the sample electron image is presented in a thick light because different parts of the sample have different scattering degrees of electrons. The image of the sample in the light mirror is presented with a difference in brightness, which is caused by the difference in how much light is absorbed by different structures of the sample to be tested.

4. Resolution:

Due to the interference and diffraction of light, the resolution of light microscope can only be limited to 02-05um. Because electron microscope uses electron beam as light source, its resolution can reach between 1-3nm, so the tissue observation of light microscope belongs to micron-level analysis, and the organizational observation of electron microscope belongs to nanoscale analysis.

5. Depth of field:

Generally, the depth of field of optical microscopes is between 2-3um, so it has extremely high requirements for the surface smoothness of the sample, so the sample preparation process is relatively complicated. SEM can be as deep as a few millimeters, so there is almost no requirement for the smoothness of the sample surface, sample preparation is relatively simple, and some samples require little to no sample preparation. Although the asana microscope also has a relatively large depth of field, its resolution is very low. Magnification: The effective magnification of the optical microscope is 1000X. Electron microscope effective magnification.

6. The specimen preparation method used is different:

E-sports The preparation procedure of tissue cell specimens used in observation is more complicated, technical difficulty and cost are high, special reagents and operations are required in the links of material collection, fixation, dehydration and embedding, and finally the embedded tissue blocks need to be cut into ultrathin specimen pieces 50~100nm thick by ultra-thin microtome. Specimens observed by light microscopy are generally placed on glass slides, such as common tissue section specimens, cell smear specimens, tissue compression specimens, and cell drop specimens. Up to 1000,000X

7. Application fields:

Light microscope is mainly used for micron-level tissue observation and measurement of smooth surfaces, because visible light is used as a light source, so not only the surface tissue of the sample can be observed, but also a certain range of tissue below the surface can also be observed, and light microscope is very sensitive and accurate for color recognition. Electron microscopy is mainly used for nanoscale sample surface topography observation, because scanning electron microscopy relies on the strength of physical signals to distinguish tissue information, so the images of scanning electron microscopy are black and white, and scanning electron microscopy is powerless for the recognition of color images. Scanning electron microscopy can not only observe the topography of the sample surface, but also further expand the use of electronic microscopy by using different accessory equipment such as EDS, WDS, EBSD, etc. By using EDS and WDS auxiliary equipment, scanning electron microscopy can analyze the chemical composition of micro-regions, which is important in the field of failure analysis research. Using EBSD, SEM can study the lattice orientation of materials.

Application of electron microscopy in agriculture

First, the application and progress of electron microscope in agriculture

1. Application of scanning electron microscope in agriculture

Scanning electron microscope (SEM) has the advantages of large depth of field, strong image stereoscopic sense, high resolution, large image range and relatively simple sample preparation process, which has attracted great attention and favor from agricultural researchers. Scanning electron microscopy in agricultural research mainly studies the surface morphology and internal structure of different tissues and micro-organs such as animals, plants, microorganisms, insects, etc., so as to deepen the understanding of their application in physiological functions and explore the life rules of organisms. For example, in insects, it is convenient to improve the ability to distinguish their tiny organs, improve the classification level, and at the same time further clarify the role of organs, characterize and compare the external morphology of insects, and study their shape changes and their laws and structural characteristics in order to have a deeper understanding. In plants, it is of taxonomic significance to study the characteristics of crops such as pollen, peel, seed coat surface pattern and seed internal structure. In terms of microorganisms, it plays a greater role in studying the classification of fungi, actinomycetes, bacteria, identifying family genera, and judging the source of disease, especially for the activity of pathogens, spore germination, invasion of hosts, etc., and can obtain satisfactory results with scanning electron microscopy.

2. Application of transmission electron microscopy in agriculture

Transmission electron microscopy (TEM) is mainly composed of electron optical system, vacuum system, power supply system and auxiliary system. The principle of transmission electron microscopy imaging is to use electron rays without information, which interact with the sample when passing through the sample, and when the electron rays reappear on the other side of the sample, they already have the information of the sample, and then magnify the processing so that people can see the microscopic information inside it and interpret it. When the electron beam interacts with the sample substance, it can produce a lot of information with the sample, such as transmitted electrons, scattered electrons, secondary electrons, etc. The contrast of transmission electron microscopy is determined by the scattering absorption, diffraction difference, and aberration that occur when the incident electron passes through the sample. In the observation of agricultural biological samples, with the continuous improvement of the resolution of electron microscopy, the clarity of electron microscope images will not completely depend on the resolution of electron microscopy, but largely depend on the sample production technology. The commonly used preparation techniques for agricultural biological samples under transmission electron microscopy include: ultra-thin sectioning technology, immuno-electron microscopy technology, negative staining technology, biological macromolecular electron microscopy technology, etc. As an important pathogen that erodes angiosperms, gymnosperms and ferns, plant viruses cause diseases of crops, fruit trees, flowers, pastures and medicinal plants worldwide, resulting in a decline in yield and quality, and seriously affecting human production and life. The application of electron microscopy technology in determining the morphological structure, gene organization structure and function, virus replication process, in-depth understanding of the relationship between virus and host, and observing cell supermicrostructure lesions has an irreplaceable role in other methods, laying a foundation for gradually revealing the nature of virus and finally solving the problems of viruses and diseases.

The above is the principle, structure, disadvantages, application fields, differences between optical microscopes, and applications in agriculture that I introduce to you. With the continuous in-depth development of modern science and technology, the application technology of electron microscope is becoming more and more extensive, as the “scientific eye” of observing the microscopic world – electron microscope has high resolution, intuitive characteristics can not be replaced by any other scientific instruments, electron microscope for medicine, biology, physics, chemistry, metallurgy and materials science discipline development has played an excellent role, electron microscope in many disciplines of research work has become an indispensable conventional instrument.