留学生nursing代写:折射显微镜

2020-01-22 12:20

与SEs不同，BSEs不是来自样本。BSEs是折射入射电子。一些入射电子能够穿透样品表面以下的地方，在那里它们与样品中的原子发生碰撞。样品中的原子比入射电子大得多，因此，入射电子被“推”或“弹”出样品。一个元素的质量越大，也就是说，原子序数越大，入射电子在碰撞时散射的就越多。从视觉上看，质量大的原子比质量小的原子更亮。因此，当与已知样品比较时，可以初步确定未知样品的组成。当入射电子从样品中的原子中移除一个内壳层电子时，就形成了特征x射线。根据Aufbau原理，最外层的电子取代了被取代的内层电子。在这过程中，最外层的电子必须释放能量。它以x射线的形式存在。x射线的能量和波长与样品中的元素直接相关，可以用来对样品进行成分分析。x射线由能量色散光谱仪(EDS或EDX)或波长色散光谱仪(WDS)检测。样品制备取决于样品材料、所需信息和使用的SEM类型。与非生物样品相比，大多数生物样品通常具有更高的水浓度。“湿”样品必须在观看前烘干，因为SEM真空会在抽真空过程中使样品脱水不当而损坏样品。此外，这种对水的吸吮会污染仪器。干燥样品时应注意保持样品的自然完整性。样品还必须是导电的，无论是固有的或通过导电涂层的方式。
留学生nursing代写:折射显微镜
 Unlike SEs, BSEs do not originate from the sample. BSEs are refracted incident electrons. Some incident electrons are able to pierce just below the sample’s surface where they collide with atoms within the sample.  Atoms within the sample are much larger than the incident electrons, and consequently, the incident electrons are “pushed” or “bounced” out of the sample. The more massive an element is, that is to say, the larger the atomic number, the more the incident electrons are scattered upon impact. Visually, more massive atoms appear brighter than less massive atoms. Thus, when compared to a known sample, an unknown sample’s composition can be tentatively determined.Characteristic X-rays are formed when incident electrons remove an inner-shell electron from an atom in the sample. True to the Aufbau principle, electron(s) from the outer-most shell replace the displaced inner-shell electron(s). In doing so, the outer-most electron must release energy. It does so in the form of X-rays. The energies, and consequently wavelengths, of the X-rays are directly related to the element(s) present, and can be used to perform compositional analysis on the sample. X-rays are detected by an energy-dispersive spectrometer (EDS or EDX) or by a wavelength-dispersive spectrometer (WDS).Sample preparation varies depending on the sample material, desired information, and type of SEM used. Most biological samples typically have a higher concentration of water compared to non-biological samples. “Wet” samples must be dried prior to viewing, as the SEM vacuum can damage the sample during evacuation by improperly dehydrating the sample. Moreover, this leeching of water can contaminate the instrument. Drying of the sample should be done in such as way as to preserve the sample’s natural integrity. Samples must also be conductive, either inherently or by means of conductive coating.

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