Can you answer these questions too ?? IV. Basic Principles What happens when a s

Question

Can you answer these questions too ?? IV. Basic Principles What happens when a source X-Ray beam hits a substance, such as a fused disk or an SEM sample? Plan your answer so that it explains all the different K and L and M spectral lines. How is this effect even useful in the analysis of a substance? V. Explain the limitations and strengths of two of these instrumental techniques XRD XRF SEM PLM VI. Propose modifications to either the Frantz Isodynamic Separator or the Specific Gravity apparatus To be complete -note: 1) why the modifications are needed 2) what the modification are specifically 3) how you could quantify the differences in either accuracy or precision before and after the modifications were added. VII. Light versus X-Rays We began the course dealing with properties of light and how they have been used in Polarized Light Microscopy to identify substances. Some of these effects are sort of odd. So- explain some of the effects of monochromatic "light" and how they are identified. Then - explain how X-Rays are different. The wavelengths and effects and uses, etc.

Explanation / Answer

In the microscope when a sample is placed and it is bombarded with high energy electrons, besides the generation of X-rays many more things also happen. Of the elements present in the sample the characteristic emission spectra are these X-rays. Energy-dispersive analysis by X-rays (EDAX) is an analytical technique that is used for the investigation of the interaction between some source of X-ray and a sample. The characterization is done by the fundamental principle that each and every element has a unique atomic structure which allows a unique set of peaks on its X-ray spectrum. For the identification of the emission of characteristic X-rays from a specimen, a high-energy beam of charged particles like electrons or protons, or a beam of X-rays, is introduced into the sample being studied. Within the sample, an atom contains ground electrons in discrete energy levels or electron shells which are bounded to the nucleus. The incident beam may excites an inner shell electron by ejecting it from the shell and creating an electron hole in the place where the electron was. Then the electron from an outer higher-energy shell fills the hole, and this difference in energy between the higher-energy shell and the lower energy shell is an X-ray that is released in this process. The energy of the X- 80 rays are characteristic of the difference in energy between the two shells. So by the scanning of either the wavelength or energy of the emitted X-rays the identification of the element present is possible.    

                  

                 

  

         Strengths and Limitations of XRF and SEM:

Strengths

X-Ray fluorescence technique is used for the investigations like:

Limitations

Theoritically, by the use of XRF the detection of X-ray emission from virtually all elements, can be done depending on the wavelength and intensity of incident x-rays. But,

Strengths

For the characterization of solid materials the SEM is used. But it is most concerned with geological applications. Having user-friendly "intuitive" interfaces, most SEM's are easy to operate in comparision to other techniques. Many applications in SEM require sample preparation minimal. Data acquisition is rapid for many applications. In modern time the generated data by SEMs is in digital formats, which are highly portable.

Limitations

For SEM samples must be solid and must fit into the microscope chamber. Maximum size of sample in horizontal dimensions is usually of the order of 10 cm, and vertical dimensions are such that they rarely exceed 40 mm. This technique is unsuitable for the examination of the samples which are likely to outgas at low pressures like rocks saturated with hydrocarbons,in conventional SEM's. EDS detectors on SEM's are not able to detect very light elements like H, He, and Li, and many instruments are unable to detect the elements with atomic numbers less than 11 (Na). To electrically insulating samples for study in conventional SEM's,the application of an electrically conductive coating is must, till the time the instrument is capable of operating in a low vacuum mode.

Specific gravity or density of a solid is a property that help us for identifying a material. By following physical changes in a sample, to indicate degree of uniformity among different sampling units or specimens, the average density of a large item can be indicated. Changes in density of a single material are due to the differences in crystallinity, loss of plasticizer, absorption of solvent,. It is possible that etc. Portions of a sample differ in density because of their differences in crystallinity, thermal history, porosity and composition. Density is useful for calculating strength-weight and cost-weight ratios.

X-rays have a wavelength that range from 0.01 to 10 nanometers, X-ray wavelengths are shorter than those of UV rays and longer than gamma rays. These are used in flouroscopy,radiotherapy, X ray astronomy, industrial radiography, X-ray photoelectron spectroscopy

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