[MUSIC] [MUSIC] Now sometimes, forensic scientists need to analyze objects that are extremely small. For instance, gun shot residue. This is particulate matter given out when firearms are fired, and these are particles which are maybe five micrometers across. Far below what can be seen normally. So in order to visualize these objects, you have to use a Scanning Electron Microscope. Now, a Scanning Electron Microscope uses electrons instead of light, which is what normal microscopes use. So in a Scanning Electron Microscope, there is an electron gun which fires a beam of electrons at the object of interest. These electrons hit the object and then they bounce off, where they are detected, and it's the signals that are from the detector that are used to generate the image like the image shown here. Now, when these electrons hit the sample, they cannot just bounce off. They can also hit the atom and cause an electron to be ejected from that atom. When an electron is ejected from low down in the energy levels of the atom, it means that an electron higher up in the atom is going to drop down to fill that vacancy. And of course, as we know, when the electron drops down to this lower energy level, it must give out energy in the form of electromagnetic radiation. Now typically, the kinds of drops that the electrons will do with this technique, these are big drops. They're dropping from a high energy level down to a much lower energy level. That means the radiation given out is high energy and it typically corresponds to x-rays, and of course these x-rays are characteristic of the particular element. So this technique is called Energy Dispersive X-ray fluorescence or EDX. And because it is combined with the Scanning Electron Microscope, we have what we sometimes call a hyphenated technique. And this technique, SEM-EDX, is very powerful for forensic science because the SEM part, the microscopy, allows forensic scientists to look at extremely small objects, and the EDX part allows them to analyze the objects at the same time. So a SEM-EDX spectrum looks something like this, where on the x-axis we're looking at the energy of the emitted x-rays and you can see we get peaks for the different elements. So this particular sample here has relatively large amounts of nickel and chromium, and smaller amounts of a range of other elements. [BLANK_AUDIO]
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