Week 2A - 4 Analysis of Microscopic Objects

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From the course by Nanyang Technological University, Singapore

Introduction to Forensic Science

361 ratings

Nanyang Technological University, Singapore

Introduction to Forensic Science

361 ratings

We have all seen forensic scientists in TV shows, but how do they really work? What is the science behind their work? The course aims to explain the scientific principles and techniques behind the work of forensic scientists and will be illustrated with numerous case studies from Singapore and around the world. Some questions which we will attempt to address include: How did forensics come about? What is the role of forensics in police work? Can these methods be used in non-criminal areas? Blood. What is it? How can traces of blood be found and used in evidence? Is DNA chemistry really so powerful? What happens (biologically and chemically) if someone tries to poison me? What happens if I try to poison myself? How can we tell how long someone has been dead? What if they have been dead for a really long time? Can a little piece of a carpet fluff, or a single hair, convict someone? Was Emperor Napoleon murdered by the perfidious British, or killed by his wallpaper? *For Nanyang Technological University (NTU) students, please be noted that this course will no longer be eligible for credit transfer.

From the lesson

Chemical Analysis in Forensic Science

Opinion Poll 20:00

Week 2A - 1 Introduction to Atomic Structure16:35

Week 2A - 2 Structure of the Atom15:26

Week 2A - 3 Elemental Analysis3:14

Week 2A - 4 Analysis of Microscopic Objects3:36

Week 2A - 5 Napoleon Case9:16

Week 2A - 6 JFK Assassination Case4:33

Week 2A - 7 "Adam" Case; Summary7:36

Week 2B - 1 Introduction to Chromatography15:25

Week 2B - 2 GC & HPLC11:18

Week 2B - 3 Infrared Spectroscopy7:52

Week 2B - 4 Mass Spectrometry; Summary9:41

Meet the Instructors

Roderick Bates
Associate Professor
Division of Chemistry & Biological Chemistry | School of Physical & Mathematical Sciences

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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.

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