Forensic science - The chemistry of almost everything (15/31)

Uploaded by OUlearn on 03.09.2009

In Washington FBI ballistic experts establish...
For over 40 years forensic chemistry has been used
to try to find out whether a suspect has fired a gun.
But the old, simple methods have given way
to a technique which goes into the invisible.
As a gun is fired,
it sends out a complex cocktail of chemical compounds and elements -
lead from the bullet but also more unusual elements
like barium and antimony from the propellant.
For decades forensic labs around the world
have applied simple chemical tests developed in America.
They test for the presence of these elements.
A simple colour change on the filter paper
or a change in the light spectrum from a flame.
These were the telltale signs
of the presence of each of the three key elements.
But all the tests were for each element in turn.
They all suffered from the same fundamental limitation
in that they could tell you that there were certain things present
in a sample taken from somebody's hands
but they couldn't prove that that material came as a result
of an association with the firing of a gun or whatever
because the substances which were being detected,
lead, barium, things like that, are very common in the environment.
As a gun is fired, the heat energy vaporises the lead,
barium and antimony.
There is only one place where all these three elements come together,
near a gun.
The surface of anything near gunfire - clothes, skin, hair -
becomes covered with tiny particles.
As the vapours cool, they condense and form small, metal pellets.
These are collected as part of normal forensic practice
and would be tested for the three elements.
But even if you found them all,
each might have come from a different source.
What you need is to show whether the three elements found in a sample
really do originate from the same place at the same time.
When it came, the answer was found
using a common tool of the forensic scientists.
The scanning electron microscope.
This device allows the forensic scientist
to see objects only a few microns across.
The sample is locked into a vacuum chamber and when inside
it's going to be inspected not with light but with a beam of electrons.
While using this for standard lab work,
Robin Keeley hadn't realised it would be the tool
which would offer up a solution
to the problem of dealing with the original source of gun residues.
The solution came by chance.
A colleague brought me some air filters
which he'd collected on the laboratory firing range
and asked me to examine them for the presence of lead
because they were worried about inhaling this lead
when they were doing the test shots.
It was only when I started to look at the filters
that I noticed these rather strange particles which I'd not seen before.
Scanning through the dust
he came across the mysterious, tiny, round particles.
They came from a firing range but were they from a gun?
The electron microscope beam makes the particles give off x-rays.
Attached to his microscope, Robin Keeley had an x-ray detector.
Using this he could see from the spectrum of rays
which elements were present in the sphere.
Lead, barium and antimony.
A clear indication that the particle had originally come from a hot gun.
As different as guns and granites are,
those last two stories were really about the same thing.
Peering into the invisible and coming up with an answer to a problem
that you really couldn't solve in any other way.
Think of the power that analytical techniques like those give you
and what you can do with them.
You could identify the atmospheres of distant planets,
check the balance of critical chemicals in the body
or even quantify the tiny traces of pollutants in rivers.
The list is almost endless.
Think on.