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


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.