Examination of Chemical Evidence
Introduction
What is chemical evidence? Chemical evidence refers to “any chemicals found on people, various
objects, or in solutions, generated as part of a crime and recovered at the scene or a related
location” (Lee, 2014). Chemical evidence can be used to determine why an individual has acted in a
certain way, who fired a weapon and determine what a substance is and its concentration. The three
main areas of chemical evidence look at in this report are gunshot residue, drugs and poisons and
the analytical techniques used to examine them.
1.Gunshot Residue Analysis
Gunshot residue is formed as soon as a weapon is fired, and the vapours from the primer in the gun
solidify into particles and attach to people or objects. Primer is an explosive chemical compound that
ignites the gunpowder when hit with a firing pin. The chemicals in the primer consist of lead
styphnate, antimony sulphide and barium nitrate. Therefore, when testing for gunshot residue the
presence of metals is found due to the lead and nitrates are found due to the barium. When fired,
the gun releases the gunshot residue back towards the shooter (usually at the hands, sleeves or
chest area) due to close proximity to the weapon; residue can also settle in surrounding
areas/surfaces. The quality and quantity of the GSR are affected by factors such as the type of gun
used, type of bullets used, environmental conditions and caliber (the internal diameter of the gun
barrel).
Like most forms of evidence, gunshot residue is first put through a presumptive
test before sent for further analysis. The presumptive tests for gunshot residue
include a test for metals, using sodium rhodizonate, and a test for nitrates, using
diphenylamine. When carrying out this technique it is beneficial to conduct a
test of a controlled sample of a metal nitrate, as seen in figure 1.1. To do so, an
alcohol swab is placed into a petri dish that then has a few drops of positive
control lead nitrate. Afterwards, one drop of sodium rhodizonate is placed on
one side of the swab and should show a colour change of deep red indicating a
presence of metals. While the other side has one drop of diphenylamine that
should show a colour change of yellow/green/blue indicating a presence of Figure 1.1 – Control sample
test
nitrates. Once the control sample has been tested and the investigator is
satisfied that the test is accurate, a sample can be taken either from the suspects skin directly, or
from an item of clothing they were wearing when the gun was fired. The
alcohol swab needs to be rubbed against the area suspected of containing
the gunshot residue and then placed in a petri dish. Like with the control
sample, sodium rhodizonate is added to one side to detect for metals
present and
diphenylamine is added
to the opposite side to
detect nitrates present,
Figure 1.2 – Suspect
sample tested for GSR
the tested suspect
sample can be seen in
figure 1.2. The comparison between the control
sample and the suspect sample can be seen in
figure 1.3. Both reactions need to be positive to
Figure 2.3 – Control sample (left) and suspect sample (right)
