Examination of chemical forensic evidence
Luke David Farrar
Introduction
Chemical evidence can be extremely valuable when linking certain objects, people,
and motives together. From the analysis of gunshot residue in a potential murder, to the use
of recreational drugs in a household setting, this type of evidence can be found at a large
majority of crime scenes—if one looks hard enough.
Gunshot residue
Sodium rhodizonate
A colour producing test that
detects the presence of leads in any
form—for this test, we’re looking for
the presence of residue. When lead is
present, the sodium rhodizonate
solution changes to a deep, brown/red
colour. Most commonly used on
surfaces where a gun has likely been
fired through or near—like a small
impression or hole in a wall from a
suspected gun shot, or on the clothing Left: Sodium Rhodizonate reacting to the presence of
of a suspect or victim. lead, changing to a muddy red colour.
This technique has the Right: Diphenylamine/sulphuric acid reacting to the presence of
possibility of giving false positives due nitrates, the yellow/green solution becoming blue.
to the highly acute nature of analysis
used—it may pick up a metal unrelated to guns entirely, therefore, it is only used as a
presumptive spot test. However, it is very quick and easy to do.
Diphenylamine/sulphuric acid solution
A compound used in the detection of nitrates in
any form. The solution itself is colourless, though, due to
preservative techniques can appear to be “a light tan to
brown solid with a pleasant odor.” (Diphenylamine, 2020)
Forms a yellow/green colour—also producing a
blue mixture—in the presence of nitrates. Can also give
false positives, though is equally as quick and inexpensive
to do as a sodium rhodizonate test. This occurred during
the infamous shooting of Jill Dando, where a man was
arrested due to the presence of gunshot residue on his
clothing but was later revealed to be an avid hunter.
Additionally, spot tests were taken on individuals at
, random which also came back positive for gunshot residue—this was due to the long
distances that the residue can carry,
Drug analysis
Marquis reagent
A presumptive spot test that reacts differently—with a positive colour change—in the
presence of certain drugs with alkaline traces.
COLOUR CHANGES USING MARQUIS REAGENT
Cocaine Blue
Heroin Red/pink
Amphetamine Mustard yellow/brown
Ecstasy Yellow/green
Marquis reagent is very
useful when drug driving is
suspected as a spot test can be
completed on scene, producing
results very quickly. The majority
of tests also come in some form of
briefcase, so they are easy for a
forensic practitioner to have on
hand. However, when used at a
scene, it is possible for the
marquis reagent to react with
household items containing
alkaline traces such as bleach.
Alkaloid cuttings agents may also
Analysis of Cocaine, Heroin, Amphetamine, and Ecstasy to
affect the outcome.
compare against suspect samples 0001 and 0002.
A TLC
The four 'fake' drug samples analysed above, all producing the samesample
reaction from
as the agenuine
wine bottle
article. (Left)
against a sample from a recovered glass of
wine suspected of being spiked (Right).
