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Fig. 3. Physical evidence. (a) Crushed vegetable; (b) Cis-delta-9tetrahydrocannabinol, the main psychoactive constituent of cannabis; and (c)
The detection with the Duquenois-Levine test, a screening test for marijuana [5].

When an unknown substance is submitted to a crime laboratory, the Forensic Chemist observes the properties of the
substance. As an example, in the presence of a crushed and
dried green-leafy material, the chemist can make hypothesis
that the unknown substance is marijuana. He devises a plan
to test the hypothesis: to view the substance under the microscope looking for properties of crushed marijuana leaves, and
if the microscopic observations validate the hypothesis, he
will make the reaction with the Duquenois-Levine reagent, a
widely used presumptive test for tetrahydrocannabinol and
other cannabinoids. The Duquenois-Levine reagent is a solution containing 2% vanillin and 1% ethanal, and after the
addition of concentrated hydrochloric acid and chloroform,
the chloroform layer becomes red purple if the drug is present,
as shown in Fig. 3. If the result is negative, the Forensic Chemist makes another hypothesis, such as the unknown substance
could be oregano, and tests this hypothesis with a proper analysis. If the hypothesis is not verified, the Forensic Chemist goes
on with another hypothesis and so on, until a positive match
is obtained.
Over the past decade, new and more dangerous synthetic
drugs entered the market. Their identification is a noteworthy
challenge because their chemical composition is continuously
altered by producers in the attempt to circumvent the law and
to avoid prosecution.

The Important Case of Blood Stains and
Urine Analysis
Forensic Chemistry considers a wide range of evidence, and
one of the most important and frequently performed types of
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analysis is the search for traces of drugs or poisons in samples
of blood stains or urine [6]. Considerable effort is spent on drug
screening of people, as employees or athletes for example, to
detect the presence of performance-enhancing drugs. In this
case, the role of Forensic Chemists is to discriminate, by means
of accurate and cost-effective tests, between the external addition of illegal drugs and metabolites coming from foods like
poppy seeds or from drugs intentionally taken at the suggestion of a doctor. Moreover, from a scientific point of view, drug
metabolites exist in hair as the result of complex processes that
probably include combination of incorporation and metabolism in hair/hair bulb.
These tests may be simple color tests or may involve different instrumental techniques more or less complex [7]. Paper or
thin-layer chromatography, gas chromatography, or electrophoresis may be employed for the analysis of different toxins.
In cases of deaths from unknown causes, samples of blood,
lungs, urine, stomach contents and vitreous humor of the victims are examined for traces of poisons. Furthermore, insects
found on dead bodies are another source of information, because they not only may absorb drugs or poisons from the
body in trace amounts but maintain them even when the concentrations in the body have decreased.
From a single drop of blood, by analyzing the chemical
compounds as well as measurable morphological characteristics of the resulting bloodstains, interesting information about
the source of bleeding, impact surface and mechanisms of the
formation of bloodstains can be obtained.
Long before DNA analysis, blood samples were collected
and analyzed in crime laboratories with tests based on peroxidase, an enzyme found in blood, that acts as a catalyst for
the reagent added to the blood and gives a characteristic color
to the solution of dried blood and water: blue for the addition
of benzidine or pink for the addition of phenolphthalein, as
shown in Fig. 4.
An immunochromatographic procedure for the qualitative
indication of human blood is intended for selective use when
human origin of suspected bloodstains is questionable and/or
when the determination of the indication of human origin is
time sensitive. Limited sample size may preclude the use of
this test to allow future DNA testing.
More specific tests are then applied to determine if the
blood is human. The evidence available through blood typing
is not as convincing as genetic fingerprinting, but it can readily
prove innocence or increase the probability of a defendant being guilty. All humans belong to one of four blood groups (A,
B, AB, O), based on genetically determined antigens (A and/or
B) that may be attached to the red blood cells. These antigens
may be present or absent in blood, and by adding specific antibodies (anti-A or anti-B), the presence or absence of A and B
antigens can be determined.
If a person accused of a homicide has type AB blood and
it matches the type found at the crime scene of a victim, the
evidence for guilt is more convincing than if a match was
found for type O blood, because only 4% of the population
has type AB blood. The higher the number of rare factors in

IEEE Instrumentation & Measurement Magazine

February 2021

Instrumentation & Measurement Magazine 24-1

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