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Unit 4 Assignment B - Analysis of Evidence (Biological)
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Analysing Forensic EvidenceUnit 4
Assignment B
Within this assignment, analytical techniques undertaken by forensic scientists and their
reliability and accuracy will be discussed and evaluated based on evidence collected from a
mock crime scene. Analytical techniques are used to determine facts surrounding a crime,
and find out what actually happened. A variety of physical, biological and chemical evidence
is used to do this. This report will discuss two biological pieces of evidence from a crime
scene and justify why the method was used, as well as disadvantages and advantages of
the methods, and the validity of the results obtained.
Biological Evidence
Biological evidence is important as it can help conclude things such as DNA, or fingerprinting
to be matched to a worldwide database.
Blood analysis - using Kastle Meyer
In our crime scene, blood was found located on the mock deceased body. It was packaged
into evidence-holding tubes after it had been swabbed - both a wet and dry sample had
been swabbed. Kastle-Meyer is a common presumptive test used - there are two more
presumptive tests that can be done alongside this.
The swabs are taken out of the evidence tubes, ensuring that the chain of custody is
followed (the date and time it is unsealed is marked on the evidence labels, as well as the
evidence reference number, and then what time is is repackaged), and phenolphthalein is
added to the swab - phenolphthalein is an indicator solution which is colourless - as well as
this, a drop of hydrogen peroxide is added. If the swab is in fact the compounds in the
solutions will begin to oxidise, and produce a pink stain. The faster the stain occurs, the
more conclusive it is that the presumed blood stain is in fact blood. The results aren’t fully
conclusive however, and more further testing may have to be done.
Other presumptive blood testing:
● Leucomalachite green - Leucomalachite green is a presumptive test that produces a
green/blue stain if blood is present. It is also tested using hydrogen peroxide, which
catalyses with haemoglobin. The sample can be placed onto a clean white cloth, and
a few drops can be added to the sample, 30 seconds-2 minutes is then waited until
the positive change in colour may occur. However, Leucomalachite green can also
be carcinogenic and toxic (increasing over the amount of time it is left after
developing) however, therefore in evaluation, presumptive testing such as Kastle-
Meyer can be used, as it has less health hazards than Leucomalachite. As well as
this, Leucomalachite green should not be used as a single way of determining if
blood is present - and false positives can be obtained if plant materials or substances
that contain the same compounds used in the reaction are on a sample.
● Luminol - Luminol is effective as it can help detect blood stains that have been long
cleaned away or diluted. It works by reacting with the iron found within haemoglobin,
, and produces a blue/green light on a substance. It can also produce a glowing effect
if sprayed onto an area. It is not a definitive test - further testing needs to be done on
a sample such as DNA analysis. As well as this, Luminol can produce false positives,
if it interacts with another sample containing iron - like cleaning supplies. Therefore,
using luminol can be unreliable and should not be the only method used.
Presumptive testing is used as it is cost/time effective at a crime scene. It means areas of a
crime scene can be narrowed down (such as in the case of a spray like luminol) to where
blood may be present, and blood can be identified before most costly analysis is done in the
swabs. They also produce a fast result, usually within 2 minutes of a test being done. This
means initial evidence can be analysed quickly and speed up the time that can be used
before a sample may start to break down and decompose. However, presumptive tests can
produce many false positives and are not conclusive tests, therefore, just because a test
says there is blood present - doesn’t mean there is blood present. They all need further
testing done to conclude that they are in fact blood samples.
Genetics Analysis
Genetics analysis is useful as it can help link the DNA of a suspect to the crime, this can be
used in things such as sexual assault cases, where semen is present, and is found to be a
pretty conclusive way of identifying a suspect.
When only a little of a sample is available, polymerase chain reactions can be used to
replicate the strands of DNA that pertain to genetics. It can be done with specific regions.
This may be done alongside another technique, called short tandem repeat analysis, which
compares the individual specific strands of DNA and compares potential suspects DNA to
the ones found at the crime scene. Using these two methods together makes a pretty
reliable case for conclusively identifying a suspect based off of their DNA, however having a
bigger sample initially may be better for the case.
Databases can also be used, which already contain the DNA of previous convicts and allows
for the potential suspect to be identified, then you can use the previous two methods to
confirm. If a suspect is male, Y-chromosome analysis can be done, this can be done on
things such as semen, and can also confirm a suspect was male. It can help even find out
paternity in some cases such as if a case goes to a family court.
Fingerprint analysis
Fingerprints are left due to oils and sweat on the fingertips, which leave a mark on a surface.
These marks are unique to every individual - this is important for a criminal case as they can
be matched to one specific individual using an international database (you can then also see
if an offender has offended before). Fingerprints can also be physical evidence - however
turn biological when lifted from a surface. Fingerprints can either be latent or patent - patent
can be seen without the need for analysis/extra help, compared to latent, which need
enhancing in order to be analysed or seen. The quality of a fingerprint can change, this can
depend on a variety of things such as pressure, the amount of time a surface was interacted
with, or other environmental conditions. There are different types of methods, using
powders, gentian violet, iodine fuming, superglue fuming and ninhydrin fuming.
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