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Writeup for Unit 3: AC1.3
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Complete writeup for Unit 3 for AC1.3
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AC1.3 WriteupLocard’s exchange principle states that every time someone enters an environment,
something is added, and something is removed. The principle is sometimes stated as ‘every
contact leaves a trace’ and applies to contact between individuals as well as between an
individual and a physical environment. Scenes of crime officers always work on the principle
that physical evidence is left behind at every scene.
Physical evidence is tangible evidence that can be touched or held such as
weapons/hair/biohazards/fibres/etc, at the crime scene. This will mostly need
testing/sending to a forensic laboratory for processing.
One example of physical evidence is blood stains, these will mainly be found at the scenes of
aggravated assault, murder, rape, or other crimes where brutal physical contact has
occurred. Stains can be found anywhere, on the walls, floor, bedding, and clothing. If the
blood stains aren’t dry then it can be collected using something sterile such as a gauze, this
will then need to dry before it can be placed in an airtight container; however, if the stain is
dried then the material it is on will be cut and the section with the stain will be collected.
The stain must be transferred to the lab within the initial 24 hours of collection and should
be transferred from the crime scene in a scientific evidence van which will be completely
sterile to avoid any chance of cross contamination. Once in the lab, all samples must be
stored in an airtight container with labels to prevent confusion between specific samples,
the samples will all be in their own containers to prevent cross contamination. Before
anything is analysed in terms of DNA, a presumptive test kit must be performed in order to
decide whether the stain is really blood; next, a confirmation test will determine whether it
is from an animal or a human, and once all these variables have been confirmed, tests can
extract DNA and Raman spectroscopy can detect how long the stain has been there.
Personnel involved will mainly be CSI’s as they collect the blood stains with forensic
scientists analysing it.
Another example of physical evidence are hair samples, these are usually singular strands
which have accidentally been left at the scene of the crime while the crime was taking place,
the victim may have also fought against the offender and grabbed a fistful of hair which can
also be used for analysis. Hair samples are likely to be on clothing so the clothing should be
seized and given to the lab for easier transport; however, if loose strands are found then
these will be placed in paper. These samples should be transferred within 24 hours and will
be transferred via a scientific evidence police van. Once at the lab, if it is just loose hair
strands then these will be rewrapped in paper and packed in a bag, sealed, and labelled in
order to avoid confusion; if it is on a piece of clothing then the clothing will be packaged in a
paper bag similarly sealed and labelled. When it is ready to be analysed, the hair will be
inspected with a microscope and will be compared with hair from the suspect after it’s been
confirmed the hair is human. If the hair root is still attached, then DNA may be taken if it is
unclear. Pathologists may collect samples at the autopsy if the identity of the victim is
unknown. Otherwise, the CSI collect the samples for the forensic scientists to analyse.
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